Tunnel Engineering

20 Lars Christian F. Ingerslev, Arthur G. BendeliusParsons BrinckerhoffNew York, New York

TUNNEL ENGINEERING

Tunnel engineering makes possible manyvital underwater and undergroundfacilities. Unique design and construc-tion techniques are involved because of

the necessity of protecting the constructors andusers of these facilities from alien environments.These facilities must be built to exclude thematerials through which they pass, includingwater. Often, they have to withstand highpressures. And when used for transportation orhuman occupancy, tunnels must provide adequatelighting and a safe atmosphere, with means forremoving pollutants.

Tunnels are constructed using many methods,depending upon the kind of soil and/or rockthrough which they will pass, their size, how deepthey need to be, and the obstructions that may beencountered along the route. These methodsinclude cut-and-cover construction, drill and blast,tunnel boring machine (TBM), immersion ofprefabricated tunnels, and sequential excavationmethods (SEM). More specializedmethods, such asground freezing and tunnel jacking, are used lessfrequently and often under very difficult con-ditions. Compressed air working has becomeuneconomical because of working hour restric-tions, time for decompression that results fromhigh working pressures (over 40 psi is notunusual), union labor agreements for work undercompressed air, and high workmen’s compen-sation and health benefit rates. Occasional entryunder compressed air may still be required, such asto clear obstructions ahead of a tunnel boringmachine, or to perform essential maintenance onparts of such a machine.

The design approach to underground andunderwater structures differs from that of mostother structures. Internal space, design life, and

other requirements for the tunnel must first bedefined. Geological and environmental data mustthen be collected. Critical design loading con-ditions must then be established, includingacceptable conditions of the tunnel followingextreme events (for example, how long before thetunnel is reusable). Appropriate constructionmethods are then evaluated to determine the mostappropriate to meet the established criteria,conditions, and cost. The methods under consider-ation should include both temporary and perma-nent excavation support systems as well as thestructures itself. Design standards and codes ofpractice apply primarily to above-ground struc-tures, so that care should be used in theirapplication to underground and underwaterstructures.

20.1 Glossary

Adit.A short, transverse tunnel between paralleltunnels or to the face of the slope in a sidehilltunnel.

Air Lock. A compartment in which air pressurecan be varied between that of the compressed airused in shield tunneling and that of the outside air,to permit passage of workers or material.

Bench. Top of part of a tunnel section, withhorizontal or nearly horizontal upper surface,temporarily left unexcavated.

Blowout. A sudden loss of a large amount ofcompressed air at the top of a tunnel shield.

Breast Boards. Timber planks to hold the face oftunnel excavation in loose soil.

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Source: Standard Handbook for Civil Engineers

Dry Packing. Filling a void with a stiff mortar,placed in small increments, each rammed intoplace.

Evase Stack. An air-exhaust stack with a crosssection increasing in the direction of air flow at arate to regain pressure.

Face. The surface at the head of a tunnelexcavation. A mixed face is a condition with morethan one type of material, such as clay, sand, gravel,cobbles or rock.

Grommet. A ring of compressible materialinserted under the head and nut of a boltconnecting tunnel liners to seal the bolt hole.

Heading. A small tunnel, or tunnels, excavatedwithin a large tunnel cross section which will beenlarged to the full section.

Jumbo. A frame that rolls on tracks or rubberwheels and carries drills for excavation of rocktunnels.

Lagging. Timber planks or steel plates insertedabove tunnel-supporting ribs to hold back rocksor soil.

Liner Plate. A steel segment to support theinterior of a tunnel excavation.

Lining. A temporary or permanent structuremade of concrete or other materials to secure andfinish the tunnel interior or to support anexcavation

Mucking. Removal of excavated or blastedmaterial from face of tunnel.

Pilot Tunnel. A small tunnel excavated over partor the entire length to explore geological conditionsand assist in final excavation.

Pioneer Bore. (See Pilot Tunnel.)

Poling Boards. Timber planks driven into softsoil, over timber supports, to hold back materialduring excavation.

Scaling. Removal of loose rocks from tunnelsurface after blasting.

Shield. A steel cylinder of diameter equal to thatof the tunnel, for excavation of tunnels in softmaterial to provide support at the face of the tunnel,to provide space for erecting supports, and toprotect workers excavating and erecting supports.

Spiling. (See Poling Boards.)

20.2 Clearances for Tunnels

Clearance in a tunnel is the least distance betweenthe inner surfaces of the tunnel necessary toprovide space between the closest approach ofvehicles or their cargo or pedestrian traffic andthose surfaces. Minimum tunnel dimensions aredetermined by adding the minimum clearancesestablished for a tunnel to the dimensions selectedfor the type of traffic to be accommodated in thetunnel and the space needed for other require-ments, such as ventilation ducts and pipelines.

Clearances for Railroad Tunnels n Indi-vidual railroads have different standards to suittheir equipment. But on tangent tracks, clearancesfor single- and double-track tunnels should not beless than those shown in Fig. 20.1. (Clearancesshown are those in the “AREMA Manual”American Railway Engineering and Maintenance-of-Way Association, 8201 Corporate Drive, Suite1125, Landover, MD 20785, (www.AREMA.org).

In rail tunnels, clearances for personnel arerequired on both sides where niches are notprovided. These clearances should be at least 6 ft8 in or 2 m high and 30 in wide each side of thevehicle clearance diagram, although a 24-inminimum is permitted on some lines. In highwaytunnels, a 3 ft or 0.9 m clearance from face of curbis used where walkways are provided. In bothroad and rail tunnels, it is common practice toprovide a walkway along the common wallbetween adjacent ducts to facilitate emergencyevacuation between ducts and to prevent peoplefrom emerging directly into the path of oncomingtraffic.

On curved tracks, the clearances should beincreased to allow for overhang and tilting of an 85-ft-long car, 60 ft c to c of trucks, and a height of 15 ft1 in above top of rail. (Distance from top of rails totop of ties should be taken as 8 in.)

The track should be superelevated at curvesaccording to AREMA standards.

Clearances for pantograph, third-rail, or caten-ary construction should conform to diagramspublished by the Electrical Section, EngineeringDivision of the Association of American Rail-roads.

The latest clearance standards of AREMAshould be checked for new construction. Locallegal requirements should govern if they exceedthese standards.

20.2 n Section Twenty



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Circular tunnels should be fitted to the clearancediagrams, with such modifications as may bepermissible.

Clearances for Rapid-Transit Tunnels n

There are no general standards for clearances in

rapid-transit tunnels. Requirements vary with sizeof rolling stock used in the system.

Figure 20.2 shows the normal-clearance dia-gram of the New York City BMT and IND Division267 ft cars. Figure 20.3 gives the clearancesestablished for the San Francisco Bay Area Rapid

Fig. 20.1 Clearances specified by AREMA for railway tunnels on a tangent.

Tunnel Engineering n 20.3



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Transit System, which has cars 10 ft wide and 75 ftlong on a 5-ft 6-in gage track. The clearances allownot only for overhang of cars, tilting due tosuperelevation, and sway, but for a broken springor defective car suspension.

Clearances for Highway Tunnels n TheAmerican Association of State Highway andTransportation Officials (AASHTO) has establishedstandard horizontal and vertical clearances forvarious classes of highways. These have beenmodified and expanded for the Interstate HighwaySystem under the jurisdiction of the FederalHighway Authority (FHWA) (Fig. 20.4).

For rural and most urban parts of the InterstateHighway System, a 16-ft vertical clearance isrequired.

Since construction costs of tunnels are high,clearance requirements are usually somewhatreduced. Although some older 2-lane tunnels haveused roadway widths of 21 ft between curbs forunidirectional traffic and 23 ft for bi-directionaltraffic, usually with speed restrictions, these widthsno longer meet current standards for 12 ft or 3.6 mlanes. Full width shoulders are rarely provided dueto cost, but at least an additional 1 ft is providedadjacent to each curb. Wider shoulders or sightshelves may be required around horizontal curvesto comply with sight distance requirements. Aminimum distance between walls of 30 ft is acommon requirement. Resurfacing within tunnelsis rarely permitted without first removing the oldsurfacing, so no allowance for resurfacing isrequired for overhead clearance. It is usual in

Fig. 20.2 Clearance diagram for 670 car (BMT & IND Divisions). New York City Subway System.




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tunnels to provide overhead lane signals to showwhich lanes are open to traffic in the direction oftravel, so extra overhead allowance is required forthese, and when appropriate also for lighting,

overhead signs, jet fans for ventilation, and anyother ceiling-mounted items. Minimum overheadtraffic clearances depend upon which alternativeroutes are available for over-height vehicles andthe classification of the highway, but acceptedvalues usually lie between 14 ft and 5.1 m.Additional height may be required on verticalcurves to allow for long trucks. Additional spacemay be required for ventilation, ventilation equip-ment, and ventilation ducts.

20.3 Alignment and Gradesfor Tunnels

Alignment of a tunnel, both horizontal and vertical,generally consists of straight lines connected bycurves. Minimum grades are established to ensureadequate drainage. Maximum grades depend on

Fig. 20.3 Clearance diagram for San Francisco Bay Area Rapid Transit System.

Fig. 20.4 Clearance diagram for interstate high-way tunnels.




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the purpose of the tunnel. Construction of a tunnelin the upgrade direction is preferred wheneverpossible, since this permits water to drain awayfrom the face under construction.

Alignment and Grades for RailroadTunnels n Straight alignments and grades as lowas possible, yet providing good drainage, aredesirable for train operation. But overall construc-tion costs must be taken into account.

Grades in curved tunnels should be compen-sated for curvature, as is done for open lines. Ingeneral, maximum grades in tunnels should notexceed about 75% of the ruling grade of the line.This grade should be extended about 3000 ft belowand 1000 ft above the tunnel.

Short (under 2500 ft), unventilated tunnelsshould have a constant grade throughout. Long,ventilated tunnels may require a high point nearthe center for better drainage during construction ifwork starts from two headings.

Radii of curves and superelevation of tracks aregoverned by maximum train speeds (Art. 19.9).

Alignment and Grades for Rapid-TransitTunnels n Radii of curvature and limiting gradesare governed by operating requirements. TheNew York City IND Subway has a 350-ft minimumradius, with transition curves for radii below2300 ft. Maximum grades for this system are 3%between stations and 1.5% for turnouts andcrossovers. The San Francisco BART system isdesigned for train speeds of 80 mi/h. Relation ofspeed to radius and superelevation of track forhorizontal curves is determined by

E ¼ 4:65V2

R�U (20:1)

where E ¼ superelevation, in

R ¼ radius, ft

V ¼ train speed, mi/h

U ¼ unbalanced superelevation, whichshould not exceed 23⁄4 in optimum or4 in as an absolute maximum

For 80 mi/h design speed, the radius with anoptimum superelevation would be 5000 ft. For amaximum permissible superelevation of 81⁄4 in, aminimum radius of 3600 ft would be required. Theabsolute minimum radius for yards and turnouts is

500 ft. Maximum line grade is 3.0% and 1.0% instations. To ensure good drainage, grade shouldpreferably be not less than 0.50%.

Alignment and Grades for HighwayTunnels n For tunnels under navigable watercarrying heavy traffic, upgrades are generallylimited to 3.5%; downgrades of 4% are acceptable.For lighter traffic volumes, grades up to 5% havebeen used for economy’s sake. Between governingnavigation clearances, grades are reduced to aminimum adequate for drainage, preferably notless than 0.25% longitudinally and a cross slope of1.0%. For long rock tunnels with two-way traffic, amaximum grade of 3% is desirable to maintainreasonable truck speeds. Additional climbing lanesfor slower traffic may be required when gradesexceed 4%.

Radii of curvature should match tunnel designspeeds. Short radii require superelevation andsome widening of roadway to provide foroverhang and sight distance.

20.4 Pavements andEquipment forHighway Tunnels

Roadway base is a reinforced concrete slab; on thisis placed a renewable pavement. Well-designedbitumastic concrete has given good service and hasgood riding qualities.

Average daily traffic capacity of a two-lane two-directional tunnel is about 20,000 vehicles with amaximum of 1200 to 1500 vehicles per lane perhour. For single-direction traffic in both lanes,capacities are 10 to 15% higher.

Red-amber-green traffic lights are installed atabout 1000-ft intervals, or at such spacing that thedriver always sees at least one light. Telephones areplaced in recesses about 500 ft apart for service andemergency calls.

Most tunnels, particularly those under water,are equipped with fire mains and hose outletsevery 300 ft. Booster pumps in ventilation build-ings raise supply pressure to 120 psi for use offoam. Fire extinguishers are mounted in recesses ofhose outlets. Fire-alarm stations and phones are atthe same locations. Emergency trucks with heavyhoists, fire hose, foam equipment, and emergencytools are kept in readiness at each portal.




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20.5 PreliminaryInvestigations

Surveys should be made to establish all topogra-phical features and locate all surface and subsur-face structures that may be affected by the tunnelconstruction. For underwater tunnels, soundingsshould be made to plot the bed levels.

Knowledge of geological conditions is essentialfor all tunnel construction but is of primaryimportance for rock tunnels. Explorations byborings and seismic reflection for soft ground andunderwater tunnels are readily made to the extentnecessary. For rock tunnels, particularly long ones,however, possibilities for borings are often limited.A thorough investigation should be made by ageologist familiar with the area. This study shouldbe based on a careful surface investigation andexamination of all available records, includingrecords of other construction in the vicinity, such asprevious tunnels, mines, quarries, open cuts,shafts, and borings. The geologist should preparea comprehensive report for the guidance ofdesigners and contractors.

For soft ground and underwater tunnels,borings should be made at regular intervals. Theyshould be spaced 500 to 1000 ft apart, dependingon local conditions. Closer spacing should be usedin areas of special construction, such as ventilationbuildings, portals, and cut-and-cover sections.Spoon samples should be taken for soil classifi-cation, and undisturbed samples, where possible,for laboratory testing. Samples not needed in thelaboratory, boring logs, and laboratory reportsshould be preserved for inspection by contractors.Density, shear and compressive strength, andplasticity of soils are of special interest.

All borings should be carried below tunnelinvert. For pressure face tunnels, borings should belocated outside the tunnel cross section.

For rock tunnels, as many borings as practicableshould be made. Holes may be inclined, to cut asmany layers as possible. Holes should be carriedbelow the invert and may be staggered on eitherside of the center line, but preferably outside thetunnel cross section to prevent annoying waterleaks. Where formations striking across the tunnelhave steep dips, horizontal borings may give moreinformation; borings 2000 ft in length are notuncommon. All cores should be carefully catalogedand preserved for future inspection. The ratio ofcore recovery to core length, called the rock quality

designation (RQD), is an indicator of rock problemsto be encountered.

Groundwater levels should be logged in allborings. Presence of any noxious, explosive, orother gases should be noted.

Where lowering of groundwater may beemployed during construction of cut-and-cover orbored tunnels on land, the permeability of theground should be tested by pumping tests in deepwells at selected locations. Rate of pumping anddrawdown checked in observation wells at variousdistances should be recorded; as well as recovery ofthe water level after stopping the pumps.

Geophysical exploration to determineelevations of distinctive layers of soil or rocksurfaces, density, and elastic constants of soil maybe used for preliminary investigations. The find-ings should be verified by a complete boringprogram before final design and construction.

20.6 Tunnel Ventilation

Tunnels will be required to be ventilated to diluteor remove contaminants, control temperature,improve visibility and to control smoke and heatedgases in the event of a fire in the tunnel.

20.6.1 Ventilation Requirementsfor Construction

Occupational Safety and Health Administration(OSHA) establishes standards, regulations, andprocedures necessary to maintain safe, sanitaryconditions for all workers on construction sites.Employers are required to initiate and maintainprograms that will prevent accidents. Also,employers are advised to avail themselves of safetyand health programs provided by OSHA and arerequired to instruct and train employees torecognize and avoid unsafe, unsanitary conditions,including prevention and spread of fires. OSHArequirements also cover underground construc-tion. Following are some of the requirementsapplicable to ventilation.

Fresh air should be supplied to all undergroundwork areas in sufficient quantities to preventdangerous or harmful accumulation of dusts,fumes, mists, vapors, or gases. Unless naturalventilation meets this requirement, mechanicalventilation should be supplied. At least, 200 ft3 of




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fresh air should be provided for each employeeunderground. The air flow should be at least 30 ft/min where blasting or rock drilling is conducted orwhere polluted air is likely to be present ordeveloped. The direction of air flow should bereversible. After blasting, smoke and fumes shouldbe immediately exhausted to outdoors before workis resumed in affected areas.

Underground operations are classified as gassyif air monitoring discloses for three consecutivedays 10% or more of the lower explosive limit formethane or other flammable gases, measuredabout 12 in from work-area enclosure surfaces.Where such conditions occur, operations etherthan those necessary for correcting the conditionsshould be discontinued. Ventilation systems shouldbe made of fire-resistant materials. Controlsfor reversing air flow should be located aboveground.

At normal atmospheric pressure underground,the air should contain at least 19.5% but not morethan 22% oxygen. Test should be made frequentlyfirst for oxygen, then for carbon monoxide,nitrogen dioxide, hydrogen sulfide, and otherpollutants. If hydrogen sulfide concentrationreaches 20 ppm or 20% or more of the lowerexplosive limit for flammable gases is detected,precautions should be taken to protect or evacuatepersonnel.

Mobile diesel-powered equipment used under-ground in atmospheres other than gassy operationsmust either be approved byMSHA or the employermust demonstrate that it is fully equivalent to suchMSHA-approved equipment. (30 CFR Part 32MSHA).

For construction in compressed air, seeArt. 20.16.

[“Construction Industry: OSHA Standards forthe Construction Industry (29 CFR 1926/1910),”Superintendent of Documents, Government Print-ing Office, Washington, DC 20402 (www.gpo.gov)].

20.6.2 Ventilation for Railroadand Rapid-Transit Tunnels

Short tunnels generally have no forced ventilation.Longer tunnels for diesel trains may needventilation to purge smoke and exhaust gases.Tunnels for electric traction are adequately self-ventilated by piston action but may requireemergency ventilation.

A ventilation system dilutes and purges smokeand combustion and exhaust gases. Its capacitymust be adequate to prevent irritating smoke or gasconcentrations while a train passes through and toclear the air between train passages. Diesel contentof nitrogen oxides may form corrosive acids inlungs when inhaled for long periods. The followingsystems are used by American railroads:

Injecting a stream of air at high velocity in thedirection of train movement to keep smoke aheadof the train.

Injecting a high-speed high-volume air streamfrom the opposite end against the train motion todilute smoke and clear the tunnel.

Addition of portal doors with the first injectionsystem, to increase efficiency and prevent backflowin case of a stalled train. Doors are interlocked withsignal systems (Moffat Tunnel).

Because of absence of smoke or exhaust gaswhen electric traction is used, ventilation bypiston action of trains is adequate for tunnels forelectric trains except under emergency conditions.Auxiliary exhaust fans should be installed toremove smoke in case of fire and to draw fresh airinto the tunnel from the stations or portals. Fansmay be installed in exhaust shafts betweenstations or in separate ventilation buildings inlong underwater tunnels equipped with exhaustducts. High-speed rapid-transit tunnels requireair-relief shafts ahead of stations to prevent airblasts from entering the stations. In hot climates,heat dissipation in tunnels and stations requiresspecial ventilation capacity and air conditioning.A computer program, the Subway EnvironmentSimulation (SES), for system design has beendeveloped. (“Subway Environmental DesignHandbook,” Urban Transportation Adminis-tration, Washington, DC 20590.)

20.6.3 Emission Contaminantsin Road Tunnels

Exhaust gases of gasoline internal combustionengines contain deadly carbon monoxide andirritating smoke and oil vapors. Diesel engineswill also produce dangerous nitrogen oxides andaldehydes. The components of exhaust gases varyover a wide range.




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The ventilation system also must be capable ofcontrolling smoke and hot gases in case of fire (seeVentilation Systems for Road Tunnels following).

The Federal government or health authorities ofstates place restrictions on permissible carbonmonoxide (CO) content. With new standardslimiting contaminants in vehicle exhaust gases,however, it may eventually be possible to meet theCO limitations without extensive increase inventilation. Engineers should check current rulesat time of design.

Haze from vehicle exhaust gases, particularlyfrom Diesel engined vehicles, does reduce visibilityin the tunnel. In practice, when the CO level withinthe tunnel is maintained at levels as proposed inTable 20.1, adequate dilution of the irritating partsof exhaust gases and adequate visibility is assured.

New road tunnels built in the United Statesmust comply with the time-weighted limits forconcentration of CO established by the U.S.Environmental Protection Agency and the FederalHighway Administration. These limits are listed inTable 20.1.

Other countries may set other standards forcarbon monoxide (CO) concentrations within theirtunnels. The World Road Association (PIARC)publications provide documentation on thesubject.

For tunnels in which traffic may incorporate ahigh percentage (10% or more) of diesel vehicles,the ventilation requirements for dilution of NOx

particles of nitrogen and particulates (smoke)become significant. The NOx emitted by vehiclesconsists mainly of nitric oxide (NO), whichoxidizes in the atmosphere to form nitrogendioxide (NO2). Based on exposure limits rec-ommended by the American Conference ofGovernmental Industrial Hygienists and a typical4-to-1 ratio for NO to NO2, the maximumpermissible concentration of NOx is about 10 ppm.

Carbon Monoxide (CO) has been proven to bethe umbrella pollutant in most road tunnels. Thatis, when the CO level in any given road tunnel ismaintained at or below the levels shown in Table20.1, all of the other vehicle pollutants will bewithin appropriate levels. The only exception tothis is the case of particulate matter emitted byDiesel engined vehicles when the tunnel trafficstream contains on an average more than 15%Diesel engined vehicles.

The current method of determining the vehicleemissions to be considered for a road tunnelventilation system design is to apply the UnitedStated Environmental Protection Agency’s MOBILseries of computer programs. Mobil5B is thecurrent version in use today.

20.6.4 Ventilation Systems forRoad Tunnels

In straight tunnels up to about 1000 ft in length,natural air flow is usually sufficient, particularlywith traffic in one direction. If a tunnel is exposedto heavy traffic congestion at times, installation ofexhaust fans in a shaft or adit near the center foremergency ventilation is advisable if the lengthexceeds 500 ft.

Natural Ventilation n Naturally ventilatedtunnels rely primarily on atmospheric conditionsto maintain airflow and a satisfactory environmentin the tunnel. The piston effect of traffic providesadditional airflow when the traffic is moving.Naturally ventilated tunnels over 1,000 feet (305meters) long require emergency mechanical venti-lation to extract smoke and hot gases generatedduring a fire as defined by NFPA 502 “Standard forRoad Tunnels, Bridges, and Other Limited AccessHighways”. Tunnels with lengths between 800 and1,000 feet (240 and 305 meters) will require theperformance of an engineering analysis to deter-mine the need for emergency ventilation. Becauseof the uncertainties of natural ventilation,especially the effect of adverse meteorological andoperating conditions, reliance on natural venti-lation, to maintain carbon monoxide (CO) levels,for tunnels over 800 ft (240 m) long should bethoroughly evaluated. If the natural ventilation isdemonstrated to be inadequate, the installation of amechanical system with fans should be consideredfor normal operations.

Table 20.1 Limits on CO in Road Tunnels

Exposure time,min

Maximum COconcentration, ppm

0–15 12016–30 6531–41 4546–60 35




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Smoke from a fire in a tunnel with only naturalventilation moves up the grade driven primarily bythe buoyant effect of the hot smoke and gases. Thesteeper the grade the faster the smoke will movethus restricting the ability of motorists trappedbetween the incident and the portal at the higherelevation to evacuate the tunnel safely.

Mechanical Ventilation n A tunnel that issufficiently long, has heavy traffic flow, orexperiences adverse atmospheric conditionsrequires mechanical ventilation with fans. Mech-anical ventilation layouts in road tunnels are eitherof the longitudinal or transverse type.

Longitudinal Ventilation n This type ofventilation introduces or removes air from thetunnel at a limited number of points, thus creatinga longitudinal flow of air along the roadway.Ventilation is either by injection, or by jet fans.

Injection Longitudinal Ventilation is fre-quently used in rail tunnels and is also found inroad tunnels. Air injected at one end of the tunnelmixes with air brought in by the piston effect of theincoming traffic. This type of ventilation is mosteffective where traffic is unidirectional. The airspeed remains uniform throughout the tunnel, andthe concentration of contaminants increases fromzero at the entrance to a maximum at the exit.Injection longitudinal ventilation with the supplyat a limited number of locations in the tunnelis economical because it requires the least num-ber of fans, places the least operating burdenon these fans, and requires no distribution airducts.

Jet Fan Longitudinal Ventilation has beeninstalled in a significant number of tunnels world-wide. Longitudinal ventilation is achieved withspecially designed axial fans (jet fans) mounted atthe tunnel ceiling. Such a system eliminates thespaces needed to house ventilation fans in aseparate structure or ventilation building; however,it may require a tunnel of greater height or width toaccommodate jet fans so that they are out of thetunnel’s dynamic clearance envelope. This envel-ope, formed by the vertical and horizontal planessurrounding the roadway pavement in a tunnel,define the maximum limits of predicted verticaland lateral movement of vehicles traveling on theroad at design speed. As the length of the tunnelincreases, however, the disadvantages of longi-

tudinal systems, such as excessive air speed in theroadway and smoke being drawn the entire lengthof the roadway during an emergency, becomeapparent.

The longitudinal form of ventilation is the mosteffective method of smoke control in a road tunnelwith unidirectional traffic as was determined in theMemorial Tunnel Fire Ventilation Test Program. Alongitudinal ventilation system must generatesufficient longitudinal air velocity to prevent thebacklayering of smoke. Backlayering is the move-ment of smoke and hot gases contrary to thedirection of the ventilation airflow in the tunnelroadway. The air velocity necessary to preventbacklayering of smoke over the stalled motorvehicles is the minimum velocity needed for smokecontrol in a longitudinal ventilation system and isknown as the critical velocity.

Transverse Ventilation n Transverse venti-lation includes systems that distribute supply airand collect exhaust air uniformly along the lengthof the tunnel. There are several such systemsincluding the full transverse system whichincludes both supply and exhaust air uniformlydistributed and collected. The semi- or partialtransverse systems incorporate only one, eithersupply or exhaust air.

Semi transverse ventilation can be configuredas either a supply system or an exhaust system.Semi transverse ventilation is normally used intunnels up to about 7,000 feet (2,000 meters);beyond that length the tunnel air velocity speednear the portals may become excessive.

Supply semi transverse ventilation applied toa tunnel with bi-directional traffic produces auniform level of contaminants throughout thetunnel because the air and the vehicle exhaustgases enter the roadway area at the same uniformrate. In a tunnel with unidirectional traffic,additional airflow is generated in the roadway bythe movement of the vehicles, thus reducing thecontaminant level in portions of the tunnel.

Because the tunnel airflow is fan-generated, thistype of ventilation is not adversely affected byatmospheric conditions. The supply air travels thelength of the tunnel in a tunnel duct fitted withsupply outlets spaced at predetermined distances.If a fire occurs in the tunnel, the supply air initiallydilutes the smoke, which was shown in theMemorial Tunnel Fire Ventilation Test Program tobe an ineffective method for controlling smoke




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from larger fires. Supply semi transverse venti-lation should be operated in a reversed mode forthe emergency so that fresh air enters the tunnelthrough the portals to create a longitudinal flowof air equivalent to the critical velocity. It alsoprovides a tenable environment for fire-fightingefforts and emergency egress.

Exhaust semi transverse ventilation installed ina unidirectional tunnel produces a maximumcontaminant concentration at the exit portal. In abi-directional tunnel, the maximum level of con-taminants is located near the center of the tunnel.

In a fire emergency both the exhaust semitransverse ventilation system and the reversedsemi transverse supply system create a longitudi-nal air velocity in the tunnel roadway thusextracting smoke and hot gases uniformly alongthe tunnel length.

Full Transverse Ventilation n Full trans-verse ventilation has been used in extremely longtunnels and in tunnels with heavy traffic volume.Full transverse ventilation includes both a supplyduct and an exhaust duct to achieve uniformdistribution of supply air and uniform collection ofvitiated air throughout the tunnel length. During afire emergency the exhaust system in the incidentzone should be operated at the highest availablecapacity while the supply system in the adjacentincident zone is operated. This mode of operationcreates a longitudinal airflow (achieving the criticalvelocity) towards the incident zone and allows thesmoke and heated gases to be extracted as close aspossible to the fire and keep the upstream stoppedtraffic clear of smoke.

Other Ventilation Systems n There aremany variations and combinations of the systemsdescribed previously. Most of the hybrid systemsare configured to solve a particular problem facedin the development and planning of the specifictunnel, such as excessive air contaminants exitingat the portal(s).

Ventilation System Enhancements n Afew enhancements are available for the systemsdescribed previously. The twomajor enhancementsare single point extraction and oversized exhaustports.

Single point extraction is an enhancement to atransverse system that adds large openings to the

exhaust duct. These openings include devices thatcan be operated during a fire emergency to extracta large volume of smoke as close to the fire sourceas possible. Tests conducted as a part of theMemorial Tunnel Fire Ventilation Test Programconcluded that this concept is extremely effectivein reducing the temperature and smoke in thetunnel. The size of openings tested ranged from 100to 300 ft2 (9.3 to 28 m2).

Oversized exhaust ports are simply an expan-sion of the standard exhaust port installed in theexhaust duct of a transverse or semi-transverseventilation system. Two methods are used to createsuch a configuration. One is to install on each portexpansion a damper with a fusible link; the otheruses a material that when heated to a specifictemperature melts and opens the airway. Severaltests of such meltable material were conducted aspart of the Memorial Tunnel Fire Ventilation TestProgram but with limited success.

20.6.5 Elements of Road TunnelVentilation Systems

Major components of ventilation systems com-monly used for road tunnels are described in thefollowing.

Ventilation Buildings (Figs. 20.5 and20.6) n Fans, electrical transformers and switch-gear, control board, and auxiliary equipment arehoused in ventilation buildings. In short- andmedium-length tunnels, one building at eitherportal is sufficient. Longer tunnels should have abuilding at each portal. A few of the longest havethree or four buildings. For underwater tunnels,ventilation buildings may be at the water’s edge,each building controlling a land and a river sectionof the tunnel.

Fresh air is taken in through large louver areasin the walls of the building. The louvers should beprotected by bird screens. Louvers are usuallyaluminum and arranged for shedding water.Adequate drains should be provided in the fanroom to remove rainwater, which may blow inthrough the louvers. Vitiated air is dischargedthrough vertical stacks, which also should becovered by screens.

Tunnel Ducts are usually of constant areathroughout their length. Concrete surfaces shouldbe smooth for minimum friction. Obstructions,




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such as ceiling hangers, should be streamlined or atleast rounded. Turns in ducts and shafts leading tothe tunnel should be equipped with noncorrosiveturning vanes for smooth air flow.

Flues spaced about 15 ft apart, extended fromthe ducts, supply fresh air slightly above roadwaylevel. Ceiling ports are slanted at 458 in thedirection of air flow in the ducts. All air openingsshould provide the means to adjust size to balancethe air flow over the length of the tunnel.

Fans n Two types of fans are available:centrifugal fans, used in all tunnels up to about

1938, and vane axial fans, a later development.Centrifugal fans have backward-curved blades andare nonoverloading. The efficiencies of well-designed fans of either type are about the same.For underwater tunnels, with vertical air shafts inthe ventilation buildings, the vane axial fansrequire considerably less space and avoid theefficiency loss through the fan chambers usuallyassociated with centrifugal fans. Blades for vaneaxial fans may have a fixed pitch or may beadjustable during operation. When reversed, theformer type provides 80% of maximum capacity.The latter type may be adjusted from 0 to 100% ofcapacity for supply and exhaust, thus permitting

Fig. 20.5 Sections through Hampton Roads Tunnel Ventilation Building. (a) Fresh-air supplysystem.




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adjustments to meet variable demands for venti-lation with fewer fans. The noise level of vaneaxial fans at maximum speed is somewhathigher than that for centrifugal fans because ofgreater tip speed. In sensitive surroundings, thenoise from supply and exhaust fans can bedampened by sound baffles. Vane axial fans mayhave external drives or motors built into the hub ofthe impellers.

Centrifugal fans are operated by squirrel-cagemotors through chain or multiple V-belt drives.The latter eliminate lubrication problems and wearon a multiplicity of parts (inherent in chain drives),give excellent service, and can be easily replaced.

Chains are enclosed in solid housings; belts areprotected by wire guards.

For flexibility, the load is divided betweenseveral fans—at least two, sometimes as many assix—for each system. Four is a good number fordemands exceeding about 600,000 ft3/min.

To further adjust supply to variable demand, fanmotors are equipped with two-speed windings.Three speeds with two motors have been used inearlier installations but are not necessary with anadequate number of fans. Spare fans may beprovided as protection against breakdown, or totalfan capacity may be increased by 10 or 15%. Withgood maintenance, fans are seldom out of

Fig. 20.5 (Continued) (b) Exhaust-air system.




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commission, and the extra capacity of the system issufficient to maintain acceptable conditions forlimited periods with one unit out of service.

To protect the exhaust fans in case of a seriousfire in the tunnel, automatic deluge sprinklersystems should be installed to cool the exhaust air.

Dampers n All fans should be equipped withshutoff dampers to prevent short circuiting of air.Their operating motors should be interlocked withthe control of the fan motors for automatic openingand closing. Trapdoor-type or multiblade dampers

are in use; the latter take less space and time tooperate.

Fan Control n In short, unattended tunnels,fans can be controlled automatically with carbonmonoxide analyzers. Larger tunnels with heavytraffic may have operators stationed in the controlroom. They operate the fans to control conditions inthe tunnel. At least two independent sources ofelectric power must be available, usually throughfeeders from different parts of the utility system. Ifthese are not available, a diesel-engine emergency

Fig. 20.6 Section through ventilation building of the Holland Tunnel.




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generator sufficient for minimum requirementsshould be installed.

Carbon Monoxide Analyzers n These takecontinuous air samples from the tunnel andanalyze them for CO content. The results arevisually indicated and also recorded on paper tape,with time gradations. The recorders are mountedon the face of the control board, to guide theoperator in selection of number of fans and speednecessary.

In a longitudinal or semitransverse supplysystem, air samples are taken from the tunnelproper at points of maximum concentration.In transverse systems, the samples may be takenfrom the exhaust ducts.

Haze Control n To measure visibility intunnels affected by haze from exhaust gases,instruments have been developed that give areliable indication without excessive maintenance.Equipment manufactured for the Port Authority ofNew York and New Jersey uses the scattering ofultraviolet light by dust particles. Instrumentsprotect the optics by recessing them in tubesthrough which filtered air is exhausted. Anothertype of instrument compares the intensities of twobranches of a split light beam passing through thesame optics, one going through a tube filled withclean air, the other through tunnel air.

Ventilation Power Requirements n Thepower requirements and pressure losses are bestevaluated using the prodedures contained in theTunnel Engineering Handbook (J.O. Bickel andT.R. Kuesel, ‘‘Tunnel Engineering Handbook,’’Kluwer Academic Publishers, New York).

20.7 Tunnel Surveillanceand Control

Emergency exhaust ventilations systems in shorttunnels or tunnels with very light traffic may beactivated by such instruments in the tunnel ascarbon monoxide analyzers or fire-alarm ortelephone boxes connected to the nearest fire andpolice departments. Emergency operation for othertypes of tunnels should be supervised by personnelin control centers.

Control of many newer tunnels is programmedfor computer operation. The computers, however,may be bypassed for manual operation in anemergency.

To permit surveillance of tunnel traffic bypersonnel in the control room, monitors may beinstalled in that room to display views of the entirelength of the roadways as transmitted by televisioncameras mounted in the tunnel. In a short tunnel,each camera covers a specific stretch of roadwayand transmits to a specific monitor. For a longtunnel, to limit the number of monitors required toa convenient number, groups of cameras may beoperated in sequence to transmit to their monitors.In an emergency, the sequence can be interruptedto permit a specific camera to focus on the region ofconcern.

Traffic Control n Signal lights generally aremounted at the portals of a tunnel and at intervalsin the interior such that at least one traffic light isplainly visible within a safe stopping distance. In atunnel with two-way traffic, the signals facingtraffic may incorporate red, amber, and greenlights. Lights on the reverse side of those signalsmay be red and amber, to permit lane alternationin an emergency. In a tunnel with two-lane, one-way traffic, the signals facing traffic carry red,amber, and green lights, whereas lights on thereverse side of signals for the left lane may beamber and red, to permit two-way traffic in anemergency.

Traffic flow may be monitored by pairs ofelectric induction coils that are embedded in thepavement of each lane and that report the flow onindicators in the control room. If traffic velocity istoo slow, for instance, less than 5 to 10 mi/h, thetraffic lights are changed to amber, for caution. Iftraffic stops, the lights are changed to red. Ifnecessary, for slow traffic, the traffic lights may bealternated between stop and go to space traffic flowinto the tunnel.

Fire Control n Automatic fire detectors may beinstalled in the ceiling throughout a tunnel. When afire occurs, they indicate the location and send analarm to an operator who alerts an emergencycrew. If the operator verifies the alarm (whichmight have been activated by the heavy exhaust ofa diesel engine rather than by a fire), an emergencyprogram can be started: The emergency crew and




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vehicles are mobilized. For traffic moving toward afire, signal lights are turned to red, while for trafficmoving away from the fire, signals remain green, topermit evacuation. And the ventilation system forthe affected part of the tunnel is converted toexhaust.

Hydrants generally are installed about 300 ftapart, in niches in the tunnel walls, to providewater for fire fighting. Water may be obtained frommunicipal water supplies, if available. Otherwise,the water mains may be connected to tanksproviding about 10,000 gal of storage. The tanksmay be located near each portal and supplied bypumps from local sources or from groundwater.Booster pumps may be installed to provide atleast 125-psi pressure for application of wateron fires. Fire alarms and fire extinguishers forcontrol of minor fires may be installed next to thehydrants.

Communications n Emergency telephonesmay be placed along the tunnel side walls forcommunication with an operator in the controlroom. An aerial in the tunnel will permit theoperator to transmit messages to motorists throughtheir car radios and allow them to receive otherbroadcasts while in the tunnel.

Power Supply n Power should be suppliedfrom two independent sources, for example, fromtwo different utilities or independent substationsof one utility. An alternative is a standby dieselgenerating plant capable of supplying powerat least for ventilation and emergency lightingto keep the tunnel in operation. This equip-ment should be supplemented by storage bat-teries to supply instant power for the emergencylighting.

20.8 Tunnel Lighting

The Occupational Safety and Health Adminis-tration sets minimum requirements for illumina-tion on construction sites:

5 ft-c—general construction-area lighting, ware-houses, corridors, exitways, tunnels, and shafts

3 ft-c—concrete placement, excavation and wasteareas, accessways, active storage areas, loadingplatforms, refueling, and field maintenance areas

10 ft-c—batch and screening plants, mechanicaland electrical rooms, indoor work rooms, rigginglofts, indoor toilets, and tunnel and shaft headingsduring drilling, mucking, and scaling.

For other areas, follow illumination recommen-dations in “Practice for Industrial Lighting,” IESRP7, the Illuminating Engineering Society of NorthAmerica.

For emergency use, every employee under-ground should be equipped with a portable handor cap lamp unless sufficient natural light or anemergency lighting system provides sufficientillumination along escape paths. Only portablelighting meeting OSHA requirements may be usedwithin 50 ft of any heading during explosivehandling. (See also Art. 20.16.)

[“Construction Industry: OSHA Safety andHealth Standards 29 CFR 1926/1910,” Superinten-dent of Documents, Government Printing Office,Washington, DC 20402.]

Lighting for Tunnels in Service n Sincelocomotives are equipped with strong headlights,railway tunnels are generally not lighted except foremergency evacuation. Subaqueous tunnels andother tunnels on electrified lines, particularly incities, are equipped with a nominal amount oflights, especially in refuge niches.

Rapid-transit tunnels are lighted sufficiently tomake obstructions on tracks visible and to facilitatemaintenance work. The lights are installed and/or shielded to prevent glare in the motorman’seyes. Luminaires are installed in tunnels foremergency use.

For highway tunnels, the most troublesomelighting condition is the transition from bright lightin the approach to the tunnel, the entrance(threshold zone) luminance, to the luminance inthe interior. Guidelines for alleviating this con-dition have been issued by the American Associ-ation of State Highway and TransportationOfficials (AASHTO) and the Illuminating Engin-eering Society of North America. Threshold zoneillumination varies greatly with topography, orien-tation, sun exposure, and season and should beevaluated for the most critical condition. Daylightpenetration through the portal into the thresholdzone may assist the transition. In addition to thethreshold zone, two or three transition zonesgradually reduce the luminance to that of the




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interior. The length of each of these zones shouldbe approximately one safe-stopping sight-distance(SSSD) at design speed. Reduction between zonesshould not exceed 3:1.

At night, a pavement luminance of 2–5 cd/m2

minimum is recommended for the entire length ofthe tunnel. The approach and exit roadways shouldhave a luminance level of no less than one third thetunnel interior level for a distance of a SSSD.

There are four viable types of light sources usedin tunnels, fluorescent, low-pressure sodium (LPS),high-pressure sodium (HPS), and metal halide(MH). The advantages and disadvantages of eachare discussed in greater detail in ANSI/IESNA RP-22-96 8.1. These include restrike time in the event ofmomentary power interruption, linearity of sourceto reduce flicker, cost, color rendering, lamp size,lamp efficacy, control of light distribution, effects ofair temperature, lumen depreciation with time,glare, the risk of lamp rupture, and keepingenclosures dust-tight and water tight. Florescentlamps frequently provide the lower illuminationlevels, combined with LPS at threshold andtransition zones. Lower wattage LPS sources arealso used in interior zones. HPS and MH lampscome in a wide selection of sizes, better lamp life,compact size and are easily optically controlled.

20.9 Tunnel Drainage

Most tunnels through hills and mountains havewater problems. Surface water penetrates throughfissures and percolates through permeable soils.Attempts to seal off the rock by grouting, witheither cement or chemicals, usually are notcompletely successful since very high pressuresmay build up even if flows are low. Cast-in-placeconcrete linings may not be completely watertight.Water may find its way through shrinkage cracks inthe linings into the interior of tunnels. There, it mayfreeze in cold weather and produce an unsightlyappearance, objectionable in highway tunnels.Consequently, provision must be made to drainwater from tunnels.

Fire fighting, washing of tunnel interiors, andflushing of pavements also introduce water thatmust be drained.

Although cut-and-cover tunnels can be water-proofed, this is difficult with bored tunnels. If thewater problem is not serious, the most economicalsolution is to seal cracks in the lining that leak.With

good concrete control, the number of these shouldbe small. It is good practice to design tunnelsassuming that they will leak and therefore provideappropriate drainage paths.

If water appears in considerable quantity duringrock tunneling operations, tight steel lagging overthe tunnel supports and grouting may preventleakage. In serious cases, it may be necessary todry-pack between the rock and the tunnel laggingto drain water. This is a slow, costly methodrequiring much manual labor. Dry pack behind theside walls can easily be placed and is effective inpreventing the buildup of a hydrostatic headbehind the lining. Longitudinal drain pipes shouldbe installed behind the base of the side walls, withthe laterals at regular intervals leading to the maindrain (this is a large drain installed under theroadway, for roadway drainage). Water will flowthrough the dry packing and into the base drains.

In a rock tunnel, heavy flow of water comingthrough a drill hole indicates a water-bearing faultor seam. The flow may be stopped by drillingadditional holes and injecting cement grout. Someholes should be slanted to reach beyond theperiphery. If dense sand or rock flour in the faultprevents proper penetration of cement grout,chemical grouting may give satisfactory results.In special cases, it may be necessary to drill a pilothole well ahead of the face to detect severe waterconditions, especially substantial quantities underheavy pressure. This must be done for rocktunneling under deep bodies of water.

In highway tunnels, drainage inlets should beinstalled at regular intervals along the curbs, withcross connections to the main drain. The lattershould be of generous size, in longer tunnelspreferably large enough to provide crawl spaceto remove silt accumulations, particularly whengrades are near horizontal. Traps at drainage inletsare undesirable, because of the danger in the eventof a fuel spill.

Leakage in well-constructed underwater tun-nels, either shield-driven or immersed, is usuallyminor. It can be controlled by calking joints insegmental liners or by injecting cracks where leaksappear. Main sources of water are washing oftunnel interior, fire fighting, drippings fromvehicles, and rain collected in open approaches.Pumps are usually sized to handle the full flowfrom one fire hydrant.

Continuous open gutters recessed into the curbshave been used in many subaqueous tunnels. The




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gutters lead water to a low point, where it iscollected in a sump. Drainage inlets, spaced about50 ft apart along each curb and connected tolongitudinal drain lines embedded in the concretebelow the curbs, are desirable because they preventpropagation of fire by burning fuel in case of aserious accident. Drain lines should be at least 8 inin diameter. They should be equipped withcleanouts every 500 ft.

In straight, open approaches, transverse inter-ceptors about 300 ft apart are most effective inpreventing water from entering a tunnel. They are18 in wide, extend from curb to curb, and arecovered with gratings, with slots parallel to thecenter line of the roadway. An interceptor is placedin front of the tunnel portal and another about 10 ftinside.

In curved, superelevated approaches, drainageinlets should be installed at regular intervals alongthe low curb.

All drainage from open approaches should becollected inside the portals in sumps below theroadway. Each sump should be divided into asettling basin and a suction chamber. Easy accessmust be provided for cleaning out sediments. Aminimum of three electrically driven, large-clearance drainage pumps should be installed, oneas a standby. Alternating automatic controls rotatethe pumps in service. High-water-level alarmcircuits should be extended to the control room.Sump and pump capacity, with two pumpsoperating, should be designed for maximum,short-duration rainfall for the locality. An intensityof 4 in/h, based on a 15-min downpour at a rate of8 in/h, is ample for most areas.

A smaller sump should be located at the lowpoint of the tunnel. This sump also should bedivided into a settling and a suction chamber. Twoautomatically controlled drainage pumps, with acapacity of 250 gal/min each, may be adequate.Their discharge should be carried to one of theportal sumps.

20.10 Water Tunnels

These may be diversion or intake tunnels forhydropower plants, or aqueducts bringing water tocity and municipal distribution systems.

Diversion tunnels carry river water around damsites during construction. They are designed tocarry the maximum expected runoff during this

period. They may also discharge excess water afterthe reservoir has been filled, or be converted tointake tunnels to a powerhouse located in the sideof the valley below the dam. If they are not neededafter completion of the project, the diversiontunnels are closed with concrete plugs. Extensivediversion tunnels have also been built to collectwater from several watersheds for a central powerplant.

Intake tunnels bring water from reservoirs toturbines or the heads of penstocks. The tunnels aremostly in rock and operate under a positivehydrostatic head. In pervious and fissured ground,they are lined with reinforced concrete or steelplate; in sound rock, a sprayed-concrete lining maybe adequate to provide a smooth surface as long asthere is sufficient overburden to exceed the internalpressure.

Many miles of aqueduct tunnels have been builtfor municipal or area water-distribution systems.These tunnels are, for the most part, in rock butmay also contain stretches of soft-ground tunnel-ing. They may be under large hydrostatic pressure,such as the New York City aqueduct, which crossesthe Hudson River 600 ft below sea level.

Tunnels with small or no interior pressuregenerally have a horseshoe section; pressuretunnels are circular. Lining is concrete, 6 to 36 inthick, depending on size, pressure, and nature ofrock. Welded steel tubes may be needed whenpressures are particularly high. Grade tunnels maybe lined with plain concrete, pressure tunnels withreinforced concrete. Diameters range from 7 ft forsmall aqueducts to 50 ft for Hoover Dam diversiontunnels. In very sound rock, sprayed-concretelining has been used. Parts of the Colorado Riveraqueduct are lined with continuous steel shellsagainst concrete backing, and the inside isprotected by 2 in of reinforced sprayed concrete.

To expedite construction, long tunnels aresubdivided into several headings by shafts oradits, about 2 to 5 mi apart.

20.11 Sewer and DrainageTunnels

Large cities require miles of tunnels to carry offstorm runoff and to conduct wastewater totreatment plants. These tunnels are built in avariety of soils. Some are constructed as boxculverts by the cut-and-cover method, but most are




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tunneled with tunnel boringmachines (TBMs). Sizevaries from about 7 to 15 ft. Drainage tunnels forstorm water are usually less extensive since theycan discharge into nearest open waters.

The cross section of sewer and drainage tunnelsis usually horseshoe or circular, with concretelining. Quality of concrete is of special importanceto resist the detrimental effect of wastewater.Generally, they are grade tunnels, except forsiphons under rivers, which are under pressure.A circular or egg-shaped section maintains velocityat low flow to prevent excessive settling of solids.

Alignment is dictated by location of treatmentplants, soil conditions, and the street plan of thecity. Continuous grades should be maintainedexcept for siphons. A minimum grade should bemaintained for gravity flow.

20.12 Cut-and-Cover Tunnels

Shallow-depth tunnels, such as rapid-transit linesunder city streets, underpasses, land sections ofunderwater tunnels, and end sections of tunnelsthrough hills, are built by cut-and-cover methods.A trench is excavated from the surface, withinwhich a concrete tunnel is constructed. Withbottom-up construction, the completed tunnel iscovered up, and the surface reinstated. With top-down construction, the walls are constructed first,perhaps using bentonite slurry in narrow trenches.The roof is constructed next, backfilled and thesurface reinstated. Excavation and construction ofthe floors below roof level then follow, using accessfrom the ends or from glory holes. Both bottom-upand top-down construction almost always use cast-in place concrete. Depth of invert on subways andunderpasses usually does not exceed 35 to 40 ft. Forconnections to subaqueous tunnels, cuts up to100 ft have been used under special circumstances,and depths to 60 ft are not uncommon.

The Occupational Safety and Health Adminis-tration (OSHA) sets standards, regulations, andprocedures for protection of personnel duringexcavation. OSHA requires that all surface encum-brances and underground utility installations, suchas sewers, electrical and telephone conduits, andwater pipes, be protected, supported, or removedas necessary to safeguard the workers. Also,structural ramps used for access or egress shouldbe designed by a structural engineer and con-structed in accordance with the design. For trench

excavations that are 4 ft or more deep, a stairway,ladder, or ramp should be provided for egress so asto require no more than 25 ft of lateral travel forworkers.

Among the measures that OSHA specifies forsafeguarding personnel in excavations are thefollowing: Precautions should be taken to preventexposure of personnel to harmful levels ofatmospheric contaminants (Art. 20.6). If naturallighting is inadequate for safe working conditions,illumination to meet OSHA requirements forexcavation should be provided (Art. 20.8). Person-nel should not be allowed to work in excavations inwhich water accumulates unless the workers areprotected by safety harnesses and lifelines, water isbeing removed to control the water level withinsafe limits, and special supports or shields are usedto protect against cave-ins.

To avoid exposure to falling objects, personnelshould not be permitted below loads carried by

Fig. 20.7 Taipei Metro Cut-and-Cover.




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lifting or digging equipment. Unless excavationsare entirely in stable rock or are less than 5 ft deepin stable soil, protection should be providedagainst cave-ins. Retaining devices may berequired to prevent excavated or other materialsor equipment from falling or rolling into theexcavation.

Where space and depth of excavation permitand the ground is sufficiently firm, open slopesmay be used along the sides of the excavation. Forexcavations up to 20 ft deep, OSHA limits the slopeto a maximum of 1:11⁄2 (348 with the horizontal),unless soil tests and analyses indicate steeperslopes will be stable. A registered professionalengineer must design excavations deeper than20 ft, and excavations must be monitored by acompetent person as defined by OSHA. Personnelshould be protected from loose rock or soil fallingor rolling from the excavation face. For thepurpose, loose material may be removed by scalingand protective barriers may be installed at intervals

along the face. If the lower portion of theexcavation has vertical sides, that region shouldbe shielded or supported to a height at least 18 inabove the vertical sides.

Groundwater may be lowered, as needed, bytiers of wellpoints. This may lower the ground-water outside the excavation considerably andcause settlements. The lowering of the externalgroundwater can be reduced by the use of slurrywalls, contiguous or overlapping bored piles, orsteel sheet piling. Adjacent structures with a risk ofsettlement may require underpinning. Further-more, where lowering of groundwater exposeswooden piles to air, deterioration may follow.

Where space permits, the sides of the trenchmay be sloped back to reduce the need to providesupport to them. In confined or deep areas, supportof excavation may be required. The excavationsupport may be temporary walls that are not partof the final structure, or they may form part of thefinal structure, especially when excavations are

Fig. 20.8 63rd Street Tunnel Strutting and Tie-backs.




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deep. The ease and simplicity of constructing thefinal structure within temporary walls must bebalanced against cost savings when they areincorporated into the final structure. Temporaryexcavation support may be steel sheet piles, soldierpiles and lagging, and tangent or secant piles. Fordeeper excavations, concrete slurry walls may beconstructed, usually 2 ft, 3 ft, or 4 ft thick, some-times incorporating soldier piles or beams (SPTCwalls), and may form part of the final structure.

Steel sheetpile walls, for depths to about 30 to40 ft, supported by wales and cross bracing. Thewalls keep loss of ground to a minimum.

Soldier piles and lagging, made of steel Hbeams with wood or concrete lagging. These areused for greater depth. Lagging must be blockedtight against the earth to control loss of ground.Soldier piles may be combined with sheetpiles,instead of wood lagging, if tight bulkheads arerequired. Wales and cross bracing support thewalls.

Concrete slurry walls built in bentonite-slurrytrenches have been used to prevent loss of ground

and eliminate or reduce groundwater lowering.Sections of trenches about 20 ft long are excavated.The trenches are kept filled with bentonite slurry.Then, reinforcing cages are lowered into them, andconcrete is placed to fill the trenches, displacing theslurry. Key sections are formed at the ends of thetrenches. The walls serve as part of the finalstructure or as impervious bulkheads.

SPTC or California Wall, a combination ofsoldier piles and slurry wall. This was used onsome stations of BART, and as part of the finalstructure for much of the Central Artery Project inBoston. Large wide-flange steel beams are insertedin slurry-filled bored holes, the space between thebeams is excavated under slurry, and excavationand pipe holes are filled with concrete. Care mustbe used in excavation to have the concrete solidlykeyed into the space between the flanges. The steelpiles in the composite wall act as reinforcing andpermit easy attachment of interior bracing.

The fundamental basis for the design ofexcavation support systems is consideration ofhow the soil being supported behaves, and perhapsalso how the floor of the trench behaves, sincesubstantial heave can occur under adverse con-ditions. Any movement of the support system cancause soil movement and hence settlement ofadjacent structures. It is the amount of movementthat can be tolerated at the adjacent structures thatoften dictates the type and stiffness of theexcavation support system, with control ofground-water levels often being crucial. Structuresmay have to be designed for both short-term andlong-term effects. One of the primary tools forthis purpose is soil-structure interaction analysis.Frame structures supported by beam-on-an-elastic-foundation analysis are also commonlyused. In most cases, a two-dimensional analysis issufficient, although complex areas may requirethree-dimensional modeling, perhaps using finiteelement analysis.

Many subway and highway structures havebeen built using steel columns and beams with jackarches, but this is uncommon today. New struc-tures are generally reinforced concrete box struc-tures but when the excavation support system alsoforms part of the final structure, it may in practicebe difficult to obtain full fixity between the wallsand the slabs. Partial fixity may then be specified,and perhaps shear connections also provided. Forhigh load on the roof or large spans, the compositeaction of a thin concrete slab on top of steel beams

Fig. 20.9 Tangent Piles.




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has been used. Haunches may help to reduceeffective spans. Arched tunnel roofs are rare today.

Design loads include weight of overburden, selfweight, live load surcharge, potential futureconstruction, horizontal earth pressure, hydrostaticloads if below the water table, and seismic loads.Weight of submerged structures must be adequateto prevent floatation excluding all removable itemsfrom within the tunnel and above it while ignoringfriction on the sides.

Tunnel Waterproofing n Tunnels in dry soilneed no waterproofing on base and walls, but roofslabs should have at least minimumwaterproofing.Tunnels below groundwater level should bewaterproofed all around.

Tunnels that are not waterproofed should bedesigned assuming that they will leak, and pathsprovided to remove any leakage water. Manytunnels have drainage channels adjacent to exteriorwalls and a second “false” wall a few inches fromthe first onto which the final tunnel finishes areattached. Joints are more likely to leak than otherlocations, so that special attention should be paid towaterproofing joints even if surface waterproofingis not applied.

Methods of surface waterproofing includemembranes supplied in roll form with overlappingor welded joints, spray-applied membranes, blind-side waterproofing designed for the outside face ofwalls cast against existing ground or the supportsystem, clay-based panels that swell on contactwithwater, and chemical additives to the concrete. Somemethods of waterproofing require safety precau-tions during application, such as ventilation.Water-proofing membranes that adhere to the surface towhich they are applied help to prevent the spreadbeneath the membrane of any water that could leakthrough a puncture. The repair of leaks appearingon the inside of the tunnel may then be as simple asinjecting offending cracks locally, otherwise newleaks may appear as previous leaks are repaired.

For joints, a large number of extrusions areavailable that are designed to be buried in theconcrete, half each side of the joint, some of whichcan be injected later if leaks still occur. Joints mayalso be waterproofed using hydrophilic materials,gaskets in compression within the joint or boltedto the surface each side of the joint, and surfaceapplied injectable and reinjectable tubes.

Most waterproofing must be protected againstmechanical damage (for example, during back-

filling or the placement of reinforcement), andagainst noxiousgases (vehicle exhaust, for example)and fire within the tunnel. Protection methods haveincluded a layer of concrete, plywood boards,and brick (against vertical faces). Heat resistingmaterials and cover plates with gaskets have beenused to protect joints within tunnels.

To save on excavation width, waterproofing forwalls may be applied to trench bulkheads andconcrete placed against it.

20.13 Rock Tunneling

Standards, regulations, and procedures of theOccupational Safety and Health Administrationshould be adhered to in rock excavations, as in allconstruction operations. (See Arts. 20.6, 20.8, and20.12.)

Tunneling in rock today is primarily by drill-and-blast or by using a TBM (tunnel boringmachine). Drill-and-blast tunnels can be any shape,whereas most TBMs are only capable of drillingcircular holes. A rule of thumb is that a tunnelrequires at least one diameter of cover, althoughless may be possible. Where rock quality isparticularly good, the tunnel may be unlined ormay only need mesh, sprayed concrete and rockbolts or dowels. More fractured rock may requiresignificant temporary ground support such as steelsets and lattice girders until a final lining iscompleted. Fracture zones may be particularlydifficult to cross due to high flows of water underhigh pressure, and due to the quantities of loosematerial. For some purposes, lining may berequired to promote flow or to prevent ingress ofwater.

(Kuesel, T. R., Tunnel Stabilization and Lining,in ‘‘Tunnel Engineering Handbook,’’ Bickel, J. O.,Kuesel, T. R., and King E. H., Editors, Chapman &Hall, 1996. U.S. Army Corps of Engineers Manual,1997, Design of Tunnels and Shafts in Rock, EM1110-2-2901.)

For rock excavation, the most importantgeological conditions to be anticipated are thepresence of faults, usually involving areas of badlyfractured rock; direction and degree of stratifica-tion; fissures and seams; presence of water, whichmay be cold or hot or contain corrosive or irritatingingredients; pockets of explosive or toxic gas; androck strain. The petrography is of lesser importance




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unless the rock is highly abrasive, causingexcessive wear of drills.

Too much information can never be providedfor the engineer, to produce a realistic design, andfor the contractors, to prepare sound bids. Even atbest, unforeseen difficulties must be expected.

In addition to geological surveys and borings(Art. 20.5), engineers may use electric-resistivitymeasurements and gamma-ray absorption forinformation on depth and characteristics of rockformations. Information also may be obtained fromthe U.S. Geological Survey, which has extended itsscope and geophysical studies beyond the miningfield. Where geological conditions are particularlyhard to evaluate or are especially severe, explora-tory pilot tunnels, about 10 � 10 ft, may be drivenpart way from each end or for the entire length of atunnel, prior to final design and advertising ofconstruction. TBMs may be used for pilot tunnels

either used as a seperate service tunnel, orabandoned, or enlarged to form the final tunnel.In these pilot tunnels, internal rock stresses can bemeasured by pressure cells and strain gagesinserted in transverse drill holes, and the natureof the rock, foliation, blockiness, and pressure offaults and water can be inspected.

Tunnel Boring Machines (TBM) n The highinitial cost of hard-rock TBMs tends to restrict theiruse to longer tunnels. Although ideally suited tocircular tunnels due to the rotary motion of thecutting heads, variations of the excavated shapemay be technically feasible, as have been done witha few soft-ground TBMs. Large grippers are jackedoutwards, either to each side or top and bottom,and hold the main part of the machine and transferthe applied thrust to the adjacent firm rock. Disk

Table 20.2 Load Hp in Feet on Rock on Support in Tunnel*

Rock condition Hp, ft Remarks

1. Hard and intact Zero Light lining on rock bolts only if spalling orpopping occurs

2. Hard, stratified, or schistose 0 to 0.5B Light supports. Load may change erraticallyfrom point to point

3. Massive, moderately jointed 0 to 0.25B

4. Moderately blocky and seamy† 0.35(B þ Ht) to1.10(B þ Ht)

No side pressure

5. Very blocky and seamy 0.35(B þ Ht)to 1.10(B þ Ht)

Little or no side pressure

6. Completely crushed, chemicallyintact†

1.10(B þ Ht) Considerable side pressure. Requirescontinuous support of lower ends of ribsor circular ribs

7. Squeezing rock, moderate depth 1.10(B þ Ht) to2.10(B þ Ht)

Heavy side pressure; invert struts required.Circular ribs recommended.

8. Squeezing rock, great depth 2.10(B þ Ht) to4.50(B þ Ht)

Same as for Type 7

9. Swelling rock Up to 250ft,regardless ofvalue (B þ Ht)

Circular ribs. In extreme cases, use yieldingsupports

* If depth of rock over tunnel is more than 1.5(B þ Ht), where B is width andHt is height of tunnel. FromR. V. Proctor and T. L.White,“Rock Tunnels and Steel Supports,” Commercial Shearing & Stamping Co., Youngstown, Ohio.

† If roof of tunnel is permanently above the water table, values for Types 4 and 6 can be reduced by 50%.




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cutters mounted in the cutter-head face roll at highpressure on the exposed rock face and crush therock, the tailings from which are mechanicallyremoved. After an advance of up to six feet or so,the grippers must be retracted, moved forwardsand jacked out again. Advance rates are verydependent on the hardness of the rock and itsintegrity, and the wear and tear that the rock maycause. In swelling rock, extra precautions must betaken to ensure that the machine does not get stuck.As the rock quality diminishes, the machine willneed to resemble a soft ground TBM more andmore. Rock TBMs tend to be launched from amined chamber in which the whole machine can beassembled.

Headings n In the past, when mucking wasdone by hand loading into mine cars, and drillequipment was cumbersome, excavation wasadvanced in drifts or headings. In weak rock orfor very wide tunnels, this method is still used. Atop heading may be advanced first. This permitsinstallation of crown supports if needed. The rest isexcavated by benching down from the top heading.These different levels make transportation ofexcavated material inconvenient. In wide tunnels,side headings may be advanced. In that case, legsof steel sets (supports for side walls and roof) areplaced, where necessary. The side headings arefollowed by a top heading and erection of the archsupports. The remaining block can be attackedfrom the face or from the side drifts.

A bottom heading or pilot tunnel may be usedinstead. Enlargement proceeds at several placesalong the heading simultaneously. The pilot tunnelhas to be large enough to allow in and out trafficand should be timbered to protect it.

In very long tunnels, a parallel heading, 40 ft ormore from the tunnel axis, expedites excavation byproviding access to several working faces throughcross drifts. From this pilot tunnel, transverseheadings are driven at several points to the maintunnel axis, from which tunnel excavation canproceed in both directions. The parallel headingcarries all traffic to the different faces and serves asa drainage and ventilation tunnel. This methodwasused in the 12-mi Simplon tunnel, where theparallel drift was later enlarged to a full-sizedsingle-track railroad clearance, and in the Moffatand New Cascade tunnels.

A center heading may also be used in large rocktunnels. From it, the section is enlarged to full sizeby radial drilling.

Full-Face Tunneling n To save time andlabor, full-face rock excavation is used whereverfeasible, for efficient mechanization of the oper-ation. Large track or rubber-wheel-mounted jumboframes carry high-speed drills. As an alternative todrill and blast, roadheaders are sometimes used onweaker rock. They are smaller excavators equippedwith ripper or point-attack teeth mounted onrotating balls attached to slewing and elevatingarms. Being more maneuverable, roadheaders canexcavate openings of almost any shape. Mucking(removal of excavation) is done by large, mechan-ized loaders. Muck is carried in diesel trucks,where permissible, or in trains of large mine carspulled by battery-powered locomotives if lawsprohibit use of internal combustion engines.

Excavation Limits n Contract plans prescribeexcavation profiles. An inner A line is theminimum theoretical section to be excavated; tothis is added a tolerance, usually 6 in to the B line orpayment line. Any overbreak beyond this is at thecontractor’s risk and has to be filled at thecontractor’s expense.

Blasting n Drilling pattern and blastingcharges are governed by the rock characteristics,fragmentation desired for mucking, and externalconditions, such as proximity of sensitive struc-tures. The procedure should be worked out by anexperienced blasting expert and may have to bemodified during construction. The center group ofholes, fired first, are drilled convergent, so that aconical shape is blasted. Blasting proceeds towardthe periphery with short-time delays. A 6- or 8-in-diameter center, or “burn” hole, without charge,acts as a relief opening, improving blasting effect.Rounds are usually about 10 ft deep but may bemore or less, depending on the rock. Line drilling,a ring of straight holes, fairly closely spacedaround the periphery, is used if as smooth a sectionas possible is desired.

Temporary Supports n Practically all rocktunnels need some temporary supports. Timbermay be used in pilot tunnels and small headings.For larger tunnel cross sections, steel sets are




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more economical because of their strength andease of installation. These are made of I beamscold-rolled into shape. For small tunnels withcircular arches, the sets may be continuousframes. In larger tunnels or for flat arches, thesets consist of separate posts and arches (Fig. 20.11).Where roof supports only are necessary, thearches may be supported on plates resting onrock ledges. Steel sections are usually uniform forthe entire tunnel, and spacing of sets is variedaccording to rock loads. Normal spacing is 4 ft.but spacing may be reduced to 2 ft or increased toas much as 6 ft.

The sets should be erected as soon as scaling ofloose rock has been completed. Blocking shouldimmediately be wedged between the steel and therock surface at 3- to 5-ft intervals to prevent rockmovement from starting. The steel frames shouldallow space at the crown, between the lower flangeand the concrete surface, for a pipe for placingconcrete.

Timber or steel lagging should be placedbetween the sets. The amount of lagging dependson rock conditions. Lagging may be practicallysolid, or there may be gaps of various widthsbetween the sheets, as required by circumstances.Badly fragmented rock may require metal panningbetween sets if water is present. The pans are made

of interlocking channels. The space between pansand rock should be dry-packed to allow water torun off into the drainage system.

The concentrated loads on the sets at blockingpoints produce bending moments in the frames.Table 20.2 presents formulas for loads on supportsin rock tunnels (R. V. Proctor and T. L.White, “RockTunnels and Steel Supports,” Commercial Shearingand Stamping Co., Youngstown, Ohio).

Through badly faulted rock or pressure areas,circular tunnel sections and ring supports arepreferable, particularly in seismic areas (Fig. 20.12).

Rock Bolts n In good rock, but also for somerock that may be classified as poor, rock bolts maybe used to secure the excavation. They are usually1 in in diameter and 8 ft long. They may becoupled, however. The bolts provide anchorage insound rock, where they are held by wedges driveninto split ends when the bolts are inserted or byexpansion sleeves gripping the sides of the holewhen the bolts are threaded in. The bolts are testedfor pull-out and prestressed by nuts bearingagainst face plates on the rock surface. Unten-sioned deformed bars, bolts, or steel or glass-fibertubes are used as rock reinforcement and fullygrouted with cement or high-strength resin grout.

Fig. 20.10 Drill and Blast (TARP).




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They are stressed by deformation of the rock,which is monitored by extensometers and conver-gence measurement until equilibrium is reached. Ifnecessary, additional untensioned or tensionedbolts are inserted. All rock bolts in permanentinstallations should be grouted as protectionagainst corrosion.

Another type of bolt has a perforated sleeve,which is placed in a hole in the rock and filled withgrout. As the bolt is pushed into the hole, the groutis squeezed through the perforations and againstthe rock. Bond between bolt, grout, and rockprovides the holding force.

Shotcrete n Use of sprayed concrete (gunite orshotcrete) as preliminary tunnel support for rocktunnels was developed in Europe and has also beensuccessful in North America. As soon as possibleafter blasting, while mucking is going on, a layer of

concrete is sprayed on the roof. The concrete ismadewith awell-graded aggregate, up to 3⁄4-in size,which is frequently dry-mixed with cement and anaccelerating agent. The mixture is ejected through anozzle under pressure by special pumps. Mixingwater is added at the nozzle. Initial set takes placein about 30 to 120 s, final set in 12 min.

Also often used is a wet mix, for whichaggregate, cement, and water are placed in themixer and additive is injected as a liquid at thenozzle. Addition of about 5% by volume ofmicrosilicate greatly improves adherence of shot-crete to the rock and reduces reinforcing steelrequirements. Addition of 1 =

2 - to 1 1 =

2 -in steel fiberto a mix in the amount of 1⁄2 to 1% by weightconsiderably increases the ultimate strength andtoughness of the shotcrete.

Thickness of the initial layer may vary from 2 to4 in, depending on rock conditions. Additional

Fig. 20.11 Typical cross section through the Lehigh Tunnel on the Pennsylvania Turnpike Extension.Half section B shows the bracing, or sets.




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layers may be sprayed on as needed. Totalthickness may be as much as 8 in.

The nozzle may be held directly by an operatoror attached to a boom manipulated by a workerstationed under the protective roof of the jumbo.Automatic application has been successful in amachine-bored tunnel (Heitersberg Tunnel inSwitzerland). Robots, controlled by an operatoron the jumbo, can be used to apply either dry orwet mix shotcrete.

Shotcrete is sprayed on the sidewalls aftercompletion of mucking. Heavy water inflow mustbe intercepted and drained through inserts in theshotcrete. Well-trained operators and carefulsupervision and control are essential for goodresults. Properly executed, the method can be usedsuccessfully for fractured rock.

Strength of concrete in place reaches 200 to250 psi in 2 h, 1400 to 1500 psi in 12 h. The ultimatecompressive strength of 4000 to 5500 psi is about15% less than that of the same concrete withoutaccelerator.

Waterproofing n Above the groundwatertable, waterproofing is usually applied to ceilingsin transportation tunnels to prevent dripping.Drainage paths may be provided along the baseof the walls to handle any water that does appear.False walls with finishes are often used to hidewalls where leakage is expected. For most tunnelsbelow the groundwater table, a waterproofingmembrane enveloping the tunnel is used betweenthe initial ground support and the final lining. If the

Fig. 20.12 Typical section through Berkeley Hill Rock Tunnel (heavily faulted rock) for theSan Francisco Bay Area Rapid Transit.




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tunnel lining is undrained, the final lining willcarry the full groundwater pressure and should bedesigned accordingly. Where drainage is providedoutside the waterproofing membrane, the finallining may be designed for a reduced groundwaterpressure. Waterproofing membranes may alsoneed to resist deleterious gases or liquids expectedto be present.

Leakage n See Art. 20.13.(J. O. Bickel and T. R. Kuesel, “Tunnel

Engineering Handbook,” Van Nostrand ReinholdCompany, New York.)

20.14 Tunnels in FirmMaterials

Occupational Safety and Health Administrationrequirements should be satisfied in undergroundexcavations. (See Arts. 20.6, 20.8, and 20.12.)

Materials, other than rock, that may beencountered in tunneling are sands of variousdensities and grain sizes; sands mixed with silt orclay; clays, either pure or containing silt or sand,and varying from relatively plastic with high watercontent to firm and dry; and alluvial mixtures ofsand and gravel or glacial till. To improve theproperties of poorer ground, or to reduce waterinfiltration, ground improvement prior to miningmay be undertaken. This may take the form of theinjection of cementitious or chemical grout, or itmight physically mix soil with these materials.Bentonite has also been used; environmental issuesassociated with the use of the selected materialshould be considered. If not subject to hydrostaticpressure of free water, materials may be excavatedby mining. Temporary support is given by timberor steel framing in headings whose size andnumber depend on local conditions.

Mining of headings in all these materialsrequires the driving of poling boards, supportedby cross timbers and posts to hold the roof. Asexcavation is advanced on a face as steep as thematerial will stand, these boards are driven further,with the rear supported by the frame, the front bythe soil. A new support is set under the forwardend of the poling boards and the process repeated.The sides of the heading are held by boardssupported by the posts, as required. Figure 20.13illustrates the basic procedure for this type ofexcavation.

Steel supports are often used instead of timber,particularly for large headings. Steel lances, madeof small wide-flange beams with wedge-shapedpoints, may be used instead of wood poling boards.The lances are long enough to be supported on twoframes and driven by jacks or air hammers into thesoft face for a distance equal to the support spacing.

In loose soil or running sand, the face issupported by breast boards. A shallow slot about2 ft deep and one or two poling boards wide isexcavated in the top of the face, and a short verticalbreast board is placed immediately, to hold the faceand support the forward end of the poling. Afterthis slot has been excavated across the heading andall vertical breast boards set, a cap is installed,supported by short posts. The rest of the face maythen be excavated downward and held byhorizontal breast boards (see Fig. 20.14).

The size of the heading should be as large as soilcharacteristics allow, but not less than 5 ft wide by7 ft high. Steel bents shaped to the tunnel arch arepreferable to timber framing, if economical,considering both price and speed of operation.Poling may be timber or steel.

Fig. 20.13 Timber bents support poling boardsin basic earth mining.

Fig. 20.14 Mining in running ground requiresbreast boards.




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Steel liner plates are available in various shapesand sizes. They may be used to support the groundif a limited excavated area of the roof or arch willstand long enough for insertion of the liner plates,starting at the top of the arch and working down.The flange of each plate is bolted to the previouslyerected liner.

In small tunnels, ribbed or corrugated linerplates may give adequate support. In large tunnelsor under heavier loads, the plates are backed up bysteel ribs, against which they are blocked. Linerplates without flanges may also be used as laggingor poling. See also Art. 20.17.

To prevent settlement or unbalanced load, allvoids behind the liner plates should be filled byinjection of pea gravel or cement grout.

Small tunnels may consist of a single heading.For large tunnels, various combinations of head-ings are used. Some of these are known by thecountry of their origin, as American, Austrian,Belgian, English, German, or Italian methods, butare used in many variations. Originally, themethods required wood supports, but now steelsupports are favored, where economical.

Sequential Excavation Method (SEM) n

Also known as the New Austrian TunnelingMethod (NATM), SEM was developed in Austriabut is now used worldwide. It is a tunnelingmethod adapted to the excavation of variable andnon-circular cross-section reaches of tunnel, suchas highway ramps and subway stations. Thisunderground method of excavation divides thespace (cross-section) to be excavated into segments,then mines the segments sequentially, one portionat a time. Excavation sequencing by the Americanmethod, Austrian system and Belgian method areoutlined below.

The excavation can be carried out with commonmining methods and equipment (often a backhoe),chosen according to the soil conditions; tunnel-boring machines are not used. Ground conditionsare assessed at the face of the tunnel or from theside of a small tunnel, which helps to decide how toproceed in the best way and determines the choiceof equipment and lining. It should be noted that thecombination of ground treatment and SEM for theexcavation of uniform cross-section tunnels wouldgenerally be more expensive than the use ofpressurized face TBM construction under Amer-ican underground construction labor and economic

conditions. Thus the application of SEM wouldbe limited economically to variable geometrystructures. However for shallow tunnels, suchstructures could probably be more economicallyconstructed using cut-and-cover techniques.

SEM requires extremely dry conditions; de-watering is often necessary before the excavationcan proceed. SEM involves careful sequencing ofthe excavation as well as installation of supports.Shotcrete (a kind of concrete sprayed from high-powered hoses) may be used to line the tunnel orsupport the face, and grouting (the injection of acementing or chemical agent into the soil) may beused to increase the soil’s strength and reduce itspermeability. Because of the requirements of thismethod, the rate of excavation is slow. Use of thismethod in saturated, non-cohesive granular soilswould require the use of groundwater control andground improvement techniques. One real concernwith the use of SEM in granular soils is suddenuncontrollable ground loss, often resulting insurface sinkholes. This can happen when theselected ground improvement method is unsuc-cessful because of localized variation in groundconditions.

One method often used to control groundwateris compressed air. However, the high air pressuresoften required might make the use of compressedair tunneling uneconomical in comparison to otherpossiblemethods. It is for similar reasons that shieldtunneling using compressed air has been replacedby tunneling with pressurized face tunnel boringmachines. Another commonly used method ofcontrolling groundwater is dewatering. However,unrestricted dewatering can have a significanteffect on adjacent foundations. An approach thathas been used with variable success overseas hasbeen to install groundwater cut-off walls (slurrywalls, etc.) along both sides of the right-of-way andthen dewatering inside the cut-off. When dewater-ing sands, running or fast-raveling ground, con-ditions may result so that some form of groundimprovement, such as closely spaced groutablespiles or horizontal jet grouting above the crown ofthe excavation could be required. Two other groundimprovement methods that could be used are jetgrouting and chemical grouting. Each methodwould be used to create a block of stabilized groundthrough which the tunnels could be excavated.

American Method n As shown in Fig. 20.15a,excavation starts with (1) a top heading at the




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tunnel crown, which is supported by poling, posts,and caps. Next, the excavation is widened betweentwo bents and the top arch segments adjoining thecrown are set, supported by extra posts or struts.(2) The excavation then is benched down along thesides, and another segment of ribs is set on eachside. (3) and (4) These are doweled to the upperpart and supported by temporary sills. Thisprocess is repeated to the invert sill. The benchfinally is excavated to full section. (5) Groundbetween ribs is held by lagging, and voids arepacked. This method is suitable in reasonably firmmaterial.

Austrian System n As shown in Fig. 20.15b, afull-height center heading is advanced. It eitherstarts with a top heading and is cut down to theinvert in short lengths or starts as separate bottomand top headings. (1) and (2) In the latter case, thecore between the two is excavated for shortdistances, and the short posts replaced with longones. (3) The arch section is widened in shortlengths and is held by segmental arch ribs andlongitudinal poling boards. (4) The arch ribs aresupported by struts from the center-cut framingand sills at the spring line. The rest of the excavationis advanced to full face in short increments, andposts are set to support the sills. (5) This method issuitable for reasonably stable soil.

Belgian Method n As shown in Fig. 20.15c, infirm ground, the upper half of the tunnel isexcavated, starting with a center heading from thecrown to the spring line. (1) This is widened to bothsides, the ground being held by transverse polings.

These are supported by longitudinal timbers, inturn supported by struts extending fanlike from asill in the center heading. (2) Next, a center cut isexcavated to the invert (3), leaving benches tosupport the arch of the tunnel lining. Slots are cutinto the benches at intervals to underpin the arches.The rest of the bench then is removed to completethe sidewalls (4), afterwhich the invert is concreted.The excavation may be advanced a considerabledistance before the tunnel lining need be inserted.

(J. O. Bickel and T. R. Kuesel, “TunnelEngineering Handbook,” Van Nostrand ReinholdCompany, New York.)

20.15 Shield Tunnel inFree Air

This section describes shield tunneling where theface is essentially open and exposed at ambient airpressure, and Section 20.16 when exposed undercompressed air. Both these methods are lesscommon today than tunneling by the tunnel boringmachines (TBM) described in Section 20.19, nowwidely used.

Shield tunneling is generally used in noncohe-sive, soft ground composed of loose sand, gravel,or silt and in all types of clay, or in mixtures of anyof these. It is indispensable for tunneling in thesematerials below the water table.

Requirements of the Occupational Safety andHealth Administration (OSHA) for undergroundconstruction should be complied with in oper-ations with shields. OSHA requires the following,in particular: Lateral or other hazardous movementof a shield when subjected to a sudden lateral load

Fig. 20.15 Some excavation procedures for large tunnels: (a) American method. (b) Austrian method.(c) Belgian method.




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should be restricted. Personnel accessing shieldedareas should be protected against cave-ins. Person-nel should not be permitted in shields when theyare being installed, removed, or moved vertically.Excavations may extend up to 2 ft below a shieldbottom, if the shield is designed to resist the forcesfrom the full depth of the trench and if soil will notbe lost from behind or below the bottom of theshield. (See also Arts. 20.6 and 20.8.)

The shield is a cylinder made of welded steelplate (Fig. 20.16). It has a diameter slightly largerthan the outside of the tunnel lining. The plate isstiffened by two interior ring girders, the first oneinstalled a short distance behind the cutting edge.

Depending on the diameter and loads, thegirders are braced by horizontal and vertical steelstruts. The cutting edge is beveled and reinforcedby welded steel plates to a thickness of up to 3 or4 in. For loose ground, the upper half of theshield is extended forward 12 to 18 in to form aprotective hood.

The tail of the shield overlaps slightly the end ofthe finished lining and provides space for at leastone liner ring, and for underwater tunnels isusually long enough to accommodate two rings.The inside of the tail clears the lining by about 1 inall around. For working in soft clay, the front of theshieldmay be closed by a steel bulkheadwith door-

Fig. 20.16 Longitudinal section through a shield used for tunneling through soft ground in free air.




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equipped openings through which material isexcavated. Soft clay may be extruded through theopenings while the shield advances.

Working platforms that can be advanced andretracted by hydraulic jacks are mounted on theshield bracing (Fig. 20.16). They give access to allparts of the face and, by keeping in contact with it,support it if necessary during shoving. Additionalbreasting jacks can be mounted in the bracing tohold breast boards against the face if it needsextensive support.

Shield Advancement n Hydraulic jacks (Fig.20.16) for advancing the shield are set on the websof the ring girders close to the periphery of theshield. The shove jacks are evenly spaced aroundthe perimeter and exert pressure against theforward ring girder, which is stiffened by bracketswelded to the skin of the cutting edge. Jackplungers are equipped with shoes bearing againstthe tunnel lining. The stroke of the jacks is slightlymore than the width of a liner ring.

A rotating erector arm is mounted inside the tailto pick up and place liner segments. Hydraulicpumps mounted behind the shield supply 5000- to6000-psi pressure to the jacks, erector arm motor,and other hydraulically operated equipment.Control valves for these devices are mounted on apanel in the shield.

The method of operation, excavation, and speedof advance vary greatly according to the type ofsoil. In sand and gravel, the face usually has to beheld by breast boards (Fig. 20.16), which are bracedby telescoping struts, breasting jacks, or theworking platforms. The breasting may have to becarried down to the invert of the face, which isexcavated to the cutting edge of the hood. Ifcompressed air is used, the breasting may becarried part way down to where the air pressurebalances the hydrostatic head, the lower part of theface taking its natural slope. In firm materials, siltysand, or stiff clay, the full face may be excavatedwithout breasting. Average progress for the 31-ft-diameter Queens Midtown Tunnel, New York City,in these materials was between 7 and 8 ft in 24 h.

Shields are not well-suited for rock tunneling,but rock or mixed faces, partly rock and partly soil,may be encountered in parts of soft-groundtunnels. If the rock is high enough, a bottomheading may be excavated ahead of the shield anda concrete cradle placed, with steel rails embedded,

to exact line and grade to support the shield as itadvances. A similar bottom heading may be usedin a full rock face if the full cross section cannot beexcavated. Then, the rock may be blasted aroundthe periphery of the rest of the cutting edge topermit advancing the shield. Progress in mixedface in the Queens Midtown Tunnel averagedabout 3 to 4 ft per 24-h day.

Best progress is made in plastic materialthrough which the shield may be shoved blind,that is, without taking any soil into the inside, thevolume being displaced by compressing orheaving of the surrounding material. To counter-act the tendency of the tunnel to heave behind theshield, because of buoyancy, enough soil may beadmitted through small openings in the facebulkhead and left in the invert to balance theforces until the interior lining is placed. Thismethod is called shoving half-blind. In the firsttube of the Lincoln Tunnel, New York City, about20% of the material was taken in. If displacementor heaving of the soil may cause disturbance ofadjacent structures, such as buildings or anothertube nearby already in place, the openings shouldbe adjusted to admit nearly all the displacedmaterial. This was done in the second and thirdtubes of the Lincoln Tunnel, through openingsaggregating 5 to 20% of the face area. Averagedaily progress was about 30 ft.

Shields are usually started from shafts sunk tothe invert grade. These shafts may be speciallyconstructed for this or may later be part of aventilation building. An opening is provided in theshaft wall to fit the shield and is closed by a timberbulkhead during sinking operations. The shield iserected on a concrete cradle at the base of the shaft.The opposite shaft wall forms the abutment for thejacking forces. A few rings are erected behind theshield, which is advanced through the openingafter removal of the bulkhead.

The shield is steered by varying the pressure ofthe shoving jacks around the periphery. On largetunnels, the total jacking force may be 3000 to5000 tons. If the shield has a tendency to rise, morepressure is applied at the top than at the bottom.Similar corrections are made for other directions.

If the soil is relatively loose, it is excavated at theface by hand tools. In hard-packed silty sands orvery stiff clay, air spades are used. Relatively softclay may be cut by clay knives. The muck may beshoveled by hand on a short conveyor in the shield,but more commonly, a large hydraulic scraper or




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hoe is used to loosen the soil and scoop it onto theconveyor. From there, it is discharged on a loadingconveyor mounted on a movable carriage behindthe shield. The loading conveyor dumps it intomine cars, usually of about 4-yd3 capacity in largetunnels. The muck trains are rolled back throughthe tunnels to an access shaft. The individual carsare hoisted up the access shaft and dumped intohoppers for discharging into trucks.

Tunnel Linings n Except in very stiff orcompact soils, segmental ring liners are used inshield tunnels. These used to be of cast iron buttoday steel or precast concrete is used. Thesegments are brought in by mine cars, unloadedby hoists mounted on the conveyor carriage, anddeposited within reach of the erector arm. This is atelescoping, counterweighted arm pivoted on thecenter line of the tunnel for full rotation by ahydraulic motor (Fig. 20.17). A gripper at itsouter end engages lugs or bars in the segmentsand places these, starting at the bottom. Ashort, tapered segment forms the key. See alsoArt. 20.17.

Packing n Since the shield has a largerdiameter than the lining, a void exists around theliner rings. This may permit a cave-in and causesettlement. The usual practice when segmentalliners are used is to inject pea gravel into this voidthrough grout holes in the liners immediately afterthe shield has been advanced (Fig. 20.16). Cementgrout is later injected into the gravel to solidify it. Ina section of the Victoria line of the London subwayin deep, very stiff clay, an articulated cast-ironlining was installed and expanded against the claybehind the shield. The adjacent rings were pressedinto contact by the jacking forces but werenot bolted. Expansion of steel ribs with woodlagging has also been used to achieve tight fitagainst the soil.

Semicircular or semielliptical shields havebeen used as temporary supports for the roof orarch of excavations, mostly in dry or dewateredsoils, for example, for tunnels at shallow depthwhere open-cut operations are prohibited bycircumstances. They are advanced in a mannersimilar to that for circular shields.


Fig. 20.17 Section through a conventional shield (used in 1930 for the Detroit, Mich.—Windsor, Ont.,Tunnel) for tunneling with compressed air.




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20.16 Compressed-AirTunneling

Although tunnel shields in free air are effective innaturally dry soil or ground that can be dewatered(Art. 20.15), compressed air is needed whiletunneling below the water table, particularly insubaqueous tunnels. The air pressure counteractsthe hydrostatic head. Also, the pressure reduces thewater content of the soil at the face, making it morestable and safer to excavate.

Legal issues surrounding safety and healthissues of compressed air tunneling have reduceduse of this method. It still has to be used from timeto time to remove obstructions in front of tunnelboring machines that they cannot handle, and tocarry out maintenance and repairs on some of themachines.

Air Pressure n Theoretically, the air pressurerequired to balance the hydrostatic head is 0.43 psifor fresh water and 0.44 psi for seawater per foot ofdepth. Actually, the pressure depends on theproperties of the soil as well as on the method ofexcavation. In open material, such as perviouscoarse sand and gravel, the full air pressure wouldbe required, whereas in impervious soils, such asstiff clay, no pressure at all may be needed. Acareful analysis of the soil at regular intervals alongthe alignment is needed to estimate the maximumair pressure and air quantities required. Closedshields for tunnels in the Hudson River siltoperated with as little as 16-psi air pressure indepths up to 100 ft. In the sand and gravel underthe East River in New York, the hydrostatic headwas balanced for about one-quarter or one-thirdthe diameter above the invert. To reduce loss of airat the top of the face, breast boards were plasteredwith clay.

Blowout Prevention n With the air pressurebalancing the head at the bottom of the face, there isan excess of pressure at the top. If the weight ofcover over the tunnel is insufficient to hold theexcess air pressure safely, a heavy clay blanket maybe placed on the river bottom over the tunnelheading to prevent a blowout at the top of the face.If the air pressure equals the water pressure at theinvert, the excess pressure at the top of a 30-ft-diameter face would be 13 psi in seawater, or1870 lb/ft2. For a 10-ft natural cover of 50-lb/ft3

material, the blanket would have to make up thedeficiency of 1370 lb/ft2. At 60 lb/ft3 submergedweight, 23 ft of clay would be required. Navigationrequirements may make it necessary to remove theblanket after completion of the tunnel. Clay for thisblanket should be relatively soft so that it willreadily coalesce into an impervious layer.

Bulkheads n When the shield is started from ashaft, an airtight deck is built above the tunnel, tohold the pressure until the shield is advanced somedistance. An airtight bulkhead is then built into thetunnel a sufficient distance behind the shield toprovide space for the loading conveyor and a fewmine cars. To keep the volume filled withcompressed air within reasonable limits and tocomply with safety laws, new bulkheads are builtas the tunnel advances and the old bulkheads areremoved. Usually, regulations permit a maximumdistance of 1000 ft between the face and thebulkhead, which may be constructed of steel orconcrete.

Air Locks n Worker and material locks arebuilt into the airtight bulkhead. The locks areairtight cylinders at least 5 ft in diameter and havegasketed doors (Fig. 20.17). Worker locks shouldprovide at least 30 ft3 of air space per occupant.Compressed air is admitted to the lock from thehigh-pressure side or from the compressed-air lineand is exhausted through a connection to the free-air side. Valves of these connections are controlledfrom the inside in worker locks and generally fromthe outside in material locks. The door at the high-pressure side opens from the lock into the tunnel;the door at the free-air end opens into the lockchamber. Doors are held tight by the air pressureand cannot be opened until pressures on both sidesare equalized. Pressure gages are provided in thelocks as well as in the tunnel.

The material lock is at the level of the mine-cartrack. The lock should be large enough toaccommodate several mine cars.

The worker lock is at a higher level and musthave not less than 5 ft clear head space. This lock isequipped with benches for workers to sit down on.In large tunnels, two sets of locks may be used tospeed up operations. If there is danger of rapidflooding, an extra worker lock may be placed ashigh as possible, and a hanging safety walkextended at this level from the lock to the shield.




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A safety screen placed in the upper part of thetunnel near the heading will trap air above thissafety walk in case of flooding and permit workersto escape. Some safety laws require installation oftwo worker locks.

A special decompression chamber capable ofaccommodating an entire shift of workers shouldbe available when decompression time required ismore than 75 min. A passageway should beprovided to give workers in a man lock access tothe special chamber.

Safety and Health n For all compressed-airwork, a well-equipped first-aid station anddecompression chamber are required, staffed by atrained attendant at all times. A physician must beavailable at all times for emergency calls whilework is in progress.

Most states and countries have laws regulatingthe working hours and locking rates for com-pressed-air work. Regulations of the U.S. Occu-pational Safety and Health Administration forwork in compressed air as well as for constructionin general and all underground operations shouldbe observed. (See also Arts. 20.6, 20.8, and 20.15.)OSHA requires that a record be kept outsideworker locks of the time in each shift that workersspend in compression and decompression. A copyshould be given to the supervising physician.

During the first minute of compression in a lock,pressure may be increased up to 3 psig and shouldbe held at that level, and again at 7 psig, longenough to determine if anyone in the lock is beingadversely affected. After the first minute, pressuremay be raised gradually at a rate up to 10 psi/min.If personnel experience discomfort, pressureshould be reduced to atmospheric and distressedpersonnel should be evacuated from the lock.Except in emergencies, pressure in a lock shouldnot exceed 50 psi. Temperature in a lock should beat least 70 8F but not more than 90 8F., whereastemperature in compressed-air working areasshould not exceed 85 8F.

Unless air pressure in the working chamber isless than 12 psig, decompression in a worker lockshould be automatic. Manual controls, however,should be provided inside and outside the lock tooverride the automatic mechanism in emergencies.The lock should have a window at least 4 in indiameter to permit observation of the occupants

from the working chamber and free-air side ofthe lock.

OSHA requires also that at least 30 ft3/min ofventilation air be supplied per worker in theworking chamber. In addition, OSHA specifies thatat least 10 ft-c be provided by electric lights onwalkways, ladders, stairways, or working levels.Two independent supply sources should be used,including an emergency source that becomesoperative if the regular source should fail. Externalparts of electrical equipment, including lightingfixtures, when installed within 8 ft of the floor,should be made of grounded metal or noncombus-tible, nonabsorptive, insulating material.

OSHA also requires that sanitary, comfortabledressing and drying rooms be provided forworkers employed in compressed air. Facilitiesshould include at least one shower for every 10workers and at least one toilet for every 15 workers.Fire-fighting equipment should be available at alltimes in working chambers and worker locks.

OSHA requirements for total decompressiontime, which depends on the air pressure in theworking chamber and the time of worker exposureto that pressure, are listed in Table 20.3. Decom-pression should take place in two or more stages,but not more than four. (Four stages are requiredfor pressures of 40 psig or more.) In Stage 1,pressure may be reduced at a rate up to 5 psi/minfrom 10 to 16 psi, but not to less than 4 psig. In laterstages, the rate of pressure reduction may notexceed 1 psi/min. Local rules, however, alsoshould be checked. Limits of union contracts,though, are sometimes more stringent than legalrequirements.

The amount of air required for compressed-airtunneling depends on so many variables that exactrules cannot be given. To determine the size of thecompressor plant for a given job requires a greatdeal of judgment by the engineer, based on pastexperience. Low-pressure machines are installedfor the tunnel air and high-pressure units for airtools. Adequate standby capacity must be providedby using a number of compressors. High-pressureair may be used as an emergency tunnel supply byinterconnecting the compressors through reducingvalves.

Shieldless Tunneling n Some tunnels havebeen built in water-bearing ground by usingcompressed air in conjunction with liner plates,




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without a shield. Considerable lengths of 7- to 12-ft-diameter interceptor sewers in New York wereconstructed this way. A few miles of Chicagosubway were built in soft clay with steel ribs andliner plates under compressed air.

[J. O. Bickel and T. R. Kuesel, “TunnelEngineering Handbook,” Van Nostrand ReinholdCompany, New York; “Construction Industry:OSHA Safety and Health Standards (29 CFR1926/1910,” Superintendent of Documents,Government Printing Office, Washington, DC20402 (www.gpo.gov).]

20.17 Tunnel Linings

Unlined Tunnels n Tunnels in very soundrock, not affected by exposure to air, humidity, orfreezing, and where appearance is immaterial, areleft unlined. This is the case with many railroadtunnels.

Unlined water tunnels in rock are susceptible toleakage either into or out of the tunnel, dependingupon the relative pressures. There is therefore arisk that material could be washed out of weakzones and fissures, potentially leading to instabilityunless lined. However, Norwegian hydropowertunnels in good crystalline rock are often unlinedfor most of their length.

Shotcrete Lining n Where rock is structurallysound but may deteriorate through contact withwater or atmospheric conditions, it can beprotected by coating with sprayed concrete,reinforced with wire fabric or fibers, or unrein-forced (Art. 20.13). Such a lining may also be usedin water tunnels in good rock to provide a smoothsurface, reducing the friction factor and turbulence.

Cast-in-Place Concrete n Most tunnels inrock, and all tunnels in softer ground, require asolid lining. Highway tunnels of any importanceare always lined for appearance and better lighting

Table 20.3 Total Decompression Time, Min, after Construction Work in Compressed Air*

Working-chamber,

psig

Working period, h

1⁄2 1 11⁄2 2 3 4 5 6 7 8 Over 8

9 to 12 3 3 3 3 3 3 3 3 3 3 314 6 6 6 6 6 6 6 6 16 16 3316 7 7 7 7 7 7 17 33 48 48 6218 7 7 7 8 11 17 48 63 63 73 8720 7 7 8 15 15 43 63 73 83 103 11322 9 9 16 24 38 68 93 103 113 128 13324 11 12 23 27 52 92 117 122 127 137 15126 13 14 29 34 69 104 126 141 142 142 16328 15 23 31 41 98 127 143 153 153 165 18330 17 28 38 62 105 143 165 168 178 188 20432 19 35 43 85 126 163 178 193 203 213 22634 21 39 58 98 151 178 195 218 223 233 24836 24 44 63 113 170 198 223 233 243 253 27338 28 49 73 128 178 203 223 238 253 263 27840 31 49 84 143 183 213 233 248 258 278 28842 37 56 102 144 189 215 245 260 263 268 29344 43 64 118 154 199 234 254 264 269 269 29346 44 74 139 171 214 244 269 274 289 299 31848 51 89 144 189 229 269 299 309 319 31950 58 94 164 209 249 279 309 329

* For normal conditions, as specified by the Occupational Safety and Health Administration in “Construction Industry: OSHASafety and Health Standards (29CFR 1926/1910),” revised 1991.




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conditions. Stone or brickmasonry has been used toa great extent in the past, but currently concrete ispreferred. The thickness of the permanent concretelining is determined by the size of the tunnel,loading conditions, and the minimum required toembed the steel ribs of any primary lining.

The lining is placed in sections 20 to 30 ft long.Segmental steel forms are universally used andmust be properly braced to support the weight ofthe fresh concrete. The walls are usually concretedfirst, up to the spring line. Next come the archpours. It is important that the space between theforms and the rock or soil surface be completelyfilled. Grout pipes should be inserted in the archconcrete to permit filling any voids with sand-and-cement grout.

Concrete is placed through ports in the steellining or pumped through a pipe introduced in thecrown, a so-called slick line. Placement starts at theback of the pour, and the pipe is withdrawn slowly.A combination of both methods may be used.Concrete is either pumped or injected by slugs ofcompressed air. Admixtures are added to get aneasily placed mix with low water content and toreduce concrete shrinkage. If there is leakageof water, it usually occurs at shrinkage cracks,which may be sealed with a plastic compound. Orthe water may be carried off by copper drainagechannels installed in chases cut in the concrete(Art. 20.9).

Footings for side walls in rock tunnels are cutinto the rock below grade. They give adequatestability unless squeezing ground is encountered,in which case a concrete invert lining is placed. Insoft ground, a concrete slab is placed, to serve aspavement in highway tunnels. If heavy sidepressure exists, this slab may have to be madeheavier to prevent buckling.

Unreinforced Concrete Lining n A con-crete lining is placed to protect the rock andprovide a smooth interior surface. Where theconcrete lining is exposed to compression stressesonly, it may be unreinforced. Most shafts notsubject to internal pressure are lined withunreinforced concrete. Shrinkage and temperaturecracks are probable and may cause leakage. Wherethere is a risk of non-uniform loading, unreinforcedliners are not used, such as in squeezing groundand through soil overburden.

(Recommendations in Respect of the Use ofPlain Concrete in Tunnels, AFTES c/o SNCF, 17Rue d’Amsterdam, F75008 Paris, France.)

Reinforced Concrete Lining n In mostcases, reinforcing steel will be required to with-stand tension and bending stresses. Reinforcementis usually required at least on the inside face toresist temperature stresses and shrinkage, althoughreinforcement elsewhere may be needed to resistmoments.

Linings for Shield Tunnels n Linings forshield tunnels may be one-pass or two-pass. A one-pass lining system is when the final lining is alsothe initial lining, usually for tunnels in soil. With atwo-pass lining system, an initial lining is installedbehind the shield just sufficient to allow the shieldto advance while a waterproofing membrane isinstalled and the final cast-in-place reinforcedconcrete lining is prepared. The advance rate isthus usually faster and costs fall. The initial liningmay be segmental rings with minimal bolting forease of erection (Fig. 20.18), or steel ribs withlagging. Precast concrete segments are now widelyused and the use of cast iron and fabricated steelare rare due to their high cost. Although the initiallining may be designed as part of the final lining,any leakage through the seals would result in thefull hydrostatic pressure acting on the inside finallining for which it should be designed.

Pipe in Tunnel n Water and sewer tunnels upto 14 ft diameter are often provided with aninternal pipe that forms the inner lining. After thepipe is secured against movement, the spacebetween the initial ground support and the pipeis filled with cellular or mass concrete. Sewer pipesmay require a further interior lining to protectagainst corrosive liquids and gases. Water tunnelswith a high internal pressure exceeding theexpected external pressures are usually providedwith a steel lining if a reinforced concrete lining isinsufficiently strong. Since the pipe may bedewatered, it must also be designed for the externalpressure, which, if the pipe has leaked, may equalthe internal pressure.

(U.S. Army Corps of Engineers Manual, 1997,Design of Tunnels and Shafts in Rock, EM 1110-2-2901.)




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In stiff soils, steel ribs, usually 4-in H beams, andwood lagging may be used as primary lining. Theribs are usually spaced 4 ft c to c and are erected inthe tail of the shield. Precut and dressed woodlagging is placed solidly around the circumferencebetween the flanges of the ribs. This lagging alsotransfers the jacking forces to the tunnel lining.Precast-concrete lagging has also been usedsuccessfully.

Segments are made as long as convenienthandling permits, usually 6 to 7 ft. The width ofthe rings depends on the distance the face can besafely excavated ahead of the shield and weight tobe handled. The wider the rings, the longer the tailof the shield and hence the more difficult thesteering of the shield. Early tunnels had 18-in-widerings. Recent tunnels have gone to 30 or 32 in.

Segments are made to close tolerances on allsides. They are connected by high-strength bolts.Longitudinal joints are offset in successive rings.

The flanges have recesses along their matchingedges for calking. These grooves used to be filledwith lead or impregnated asbestos calking strips,pounded in manually. Synthetic sealers, such assilicone rubber and polysulfides, can be injectedinto the grooves by calking guns. These compoundsadhere to the metal sufficiently to form an effectiveseal under pressures usually encountered inunderwater tunnels.

Each cast-iron segment is provided with a 2-ingrout plug for injection of pea gravel and grout intothe space between the lining and the soil. Bolt holesare sealed with grommets of impregnated fabric orplastic grommets, the latter being particularly

Fig. 20.18 Cross section shows typical segmental cast-iron lining for a tunnel (Lincoln Tunnel underthe Hudson River).




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effective. Bolts are tightened with hydraulic orpneumatic wrenches where possible otherwisewith hand wrenches.

Welded steel segments, similar in shape to cast-iron segments, have been used for economicreasons in some subaqueous tunnels. They werewelded in jigs to tolerances as close as practicable,but flanges were not machined and no calkinggrooves were provided. Difficulties were experi-enced in making them watertight with gaskets.An improved design includes calking groovesand fabrication tolerances similar to those forcast iron.

Precast Concrete n Precast segments areessential to increasing the speed of machinetunneling. A compromisemust be reached betweenthe segment size and the number of segments to beinstalled, directly affecting the weight of thesegments, the size of the equipment needed tohandle the segments, and the number of operationsto be carried out. The width of the segments isgoverned by the stroke of the jacks pushing thehead of the shield, usually in the range of three tofive feet. Tapered rings, narrower on one side, areused on bends. At least three segments per ring arerequired, with five to eight being more common.The closing segment in a ring is usually smaller andwedge shaped to facilitate insertion. Joints inadjacent rings are usually staggered so that alljoints are discontinuous, helping to stiffen therings.

Connection details to adjacent segments varywidely and can be flanged (Fig. 20.19). Straightbolts with nuts, washers and grommets are themost common, but the use of curved recessed boltsresult in smaller pockets. Gaining popularity arestraight bolts placed at an angle to minimizerecesses; the bolts couple into sockets cast into theadjacent section. Dowels may also be used betweenadjacent rings. The bolts ensure that the rubber orneoprene seals between segments are compressed.The addition of a hydrophilic seal near the outsideface may reduce leakage even further. Due to thevery close tolerances needed to ensure seals remainwatertight and that the diameter remains constant,a high degree of mechanization with steel forms isused. The segments must be installed within theshield tail and the space behind them (the tail void)grouted at a pressure at least equal to the externalpressure, making lateral alignment modifications

very difficult. It is not uncommon for most bolts tobe retrieved once the grout is set. Secondary liningsare not essential.

Heavy, interlocking concrete blocks have beenused successfully in relatively dry or impervioussoil. They present difficulties when exposed towater pressure due to leakage.

Except where steel rings and lagging or concreteblocks are used as primary lining, no secondaryconcrete lining is used, unless required forappearance and interior finish of highway tunnels.In this case, a concrete lining of the minimumthickness practicable is placed. When the tunnel isto be faced with tile, provision should be made forattaching it. (To facilitate maintenance and improvelighting, walls and ceilings of highway tunnels areusually finished with ceramic tiles.) To providegood adherence of the scratch coat, scoring wiresmay be welded longitudinally on the steel formsfor the lining to provide a rough concrete surface.Coating of smooth concrete surfaces with epoxycompound may result in satisfactory finishes atless cost.

See also Art. 20.18.

20.18 Design of TunnelLinings

Article 20.17 discusses the types of linings usuallyused for tunnels. The following paragraphsdescribe design of a liner ring.

A liner ring is statically indeterminate. A one-pass lining is designed for transport and erectionloads, loads during grouting, and ground loadsincluding seismic. In lieu of computer analyses,which might be as simple as a two-dimensionalanalysis of a grid framework supported on springs,or as complex as finite element or finite differencethree-dimensional analyses using soil-structureinteraction for each step of the constructionprocess, stresses in the liner ring may be computedafter the ring is made statically determinate by acut at the top and one end is fixed (Fig. 20.20).

For a circular ring of constant cross sectionsymmetrically loaded the thrust at the crown C is

Tc ¼ 2

pR

ðp0

M cosf df (20:2)




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Fig. 20.19 Typical liner segments used for rapid-transit tunnels.




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The vertical shear at the crown is zero, and themoment is

Mc ¼ �RTc � 1

p

ðp0

Mdf (20:3)

where R ¼ radius of ring

M ¼ bending moment at any point U due toloads on CU

f ¼ angle between U and crown C

With the thrust and moment at the crown known,the stresses at any point on the ring can becomputed, as for an arch (Art. 6.71).

(A set of equations is presented in Chapter 15B‘‘Tunnel Structures, Structural Engineering Hand-book,’’ 2000 Update for ENGnetBASE, Edited byWai-Fah Chen and Lian Duan, CRC Press, 2000(www.crcpress.com).)

Loads on a lining include its own weight andinternal loads, weight of soil above the tunnel(submerged soil for tunnels below water level),reaction due to vertical loads, uniform horizontalpressure due to soil and water above the crown,and triangular horizontal pressure due to soil andwater below the crown.

Magnitude of loads on tunnel liners depends ontypes of soil, depth below surface, loads fromadjacent foundations, and surface loads. These willrequire careful analysis, in which observationsmade on previous tunnels in similar materials willbe most helpful.

In rock, the quality of the rock will affect theloads that are carried by the tunnel, and loadscarried by any initial rock support may effect the

loads carried by the secondary lining. Compressionof competent rock due to outward displacement ofthe tunnel lining in a pressure tunnel may alsoneed to be considered. Often those linings must bedesigned to take the full internal pressure. Beyond100 psi internal pressure, reinforced concrete linersmay no longer be sufficient and steel liners may beneeded. If a tunnel is watertight, the interior liningis usually designed to carry at least the full externalwater pressure, since leaks in any outer linings willeventually lead to full transfer of the hydraulichead. If the tunnel is drained, at least some of thehydrostatic head should be considered. Blastingmay also disturb the rock locally, leading to loadsdifferent to those of a bored tunnel.

Following the derivation of moments, axialthrust and shears, the concrete cross section can bedesigned accordingly, and steel or fiber reinforce-ment placed accordingly as needed. Tension cracksin themselves do not necessarily result in failure,whereas through-cracks (often caused by shrink-age) can cause leakage and corrode exposed steel. Itis usually undesirable for cracks to extend morethan halfway through the section. Typical steelreinforcement for crack control may reach 0.28% ormore of the section area. Restraints at the exteriorface due to keying into an irregular rock surfacemay change the calculated behavior. Linings withirregular width are more likely to crack at thethinnest sections or at initial ground supportembedments. Waterstops are used at constructionjoints to reduce leakage.

Because of flexibility, tunnel liner rings can offeronly limited resistance to bending produced byunbalanced vertical and horizontal forces. Thelining and soil will distort together until a state ofequilibrium is obtained. If the deflection, in,exceeds more than 1.5D/10, where D is the tunneldiameter, ft, the lining may have to be temporarilybraced with tie rods when it leaves the shield untilthe final loading conditions and passive pressureshave been developed. In certain soft materials,when shields were shoved blind (without materialbeing excavated), initial horizontal pressuresexceeded the vertical loads, so that the verticaldiameter lengthened temporarily. Ultimately, thesection reverted to approximately its initial circularconfiguration.

When a lining is in rock, determination of theloads imposed on the lining need to be done withcare. Stable rock may distribute the stresses aroundthe tunnel, and if impervious, may leave any

Fig. 20.20 Stresses in liner ring may becomputed by assuming it cut at crown C.




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tunnel lining virtually unloaded. Following exca-vation, rock that has not yet reached stability canstill be moving, extreme examples of which aresqueezing and swelling rock. Further displace-ments may be little affected by the presence of thelining in such cases, or may depend upon therelative stiffnesses of the two, so that the liningmust be designed accordingly. Concrete castagainst irregular rock may also be keyed into therock and result in composite action. If the rock isanisotropic, material properties and movementsmay depend upon direction. Some external loads,such as groundwater pressure and some clays, areindependent of displacements. Depending uponthe porosity of the surrounding material, waterpressure can eventually build up to the fullhydrostatic load even in a rock tunnel.

Potable water supply tunnels may need to bemade watertight when passing though areas wherethe inflow of groundwater is not acceptable. Wheregroundwater contains fine silt or chemicals thatcould clog drainage facilities on which the tunneldesign is based, regular maintenance is required tokeep drainage paths clear, or else new drainagepaths must be provided or the tunnel designed aswatertight. Sewage tunnels frequently generatehydrogen sulfide and so require extra protectionagainst corrosion, such as using an internal PVC orHDPE membrane cast into the internal tunnellining.

Precast Lining Segments n Analysis anddesign must cover all aspects of fabrication,storage, transportation, installation, jacking loads,and expected loads in service. Allowance for creepand shrinkage during all stages is required. It is notuncommon for segments to be installed slightlyaskew, resulting in extra bolting forces and non-uniform loads. Curved bolts although easier toinstall, require extra reinforcement. Attention toreinforcement details at corners can help to reducedamage. Compressed gaskets tend to create tensilestresses that may require reinforcement. Durabilityof the tunnel lining is highly desirable, and may beenhanced by using low permeability crack-freeconcrete, the use of pozzolans to resist sulfateattack and microsilica to improve strength, protect-ing the bolts and reinforcement against corrosionsuch as by applying epoxy coating to the fabricatedreinforcement, and by the use of external water-proofing, including quality grout and a bitumencoating to the exterior face.

[Kuesel, T. R., Tunnel Stabilization and Lining,in ‘‘Tunnel Engineering Handbook,’’ Bickel, J. O.,Kuesel, T. R., and King E. H., Editors, Chapman &Hall, 1996 (www.wkap.nl)] [U.S. Army Corps ofEngineers Manual, 1997, Design of Tunnels andShafts in Rock, EM 1110-2-2901 (www.USACE.ARMY.MIL/inet/USACE-docs/eng-manuals).] (Thedesign, sizing and construction of precast concretesegments installed at the rear of a tunnel boringmachine, 1997, translated into English 1999, FrenchTunneling Association AFTES c/o SNCF, 17 Rued’Amsterdam, F75008 Paris, France.)

20.19 Machine Tunneling

To reduce costs and increase the speed of the ever-increasing amount of tunnel construction, anumber of tunnel-boring machines (TBM) for rockand soft ground have been developed. Universalmachines for mixed ground of rock and softmaterial (mixed face) are designed for each specificlocation and have opened up new possibilities formachine tunneling.

Hard Rock TBMs n Rock-boring machinesconsist of a rotating head, either solid or withspokes, on which are mounted cutting toolssuitable for the type of rock. The machines aremounted on large frames, which carry the drivingmachinery and auxiliaries, including a series ofhydraulic jacks to exert heavy pressure against theface. Chisel cutters serve for soft rock, disk cuttersbreak harder rock by wedge action, and toothedroller cutters with tungsten carbide inserts cut thehardest rocks. A critical factor in evaluatingproduction is the amount of down time formaintenance and replacement of cutters andtheir cost. Most long tunnels in rock use hard-rockTBMs.

Soft Ground TBM n Two main types of tunnelboring machine (TBM) are used in soft ground, aslurry TBM and an earth pressure balance (EPB)TBM. Both types operate with a sealed frontcompartment that is kept under sufficient pressureto stabilize the face and minimize ground move-ment. EPB TBMs have been limited to diametersunder 33 ft due to the high torque needed to drivethe rotating cutter head, although other forms ofdrive may overcome this limitation. Slurry TBMshave been built up to 50 ft diameter, and largersizes are planned. Settlements at the surface in soft




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ground are directly related to the percentage loss ofmaterial outside the tunnel. Typical loss of materiallies between 0.5% and 2.5%. Factors affecting theloss include the properties of the materialtraversed, the face pressure used, the design of theshield, and the rate of advance. Tunnels exist wherethe loss has been zero. Soft ground TBMs aregenerally launched from a relatively small shaft,with subsequent parts of the machine being addedas progress is made.

The sealed front compartment of a slurry TBM isusually filled with a bentonite slurry held in

equilibrium with the soil and groundwaterpressures acting at the face. The equilibrium isoften balanced by a compressed air reservoir andflow controls. The slurry also acts as a lubricant andholds loosened soil in suspension. The maindisadvantage of a slurry TBM is that the slurrymust be continuously circulated through a separa-tor, often located on the surface, to remove theexcavated material before returning the recondi-tioned slurry to the face. One main advantage isthat the underground operators never come intocontact with the excavated material. The slurry

Fig. 20.21 Preparing the Cairo Metro Shield for Assembly.




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TBM also has better face control, especially inmixed-face and bouldery ground. Many slurryTBMs also incorporate a boulder-crushing unit.

Soil excavated by the rotating cutter head of anEPM TBM falls behind the head and is removed byscrew conveyor that discharges either onto aconveyor belt or directly into muck cars. Theexcavated material may be conditioned by theaddition of water, clay or by biodegradableadditives to assist in lubrication and to providebetter pressure drop through the screw conveyor,thus preventing the direct exit of groundwater. Therate of advance must be closely coupled to the rateof advance to avoid excessive ground movement.

Flowing Ground n Tunneling machines havebeen developed for use in flowing ground, toconfine the pressure to a small space between theface and a bulkhead behind the cutting wheel. Theyuse a pressurized slurry composed of bentonite orof excavated material, to balance the pressure. Thesolids are settled out of the recirculated slurry, andtheir volume is accurately measured to determinethe advance of the machine.

In very soft materials, the volume of materialexcavated must match the advance of the machine,or else sink holes may appear or mounds may bepushed up, in both cases causing unacceptablylarge ground movements.

20.20 Immersed Tunnels

Immersed tunnels should be considered for allwater crossings. They are the shallowest form oftunnel, requiring only minimal protection againstsinking ships and dropping anchors, with 5 ft ofgranular material and scour protection often beingadequate. Because they are so shallow, they resultin the shortest combined length of tunnel andapproaches, and because they may serve theirfunction better than other choices, the overall costcan be less. Approach gradients can also be flatter.Immersed tunnels can be constructed in groundconditions that would make bored tunnelingdifficult or expensive, such as the soft alluvialdeposits characteristic of large river estuaries, butwhen rock has to be excavated under water,immersed tunneling may be less cost effective.Consequently, the ideal alignment for an immersedtunnel may not coincide with the ideal alignmentfor a bridge. Alignments do not need to be straight,

so immersed tunnels can be designed to suit designspeeds, existing land uses, topography, andconnections to existing road or rail systems.

(L.C.F. Ingerslev, “Water Crossings—theOptions,” Tunneling and Underground Space Technol-ogy, Vol. 13, No. 4, pp. 357–363, Elsevier ScienceLtd., 1998.)

Immersed tunnels consist of very large precastconcrete or concrete-filled steel tunnel elements.They are fabricated in convenient lengths onshipways, in drydocks, or in improvised floodablebasins, sealed with bulkheads at each end, and thenfloated out. They may require outfitting at a pierclose to their final destination before being towedto their final location, immersed, lowered into aprepared trench, and joined to previously placedtunnel elements. After any further foundationworks have been completed as discussed below,immersed tunnels are backfilled and the bedreinstated (Fig. 20.22). Where intrusion into thewater column is permitted, the final bed level maybe higher than the original. The side slopes of theexcavated trench depend upon the soil character-istics; often a slope of 1:1.5 is feasible undertemporary conditions, although flatter grades andan allowance for possible sloughing may berequired in softer materials.

Floating Tunnels n For particularly deepafter-crossings at a number of locations, designshave been proposed for tunnels that remain totallyexposed within the water column. These “floating”tunnels may be supported below the water surfaceon piers rising from the bed, unsupported if thedistance between ends is short, supported fromfloating pontoons, or even held down by cablesif positively buoyant. The text in Section 20.20applies to floating tunnels as well as immersedtunnels, except for foundations and backfilling.Whereas immersed tunnels need only considerdynamic loads up to time that they are finallyplaced, floating tunnels have to be designed fordynamic loads throughout their life. They appearto be particularly attractive for deep narrowwaterways, since the overall length of tunnel maybe significantly shortened compared to other formsof tunnel. While most immersed tunnels are builtfor water depths of between 5 m and 20 m,concepts for 100 m depth have been prepared.Floating tunnels could avoid the need to be so




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deep, as long as risks from ship and submarinecollision can be handled.

(“International Tunneling AssociationImmersed and Floating Tunnels Working GroupState-of-the-Art Report,” Second Edition, Perga-mon, April 1997.)

Internal Dimensions n Highway design orclient requirements should determine the requirednumber of traffic lanes, tracks, or internal spaces. Itis usual to avoid climbing lanes within immersedtunnels elements themselves, and nominal widthsof emergency lanes or shoulders have almostalways been used to minimize costs. If tunnels areparticularly long, extra width may have to beprovided at intervals to permit emergency stop-ping. Curbs, or more usually barriers, are providedto protect the walls from traffic impact. Barriersover 2 ft high may make the lane width seemnarrower and slow motorists. Emergency accessinto an adjacent tunnel should be available, say at300 ft intervals, which would require an emergency“walkway” at least 2 ft wide on top of the adjacentbarrier. Such emergency “cross-passages” mayneed to be provided at intervals of say 100 meters.Extra space may be needed for tunnel and otherutilities, construction and misalignment tolerances,lighting, lane signs, and highway signs, whilekeeping the clearance height as low as possible.Escape ducts, when provided, should be slightlyover-pressurized relative to adjacent ducts toprevent entry of noxious fumes.With theminimumspaces determined, space allowances for anynecessary ventilation system (such as for jet fansor additional ducts) can then be evaluated. Thecritical design case may be for moving or stalled

traffic, but a fuel fire usually governs. Permittedclasses of vehicles may be restricted by legislationor owner requirements to limit it the potential sizeof the fire. Finally, additional air space may beneeded for the tunnel elements to be able to justfloat when completed with bulkheads in place, andperhaps space for additional ballast, either insideor out, to stay submerged when completed andbulkheads removed. Floating tunnels that rely onbuoyancy must have sufficient compartmentalizedbuoyancy to stay afloat in case of accidentaldamage to two adjacent compartments.

[L.C.F. Ingerslev et al, Chapter 15B TunnelStructures, ‘‘Structural Engineering Handbook,’’2000 Update for ENGnetBASE, Edited by Wai-FahChen and Lian Duan, CRC Press, 2000 (www.crcpress.com).]

Construction n The technique of immersedtunneling is often less risky than bored tunneling,since tunnel element manufacture can be bettercontrolled due to the construction of the elementsin a controlled environment in the dry. As a result,immersed tunnels are nearly always much morewatertight and therefore drier than bored tunnels.

Twomain types of tunnel have emerged, knownas steel and concrete. Steel tunnels use structuralsteel, usually in the form of stiffened plate, workingcompositely with the interior concrete, whereasconcrete tunnels do not, relying on steel reinforcingbars or prestressing cables. The number of concretetunnels is a almost twice that of steel tunnels. Steeltunnels can have a draft of as little as about 8 ft,whereas concrete tunnels have a draft of almost thefull depth. Tunnel cross-sections may have flatsides or curved sides. Historically, concrete tunnels

Fig. 20.22 Immersed tunnels are set in a trench, which is then backfilled.




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have been circular, or curved with a flat bottom, butthe predominant shape has been rectangular (Fig.20.23), which is particularly attractive for widehighways and combined road/rail tunnels. Steeltunnels have been circular, curved with a flatbottom, and rectangular (particularly in Japan), butthe predominant shape in the past in the US hasbeen a circular shell within an octagonal shape,with ventilation ducts above and below the road-way, either in single tube or binocular versions. Thisarrangement of ventilation ducts may change, sincecurrent techniques permit the use of longitudinalventilation in much longer tunnels, often obviatingthe need for separate ventilation ducts. Steeltunnels can be categorized into three sub-types:

† Single shell, where the structural shell plateworks compositely with the interior reinforcedconcrete and the shell plate requires corrosionprotection (Fig. 20.25).

† Double shell, where the structural shell plateworks compositely with interior reinforcedconcrete and is protected by external concreteplaced within a non-structural form plate (Fig.20.26). This shape has also been used in pairs forseveral tunnels, the most recent being TedWilliams in Boston, Massachusetts. Double shelltunnels are only found in the US.

† Sandwich, where structural steel plates, bothinternal and external, are connected by dia-phragms and the internal space is filled withunreinforced self-compacting concrete.

Concrete Tunnels n All but four of theconcrete transportation tunnels built have beenrectangular (Fig. 20.23). They are for road, rail, orfor both road and rail. A number of other tunnelscarrying pedestrians, utilities, sewage or waterhave also been built. Road and rail traffic arecarried in separate ducts. Of all the road tunnels,only one carries four lanes in a single duct. All theothers have three or less lanes per duct. In order tokeep profiles as shallow as possible, any air ductsare usually located at the sides, rather than aboveor below the traffic duct. Because concrete tunnelsare much heavier than steel tunnels when they arelaunched, they are usually constructed within drydocks or purpose-built casting basins (gravingdocks) capable of being flooded for removal of theelements. For many narrower tunnels, some formof catamaran lay barge has been used to supportthem during their immersion and placing, whereassome wider tunnels have had a pontoon placed ontop near each end from which the tunnel waslowered. In most cases, watertightness has beenassured by some form of exterior membrane, whichitself may require protection. Ideally, the mem-brane should adhere to the concrete to limit thespread of any leakage through it. An outer steelmembrane can yield and still remain watertightdespite significant deformations.

An element is a length of tunnel that is floatedand immersed as a single rigid unit. For a fewDutch tunnels and the Øresund tunnel (betweenDenmark and Sweden), the rigidity has beentemporary and was later released, elementsconsisting of a number of discrete segments

Fig. 20.23 Box-section concrete immersed tunnel (Deas Island Tunnel).




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stressed together longitudinally for ease of trans-portation and placing. After placing and release ofthe segments, each may act as a mini-element freeto move at the segment joints. The ability to usediscrete segments can depend upon subsurfaceconditions, acceptable displacements, and suffi-cient capacity to resist seismic effects.

Most tunnel elements are cast in bays, similar tosegments, but continuous across the joints. Typi-cally the floor slab is cast first. The walls and roofmay be cast in either one or two operations. Specialefforts must be made to reduce or preferablyeliminate cracking in the concrete during fabrica-tion. Testing and repair of leaks should becompleted before submerging elements.

(L.C.F. Ingerslev, “Concrete Immersed Tunnels:The Design Process,” Immersed Tunnel Techniques,The InstitutionofCivil Engineers,Telford,UK, 1989.)

Steel Single-Shell Tunnels n Of the eightsingle-shell tunnels with some external curvature,

three are for rail in Tokyo, Japan, and three are forrail in the US including the unique two over twoconfiguration of the 63rd Street Tunnel in Manhat-tan. The Baytown tunnel in Texas was circular withtwo lanes for highway, and the Cross HarbourTunnel in Hong Kong is similar but binocular togive two lanes in each direction. The Detroit Rivertunnel (1910) and the Harlem River tunnel (1914)may not quite fit into this category, being the firsttwo immersed transportation tunnels ever built,but do have similarities to single-shell tunnels andcarry rail.

Figure 20.25 shows the cross-section of the SanFrancisco Bay Area Rapid Transit (BART) trans-baytube with one track in each direction, separated byan exhaust air duct and a service passage. The 57elements have a total length of about 19,000 ft. Thesteel shell is welded to make it continuous acrossthe joints, as is the reinforced concrete lining, toprovide security against major earthquake loading.At the landfall junctions with the ventilation

Fig. 20.24 Western Harbour Crossing, Hong kong.




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buildings, special earthquake joints permit relativemovement in all directions. The shell plate isprotected against corrosion by a coating and acathodic protection system.

All eight rectangular single-shell tunnels arelocated in Japan. They are very similar in layout toconcrete tunnels, the difference being that the outersteel shell works compositely with the concrete,whereas even if an outer steel plate is present as awaterproofing membrane in a similar concretetunnel, it is not considered in the strength design.

Steel Double-Shell Tunnels n Fifteen ofthese tunnels have been built, of which five arebinocular, the most recent being the Ted WilliamsTunnel is Boston. Figure 20.26 shows a two-lanetunnel with a circular interior section. The steel shellplate was 31 ft diameter and about 300 ft long.Exterior diaphragms, approximately octagonal, arespaced about 15 ft apart, and longitudinal ribs of thebars and T sections stiffen the shell. Outside formplates were attached to the diaphragms and suppor-ted by angle struts extended from the shell stiffeners.

The tubes were erected on shipways. All weldswere tested for watertightness with a compressed-air stream and soap solution. Before the ends wereclosed with welded watertight bulkheads, thereinforcing steel for the interior concrete liningwas placed. The keel concrete in the space betweenthe outside form plates and the bottom of the shellwas cast before launching. The concrete lining androadway slab were cast while the element was

floating at a fitting-out pier, by pumping concretethrough hatches in the top of the shell intosegmental steel forms. Pouring sequences wereregulated to control increments of water pressureon the shell plate and longitudinal bendingmoments. The hatches were closed with weldedplates, and the exterior concrete cap was cast andenough tremie concrete placed in the side pocketsto reduce freeboard to about 1 ft. Most, if not all, ofthese tunnels have been immersed using acatamaran lay barge consisting of a barge eachside of the tunnel element connected by twotransverse beams from which the element issuspended (Fig. 20.28). Additional tremie concretecan then be added to give the required negativebuoyancy for immersing, placing, and joining.More tremie concrete may be needed to achieve thefinal factor of safety against floating.

Figure 20.27 shows a combination of two suchcylindrical sections for one of the two four-lanetubes of the Ft. McHenry Tunnel under BaltimoreHarbor.

Steel Sandwich Tunnels n Although pos-tulated elsewhere many years earlier, steel sand-wich tunnels have become a reality in Japan.Rectangular in shape, the principle behind thisform of construction is that there is a steel skininside and outside the tunnel, both actingcompositely with the concrete between them. Theplate is stiffened with flat and L-shaped ribs, andthe interior is divided up into cells by diaphragms

Fig. 20.25 Transbay Tube of the San Francisco Rapid Transit System (steel single shell).




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in two directions. Self-compacting non-shrinkconcrete is pumped into each cell though one holewhile air is released through others. Osaka SouthPort and Kobe Port tunnels are being constructedby this method, the latter being the only steeltunnel so far to carry three lanes per duct.

Foundations n There are two basic systems inuse for supporting immersed tunnels on line andgrade, a screeded foundation, and a pumped sandfoundation. In addition, a few tunnels are foundedon piles where soils are particularly soft or specialconditions prevail. Such conditions can includeearthquake where stone piles may help to dissipateexcess pore water pressure and prevent soilliquefaction.

(LC.F. Ingerslev, “Immersed Tunnel Foun-dations,” Comitato Organizzatore del Congresso,

“AITES-ITA 2001, World Tunnel Congress: Pro-gress in Tunnelling after 2000,” Proceedings pp209–216, Milan, June 2001.)

With a screeded foundation (Fig. 20.22), thetunnel is founded on a leveled bed of sand or stone2 ft to 3 ft thick, placed prior to the immersedtunnel. The leveling has been done by draggingeither a heavy grid of steel beams or a steel boxfilled with the foundation material along thealignment, suspending them from a carriage onrails set parallel to the required grade. The materialhas also been placed in narrow passes transverse tothe alignment using a pipe, the elevation of whichwas computer controlled.

For a pumped sand foundation, the tunnel isfounded on a sand or mortar foundation of similarthickness, placed after the tunnel element istemporarily supported in place. The element canbe set on two light pile bents that have been

Fig. 20.26 Cylindrical steel double-shell immersed tunnel (Hampton Roads Tunnel).




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Fig.20.27

Halfofsteelim

mersedtunnel

combines

twocylindricalsections(Ft.McH

enry

Tunnel).




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constructed to the correct grade. The sand can beplaced through movable pipes inserted and with-drawn from the sides beneath the structure (sandjetting), or through fixed pipes embedded with thestructure (sand-flow) with connections either exter-nal on the roof or sides, or internal through valves.

One method of sand jetting, invented by theDanish firm of Christiani & Nielson, is particularlyeffective: A sand slurry is injected through amovable nozzle, and the surplus water is pumpedoff by another nozzle, the rolling motion depositingthe sand in a compact layer. With sand flow, therolling current deposits the sand around thedischarge point in a firm circular layer (pancakes),often allowed to grow 20–26 ft radius. Dischargepoints are built into the underside of the elementand are connected to pipes leading to convenientpoints at which the pumped sand supply may beconnected.

As well as pile bents, elements have beentemporarily supported by jacks, penetratingthrough the base of the section. The jacks bear onpreviously placed concrete blocks. By adjusting thejacks, the section is brought to exact grade. Then,the sand foundation course is flushed in.

Immersion, Placing and Joining n Looselytermed the “sinking” operation, these threeoperations are performed with a high degree ofcontrol and therefore of accuracy. Immersion andlowering of each element is regulated by wincheson special barges or pontoons, or by cranes, fromwhich they are suspended. Alignment is controlledby instruments set on fixed points and sighting ontargets mounted on temporary towers attached tothe ends of the sections or by sonar to pre-installedtargets to avoid using temporary towers. Steel-shellelements have historically been connected withshort lengths of shell, which project beyond the endbulkheads. The gap between the ends was coveredby hood plates extended from the lower and upperhalf of the shell extensions. Form plates wereinserted into guides on the vertical edges of thebulkheads. The space around the joint was filledwith tremie concrete as a preliminary seal. Theinside of the joint was drained, and closure plateswere welded to interior ribs of the shell extensions.Finally, the concrete lining was completed.

Making rigid immersion joints today usingtremie concrete is unusual, with rubber-gasketjoints being almost universally used. Flexiblejoints are generally sealed with a temporary

immersion gasket or soft nosed gasket (Gina-type) in compression, attached to the end of oneof the elements and mating with a flat steel faceon the other. The use of a secondary independentflexible seal, capable of being replaced fromwithin the tunnel, is common practice (often anomega-shaped seal). Each seal should be capableof resisting the external hydrostatic pressure andshould allow for expected future movements.Jacks pull the tubes into contact to provide aninitial seal. The joint is then drained, activatingthe full hydrostatic pressure on the oppositeend of the tube. The pressure compresses thegaskets completely, providing a secure seal. Then,the bulkheads between the connected tubes canbe opened and the joint completed from theinside.

Depending upon the construction sequence,the last element may need to be inserted in theremaining space, rather than appended to the endof the previous element. In order to achieve this, asmall final gap will remain. This closure or finaljoint corresponds to a short length of tunnel thatwill need to be constructed in a special way.Methods used have include tremie concrete to seala rigid joint, and for flexible joints:

† dewatering to complete the joint in the dry fromthe inside;

† terminal block where a short closure section isslid out from within one side until it meets theother and any remaining gap is closed with arubber gasket in compression;

† wedge-shaped block dropped into the remain-ing gap until it is sealed against both sides.

Backfill n Up to about half the height of theelement, the trench is backfilled with well-gradedself-compacting material to lock the elementssecurely into place. Ordinary backfill is placed toa depth of at least 5 ft over the top of the tunnel. Ifany part of the tunnel projects above the naturalbottom, dikes should be built at least 50 ft away onboth sides to a height of 5 ft above the tunnel. Thespace between the dikes should be filled withbackfill, covered with a stone blanket to preventscour where necessary.

Design n Tunnel elements are designed as rigidstructures to resist dead loads, live loads, excep-tional loads and extreme loads. Dead load includes




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mean water level. Live load includes seabederosion and siltation, and variations in water level,current, storm loads and earthquakes, each with areturn period of 5 years or less. Exceptional loadsinclude loss of support (subsidence) below thetunnel or to one side, and storms and extremewater levels with a probability of being exceededonce during the design life. Extreme loads includesunken ships, ship collision, water-filled tunnel,explosion (e.g. vehicular), fire, the design seismicevent predicted for the location, and the resultingmovement of soils. Conditions to be investigatedshould include normal, abnormal, extreme, andconstruction.

20.21 Shafts

In tunnel work, shafts are starting points forexcavation in rock or firm material or shields. Forlong tunnels, such as aqueducts, several shaftsare used to divide construction into shortersections that can be worked simultaneously. Forvehicular tunnels, especially subaqueous shieldtunnels, the shafts are used as bases forventilation buildings. In construction of shafts,regulations of the Occupational Safety and HealthAdministration should be observed (Arts. 20.6,20.8, and 20.12 to 20.16).

Timber shafts are mined and braced in the samemanner as tunnels in similar material. Usually,poling boards 5 to 6 ft long are driven into theground and braced at regular intervals byrectangular timber frames. Then, the soil isexcavated to the ends of the polings and a newframe installed at this level.

A relatively shallow shaft may be startedoversize with sheeting 10 to 20 ft long drivenvertically on the outside of the frame bracing.Intermediate frames are installed as the excavationproceeds. At the bottom of the tier of sheeting, thesides are stepped in to make room for the next tierof vertical sheeting.

In rock shafts, timbering is used to prevent looserocks fromfallingoff thewalls. Itsplacementusuallylags an appreciable distance behind the excavation.

Steel liner plates alone, or in combination withhorizontal ribs, may be used in soft ground whereexcavation can be made in increments equal to thewidth of the liner plates. H beams driven verticallyas soldier piles, with wood or steel lagging andhorizontal bracing, may be used for rectangular

shafts. Enclosures of vertical steel sheetpiling, forround or rectangular shafts, are suitable for water-bearing ground.

Where ground conditions are poor and water-bearing, shafts may be constructed with a caisson(hollow box), with compressed air as needed toexclude water. Gravity pulls the caisson down asexcavation proceeds. Since its weight is relativelysmall, the caisson may have to be temporarilyballasted or jetted for sinking. The depth to which acaisson may be sunk is limited by the high cost ofcompressed-air work, which results from the shortworking hours permitted under high pressure.

Open-bottom shafts with heavy walls, oftencircular or subdivided into compartments, may bebuilt on the ground and sunk by excavating theground underneath. In dry soil, the excavation maybe done directly; if water is present, clamshellbuckets and high-pressure jets may be used toloosen the soil and remove it. On reaching theproper depth, the bottom of the shaft is closed bytremie concrete.

As an alternative method for shaft construction,water-bearing ground may be frozen in a circularring around the shaft location and the excavationmade in the dry. Closed-end pipes are drivenvertically into the ground around the periphery,and open-end smaller pipes inserted into them. Arefrigerant, usually brine, is circulated at tempera-tures as low as 230 8F from the interconnectedinner pipes into the larger ones and from themreturned to the refrigeration plant. Several monthsmay be required to freeze a deep ring solidly. Theventilation shaft of the Scheldt River Tunnel inAntwerp was built in this manner, as were anumber of mine shafts in Germany and France.


20.22 Seismic Analysis andDesign

Earthquake loads, or more correctly seismic loads,are included among the loads on a structure thatare required to be considered by most currentdesign codes. Seismic effects can occur during theconstruction phase and should therefore also beconsidered during that period; an appropriate levelof risk should be agreed with the owner. The




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seismic hazard at a tunnel site can be quantified bya project-specific seismic hazard assessment. Typi-cally, a functional evaluation earthquake (FEE),likely to occur notmore than once during the designlife, is used first to design the structure for eitherlimited or full performance following a seismicevent, as agreed with the owner. Next as appro-priate, either the safety evaluation earthquake (SEE)or the maximum credible earthquake (MCE), bothconcerned not only with life safety but also with thesurvivability of the structure under the most severeseismic event considered at the location, is checkedto ensure compliance with minimum performance.If necessary, the strength of some parts of thestructure may have to be enhanced to comply.

Structures buried in soil are generally con-strained to follow the seismic deformations of theground in which they are located. The stiffness ofthe tunnel is generally small relative to the soil, sothat in all but soft soils, tunnel deformations will

approximate to ground deformations, a conserva-tive assumption. For a tunnel in rock, tunneldeformations will match those of the rock, but insofter soils, the tunnel will resist soil pressures. Theresponse of the tunnel to the free-field soildisplacements (as if the tunnel were absent) willdepend upon both the stiffness of the tunnel andthat of the soil. While the complex seismic analysesmay be solved numerically using computers, somesimplified procedures have been published. Sim-plified beam-on-elastic-foundation analysis canalso be used to account for the soil-structureinteraction effects of soil deformations, especiallyin soft soils. Horizontal shear S-waves, dependingupon the angle of approach, cause transversebending or axial waves and produce the largeststrains that are usually governing. Compression P-waves should also be considered. At sites wherethere are deep deposits of soil, Rayleigh R-wavesmay govern the induced strains. Racking (ovaling)

Fig. 20.28 Lay Barge.




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deformations in the plane of the cross-section canoccur in tunnels, but not usually in verticalshafts, and is caused primarily by seismicwaves propagating perpendicular to the tunnellongitudinal axis. Vertically propagating shearwaves are generally considered the most criticaltype of waves for this mode of deformation. Axialand curvature deformations are induced bycomponents of seismic waves that propagate alongthe longitudinal axis.

The effects of a seismic event on a tunnel as awhole can be integrated to give an effectiveacceleration at the tunnel location, expressed as aseismic coefficient times the acceleration due togravity. Three seismic coefficients are usually

obtained, for longitudinal, lateral and verticaleffects. Internal elements, not in contact with thesoil and with a natural frequency approaching thatof the seismic waves, may need to be designed tosubstantially larger seismic coefficients. The ver-tical seismic coefficient can be reasonably assumedto be two-thirds of the design peak horizontalacceleration divided by the gravity.

Special precautions are needed for tunnels insoils that might liquefy or slip, especially so ifcrossing active faults. Liquefaction may causetunnels to float up. Since it is virtually impossibleto design against these conditions, the best policyis either to improve or replace the soil in questionor to avoid it. Faults are best avoided, but if that

Fig. 20.29 TARP Calumet Shaft.




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is not an option, then the tunnel must bedesigned to accommodate expected displace-ments, and perhaps the surrounding materialmay also need to permit relative movement of thetunnel. Locations of especially critical design in atunnel are at changes of inertia and soil proper-ties (where there will be contrasting responses),and at joints that might open up and causeflooding. Ductility in structures is particularlyimportant for structures to survive and for lifesafety.

(Wang, J, ‘‘Seismic Design of Tunnels—ASimple State-of-the-Art Design Approach,’’

Parsons Brinckerhoff Monograph No. 7, 1993.) (St.John, C. M., and Zahrah, T. F., Aseismic Design ofUnderground Structures, Tunneling and Under-ground Space Technology, Vol. 2, No. 2, 1987.)(Chapter 15B Tunnel Structures, ‘‘StructuralEngineering Handbook,’’ 2000 Update forENGnetBASE, Edited by Wai-Fah Chen and LianDuan, CRC Press, 2000 (www.crcpress.com).)(Earthquake Analysis, L. C. F. Ingerslev andO. Kiyomiya, International Tunneling AssociationImmersed and Floating Tunnels Working Group,‘‘State-of-the Art Report,’’ Second Edition, Perga-mon, April 1997.)




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