Design of Retaining Wall and Support System for Deep BasementDeep Basement
Ir. TAN Yean ChinG&P Geotechnics Sdn Bhd
www.gnpgroup.com.my
March, 2014
INTRODUCTION
� Deep basement construction� Urban areas for parking space� Infrastructures, e.g. KVMRT
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� Infrastructures, e.g. KVMRT
� Risk associated with deep basement construction high!
SOIL PARAMETERS
� Some important soil parameters related to retaining wall and support system design:
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system design:�Shear strength parameters (su, φ’ & c’)�Soil permeability�Soil stiffness
SOIL PARAMETERS
� Shear strength parameters (φ’ and c’)�Commonly obtained from CIU for effective
stress strength parameters
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stress strength parameters�Commonly obtained using in -situ test
methods (e.g. field vane shear test) for total stress strength parameters, su
�Finite element analysis – requires proper understanding of the constitutive soil models
SOIL PARAMETERS
� Soil permeability� Important in order to choose modelling in
drained or undrained condition
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drained or undrained condition� In-situ tests (rising, falling or constant head)
recommended�Values obtained from tests should always be
compared to published values (e.g. BS8004)
SOIL PARAMETERS
� Soil stiffness� Important parameters for retaining wall design
BUT difficult to obtain reliablyIn Malaysia, sometimes based on empirical
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� In Malaysia, sometimes based on empirical correlations
�Laboratory tests unreliable and values obtained significantly smaller than appropriate values for retaining wall design
�Designer should be aware of small-strainnature of retaining wall design
SOIL PARAMETERS
� Soil stiffness�Seismic tests or seismic piezocone appears
promising – future direction
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promising – future direction�Basis of empirical correlations should be
understood – e.g. local soil conditions, constitutive model used, etc.
�Example, correlations in Kenny Hill formation using hardening soil model of PLAXIS software
DESIGN CONSIDERATIONS
� Overall stability� Basal heave failure� Hydraulic failure
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� Hydraulic failure� Axial stability� Finite element analysis� Ground movement associated with
excavation
OVERALL STABILITY
� To ensure sufficient wall embedment�Overturning of wall and overall slope
stability
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stability�Adequate factors of safety (e.g. 1.4 for
high-risk-to-life structures)
DESIGN CONSIDERATIONS
� Overall stability� Basal heave failure� Hydraulic failure
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� Hydraulic failure� Axial stability� Finite element analysis� Ground movement associated with
excavation
BASAL HEAVE FAILURE
� Critical for deep excavation in soft ground� Analogous to bearing capacity failure , only in
reverse
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� Available methods:� Terzaghi (1943) – shallow and wide excavations� Bjerrum & Eide (1956) – deep and narrow and
excavations� Moment equilibrium - adequate for routine design
BASAL HEAVE FAILURE
� Required factor of safety for moment equilibrium method�1.2 (Kohsaka & Ishizuka, 1995)
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�1.2 (Kohsaka & Ishizuka, 1995)�Vertical shear resistance along retained
ground shallower than the excavation is ignored
DESIGN CONSIDERATIONS
� Overall stability� Basal heave failure� Hydraulic failure
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� Hydraulic failure� Axial stability� Finite element analysis� Ground movement associated with
excavation
HYDRAULIC FAILURE
� Base instability caused by piping�Seepage due to high groundwater level
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� Available methods�Terzaghi’s method�Critical hydraulic gradient method
HYDRAULIC FAILURE
� Terzaghi’s method recommended�Based on latest research by Tanaka &
Verruijt (1999)
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Verruijt (1999)�Factor of safety required – 1.2 to 1.5
HYDRAULIC FAILURE
� Heaving due to artesian pressure
�Factor of safety – 1.0 to 1.2
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�Factor of safety – 1.0 to 1.2�Smaller FOS sufficient as it did not
consider shear strength or adhesion strength of the ground and retaining wall
DESIGN CONSIDERATIONS
� Overall stability� Basal heave failure� Hydraulic failure
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� Hydraulic failure� Axial stability� Finite element analysis� Ground movement associated with
excavation
AXIAL STABILITY
� Simple check but is often overlooked
� Factor of safety
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� Factor of safetyrequired – 2.0
FINITE ELEMENT ANALYSIS
� Soil-structure interaction important for deep basement retaining wall design
� Commercial finite element software easily
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� Commercial finite element software easily available and very user friendly (e.g. PLAXIS, CRISP, etc.)
� Understanding and proper useimportant!!!
FINITE ELEMENT ANALYSIS
� Some important considerations in FEM:�Locations of the boundaries of the problem�Details of mesh
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�Modelling of stages of construction�Modelling of interfaces�Use of suitable constitutive soil model�Use of appropriate soil parameters, especially
empirical parameters
GEOMETRICAL DATA
� Provision for over-excavation � for cantilever walls - 10% of the wall height above
excavation level, limited to a maximum of 0.5m� for a supported walls - 10% of the distance between
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� for a supported walls - 10% of the distance between the lowest support and the excavation level, limited to a maximum of 0.5m
� Can be reduced when excavation surface can be controlled reliably throughout the excavation works
GEOMETRICAL DATA
� Water levels�Flood level should be taken into consideration
for flood prone areas
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for flood prone areas
� Surcharge�Minimum surcharge of 10kPa – for
construction loads and unforeseen circumstances
CONSTITUTIVE SOIL MODELS
� Various constitutive soil models, e.g. Mohr-Coulomb, Cam Clay, Hardening Soil, Soft Soil, etc.
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Soft Soil, etc.�Proper understanding and limitations of
each model important!� Incorrect use of soil models in Nicoll Highway!
Overestimation of shear strength!!!
Stress path of real soil
Stress path using Mohr-Coulomb
model
strength!!!
FEM ANALYSIS OF LIMIT STATE
� Eurocodes have replaced British Standards
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� Current design of retaining wall for deep basement mostly based on “working state design ”
FEM ANALYSIS OF LIMIT STATE
� Three (3) schemes to perform limit states design using FEM:
(A) Perform all the calculations with design (factored) values of ground and action parameters
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values of ground and action parameters(B) Simulate the whole stress history using ground
parameters at characteristic levels- check the safety at the relevant stages
(C) Simulate the whole stress history using ground parameters at characteristic levels and multiplying these values by the load factor (which then in fact acts as a model factor on the effects of actions).
FEM ANALYSIS OF LIMIT STATE
� Important to evaluate limit state:�Part of the reason for factors of safety is to
cover human error
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cover human error�Limit state analyses investigate unrealistic
states , especially in the Ultimate Limit State (ULS) analysis
TO ENSURE THAT LIMIT STATE IS SUFFICIENTLY UNLIKELY TO OCCUR
GROUND MOVEMENT
� Deep excavation include a substantial component of horizontal strain
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� Damage estimation should include both angular distortion and horizontal strain
Gap
GROUND MOVEMENT INDUCED BY DEEP EXCAVATION
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Gap(75mm)
Void (150mm)
Settlement
CATEGORY ANGULAR DISTORTION,
β (x 10-3)
HORIZONTAL STRAIN, εh (x 10-3)
Negligible < ~ 1.1 > 0.5
Very slight ~ 1.1 < β < ~ 1.6 0.5 < εh < 0.75
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Very slight ~ 1.1 < β < ~ 1.6 0.5 < εh < 0.75
Slight ~ 1.6 < β < ~ 3.3 0.75 < εh < 1.5
Moderate ~ 3.3 < β < ~ 6.7 1.5 < εh < 3.0
Severe > ~ 6.7 > 3.0
Before excavation
After excavation
h
∆∆∆∆β = β = β = β = ∆∆∆∆/Lεεεε = h/L
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εεεεh = h/LL = length of building
E.g.
Building length = 10m
Allowable lateral movement = 15mm
(εεεεh = 1.5 x 10-3)
Case History 1 :32m Deep Excavation for KVMRT – Blue LineKVMRT – Blue Line(Cochrane Underground Station)
Berjaya Times Square
� Excavated depth24.5m - 28.5m (6-level basement)
� Retaining wall
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1.2m thick diaphragm walls
� Support systemPrestressed Ground Anchors
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-10 0 10 20 30 40Wall Relative Lateral Displacement (mm)
-50
-40
-30
-20
-10
0
Dep
th (m
)
Measured ProfileFEM Back Analysed
Excavate to R.L.35.0m
Stage 1
-10 0 10 20 30 40Wall Relative Lateral Displacement (mm)
-50
-40
-30
-20
-10
0
Dep
th (m
)
Excavate to R.L.31.0m
Stage 2
-10 0 10 20 30 40Wall Relative Lateral Displacement (mm)
-40
-30
-20
-10
0
Dep
th (m
)
Stage 3
Excavate to R.L.27.0m
-50-10 0 10 20 30 40
Wall Relative Lateral Displacement (mm)
-50
-40
-30
-20
-10
0
Dep
th (m
)
Stage 4
Excavate to R.L.22.5m
-10 0 10 20 30 40Wall Relative Lateral Displacement (mm)
-50
-40
-30
-20
-10
0
Dep
th (m
)
Stage 4
Excavate to R.L.18.5m
-10 0 10 20 30 40Wall Relative Lateral Displacement (mm)
-50
-40
-30
-20
-10
0
Dep
th (m
)
Stage 6
Excavate to R.L.16.7m
WALL A
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Berjaya Times Square
� Hardening Soil Model of PLAXIS able to model the problem sufficiently accurate
� From FEM back-analysis, the correlations between soil stiffness (E’) and SPT ‘N’ as follows:�E’ = 2000*SPT‘N’(kN/m 2)�E’ur = 3*E’ = 6000*SPT’N’(kN/m 2)
Original CBP Wall with
Temporary Ground Anchors
Initial basement wall
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Anchors
Temporary Ground Anchor
� Soil nails subjected to verification and proof test based on FHWA, 1998�Criteria on creep movement�Criteria on theoretical elongation
� Based on verification tests on preliminary soil nails�Ground -grout bond = 3 to 5*SPT -N (in kPa)
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