wong design analysis deep excavations session02
TRANSCRIPT
November 2009 ULS Design & Strut Forces
Wong Kai Sin 1
Time Session Topic
Session 2Design (Part 1)
p09:00 – 10:30 1 Overview10:30 – 11:00 Coffee Break
11:00 – 12:30 2 Design (Part 1)12:30 - 01:30 Lunch
01:30 – 03:00 3 Mohr-Coulomb Soil Model &
1
Design (Part 2)03:00 – 03:30 Coffee Break
03:30 – 05:00 4 How to reduce wall deflection
ULS Design & Strut Forces
Major Design Considerations in Deep Excavations
Excessive movements
Wall deflections
Total collapse
Overall stability
2
Wall deflections
Ground settlement
Effect on adjacent structures
y
Uplift or blow‐out failure
Piping & quick condition
Basal heave
Toe stability
Strutting system failureULS Design & Strut Forces
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Overall Stability
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Uplift Instability or Blowout Failure
FillE
UMCUMC
F2
LMC
E / F2
4
1. What is the permeability of the sand?
2. Is there a free supply of water?
Sand
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Blowout Failure
γT B d + 2 αcu d Fs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
h B
For very long excavation:B
γw h B
For rectangular shape:
γT dBL + 2d αcu(B+L) Fs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
γT
R R
5
Fs = γw h B L
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Piping in Sand
Piping is a phenomenon of water rushing up through pipe‐shaped
6
Piping is a phenomenon of water rushing up through pipe shaped channels due to upward seepage under high gradient. It can lead to total collapse of the system. Sufficient penetration of sheetpile must be used to lengthen the seepage path and to reduce the hydraulic gradient.
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Penetration Depth against Piping
(Teng, 1962)
Fs = 1.5
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Basal Heave Stability
qo
qult
8
When qo > qult, failure in imminent.
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Which method should we use?
•Terzaghi •Bjerrum & Eide•Eide et al.•Tschebotarioff•Tschebotarioff•Goh•Chang•Wong and Goh•O'Rourke•Su et al.•Ukritchon et al.•Plaxis
9
Does FOS≤1 mean failure?
ULS Design & Strut Forces
Methods of Analysis
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Terzaghi’s Method(Terzaghi, 1943)
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Terzaghi’s Method
5 7 c B
T
Hard Stratum
12
5.7 cub B1FS = ----------------------
g H B1 - cuhH
If T ≥ 0.7B, B1 = 0.7B
If T < 0.7B, B1 = T
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Modification to Terzaghi’s Method
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Bjerrum and Eide’s method (1956)
14
cu NcFS = --------------
γ H + q
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Eide et al.’s Method (1972)
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Comparison of Methods – Case 1 – Sheetpile Wall
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Comparison of Methods – Case 2 – Sheetpile Wall
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Comparison of Methods – Case 3 – Sheetpile Wall
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Effect of Depth to Hard Stratum (T)
When T ≥ 0.7B, failure surface can be developed freely.
21
, p y
When T < 0.7B, the development of failure surface is restrained. It is no longer the plane of lowest resistance. Therefore, there will be an increase in factor of safety. The correction factor β accounts for the effect of depth to hard stratum, T.
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Comparison of Methods – Case 4 – Sheetpile Wall
22
0.97
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Sheetpiles are very flexible. They tend to move along with the soil.
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Comparison of Methods – Case 5 – Diaphragm Wall
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Comparison of Methods – Case 6 – Diaphragm Wall
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Effect of Wall Penetration(Zhang and Zhang, 1994)
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Modified Terzaghi’s Method for Diaphragm
Wall
(Wong and Goh, 2001)(Wong and Goh, 2001)
Method 1:
27
Method 2:
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Modified Terzaghi’s Method for Diaphragm Wall(Wong and Goh, 2001)
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Modified Terzaghi’s Method for Diaphragm Wall(Wong and Goh, 2001)
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30
Terzaghi (1943): Fs = 0.82
Modified Terzaghi: Fs = 1.13 (Method 1)
Fs = 1.06 (Method 2)
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Narrow Excavation for all Wall Types
Modified Eide et al.’s Method
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Wide Excavation with Sheetpile Wall
cuh
cub
T
Hard Stratum
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Wide Excavation with Diaphragm Wall
cuh
cud
cub
αcud
T
Hard Stratum
33ULS Design & Strut Forces
How important is the shape factor?
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Basal Heave Failure in Taipei (1998)
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Basal Heave Failure in Taipei (1998)
B
Factor of SafetyMethod A A B B
B
36
Method A-A B-BTerzaghi 0.67 0.66
Bjerrum & Eide 0.58/0.69 0.61/0.69Wong & Goh 0.94 0.99
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What factor of safety should we use?
37ULS Design & Strut Forces
How reliable is the computed F.S.?
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What type of test should we conduct to determine cu?
cu1. Most conservative:"worst scenario"
Which strength envelope should we use ?
39
Depth
2. Best Estimate:"most probable scenario"
3. Most optimistic:"most favourable scenario"
ULS Design & Strut Forces
Basal Heave of Wall with Full Penetration to Hard Stratum
Case 7 ‐ Sheetpile Wall
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Basal Heave of Wall with Full Penetration to Hard Stratum
Case 8 ‐ Diaphragm Wall
41ULS Design & Strut Forces
Basal Heave of Wall with Full Penetration to Hard Stratum
Case 9 ‐ Sheetpile Wall
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Excavation with Full Penetration of Wall into Hard Stratum
Basal heave stability is not an issue for this case. But toe kick‐out failure may occur.
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Toe Kick‐in Stability
PaPp
σ
Scenario 1
σ
Scenario 2
45
5.29cu – σv1
σv1
5.29cu – σv1
σv1
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Toe Kick‐in Stability
MA = ?A
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How to overcome the negative net pressure?
Option 1
A
Option 2
Add JGP slab
A
A
47
Penetrate into hard stratumnegative net pressure
MA = ?
A
ULS Design & Strut Forces
How to overcome negative net pressure?
Option 3
Use shorter wall
48
A
negative net pressure
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49ULS Design & Strut Forces
A Deep Excavation in Oslo
(Aas, 1985)
30 kPa
1.13
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Basal Heave Toe Stability
If there is adequate F.S. against basal heave,
PaPp
52
If there is adequate F.S. against basal heave, toe stability is not an issue.
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Toe Kick‐out Stability
Method 1:
Pp LpFs = ‐‐‐‐‐‐‐‐‐‐
M
Pa & Pf are based on unfactored strength:
Fs = ‐‐‐‐‐‐‐‐‐‐Pa La
Method 2:
Pp Lp + MallFs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Pa La
PaPp
LaLp
P f& P f are based on factored strength:
ULS Design & Strut Forces 53
Method 3:
Pp Lp + MultFs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Pa La
Method 4:
Ppf Lp > Paf La
Ppf & Paf are based on factored strength:
Method 5:
Ppf Lp + Mall > Paf La
Toe Kick‐out Stability
M
1. How do you determine the active and
PaPp
LaLp
ULS Design & Strut Forces 54
passive earth pressures?
2. Assuming Pa and Pp are known, which of the five methods would you use?
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Earth Pressure according to Rankine’s
TheoryFS = 1.29 (Terzaghi)
AA
A A
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Effect of PenetrationDepth on
Bending MomentBending Moment
56
D = 0, 8 & 17 m
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Should theoretical earth pressures be used in the analysis?
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Soil Arching & Rowe’s Moment Reduction
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59
Net
ULS Design & Strut Forces
0
2
-300 -200 -100 0 100 200 300
Passive Pressure (kPa)
H = 8m o
Earth Pressure (kPa)
4
6
8
10
Dep
th (m
)
D = 8mγ = 18 kN/m3
cu = 25 kPa
ULS Design & Strut Forces 60
12
14
16
Theory
Sheetpile
Diaphragm Wall
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0
2
-100 -50 0 50 100 150 200
Passive Pressure (kPa)
H = 8m o
Net Earth Pressure (kPa)
4
6
8
10
Dep
th (m
)
D = 8m γ = 18 kN/m3
cu = 25 kPa
Mo(kNm/m)
Theory 838
Sheetpile 304
ULS Design & Strut Forces 61
12
14
16
Theory
Sheetpile
Diaphragm Wall
Sheetpile 304
Diaphragm 1120
0
2
4
6
-400 -300 -200 -100 0 100 200 300 400
Passive Pressure (kPa)
H = 8m o
Earth Pressure (kPa)
6
8
10
12
14
16
18
Dep
th (m
)
D = 15mγ = 18 kN/m3
cu = 25 kPa
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18
20
22
24
26
Theory
Sheetpile
Diaphragm Wall
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0
2
4
6
-100 -50 0 50 100 150 200
Passive Pressure (kPa)
H = 8m o
γ = 18 kN/m3
Net Earth Pressure (kPa)
8
10
12
14
16
18
20
Dep
th (m
)
D = 15mγ /
cu = 25 kPa
Mo(kNm/m)
Theory 2121
ULS Design & Strut Forces 63
20
22
24
26
Theory
Sheetpile
Diaphragm Wall
Sheetpile 298
Diaphragm Wall
601 (Mmax=1010)
B & D can affect PA & PP
0
2
4
-100 -50 0 50 100 150 200
Passive Pressure (kPa)
D
B
Soil‐structure interaction affects PA & PP
Net Earth Pressure (kPa)
6
8
10
12
14
16
18
20
Dep
th (m
)
D
ULS Design & Strut Forces 64
Analysis based on earth pressure theories can lead to unrealistic results!
22
24
26
Theory
Sheetpile
Diaphragm Wall
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Toe Kick‐out Stability
Method 1:
Pp LpFs = ‐‐‐‐‐‐‐‐‐‐
Methods 1 to 3 are based on unfactored strength:
1. Methods 1, 2 and yield about the same FS because Mall and Mult are negligible when compared to the other terms.
2 Methods 4 & 5 yield about the same FS for theFs = ‐‐‐‐‐‐‐‐‐‐Pa La
Method 2:
Pp Lp + MallFs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Pa La
2. Methods 4 & 5 yield about the same FS for the same reason given in (1).
3. If earth pressure theory is to be used to compute Pa and Pp, all 5 methods can be used.
4. If Pa is to be determined from FEA, only Methods 1 or 3 should be used.
ULS Design & Strut Forces 65
Method 3:
Pp Lp + MultFs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
Pa La
Method 4:
Ppf Lp > Paf La
Methods 4 & 5 are based on factored strength:
Method 5:
Ppf Lp + Mall > Paf La
Toe Kick‐in Stability
PaPp
LaLp
Pp Lp + MultFs = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
P L
1. If factor of safety against basal heave is adequate, toe stability is not an issue. No analysis is necessary.
2. Compute Pa and Pp from earth pressures theory. If the computed FS is adequate without requiring excess penetration depth no further
pPa La
ULS Design & Strut Forces 66
is adequate without requiring excess penetration depth, no further analysis is needed.
3. If the required penetration depth from (1) is excessive, try using Pafrom FEA.
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67ULS Design & Strut Forces
Lateral Earth Pressure in Braced Excavations
R di t ib ti f th d t hi
68
♦ Redistribution of earth pressure due to arching
♦ Preloading
♦ Incremental excavation and strut installation
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CIRIA’s Characteristic Pressure Diagram for Sand(CIRIA, 1996)
69
P = 0.2γH
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Strut Forces in Stiff to Very Stiff Clay (CIRIA, 1996)
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CIRIA’s Characteristic Pressure Diagram for Soft
Clay (CIRIA, 1996)
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CIRIA’s Characteristic Pressure Diagram for Soft Clay (CIRIA, 1996)
72
Soft Clay(Unstable base)
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CIRIA’s Characteristic Pressure Diagram for Soft Clay (CIRIA, 1996)
Firm Clay(stable)
73
Soft Clay
(stable)ULS Design & Strut Forces
CIRIA’s Characteristic Pressure Diagram for Soft Clay (CIRIA, 1996)
Soft Clay
(unstable base)
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Strut Forces by Tributary area method
aPPA Area A
Area B
Area C
b
b
ccd
d
PB
PC
75
e.g. PB = ( b + c ) p in kN/m run
pd
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Comparison of APD – Sheetpile Wall
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Comparison of APD – Diaphragm Wall
77
Are FE results reliable?
ULS Design & Strut Forces
Strut Forces on Diaphragm Wall in Sand
(Kastner & Lareal, 1974)
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Effect of Wall Stiffness on Strut Forces(Chang & Wong, 1996)
79
x10‐7
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Effect of Wall Stiffness on Strut Forces
(Chang & Wong, 1996)
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(m=1)
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Effect ofEffect of Temperature
on Strut Forces
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Degree of Restraint(CIRIA, 1996)
82
B = Stiff Clay C = Granular Soils D = Mixed SoilsULS Design & Strut Forces
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Degree of Restraint(CIRIA, 1996)
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Temperature Effect on Strut Forces
(Batten et al., 1996)
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Temperature Effect on Strut Forces
(Batten et al., 1996)
Tubular steel props
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Other Factors Affecting Strut
Forces
86ULS Design & Strut Forces
November 2009 Finite Element Analysis
Wong Kai Sin 1
Major Design Considerations in Deep Excavations
Excessive movements
Wall deflections
Total collapse
Overall stability
U lift bl t f il
1
Wall deflections
Ground settlement
Effect on adjacent structures
Uplift or blow‐out failure
Piping & quick condition
Basal heave
Toe stability
Strutting system failure Need Finite Element Analysis!Finite Element Analysis
What do you get from Finite Element Analysis?
Strut forces
Wall bending moment & shear forces
Finite Element Analysis 2
Wall deflections
Ground settlement
Tunnel displacements
Factor of safety
November 2009 Finite Element Analysis
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Deformation Analysis using Finite Element Programs
1-D Programs 2-D Programs 3-D Programs
Rido
Wallap
FREW
Plaxis
Sage Crisp
Sigma/W
Flac
Plaxis 3D
Flac 3D
ZSOIL
GEOFEA
ABAQUS
MidasGTS 3DMidasGTS-3D
Which program should we use?
Finite Element Analysis 3
Plaxis offers the following choices for analysis of short term performance of TERS in clay:
A. Mohr‐Coulomb: effective stress, c’‐ φ’, undrained
B Mohr‐Coulomb: effective stress c ‐ φ undrained
Method of Analysis
B. Mohr‐Coulomb: effective stress, cu‐ φu, undrainedC. Mohr‐Coulomb: total stress, cu‐ φu, non‐porous, undrained D. Mohr‐Coulomb: effective stress, c’‐ φ’, consolidationE. Mohr‐Coulomb: effective stress, cu‐ φu, consolidationF. Soft Clay: effective stress, c’‐ φ’, undrainedG. Soft Clay: effective stress, c’‐ φ’, consolidationH. Mod. Cam Clay: effective stress, c’‐ φ’, undrainedy , φ ,I. Mod. Cam Clay: effective stress, c’‐ φ’, consolidationJ. Advanced Hardening: effective stress, c’‐ φ’, undrainedK. Advanced Hardening: effective stress, c’‐ φ’, consolidation
Which one should we use?Finite Element Analysis 4
November 2009 Finite Element Analysis
Wong Kai Sin 3
Blessings & curses of commercial software
Blessings:User friendlyUser friendlyGenerates output with beautiful plotsGives user a sense of accomplishment
Curses:Sometimes it aborts without suggesting the
t f tinext course of actionSometimes it produces puzzling results
Finite Element Analysis 5
Geotechnical problem
User Must define the problem the way the program will understand
Faithful but not too intelligent
Finite Element Analysis 6
November 2009 Finite Element Analysis
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1. Attend training course!
2. Study the manual and do the tutorial problems.
Advice to Users
y p
3. Do not assume it will work the way you think.
4. When in doubt, devise a simple problem and test out how the program works.
5. Check input – mesh, design parameters……6. Study output:
• Is the mode of deformation correct?• Are the magnitudes reasonable?
Finite Element Analysis 7
2‐D Finite Element Method
σvσh
8
σvσh
Finite Element Analysis
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FE Modeling of an Excavation
9Use of half‐mesh because of symmetry
Finite Element Analysis
Half mesh or Full mesh?
10
Half Mesh Full Mesh
Finite Element Analysis
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Notes on Mesh Generation for FEA
1. Set left and right boundaries far away from area of interest.
2. Use a fine mesh.3. Include only the key elements. Exclude the details.4. Simplify the soil profile.5 S d i b d i i lid i l i
Finite Element Analysis 11
5. Set proper drainage boundaries in consolidation analysis.6. Include piles only where appropriate.
Effect of Mesh Fineness on Wall Deflection
12Finite Element Analysis
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What type of analysis should we conduct?
Total StressffEffective StressUndrainedDrainedConsolidation
Finite Element Analysis 13
It depends on the permeability of soil and duration of construction.
Effect of permeability on wall deflection
Finite Element Analysis 14
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Effect of permeability on ground settlement
Finite Element Analysis 15
Is it important to conduct consolidation analysis for deep excavation in clay?
Finite Element Analysis 16
November 2009 Finite Element Analysis
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Coefficient of Permeability “k”
k (m/s)
Clean gravels
Clean sands Very fine sands
Silts & clayey sand
Clays
Drained Transition Undrained
Finite Element Analysis 17
1‐D (Beam‐n‐Spring) Analysis by Finite Element Method
WALLAP
RIDO
18
FREW
REWARD
Finite Element Analysis
November 2009 Finite Element Analysis
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Parameters for the Beam‐and‐Spring Model
Kh = ??? c
19
Kh = ??? cu
Finite Element Analysis
Ka & Kp
20Finite Element Analysis
November 2009 Finite Element Analysis
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Calibration of Soil Modulus using 1‐D and 2‐D Programs
RIDO: 1‐D Beam‐and‐Spring
EXCAV97 ‐ 2‐D continuumHyperbolic Model
21
Ks / cu = ??? Ei / cu = ???
Finite Element Analysis
Comparison of Results
Rochor Complex
22Finite Element Analysis
November 2009 Finite Element Analysis
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Comparison of Results
Lavender Station
23Finite Element Analysis
Comparison of Results
Syed Alwi Condo
24Finite Element Analysis
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25Finite Element Analysis
Limitations of Beam‐and‐Spring Method
1. It ignored the effect of width on wall deflection.
2. It ignored the effect of clay thickness on wall deflection.
26Finite Element Analysis
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Limitations of Beam‐and‐Spring Method
1. It ignored the effect of width on strut force.
2. It ignored the effect of clay thickness on strut force.
27Finite Element Analysis
Is “Eu/cu=200” applicable to all soil models and programs ?
MOEBuilding
RochorComplex
Syed AlwiProject
Lavender Station
WALLAP, Mohr Coulomb, Eu/ cu 250 250 300 300
SAGE CRISP Mohr Co lomb E /c 100 150 300 500SAGE CRISP, Mohr Coulomb, Eu/cu 100 150 300 500
SAGE CRISP, Hyperbolic, Ei/cu 300 300 300 300
EXCAV97, Hyperbolic, Ei/cu 200 200 200 200
28Finite Element Analysis
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Major Shortcomings of 2‐D Analysis
Is 2‐D analysis yappropriate?
29
Is appropriate to model the piles as plates?
Finite Element Analysis
Is 2‐D Analysis appropriate at I1, I2 and I3?
(After Ou et al., 1996)I2
I3
I1
I5I4
30Finite Element Analysis
November 2009 Finite Element Analysis
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3‐D Effect in Braced Excavation
(After Ou et al., 1996)
I2I3
I1
I5I4
I3
31
(I4 & I5)
Finite Element Analysis
A B
Which section is closer to plane strain condition?
L = 100 m
B = 100 m
L = 40 m
B = 20 m
A B
PSR = 0.91 PSR = 0.90
32
δH,max (3‐D)Plane Strain Ratio, PSR = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
δH,max (2‐D)
Finite Element Analysis
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PSR = 0.60
PSR = 0.91PSR = 0.83
B =L=100 m
B =L=60 mB =L=40 m
PSR = 0.60 PSR = 0.50 PSR = 0.42
33
PSR 0.60
L=40 m
B=40 m
L=60 m
B=40 mB=40 m
L=100 m
Finite Element Analysis
0 8
0. 9
1
B=20m
Reduction Factor for δH,max due to 3‐D Effect
(Developed based on data from Ou et al., 1996)
0. 3
0. 4
0. 5
0. 6
0. 7
0. 8
PSR
B=100m
B=80m
B=40m
B=60m
B
L
34
0
0. 1
0. 2
0 20 40 60 80 100 120L (m)
Finite Element Analysis
November 2009 Finite Element Analysis
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1
1. 2
B=20mB=40mB=60mB=80m
Reduction Factor for δH,max due to 3‐D Effect
(Developed based on data from Ou et al., 1996)
0. 4
0. 6
0. 8
1
PSR
B=100m
B
L
35
0
0. 2
0 0. 5 1 1. 5 2 2. 5 3 3. 5L/B
Finite Element Analysis
36Finite Element Analysis