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Buried Pipelinesand SewerConstruction40D J Irvine BSc, CEng, FICETechnical Services Manager,Tarmac Construction Ltd

Contents

40.1 Routing 40/3

40.2 Materials 40/3

40.3 Pipes in trench 40/340.3.1 Structural design 40/340.3.2 Rigid and flexible pipes 40/340.3.3 Longitudinal bending 40/3

40.4 Imposed loads on pipes in trench 40/540.4.1 Installation conditions considered for

design 40/5

40.5 Pipe strength 40/1040.5.1 Rigid pipes 40/1040.5.2 Flexible pipes 40/1040.5.3 Practical pipe design 40/10

40.6 Construction of pipes in trench 40/1040.6.1 Site investigation 40/1040.6.2 Ground movement 40/1140.6.3 Groundwater control 40/1240.6.4 Battered trenches 40/1240.6.5 Trench support 40/1240.6.6 Proprietary systems 40/1240.6.7 Submarine crossings 40/1540.6.8 Trench backfilling and reinstatement 40/1540.6.9 Protection 40/1540.6.10 Testing 40/1540.6.11 Safety 40/15

40.7 Trenchless pipelaying 40/1640.7.1 Microtunnelling design 40/1640.7.2 Site investigation 40/1640.7.3 Ground Movement 40/17

40.7.4 Pipe jacking with slurry shields 40/1940.7.5 Pipe jacking with steerable borers 40/1940.7.6 Pipe jacking with non-steerable augers 40/2040.7.7 Pipe ramming 40/2040.7.8 Impact moling 40/2040.7.9 Directional drilling 40/20

40.8 Man-accessible tunnels 40/2040.8.1 Site investigation 40/2140.8.2 Headings 40/2140.8.3 Steel liner plates 40/2140.8.4 Precast concrete segmental linings 40/2140.8.5 Triple segmental block (minitunnel

system) 40/2240.8.6 Pipe jacking 40/2240.8.7 Ground movement 40/23

40.9 Working in confined spaces 40/23

40.10 Rehabilitation of pipes and sewers 40/2540.10.1 Inspection and basic strategy 40/2540.10.2 Methods of rehabilitation 40/29

40.11 Costs 40/2940.11.1 Project cost appraisal 40/2940.11.2 Construction costs 40/30

References 40/31

Bibliography 40/31

Buried pipelines are used to transport fluids which may be gases,liquids or slurries. The pipes may operate at high or lowpressure. The pipelines may form part of extensive structuressuch as large national grids or be limited in extent, such asculverts under roads. Consequently, a wide range of skill andengineering expertise is needed to construct and maintain thestructures which represent a considerable capital investmentand form a significant proportion of any developed country'sinfrastructure.

40.1 Routing

The choice of a pipeline route is usually governed by economicconsiderations but practical and legal factors will have a con-siderable influence which varies upon the substance beingconveyed. A useful checklist of matters to be considered isprovided in Part 1 of Pipelines in land}

The development of pipelines in the UK is regulated by anumber of Acts of Parliament which must be strictly followedby the pipeline promoter. The principal legislation comprises:

Water Acts 1945 and 1948Gas Acts 1948 and 1965Public Health Acts 1936 and 1961Requisitioned Land and Waterworks Acts 1945 and 1948Land Powers Act 1958

Other legislation which is relevant is:

Public Utilities Street Works Act 1950(for pipes laid along or across streets)

Coast Protection Act 1949(for parts of pipelines laid below high water)

Land Drainage Act 1961(for pipelines crossing rivers and streams)

The legal situation is frequently complex and it is essential toobtain expert advice. The Pipelines Inspectorate, the Health andSafety Executive and the relevant local and statutory authoritiesshould be approached at an early stage in the development ofany large project. It should be noted that a planning consent orpipeline authorization does not confer any rights to enter orcarry out works in land and it is up to the pipeline promoter toobtain the necessary rights from the owners or occupiers. Ifthese cannot be obtained voluntarily a Compulsory RightsOrder may be required.

The greatest problems, both practical and legal, will beencountered in congested urban areas. Pressure pipes for oil, gasand water can be laid at relatively shallow depths and withinlimits may follow the ground surface profile. Sewers, however,are generally laid to falls so that the contents flow under gravityand this may result in pipelines being constructed at greaterdepths with potentially greater problems for adjacent owners.

40.2 Materials

The material for a pipeline must be selected to suit its type andpurpose (see Table 40.1). The manufacture and use of allmaterials listed are covered by British Standards which shouldbe consulted to ensure that both the pipe material and thejointing system will comply with the desired service conditions.For work outside the UK, other relevant national standardsmay apply and should be consulted.

Most pipe manufacturers have devised their own pattern ofjoint and modern practice uses flexible joints rather than rigid.

These are easier to fix and allow a limited degree of relativesettlement between pipes without the risk of leakage or fracture.'Push in' joints using rubber sealing rings are used for low-pressure work. At higher pressures the seal may be compressedby loose flanges and bolts or screwed fittings. Steel and someplastic pipes can also be jointed by welding and this process isusual where 100% watertightness is required for safety reasons.Heat-shrink plastic and other proprietary joints are gainingfavour particularly for repair work in the smaller diameters ofpipe.

It should be noted that most flexible joints do not resistlongitudinal forces. Therefore, pressure mains utilizing suchjoints should be provided with anchors at bends, tees and endcaps to resist thrusts arising from the internal pressure.

40.3 Pipes in trench

40.3.1 Structural design

A buried pipe and the soil surrounding it are interactivestructures. The extent of the interaction and hence the magni-tude of the pipe loads arising depends on the relative stiffnesses.As a result two separate traditions of design have been evolved;one for 'rigid', and one for 'flexible', pipes.

40.3.2 Rigid and flexible pipes

A rigid pipe may be defined as likely to fracture under very smalldeformation. The pipe wall resists the external load by circum-ferential bending and the tensile strength of the pipe material isthe usual limiting factor. The tensile bending stress due to theapplied loads will be affected by the circumferential tensile stressresulting from any internal pressure caused by the pipe contents.

A flexible pipe will not necessarily crack under slow deforma-tion even when this becomes large. It will fail under verticalloading by buckling or flattening if it is not adequately sup-ported laterally. A circular flexible pipe reacts to externalloading above it by deflecting downwards at the crown andoutwards at the sides. The latter movement induces a passiveresistance in the adjacent soil. The pipe should be capable ofsustaining a crown deflection of at least 10% of the originaldiameter without damage, although it is usual to limit theworking deflection to 5% or even less if protective linings areused.2

It should be noted that in reality the behaviour of any soil-pipe system varies with the pipe diameter: wall thickness ratio,its stiffness and the modulus of the soil. Consequently, there isan intermediate category of 'semi-flexible' pipes which for con-venience are normally designed as rigid pipes.2

40.3.3 Longitudinal bending

This is also known as axial bending or beam effect and arisesdue to differential settlement along the pipeline (which is likelyin most soils) or from local concentration of support. The use offlexible joints and good workmanship will assist in reducing theeffects of longitudinal bending but special attention should bepaid to small rigid pipes laid at shallow depths under heavywheel loads, and where pipes intersect with structures such asbuildings and manholes. A flexible joint as close to the face aspossible and a short 'rocker' pipe should be used to accommo-date differential movement at structures. If concrete beddingsare used for the pipe it is essential to retain flexibility at pipejoints by forming flexible joints through the bedding.

In poor ground it may be necessary to use a piled foundationwith a continuous reinforced concrete capping beam to supportthe pipeline at the required line and grade.

Application

Remarks

Sewageand

tradeeffluent

WaterSludgesand

slurries

BrineChemicalsIndustrialgases

Towngas

Naturalgas

Liquefiedpetroleum

gases

Crude oiland

petroleumproducts

Designmethod

Pipe material

Larger diametersare reinforced

Lightweight; some typescan be chemically or heatwelded

AAAA

AA

A*

A*

A

AAAA*

A*

A*

A*

A

A

AA

A

A

A

A

AAAA

A

A*

A*

A

A

A

A

A*

A*

A

A

AA

AA

A

AA

A

AA

AA

A

AA

A

AA

RIGID

FLEX

BLE

AsbestosClayConcreteGrey cast iron

Ductile cast ironPitch fibreSteel with butt-

welded jointsSteel with other

than butt-weldedjoints

Plastics

Notes:(1) Indicates pipeline may need special lining to prevent corrosion.(2) AUsed at lower operating pressures.(3) It is necessary to ensure that any jointing compounds are resistant to the fluid being carried.

Table 40.1 General pipeline applications (based on BS CP 2010)

40.4 Imposed loads on pipes in trench

The most widely used method of estimating external loads on aburied pipeline was pioneered by Marston, Spangler and Schlickin the US. It was further developed and extended for UKpractice by Clark and Young and is generally termed the'Marston' or 'computed load' method (Table 40.2).

The method is to some extent empirical and uses a soil modelbased on Rankine's theories of soil behaviour. This has been thecause of some criticism but considerable testing and successfulpractical experience has demonstrated its merits, and it remainsthe standard method in the UK, Europe and the US.

Table 40.2 Notes on imposed loads on pipes in trench

40.4.1 Installation conditions considered for design

40.4.1.1 Narrow trench condition

This is the case when the trench is narrow and deep compared tothe width of the pipe. The upper limit of narrow trench widthfor a given combination of trench depth and pipe diameter isknown as the 'transition width' (see below). The narrow trenchcondition results in smaller soil loads on pipes than otherconditions but there are practical construction difficulties whichcan prevent the design assumptions being realized.

The theory is based on an analogy with the Jansen theory of

Source of load

(1) Soil overburden

(2) Superimposed loads on surface(a) uniformly distributed load of

large extent,e.g. temporary filling

(b) uniformly distributed loads oflimited extent (permanent),e.g. foundations of structures,stacking of constructionmaterials, ground loads fromcaterpillar tracks

(c) concentrated loadse.g. vehicle loads

(3) Fluid load,i.e. load of pipe contents

(4) Internal pressure

Comment

The magnitude of the vertical load on the pipe is estimated using the Marston modeland is influenced by:

(a) the depth of the fill and its nature(b) the width of the trench(c) whether negative or positive projection(d) when the trench sheeting is removed(e) level of water table

(a) the load is expressed as an equivalent additional depth of fill in the Marston modelbut ignores any shear forces induced in the surcharge due to differential settlement

(b) for narrow trench conditions an appreciable error may result when the notionalincrease in depth approaches the same magnitude as the original depth

(a) the load on the pipe is estimated using Newmark's integration of the Boussinesqequation

(b) the soil stress at the pipe crown is calculated and assumed to be constant over 1 mrun of pipe and across the diameter

(c) the calculated load is added to the soil loads(a) the load on the pipe is estimated by the Boussinesq method for the distribution of

stress in a semi-infinite homogeneous elastic medium due to a point load at thesurface

(b) the method results in a peaked load along the pipe. This is converted to an averageload over a length of 0.9 m or less if appropriate

(c) the load thus calculated is added to the soil loads(d) the loading conditions are shown in Table 40.3 together with appropriate impact

factors(e) all pipes should be checked for the worst case arising during construction as well as

the permanent works service condition(a) weight of pipe contents causes circumferential bending in the pipe wall. (Note self-

weight of pipe usually neglected)(b) the magnitude of the bending moment depends on the manner in which the pipe is

bedded and whether the pipe is running full(c) the effect is allowed by adding an 'equivalent water load' to the other loads on the

pipe. The value can be obtained from charts or can be estimated as 0.75 times waterload

(a) pipes should be designed for the worst internal surge or test pressure which is likelyto arise

(b) in gravity pipelines the maximum static head (ignoring surge) occurs when thevelocity of flow is zero

(c) in pumping mains the maximum head will be either:(i) the sum of the maximum static head plus friction head at maximum flow plus

any other loss of head, or(ii) maximum surge pressure due to sudden stoppage of the pumps and closing of

non-return valves particularly if no surge suppression is provided in the system(d) partial vacuum conditions arising from inefficient air valves, etc. can be treated as an

additional temporary external water pressure

Condition

Main roads

Light roads

Field loading

Railway loadingRU (mainlinerailways of 1 .4 mgauge and above)

Railway loading RL(reduced loading forpassenger rapidtransit systemswhere main linelocomotives androlling stock do notoperate

Construction vehicles

Reference

BS 5400: 1975type HB

Ministry of Housing andLocal Government (1967)Working party on thedesign and construction ofunderground pipe sewers,2nd Report, HMSO,London, as quoted in:Young and O'Reilly (1983)A guide to design loadingsfor buried rigid pipes.Transport and RoadResearch Laboratory,HMSO, London

Ministry of Housing andLocal Government (1967)Working party on thedesign and construction ofunderground pipe sewers,2nd Report, HMSO,London, as quoted in:Young and O'Reilly (1983)A guide to design loadingsfor buried rigid pipes.Transport and RoadResearch Laboratory,HMSO, London

BS 5400: 1978

BS 5400: 1978

Trott and Gaunt (1976)Experimental pipelinesunder a major road:performance during andafter road construction.Report LR 692, Transportand Road ResearchLaboratory, Crowthorne

°-9mi Jk

0-9mliW

Loading

Eight wheel loads of 1 12.5 kN each(including impact factor of 1.25)distributed over circular or squarecontact area at effective pressure of1100kN/m2

Two wheel loads of 105 kN each(i.e. 70 kN static weight with impactfactor of 1.5) and contact pressure of700 kN/m2

Two wheel loads of 60 kN each(i.e. 30 kN static weight with impactfactor of 2.0) and contact pressure of400 kN/m2

per track

Loads are static and must be increased by impact factor of 2.0 for pipesup to 3 m diameter

per track

Impact factors: 1.20 for ballasted track1.40 for unballasted track

np 1.00 for temporary works with rail trafficu" limited to 25 km/h

per track

Impact included

Manufacturers' loading data for actual plant should be used whereverpossible. Alternatively, loads can be estimated from following:

Plant

Small scraperLarge scraperSmall dump truckLarge dump truckReady mix truck(6 m3 capacity)

Total Static Tyre inflationmass wheel load pressure(t) (f) (KN/m2)

23.2 6 200-400110.3 28 500-60024.3 4 350-70080.4 20 up to 650

24.0 5.5 up to 750

pressures within silos (see also Table 40.3). In a vertically sidedtrench (Figure 40.1) the load on the pipe is the weight of theprism of fill at level X-X minus the friction of the fill on theadjacent soil. The theory ignores the effect of cohesion on theshear surface and, when rigid pipes are used, any supportprovided by the fill below level X-X.

The load on the pipe is given by the expression:

[ 1_e-2W "I

-25rx<w|-y%ct

where Wc = fill load on pipe in narrow conditions in kilonewtonsper metre; y = unit weight of soil in kilonewtons per cubic metre;Bd = effective width of trench in metres; e = base of Napierianlogarithms; H= height of ground surface above top of pipe inmetres; A'=Rankine active earth pressure coefficient; and//' = coefficient of sliding friction of backfill against trench sides

KU' is a semi-empirical constant for particular soil types and it iscommon to use design charts to determine values of Cd fordifferent soil types; n' is used to distinguish the coefficient ofsliding friction from the coefficient of internal friction forbackfill used elsewhere in Young's and O'Reilly's calculations.3

It has been shown that the narrow trench condition can existin battered and stepped trenches (Figure 40.2) and the value ofKIJ. ' appropriate to the material in which shear occurs should beused for design.

40.4.1.2 Late removal of trench support sheeting in thenarrow trench condition

The narrow trench loading condition assumes that the pipe loadwill be reduced by friction at interface between the fill and thesoil. However, in close-sheeted trenches it is frequently notpracticable for reasons of safety to remove the sheets beforeplacing and compacting the backfill. In such cases it should be

Fill Soil

Friction betweenpipe fill and soil

Lateral pressureof fill on adjacentsoil

Pipe

Figure 40.1 Narrow trench conditions: diagram of forces

300 mmminimum

300 mmminimum

Figure 40.2 Effective trench widths in battered and steppedtrenches

Figure 40.3 Positive projection condition under valley fill orembankment, (a) Complete projection, i.e. shear stresses betweeninterior and exterior prisms extend to top of fill; (b) incompleteprojection, i.e. shear stresses between interior and exterior prisms donot extend to top of fill; (c) settlement ratio

p = projection ratiopBc = projection height

Sm = compression of soilin projection heightdue to overlying fill

Sf = total settlement ofpipe invert

S - settlement of naturalground surface

dc = vertical deflection(shortening) of the pipe

( Note: It is not practical to pre-determine the settlementratio and it is alwaysassessed from experimentalload measurements and practicalexperience: see Young andO' Reilly (1983) A guide todesign loadings for buried rigidpipes. Transport and RoadResearch Laboratory, HMSO,

(c) London.)

Critical plane before loadingCritical plane after loading

Natural ground before loadingNatural ground after loading

Settlement ratio rsd

= <Sm+S g ) - (S f+r f c )

No friction aboveplane of equalsettlement

Friction betweeninterior andexterior prismscontributes toload on pipe

Friction betweeninterior andexterior prismscontributesto load onpipe

Plane of equal settlement

ExteriorPrism

InteriorPrism

ExteriorPrism

top of fill

Downdrag caused by settlementof exterior prismRelative settlement of interior andexterior prisms

Critical plane before settlement

Critical plane after settlement

Natural ground

Fill load on pipeTop of fill

Plane of equal settlement

ExteriorPrism

InteriorPrism

ExteriorPrism

Downdrag caused by settlementof exterior prismRelative settlement of interiorand exterior prisms

Critical plane before settlement

Critical plane after settlement

Natural ground

assumed that the total weight of backfill will be exerted on thepipe.

40.4.1.3 Embankment or valley fill with positive projectioncondition

As the trench width increases, the prism of soil over the pipe willbe bounded on either side by deeper prisms of fill down toformation level (Figure 40.3). The latter are likely to settlerelative to the pipe and, hence, will exert downward forces onthe sides of the centre prism and increase the load on the pipe.

In a low embankment the shear stresses on the centre prismwill extend up to the surface of the fill. This is known as the'complete projection condition'.

In a sufficiently high embankment, the shear stresses do notextend to the surface but cease at an intermediate level known asthe 'plane of equal settlement'. This is the 'incomplete projectioncondition' and no frictional forces are exerted on the centreprism above the plane of equal settlement.

It should be noted that the amount of deformation of thecritical plane in Figures 40.3(a) and (b) depends on, amongother things, the 'projection ratio', i.e. the extent to which thepipe projects above the natural ground.

The total deformation of the critical plane is made up of:

(1) The settlement of the fill in the projection height pBc.(2) The total settlement of the pipe invert.(3) The settlement of the natural ground surface.(4) The vertical shortening of the pipe.(5) The 'settlement ratio' defined in Figure 40.3(c). (Actual

values of settlement ratio have been derived semi-empiri-cally.3)

The total load on the pipe is given by the expression:

WZ=CJB?

where Wc' = fill load on pipe in kilonewtons per metre; Cc = fillload coefficient for positive projection case; and Bc = externaldiameter of pipe

Cc is derived from complex expressions depending on soil and

fill characteristics and geometry, and it is usual to obtain designvalues from standard charts.3

40.4.1.4 Wide-trench condition

It has been shown that if a trench is progressively widened andother conditions do not change then the narrow trench loadingdoes not continue to increase but reaches a limiting value givenby the appropriate positive projection equation. The trenchwidth at which this limit is reached is known as the 'transitionwidth'.

The same logic applies to a narrow trench being reduced indepth. The depth at which the narrow trench load equals thepositive projection load is known as the 'transition depth'. Fordepths deeper than the transition depth the narrow trenchcondition applies.

40.4.1.5 Embankment or valley fill with negative projectioncondition

This case differs from positive projection in that the pipe is laidin a subtrench below the natural ground level (Figure 40.4). Themiddle prism of soil will tend to settle relative to the adjacentprisms on either side thus reducing the potential fill load on thepipe.

The concepts of the plane of equal settlement and settlemertratios are also used in this case.

The total load on the pipe is given by:

^V = CW

where W^ = fill load on pipe in kilonewtons per metre; Cn = fillload coefficient for negative projection case; and 2?d = width oftrench

As in the case of positive projection, the fill load coefficient Cn isdependent on soil and fill characteristics and geometry, andvalues may be obtained from standard charts.3

40.4.1.6 Loads on flexible pipes

The calculation of fill loads on flexible pipes can be simplified by

Plane of equalsettlement

Top of fill

ExteriorPrism

InteriorPrism

ExteriorPrism

Critical plane beforesettlement

Natural groundFriction reducesload on pipe

Critical plane aftersettlement

Pipe

Plane of equasettlement

Top of fill

Figure 40.4 Negative projection condition under embankment orvalley fill, (a) Complete projection, i.e. shear stresses betweeninterior and exterior prisms extend to top of fill;(b) incompleteprojection, i.e. shear stresses do not extend to top of fill

taking the total effective weight of the prism above the pipe ofwidth equal to the pipe horizontal diameter4 and extending up tothe ground surface. To this may be added the self-weight of thepipe above the midplane and the buoyancy caused by anygroundwater above the level of the midplane although theseloads will be minimal in the case of small pipes.

The effect of concentrated vehicle loads may be calculatedusing a Boussinesq distribution or by reference to design curvesin Compston et al.4 and Nath.5 The pressures due to vehicleloads will tend to be greatest at the crown and those fromexternal water pressure greatest at the invert. The maximumradial pressure is selected for design and is assumed to bedistributed as ring compression around the pipe wall4 in addi-tion to the fill load.

For the design of corrugated buried steel structures underroadways in the UK, the Department of Transport6 should beconsulted.

40.5 Pipe strength

40.5.1 Rigid pipes

The design of a rigid pipe wall is based on its resistance to thecircumferential bending moments induced by external loadsplus any circumferential tensile stress caused by internal pres-sure.

The effect of external loads on a pipe can be calculated bymathematical theory. However, due to the difficulties in accu-rately modelling the conditions which apply in practice, it isusual to design rigid pipes using an empirical method linked tostandard crushing values in the British Standards Specification.

The empirical design approach takes account of the benefitsgained by using specific bedding methods immediately aroundthe pipe (Figure 40.5). These distribute the foundation reactionaround the lower pipe periphery thus reducing the circumferen-tial bending moment in the pipe wall.

The 'bedding factor' is the ratio by which the ultimatestrength of the pipe in the ground is increased compared to itsstrength in the standard 'three-edge bearing test'. Standardtables of bedding factors are published2-3 and they normallyignore the effects of lateral restraint. When pipes are laid underembankments with positive projection, the soil will exert anactive soil pressure acting horizontally to the pipe. Since thispressure tends to produce a bending moment in the pipe ringwhich opposes that produced by the vertical load, the net effectis to produce an apparent increase in the bedding factor. Theenhanced value of bedding factor can be calculated by methodspresented by Young and Trott.7 The supporting strength of apipe must be equal to or greater than the total external load(W^. Therefore, the required strength of pipe \V.f is:

w _ External load (W6) x factor of safetyT Bedding factor

The factor of safety can be chosen at the discretion of thedesigner. It is usual practice to use a factor of safety of 1.25 inconjunction with the maximum crushing test load of a pipe, orfactor of safety = 1.0 in conjunction with the proof test load foran unreinforced concrete pipe. (All pipes must withstand thespecified maximum load and reinforced concrete pipes shouldnot crack by more than a specified amount under proof load.)

Tables are published for structural design of different types ofpipe under various load conditions.8 10

40.5.1.1 Internal pressure in rigid pipes

As previously noted, internal pressure produces circumferential

tension which reduces the available strength for resisting cir-cumferential bending tension due to the external load. In thesecases, Young and O'Reilly3 and Young and Trott7 should beused for design.

40.5.2 Flexible pipes

The design of flexible pipes is based on the yield strength of thepipe material in compression and on its resistance to circumfer-ential buckling and distortion while being restrained by thesurrounding ground. In pressure pipes the yield strength intension must also be considered.

A significant element of the strength of a flexible pipe isderived from the adjacent soil and consequently the adequacy ofthe surrounding material and the workmanship used will have asignificant effect on the performance of the pipe.

The pipe wall thickness for external loading is derived fromring compression theory.4-11 It should be noted that a minimumratio of pipe diameter to wall thickness must be provided toavoid damage to the pipe during handling.

40.5.3 Practical pipe design

The design and construction of pipes for most uses are coveredby appropriate national standards. These usually give preferredmethods of analysis and allowable stresses and deal withpractical matters. Minimum cover depths (usually not less than0.9 m) and the need to provide anchorages, air valves and otherventing arrangements may have significant effects on designloads.

Nevertheless, the designer will be required to make a numberof engineering judgements and it is essential that he shouldunderstand the basic theories involved and their limitations.Variability in soil, pipe strength and site workmanship meansthat extreme precision in design is not appropriate. Designtables are available*10 which reduce tedious calculation in alarge number of situations. In situations where site conditionsvary and the tables do not apply, the works by Young andO'Reilly3 and Young and Trott7 should be used.

The worst conditions for a length of pipeline should be usedfor design. Sources of extreme stress should be avoided ratherthan strengthening the pipe to resist them. The designer'sassumptions should not demand excessively high qualities ofworkmanship or supervision on site. In particular, the designermust recognize that trenches need to be wide enough for accessand support during construction. Experience shows that in mostcircumstances it is cheaper and more satisfactory in service touse strong pipes with cheap beddings (Class B for sewer pipes,12)and to employ the least expensive method of installation appro-priate to the site conditions.

40.6 Construction of pipes in trench

40.6.1 Site investigation

Both the pipeline designer and the constructor require know-ledge of the site conditions and the ground. A proper siteinvestigation should be carried out at an early stage in thedevelopment of a project and it should include all the relevanttopographical information and a soil profile extending to atleast 1.5 times the trench depth.

The results of the investigation should be reported formally.13

It is unlikely that extensive soil testing will be required sincevisual descriptions will be sufficient for most methods of designfor pipe strength and trench support. Particular attentionshould be given to groundwater tables and to potentiallycorrosive conditions.

Figure 40.5 Beddings for rigid pipes

40.6.2 Ground movement

All ground has existing in situ stresses and the balance of thesestresses can be disturbed by the construction of the pipeline. In atrench excavation the sides and bottom lose the restraint of theexcavated spoil and deform until either sufficient shear strengthis mobilized in the ground to restore equilibrium or failureoccurs. The insertion of the trench support assists in reducing

this potential movement and preventing collapse. However, itshould be recognized that ground movement cannot be elimi-nated.

The stress-strain relationships of soils are very complex. Thedegree of strain is dependent not only on the magnitude of theoriginal stress and the properties of the soil but also on time andother factors. In weak soils, e.g. soft clay, light loading can

Comments

Pipe laid on trimmed trench bottom

Pipe laid on a flat layer of granularmaterial with CF1 not greater than0.3

Pipe laid on flat layer of granularmaterial with CF1 not greater than0.2. Illustrated after settlement

Pipe laid on granular material tohalf diameter with CF1 not greaterthan 0.2

Pipe fully surrounded by granularmaterial with CF1 not greater than0.2

Construction as in Figures 5b, 5c,5d and 5e in this table except thatwhen the trench width exceeds 4times the outside diameter of thepipe barrel, the granular materialmay be sloped down from thatwidth to the trench formation

Figure Bedding class Bedding

Socket orjoint profile

All dimensions are in millimetres

Selected fillKey Granular material

Notes: (1) CF = Compaction fraction (see Appendix D, BS 8005: Sewerage, Part 1)

(2) Recommendations for granular pill and selected fill, are given in clauses5.5.1 and 5.5.2 of BS 8005.

(3) Where pipes with sockets are used these sockets should not be less than50 mm above the floor of the trench.

(example)B (example)

cause large deformations. Wide trenches in soft clay can heaveand fail at the base even when side support has been installed. Instrong soils, e.g. cemented sands, the same loads will cause onlysmall deformations with no failure.

The deformations are shear deflections which commenceimmediately excavation starts. There is a time lag before themaximum potential movement is achieved at each level of dig.For this reason soil can continue to creep after the trenchsupport has been installed since it may have gaps between it andthe soil and will in any case deflect under the load. Othermovements may occur due to:

(1) Inadequately compacted backfill.(2) Volume changes due to moisture variations particularly in

fine grained soils.(3) Short-term consolidation resulting from increased effective

stresses caused by groundwater lowering during construc-tion.

(4) Long-term consolidation caused by structures and geologi-cal processes.

(5) Ground heave due to groundwater pressures beneath clay.(6) Frost heave.(7) Traffic loading.

Symons14 and Attewell and Taylor15 give useful information onthe possible components of total movement and their magni-tude.

Soil movement is a cause of concern since buried pipelines willmove with the soil and hence must accommodate the movementwithout distress. Pipelines with flexible joints are better able toresist such movements than pipes with rigid joints.

The water and gas industries in the UK have recognized therisk caused by movements arising from deep excavation toadjacent buried pipes, particularly those manufactured of greycast-iron. For this reason a consultative procedure has been setup between them to investigate construction works so thatpotential problems are avoided.16

40.6.3 Groundwater control

This can be required during trench construction either to copewith groundwater flowing into the trench or to reduce upliftpressures on the bottom of the trench. Water flows may becontrolled either by driving impermeable sheets to obtain acutoff or by dewatering.

Dewatering of trenches down to 6 m is most commonly doneby sump pumping and well pointing (see Figure 40.6). Theeffectiveness depends on the nature of the soil, the trenchgeometry and the degree and rate of lowering required. Sumppumping is the cheapest and simplest method but draws waterinwards towards the base of the trench which may causeinstability due to seepage forces and erosion of fine particles.

Well points have the advantage of drawing water away fromthe trench bottom which tends to stabilize the ground byincreasing its shear resistance. Well points are most effective insands; clays and fine silts are usually too impermeable.

In these cases exclusion methods are generally employed,although ground freezing or electro-osmosis has been usedsuccessfully at great cost.17

40.6.4 Battered trenches

Where sufficient space is available, battered trenches are oftenthe quickest and cheapest method of construction. Large ex-cavators can be employed and the need for side support iseliminated by cutting the sides of the excavation at a safe slope.13

Slips due to subsequent deterioration of the exposed faces mustbe prevented by suitable measures where they are likely to causeproblems.

Figure 40.6 (a) Sump pumping; (b) single-sided well pointsystem. (This figure is reproduced by courtesy of the ConstructionIndustry Research and Information Association, Report No. 97Trenching practice by permission of the Director of CIRIA)

40.6.5 Trench support

Trench support has two functions:

(1) To provide a safe place of work for operatives workingwithin the trench.

(2) To maintain the stability of adjacent ground and hence theintegrity of structures supported by it.

Trench support systems can be considered in two groups: (1)those based on traditional timbering methods but now fre-quently employing steel sheets and adjustable struts; and (2)those methods which use proprietary components or completeproprietary systems.1318J9

In the traditional system, the sheets may be either predrivenor inserted as excavation proceeds, and then supported bywalings and adjustable struts (Figure 40.7).

The sizes of the components are estimated by simple calcula-tion.13 The use of highly mathematical theories is not justifiedsince soil characteristics are generally not accurately known andthe reuse of simple components keeps material costs low. Thesystem can be adopted readily to suit changes in conditions orgeometry but it should be installed only by men experienced inits use.

40.6.6 Proprietary systems

These may be described13-18 as:

Water piped awayOriginal ground level

Groundwater level

Invertlevel

.Reducedwater level

Pumping fromsump

Flowlines

Standing water levelGround level

About1 m Header

main

.Formationlevel

SandDrawdownat trench

Reducedwater level -Well points

Figure 40.7 Typical trench support system with struts, walingsand side sheeting. (This figure is reproduced by courtesy of theConstruction Industry Research and Information Association,Report No. 97 Trenching practice by permission of the Director ofCIRIA)

(1) Hydraulic frames and shores.(2) Boxes.(3) Slide rail systems.(4) Shields.(5) Piling frames.

Hydraulic frames (Figure 40.8) have adjustable struts perma-nently fixed to walings and may be used in place of conventionalscrew struts and separate walings. The advantage of the hy-draulic system is that in some conditions it can be placed andtightened without the need for operatives to enter the trench.The vertical shores are of similar construction and can be usedas pinchers where full face sheeting is not required.

Boxes are used to form modular strutted support walls(Figure 40.8(b)). They are either lowered into a predug trench toprovide a protective box for operatives or are buried using thedig-and-push technique. In this case, the box is progressivelypushed down and the spoil dug out between the walls. Whereefforts are made to limit overbreak, the latter method will resultin the box being a tighter fit in the ground thus giving morepositive support to the trench sides. It is normal practice whenlaying pipes to use three or four boxes in line.

Slide rail systems have vertical slide rails or soldiers struttedapart with horizontal walls spanning between them (Figure40.8(c)). The dig-and-push installation method is used and thewall panels slide vertically in the rails.

Side sheeting

Props orhangers tosupport weightof frames

Walings

Adjustable props

Progress of work

Spoil

Shields, also called drag boxes or saddles (Figure 40.8(d)), arevertical support walls permanently strutted apart. The shield islowered into the excavation and dragged forward by the excava-tor as the trench is extended. It is a loose fit in the ground andprovides a protected place of work.

The presence of existing crossing services causes practicaldifficulties for boxes, shields and slide rail systems. Conse-quently, short lengths of traditional trench support may beneeded in these areas.

Proprietary piling frames can be used to install vertical

Figure 40.8 Proprietary trench support systems, (a) Hydraulicframe system; (b) box system; (c) slide rail system (also known as aplate lining system); and (d) drag box (also known as a shield orsaddle). (This figure is reproduced by courtesy of the ConstructionIndustry Research and Information Association, Report No. 97Trenching practice by permission of the Director of CIRIA)

sheeting in the ground (Figure 40.9). The non-powered typeconsists of two sets of parallel walings forming a gate throughwhich the sheets are inserted. The walings are permanentlystrutted apart and dig-and-push methods are used to force thesheets into the ground. In the powered version, the waling gatesare equipped with hydraulic rams which enable the machine topush individual sheets up or down. The sheets are speciallymade for the machine and can be used to depths greater than6 m if intermediate waling frames are used.

A further development for installing small-diameter pipes is

Progress of work

Progress of work

Figure 40.9 Proprietary rail-mounted piling frame. (This figure isreproduced by courtesy of the Construction Industry Research andInformation Association, Technical Note 95 Proprietary trenchsupport systems)

to use a self-propelled machine which cuts a continuous slot inthe ground. It carries horizontal shields to keep the slot open sothat the pipes can be jointed continuously at the surface andinstalled down a ladder guide into the bottom of the trench.

40.6.7 Submarine crossings

Pipes to be laid under water can be installed by several methodsdepending on circumstances:

(1) In cofferdam.(2) Floating and sinking.(3) Bottom pulling.(4) Lay barge.

Cofferdams are costly and not used for pipelines of great length.Floating and sinking requires the pipeline to be fabricated onshore and fitted with buoyancy tanks before it is towed intoposition and sunk. Care must be taken not to bend the pipe andits protective coating beyond their allowable stresses.

Bottom pulling is more frequently used since it is easier tocontrol stresses. The pipe is fabricated on shore in line with thetrench and then pulled into the trench using a winch. In the laybarge method the pipe lengths are jointed on the barge as it iswinched forward. The pipe spans between the barge and thetrench bottom and frequently a ladder is used to give interme-diate support.

In all cases, sufficient weight must be provided to anchor thepipe in place when it is empty.

40.6.8 Trench backfilling and reinstatement

The backfill forms part of the load-bearing system of a pipelinestructure and must be constructed with care. The fill around thepipe needs particular attention to achieve an adequate beddingfactor while ensuring no damage is caused by over-compaction.

The materials used for backfill should be capable of densifica-tion to the required standard without undue effort. The reuse ofpoor material from the excavation can be a false economyparticularly under trafficked areas. The permanent reinstate-ment of road surfacing should be done as soon as practicable.The use of temporary surfacing may cause more problems thanit is intended to cure, particularly where it does not giveadequate support to the adjacent pavement or protect thesubgrade.

40.6.9 Protection

Pipelines may require protection both internally and externallyagainst a wide range of aggressive conditions. Steel and ductileiron pipes will tend to corrode in the presence of moisture andair. Consequently, they may require protection both internallyand externally using a wide range of materials such as reinforcedbitumen, coal tars, plastics, epoxy resin-based coatings andPortland cement mortars. Factory-applied protection is to bepreferred but, since accidental damage is likely to occur duringinstallation, ease of repair must be an important consideration.

The corrosion of ferrous materials is an electro-chemicalprocess and cathodic protection can be used to provide externalprotection in addition to coatings. This may be particularlynecessary in aggressive ground conditions and where clay soilcould promote bacteriological corrosion. The pipeline is madecathodic by using either an impressed direct current or byconnecting the pipeline to sacrificial anodes which corrode inpreference.20 It is important that cathodic protection systems areproperly engineered since corrosion may be accelerated by aninadequate design.

Concrete pipes can be damaged due to sulphate in thegroundwater and septicity in sewage. The latter generateshydrogen sulphide which can be reduced to sulphuric acid bycertain types of bacteria. Septicity can to some degree beprevented by careful design of the sewer system to ensure anadequate oxygen supply.

40.6.10 Testing

Pipelines should be inspected for line level and material defectsas they are constructed. Welded joints in steel pipelines can bechecked using radiographic or other non-destructive tests. Inter-nal proof pressure tests should be applied, preferably beforejoints have been backfilled. Air pressure is usually quicker andmore convenient than water pressure; smoke tests can be used todiscover leaks. However, air-pressure tests can be dangerous iffailure should occur on large-diameter pipes. Water testing is tobe preferred in these cases. Special arrangements may be neces-sary to anchor unbalanced forces particularly when testing largepipes. Also, the practical problems of obtaining sufficient waterand its subsequent disposal can be considerable.

40.6.11 Safety

The principal legislation dealing with construction is the Healthand Safety at Work Act 197421 which defines duties and respon-sibilities of employers, controllers of premises, manufacturersand suppliers of equipment and employees. This Act is sup-ported by the Construction Regulations22'23 together with otherlegislation. Practical advice on safety can be found else-where. l3-19-24<25

The object of the legislation is to provide a safe place of workfor operatives with safe means of entry and egress and a safesystem of work. At the same time the safety of persons outsidethe site must also be ensured.

By law, trenches must be supported properly and inspectedregularly.22 Excavators used in the work can be used as cranes

only if special exemption is obtained26 and should be inspectedweekly.

It should be noted that trenches may in some circumstancesconstitute a confined space and appropriate safety measuresshould be applied (see Table 40.4).

The legal responsibilities for safety cannot easily be subcon-tracted and the Swan Hunter case27 clarified further duties forproviding information. Contractors working alongside othercontractors (whether subcontractors, tertiary contractors orindependent contractors) have a duty to provide sufficientinformation so that each employer is aware how his operationsaffect others and how the operations of others affect his ownemployees. This duty applies not only downwards from maincontractor to subcontractor but upwards also.

40.7 Trenchless pipelaying

In urban areas the use of trenchless techniques can reduce to alarge extent the disruption caused by trench construction andalso allow obstructions such as rivers, railways, major roads,etc. to be negotiated conveniently. The techniques currentlyused may be roughly divided into microtunnelling (formation ofa new bore smaller than man-entry size) and conventionaltunnelling (larger than man-entry size). The lower limit of man-entry size is generally accepted to be about 900 mm diameter.This is the smallest diameter in which a man can work effect-ively (albeit with difficulty). Where pipes are to be inspectedonly, men can enter down to 600mm diameter if properprecautions are taken.

Since 95% of the UK sewer system is 900 mm diameter orsmaller, microtunnelling is frequently used for sewer refurbish-ment as well as new construction.

40.7.1 Microtunnelling design

In recent years there has been a considerable emphasis on thedevelopment of microtunnelling techniques, and methods areavailable for producing bores from 50 to 900mm diameter.

Figure 40.10 indicates the current techniques, which are basedmainly on pipe-jacking methods.

Little research has been done on the vertical loads exerted bythe ground on pipes installed by jacking. It is usually assumedthat a slight overbreak of soil around the pipe may occur whichwill create a soil stress system similar to a Marston narrowtrench condition. Therefore, soil load can be estimated by theequation by Young and O'Reilly (page 40/7) where Bd = effec-tive width of trench. This assumption is likely to be conservativein stable soil.7 Other soil theories may also be used; in particularTerzaghi's theory for buried pipes28 which is based on assump-tions similar to those used by Marston.

The loads on the pipe due to ground surcharge may also becalculated using the methods derived for pipes in trench.

The pipes must be strong enough to cope with the axial loadsproduced by the jacking operation as well as the external andservice loads. Experience shows that pipes which are designed tocope with the jacking forces are generally adequate for theground loads. Once the ground forces have been established, thepipe strength can be checked either using the bedding factormethod (a value of 1.9 is generally accepted for pipes installedby jacking) or by using elastic theories of ring compression.29

The jacking loads arise mainly from friction. This varies withdifferent ground conditions from 5 to 25 kN/m2 although moreextreme figures have been known. Sands and gravels causehigher values than cohesive soils and friction can be reduced bylubricating the outside of the pipe using bentonite slurry orother methods.

Jacking pipes of different materials and joint design vary inthe load accepted in end bearing, the different materials avail-able including steel, concrete, clay and GRP. Angular variationsarising between pipes during installation tend to reduce thefailure load and many types of joint have been tried to reducethis danger.28 The jacking forces must be resisted at the thrust pitby a thrust wall or a heavy foundation. These are designed byconventional soil mechanics methods.

40.7.2 Site investigation

A proper site investigation is an indispensable requirement. It is

Table 40.4 Safety checks for confined spaces

Before work starts:(1) Check ground conditions for hazards and sources of gas such as organic strata, refuse, sewers, gas mains, industrial

pipelines and the interaction between carbonate and acid (particularly in chalk, limestone and greensands)(2) Ensure personnel are fit and properly trained(3) Ensure breathing apparatus, lifelines and safety equipment is available. Define the procedures for contact with the

emergency services(4) Check gas monitoring equipment is available and working

Before entering:(5) Check atmosphere for oxygen deficiency and explosive or toxic gases(6) Check arrangements for ventilation(7) Use breathing apparatus if the atmosphere is dangerous(8) Check arrangements for entry and egress and lighting are adequate

While working:(9) Continuously monitor the atmosphere

(10) Ensure space is properly ventilated(11) Do not smoke(12) Ensure direct communication is maintained between everyone involved in the work(13) Ensure all equipment is maintained in good order and properly used

In emergencies:(14) Do not enter the space without proper equipment or without an attendant at the entrance. A lifeline and harness should be

used when entering and pulling out victims.(15) Do not attempt to purge dangerous atmospheres with pure oxygen which will cause an explosive hazard