Soil Nailing - Indian Institute of Technology...
Transcript of Soil Nailing - Indian Institute of Technology...
27-07-2016
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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
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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