wong design analysis deep excavations session02

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


Wong Kai Sin 2

Overall Stability

3ULS Design & Strut Forces

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



Wong Kai Sin 3

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


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.



Wong Kai Sin 4

Penetration Depth against Piping

(Teng, 1962)

Fs = 1.5


Basal Heave Stability

qo

qult

8

When qo > qult, failure in imminent.



Wong Kai Sin 5

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?


Methods of Analysis



Wong Kai Sin 6

Terzaghi’s Method(Terzaghi, 1943)


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



Wong Kai Sin 7

Modification to Terzaghi’s Method


Bjerrum and Eide’s method (1956)

14

cu NcFS = --------------

γ H + q



Wong Kai Sin 8


Eide et al.’s Method (1972)



Wong Kai Sin 9


Comparison of Methods – Case 1 – Sheetpile Wall



Wong Kai Sin 10






Wong Kai Sin 11

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.



22

0.97



Wong Kai Sin 12

Sheetpiles are very flexible. They tend to move along with the soil.


Comparison of Methods – Case 5 – Diaphragm Wall



Wong Kai Sin 13

Comparison of Methods – Case 6 – Diaphragm Wall


Effect of Wall Penetration(Zhang and Zhang, 1994)



Wong Kai Sin 14

Modified Terzaghi’s Method for Diaphragm

Wall

(Wong and Goh, 2001)(Wong and Goh, 2001)

Method 1:

27

Method 2:


Modified Terzaghi’s Method for Diaphragm Wall(Wong and Goh, 2001)



Wong Kai Sin 15

Modified Terzaghi’s Method for Diaphragm Wall(Wong and Goh, 2001)


30

Terzaghi (1943): Fs = 0.82

Modified Terzaghi: Fs = 1.13 (Method 1)

Fs = 1.06 (Method 2)



Wong Kai Sin 16

Narrow Excavation for all Wall Types

Modified Eide et al.’s Method


Wide Excavation with Sheetpile Wall

cuh

cub

T

Hard Stratum



Wong Kai Sin 17

Wide Excavation with Diaphragm Wall

cuh

cud

cub

αcud

T

Hard Stratum


How important is the shape factor?



Wong Kai Sin 18

Basal Heave Failure in Taipei (1998)


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



Wong Kai Sin 19

What factor of safety should we use?


How reliable is the computed F.S.?



Wong Kai Sin 20

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"


Basal Heave of Wall with Full Penetration to Hard Stratum

Case 7 ‐ Sheetpile Wall



Wong Kai Sin 21


Case 8 ‐ Diaphragm Wall



Case 9 ‐ Sheetpile Wall



Wong Kai Sin 22

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.




Wong Kai Sin 23

Toe Kick‐in Stability

PaPp

σ

Scenario 1

σ

Scenario 2

45

5.29cu – σv1

σv1

5.29cu – σv1

σv1



MA = ?A



Wong Kai Sin 24

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


How to overcome negative net pressure?

Option 3

Use shorter wall

48

A

negative net pressure



Wong Kai Sin 25


A Deep Excavation in Oslo

(Aas, 1985)

30 kPa

1.13



Wong Kai Sin 26


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.



Wong Kai Sin 27

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


M

1. How do you determine the active and

PaPp

LaLp


passive earth pressures?

2. Assuming Pa and Pp are known, which of the five methods would you use?


Wong Kai Sin 28

Earth Pressure according to Rankine’s

TheoryFS = 1.29 (Terzaghi)

AA

A A


Effect of PenetrationDepth on

Bending MomentBending Moment

56

D = 0, 8 & 17 m



Wong Kai Sin 29

Should theoretical earth pressures be used in the analysis?


Soil Arching & Rowe’s Moment Reduction



Wong Kai Sin 30

59

Net


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


12

14

16

Theory

Sheetpile

Diaphragm Wall


Wong Kai Sin 31

0

2

-100 -50 0 50 100 150 200


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


12

14

16

Theory

Sheetpile

Diaphragm Wall

Sheetpile 304

Diaphragm 1120

0

2

4

6

-400 -300 -200 -100 0 100 200 300 400


H = 8m o

Earth Pressure (kPa)

6

8

10

12

14

16

18

Dep

th (m

)

D = 15mγ = 18 kN/m3

cu = 25 kPa


18

20

22

24

26

Theory

Sheetpile

Diaphragm Wall


Wong Kai Sin 32

0

2

4

6

-100 -50 0 50 100 150 200


H = 8m o

γ = 18 kN/m3


8

10

12

14

16

18

20

Dep

th (m

)

D = 15mγ /

cu = 25 kPa

Mo(kNm/m)

Theory 2121


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


D

B

Soil‐structure interaction affects PA & PP


6

8

10

12

14

16

18

20

Dep

th (m

)

D


Analysis based on earth pressure theories can lead to unrealistic results!

22

24

26

Theory

Sheetpile

Diaphragm Wall


Wong Kai Sin 33


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.


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


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


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.


Wong Kai Sin 34


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



Wong Kai Sin 35

CIRIA’s Characteristic Pressure Diagram for Sand(CIRIA, 1996)

69

P = 0.2γH


Strut Forces in Stiff to Very Stiff Clay (CIRIA, 1996)



Wong Kai Sin 36

CIRIA’s Characteristic Pressure Diagram for Soft

Clay (CIRIA, 1996)


CIRIA’s Characteristic Pressure Diagram for Soft Clay (CIRIA, 1996)

72

Soft Clay(Unstable base)



Wong Kai Sin 37


Firm Clay(stable)

73

Soft Clay

(stable)ULS Design & Strut Forces


Soft Clay

(unstable base)



Wong Kai Sin 38

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


Comparison of APD – Sheetpile Wall



Wong Kai Sin 39

Comparison of APD – Diaphragm Wall

77

Are FE results reliable?


Strut Forces on Diaphragm Wall in Sand

(Kastner & Lareal, 1974)



Wong Kai Sin 40

Effect of Wall Stiffness on Strut Forces(Chang & Wong, 1996)

79

x10‐7


Effect of Wall Stiffness on Strut Forces

(Chang & Wong, 1996)


(m=1)


Wong Kai Sin 41

Effect ofEffect of Temperature

on Strut Forces


Degree of Restraint(CIRIA, 1996)

82

B = Stiff Clay C = Granular Soils D = Mixed SoilsULS Design & Strut Forces


Wong Kai Sin 42

Degree of Restraint(CIRIA, 1996)


Temperature Effect on Strut Forces

(Batten et al., 1996)



Wong Kai Sin 43

Temperature Effect on Strut Forces

(Batten et al., 1996)

Tubular steel props


Other Factors Affecting 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


Wong Kai Sin 2

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?


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


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


Geotechnical problem

User Must define the problem the way the program will understand

Faithful but not too intelligent



Wong Kai Sin 4

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?


2‐D Finite Element Method

σvσh

8

σvσh

Finite Element Analysis


Wong Kai Sin 5

FE Modeling of an Excavation

9Use of half‐mesh because of symmetry


Half mesh or Full mesh?

10

Half Mesh Full Mesh



Wong Kai Sin 6

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


5. Set proper drainage boundaries in consolidation analysis.6. Include piles only where appropriate.

Effect of Mesh Fineness on Wall Deflection

12Finite Element Analysis


Wong Kai Sin 7

What type of analysis should we conduct?

Total StressffEffective StressUndrainedDrainedConsolidation


It depends on the permeability of soil and duration of construction.

Effect of permeability on wall deflection



Wong Kai Sin 8

Effect of permeability on ground settlement


Is it important to conduct consolidation analysis for deep excavation in clay?



Wong Kai Sin 9

Coefficient of Permeability “k”

k (m/s)

Clean gravels

Clean sands Very fine sands

Silts & clayey sand

Clays

Drained Transition Undrained


1‐D (Beam‐n‐Spring) Analysis by Finite Element Method

WALLAP

RIDO

18

FREW

REWARD



Wong Kai Sin 10

Parameters for the Beam‐and‐Spring Model

Kh = ??? c

19

Kh = ??? cu


Ka & Kp



Wong Kai Sin 11

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 = ???


Comparison of Results

Rochor Complex



Wong Kai Sin 12


Lavender Station



Syed Alwi Condo



Wong Kai Sin 13


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.



Wong Kai Sin 14

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.


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



Wong Kai Sin 15

Major Shortcomings of 2‐D Analysis

Is 2‐D analysis yappropriate?

29

Is appropriate to model the piles as plates?


Is 2‐D Analysis appropriate at I1, I2 and I3?

(After Ou et al., 1996)I2

I3

I1

I5I4



Wong Kai Sin 16

3‐D Effect in Braced Excavation

(After Ou et al., 1996)

I2I3

I1

I5I4

I3

31

(I4 & I5)


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)



Wong Kai Sin 17

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


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)



Wong Kai Sin 18

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



wong design analysis deep excavations session02

Documents