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Soil Nailing

Amit Prashant

Indian Institute of Technology Gandhinagar

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Historical Origin of Soil Nailing

• Soil nailing technique is derived from a technique called New Austrian Tunneling

Method developed to support rock-excavation(Rabcewicz 1964a, 1964b and 1965)

• A 60 ft high cut-slope in sand was stabilized by soil nails for a railroad widening project near Versailles, France (1972). Since then soil nailing became common in France and other European countries

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Basic Concept

• To reinforce and strengthen the existing

ground by installing closely-spaced steel bars

called nails , into a slope as construction

proceeds from top to bottom. (FHWA-SA-96-069R)

• Reinforcing action developed through nail-

ground interactions as the ground deforms

during and following construction (FHWA-SA-96-

069R).

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Elements of Soil Nailed wall/Slope

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Soil Nails

• Soil Nails: Tendons +Centralizer+ Grout

Tendon

PVC Centralizer

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….

Grout

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Facing: Shotcrete

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Facing Reinforcement

• Welded Wire-Mesh (WWM): over the entire

wall hight

• Horizontal bars( Waler bars): placed around

nail heads to add bending resistance in

horizontal direction

• Vertical bearing bars: placed at nail heads to

add bending resistance in vertical direction

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Connection components

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Drainage System

• Installed behind the facing to

collect perched ground water or infiltrated surface water

direct ground water away from the wall

• Commonly comprises of:

composite , geosynthetic drainage strips( i.e. geocomposite strip drains)

• Covers only 10-20% of the wall area depending on selected strip drain spacing and commercial widths that are available

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…..

Strip drain

prior to

initial

Shotcrete

placement

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Construction Sequence

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……

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Applications: Railway/Roadway cuts

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Road widening under existing bridge

abutments

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Applications: Hybrid soil nail wall

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Shored mechanically stabilized earth

walls

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Design Aspects: Load transfer in soil

nail walls

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External Stability

Sliding Overturning

Bearing

Capacity

Deep

Seated

Stability

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Sliding Stability

• The preliminary sizing should be checked w.r.t sliding at the base layer

• Resisting force is the lesser of the shear resistance along the base

• Sliding force is the horizontal component of the thrust on the vertical face at the back of the wall

5.1forces driving horizontal

forces resisting horizontal

d

Rsliding P

PFS

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Slope Stability

• Overall stability is determined using rotational or wedge analyses which can be performed by using a classical slope stability analysis method

• For complex structures, compound failures must be considered

• If FOS < 1.35, increase reinforcement length or improve foundation soil

H1

H1/2

H1/2

H

K.H1*

La Le

Zone of maximum shear stress

or potential failure surface

Active

Zone

Resistant

Zone

Soil Nails

b

L

Soil-nail interaction and tensile force

distribution

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Maximum tensile force distribution

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Tensile forces at the wall facing

• Tensile force at the wall face, To (also called as nail

head force), is smaller than the forces along the soil

nail.

•

This variation of To is consistent with experimental

findings (Gassler and Gudehus 1981; FHWA 1993a; Holman

and Touzzolo 2009)

HvsaoHvsao SHSγ0.6K T toSHSγ0.5KT

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Design nail load (force)

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Limitations to tensile

forces in nails

Pull out

resistance

controls

Tensile

resistance

controls 26

FOS = 2

FOS = 1.8

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Shear resistance from Nail (in bending)

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Plastic axial resistance of Nail

Axial force in Nail at the

point of maximum moment

Plastic moment

resistance of Nail

Maximum allowable

moment of Nail

Maximum shear resistance

of Nail

Conservative value of coefficient, C = 2

Conservative value of shear width ls = 15.d

Dia. Of Nail

Facing failure modes

• Flexure failure

• Punching shear failure

• Headed-stud tensile failure

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Flexure failure

29 FOS = 1.5

Punching shear failure

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FOS = 1.5

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Headed stud-tensile failure

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Stability analysis of nailed slope

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Thank You