Sub - Structural

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Sub-Structure

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Section 3200

3201 SCOPE

The specifications shall apply to soil nailing works as required to be carried out under theContract or as directed by the Engineer.

3202 NAIL INSTALLATION METHODS

3202.1 Generally used soil nail installation techniques in practice are: (i) drilled andgrouted soil nails, and (ii) driven soil nails. Grouted nails are recommended for all types ofsoil nail walls applications; and in particular, for walls with vertical height more than 7 m.Driven nails shall only be used when wall heights are smaller (less than or upto 7.0 m). Themajor difference between the two is the pullout resistance of the soil nails and grouted soilnails are expected to have higher pull out resistance. Pull out test is desirable to check thevalues of pullout resistance which is useful in soil nail design.

3202.2 Drilled and grouted soil nails (or simply grouted nails), are approximately 75mm to 200 mm diameter nail holes drilled in the soil mass to be retained, which shall befollowed by placing of steel reinforcement bars (tendon) and the grouting of the drill hole.

3202.3 Driven soil nails are relatively small in diameter (20 mm to 25 mm) and aremechanically driven into the ground. They are usually spaced approximately 0.5 m to 1.0m apart.

3203 CONSTRUCTION MATERIALS

Following covers the general materials required for the construction of typical soil nailingstructure.

3203.1 Reinforcement bar (nail or tendon)

The reinforcing element (tendon) shall be high strength steel reinforcing bar conforming toIS: 1786 with a characteristic tensile strength of 415 MPa or higher which may be threadedat one end. Minimum recommended diameter of reinforcement bar (tendon) is 16 mm.

3203.2 Nail head

The nail head shall comprise of following main components: the bearing-plate, hex nut,and washers; and the headed-stud. The bearing plate with a central hole (with diametergreater than reinforcement bar) shall be of minimum Fe250 grade steel, typically square inshape with 200 to 250 mm side dimension and 19 mm thickness. Washers and nuts shallbe made of steel with a grade consistent with that of the nail bar commonly of 415 MPa orhigher. Nuts may be tightened with a hand-wrench. The headed-stud connection mayconsist of four headed studs welded near the four corners of the bearing plate to provide

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anchorage of the nail head into the permanent facing. For temporary walls, the bearingplate shall be on the outside face of the shotcrete facing.

3203.3 Grout

Grout for soil nails is required to fill the annular space between the nail bar and thesurrounding ground, and for shotcreting of the temporary facing. Grout for soil nail walls iscommonly a neat cement grout with the water/cement ratio typically ranging from 0.4 to0.5. Grout mix shall be prepared in accordance with IS: 9012. Grout shall have a minimum28 days characteristic strength of 20 MPa. For filling up nail holes, grout shall be pumpedshortly after the nail bar is placed in the drillhole to reduce the potential for hole squeezingor caving. In solid nail bar applications, the grout may be injected by tremie methods througha grout pipe, which is previously inserted to the bottom of the drillhole, until the groutcompletely fills the drillhole.

3203.4 Centralizers

Centralizers are devices made of polyvinyl chloride (PVC) or other synthetic materials thatare installed at various locations along the length of each nail bar to ensure that a minimumthickness of grout completely covers the nail bar. Centralizers shall be installed at regularintervals, typically not exceeding 2.5 m, along the length of the nail and at a distance ofabout 0.5 m from each end of the nail.

3204 FACING TYPES

Soil nail walls are generally provided with two types of facings: (a) temporary facing and(b) permanent facing.

3204.1 Temporary facing

Temporary facing shall be constructed by providing reinforcement in the form of weldedwire mesh (conforming to IS:1566) throughout the wall face, and by additional bearingplates (see Clause 3203.2.) and waler bars (rebars of smaller lengths placed in the vicinityof nail head) at the nail heads; which is, subsequently shotcreted in accordance with IS:9012. Overall temporary facing thickness shall vary from 75 mm -200 mm.

3204.2 Permanent facing

Permanent facing may be constructed as cast-in-place reinforced cement concreteconforming to IS: 456, precast concrete or any suitable material to achieve desired strengthand aesthetics. Reinforcement in the permanent facing may be adopted in the form ofwelded wire mesh or reinforcement bars in either direction. Permanent facing shall beconnected to the temporary facing by means of headed-studs (usually four numbers per

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plate) welded on the bearing plates installed during construction temporary facing. Minimumthickness of permanent facing shall not be less than 200 mm.

3205 CONSTRUCTION SEQUENCE

3205.1 Typical sequence of construction of a soil nail wall shall be in accordancewith following steps:

Step 1: Excavation of initial cut of 1.0 m -2.0 m depending upon the capacity of in-situ soilto stand unsupported for about 12-24 hrs;

Step 2: Drilling hole for nail;

Step 3: Installation of nails followed by grouting and placing of drainage strip;

Step 4: Placing of construction facing and installation of bearing plates;

Step 5: Repetition of process till final level is reached; and

Step 6: Placing of final facing.

3206 GEOTECHNICAL ASPECTS

3206.1 Soil investigation

For soil nail walls more than 30 m long, exploratory borings should be spaced between 30to 60 m along the proposed centerline of the wall. For walls less than 30 m long, at leastone boring is necessary along the proposed centerline of the wall. Borings are also necessaryin front and behind the proposed wall. Borings behind the wall should be located within adistance up to 1 to 1.5 times the height of the wall behind the wall and should be spaced upto 45 m along the wall alignment. Borings in front of the wall should be located within adistance up to 0.75 times the wall height in front of the wall and should be spaced up to 60m along the wall alignment. The depth of borings should extend at least one full wall heightbelow the bottom of the excavation.

3206.2 Bond strength

The bond strength is the mobilized shear resistance along the soil-grout interface. Thebond strength adopted for the design of soil nails is commonly based on conservativeestimates obtained from field correlation studies and local experience in similar conditions.Consequently, some percentage of the soil nails shall be load tested according to standardprocedure (pullout tests) in the field to verify bond strength design.

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3206.3 Suitable in-situ ground conditions

Following are the in-situ conditions considered favorable for the prospective use of soilnailing technique.

a) Soil shall be able to stand unsupported to a depth of about 1 m – 2 mhigh vertical or nearly vertical cut for 12-24 hours.

b) Groundwater table shall be sufficiently below level of the lowermostsoil nail at all cross-sections.

c) Favorable soils: Stiff to hard fine –grained soils, dense to very densegranular soils with some apparent cohesion, weathered rock with noweakness planes and glacial soils.

3207 ANALYSIS OF FAILURE MODES

3207.1 Analysis of various failure modes of soil nail structures shall be performedusing allowable stress design methodology in accordance with IRC publication “InterimGuidelines for Soil Nailing Technique in Highway Engineering Applications” or any otherstate-of-art standard for analysis, design and construction of soil nail walls. For rigorousanalyses, use of computational tools based on advanced numerical techniques such asfinite element method, is strongly recommended.

3208 INITIAL DESIGN CONSIDERATIONS

3208.1 Wall layout

Establish the layout of the soil nail wall, including: (1) wall height; (2) length of the wall; (3)backslope; and (4) wall face batter. Wall face batter typically ranges from 0º to 10º. Theevaluation of the wall layout also includes developing longitudinal profile of the wall, locatingwall appurtenances (e.g., traffic barriers, utilities, and drainage systems), and establishingROW limitations.

3208.2 Soil nail vertical and horizontal spacing

Typically, same nail spacing can be adopted in both horizontal Sh and vertical S-v directions.Nail spacing ranges from 1.25 to 2 m (commonly 1.5 m) for conventional drilled and groutedsoil nails, and as low as 0.4 m for driven nails. As a general rule, soil nail spacing inhorizontal and vertical direction must be such that each nail has an influence area

2h vS S 4m× ≤ .

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3208.3 Soil nail pattern on wall face

The soil nail pattern on wall face may be adopted as one of the following: (1) square (orrectangular); (2) staggered in a triangular pattern; and (3) irregular (at limited locations)depending upon the ease of construction and site-specific constraints.

3208.4 Soil nail inclination

Soil nails are typically installed at an inclination ranging from 0 to 30 degrees from horizontalwith a typical inclination of 10-25 degrees.

3208.5 Soil nail length and distribution

The distribution of soil nail lengths in a soil nail wall can be selected as either uniform (i.e.,only one nail length is used for the entire wall), or variable, where different nail lengths maybe used for individual soil nail levels within a wall cross section. Uniform nail pattern isrecommended for most applications.

3209 OTHER DESIGN CONSIDERATIONS

3209.1 Loads and load combinations

Soil nail walls used on typical highway projects are typically subjected to the followingdifferent loads during their service life: (i) Dead loads DL (e.g., weight of the soil nail wallsystem, lateral earth pressure, weight of a nearby above-ground structure); (ii) Live loadsLL (e.g., traffic loads); (iii) impact loads IL (e.g., vehicle collision on barriers above soil nailwall); and (iv) earthquake loads EQ. Following load combinations are recommended toassess the most critical loading condition:

a) DL LL+b)

D

c)

D

For earthquake loads, allowable stresses shall be increased by 133 percent from thevalues obtained with factors of safety for static loads.

3209.2 Permissible soil nail wall deformations

The maximum permissible lateral deformation at the top of the soil nail walls constructedin weathered rock and stiff soils is 0.1%H; sandy soils is 0.2%H and for fine-grained soilsis 0.3%H. Under no circumstances maximum permissible lateral deformation shall exceed0.3% H, where: H is the vertical height of the soil nail wall. Permissible vertical deformation(i.e., settlement) shall be considered to be same as the permissible horizontal deformation.

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3209.3 Drainage measures

3209.3.1 Short term drainage measures

Surface water and groundwater must be controlled both during and after construction ofthe soil nail wall. A surface water interceptor ditch, excavated along the crest of the excavationand lined with concrete, is a recommended element for controlling surface water flows.Additionally, if ground water impacts are temporary or localized, suitable dewateringmeasures may be taken for lowering the groundwater table

3209.3.2 Long term drainage measures

Long term drainage shall be provided by means of the drainage system comprising of: (i)vertical geo-composite drain strips placed suitably along the face of wall; (ii) weep holesin the form of perforated PVC pipes of typical diameter as 50-100 mm and 300- 600 mmlength, placed through the face at the location of expected localised seepage; (iii) provisionof horizontal or slightly inclined drain pipes of typical diameter 50 mm installed at thelocations where it is necessary to control the groundwater pressures imposed on the retainedsoil mass; (iv) installation of permanent interception ditch behind the wall at its crest toprevent surface water runoff from infiltrating behind the wall or flowing over the wall edge;and (v) provision of a vegetative protective cap/cover to reduce or prevent water infiltrationinto the soil.

3209.4 Corrosion protection

Corrosion potential of the soil must be evaluated for all permanent soil nail walls and, insome cases, for temporary walls.

Corrosion protection measures: (a) Specify a minimum grout cover of 25 mm betweenthe reinforcement nail bar and the soil; (b) recommend epoxy coating of minimum thickness0.4 mm on the nail bars shall be applied by the manufacturer prior to shipment of nails tothe construction site, which is, subsequently to be encased in grout cover; and (c) adoptother site-specific suitable corrosion protection measures.

3210 FIELD PULLOUT TESTING

3210.1 Field pullout testing of soil nails shall be conducted (a) to verify that the naildesign loads can be carried without excessive movements and with an adequate safetyfactor for the service life of the structure, and (b) to verify the adequacy of the contractor’sdrilling, installation, and grouting operations prior to and during construction of productionsoil nails.

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3210.1 Types of field pullout tests

Depending upon the type of test being performed, the maximum test load, the loadincrements, and the time that each load increment is held shall be determined. To preventchances of explosive failure of the steel, in no case, the soil nail tendon be stressed tomore than 80 percent of its minimum ultimate tensile strength for grade Fe415 steel, ormore than 90 percent of the minimum yield strength for grade Fe500 steel.

3210.1.1 Verification test

A verification test on soil nail is performed: (a) to determine the ultimate bond capacity (ifcarried to pullout failure); (b) verify the design bond factor of safety, and (c) to determinethe soil nail load at which excessive creep occurs. Verification tests are generally conductedon non-production “sacrificial” nails as a first order of work prior to construction.

3210.1.2 Proof test

A proof test is typically performed on a specified number of the total number of productionsoil nails installed. Typically, successful proof tests shall be performed on 5 percent of theproduction nails in each row or a minimum of 1 test per row. Proof tests provide informationnecessary to evaluate the ability of production soil nails to safely withstand design loadswithout excessive structural movement or long-term creep over the structure’s service life.

3210.1.3 Creep Test

Creep tests are generally performed as part of a verification or proof test to ensure thatnail design loads can be safely carried throughout the structure service life.

3211 MEASUREMENT FOR PAYMENT

Measurements for earthwork shall conform to IS: 1200(Part I), concrete work shall conformto IS: 1200(Part II) and steelworks shall conform to IS: 1200(Part VIII). Measurement offacia shall be in sq. metres.

3212 RATES

Rates for labour, machinery and various structural components shall conform to the localschedule of rates in practice.

3213 REFERENCES

IS:1200(Part I)-1992. Method of measurement of building and civil engineering works –EARTHWORK (fourth revision).

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IS:1200(Part II)-1974. Method of measurement of building and civil engineering works –CONCRETE WORKS (third revision).

IS:1200(Part VIII)-1993. Method of measurement of building and civil engineering works –STEELWORK AND IRONWORK (fourth revision).

IS:1566-1982. Specification for hard-drawn steel wire fabric for concrete reinforcement(second revision incorporating amendments nos. 1, 2 and 3).

IS:1786-1985. Specification for high strength deformed steel bars and wires for concretereinforcement (third revision).

IS:456-2000. Plain and reinforced concrete – code of practice (fourth revision).

IS:9012-1978 Recommended practice for shotcreting (fourth reprint).

IRC publication “Interim Guidelines for Soil Nailing Technique in Highway EngineeringApplications” (to be published).