FAILURE MECHANISM OF DEEP EXCAVATIONS IN … · 1. Introduction Collapses of deep excavations in urban area 3 Taipei Shi-Pai case Taipei Rebar Broadway case

FAILURE MECHANISM OF DEEP EXCAVATIONS IN SOFT CLAY: CASE STUDIES

Author: Tuan-Nghia Do

1

Introduction

Literature Review

Evaluation of Factors of Safety for Case Histories

Conclusions

Outlines

2

1

2

3

4

1. Introduction

Collapses of deep excavations in urban area

3

Taipei Shi-Pai case

Taipei Rebar Broadway case

4

Nicoll Highway case in

Singapore (4 casualties)

Hangzhou case in China (21

casualties)

2. Literature Review2.1 Conventional Methods

Terzaghi’s method

5

Bjerrum & Eide’s method

𝐹𝑡 =5.7𝑠𝑢2

𝑊′ + 𝑞𝑠 − 𝑠𝑢1𝐻𝑒𝐹𝑏𝑒 =

𝑁𝑐𝑠𝑢𝑊′ + 𝑞𝑠

su2

su1 He

B

qsqs

Stiff soil

45o

2/B

2/B2/B

W'

B

Stiff soil

He

qs qs

su

2/B2/B

45o

W'

Slip circle method

6

Push-in gross pressure method

𝐹𝑠𝑐 =𝑋 0

𝜋2+𝛼

𝑠𝑢𝑋𝑑𝜃 +𝑀𝑠

𝑊′𝑋/2𝐹𝑝 =

𝑃𝑝𝐿𝑝

𝑃𝑎𝐿𝑎

X

qs

Lowest strut levelHe

Hp

Stiff soil

suW'

Shear

d

NN

N

N

N

N

N

N

Ms

su

su

su

su

susu

su

su

Lowest strut level

Stiff soil

PaPp

LaLp

He

Hp

qs

Nodal displacement

Str

ength

red

uct

ion

rati

o (

SR

)2.2 Finite Element Method (FEM)

Brinkgreve and Bakker (1991)

7

Donald and Giam (1988)

Nodal displacement

Str

ength

red

uct

ion

rati

o (

SR

)

Failure based on

convergence criterion

Limited equilibrium state

FSFS

Do et al. (2013)

8

(Elastic support system)

1/Convergence criterion method: FS = SR1

2/Angle method: FS = SR2 as wall kicks out

3/Intersection method: FS = SR3

Intersection method is most reasonable at 4 real failure excavations in clay

Not kick out if 1 < 2

Kick out if 1 > 2

22

1

1

He

Hp

Lower part

Str

eng

th r

edu

ctio

n r

atio

(S

R)

Nodal displacement

Divergence of

numerical solutions

SR1

SR3Upper part

3. Evaluation of Factors of Safety for Case Histories3.1 Taipei Rebar Broadway Case

9

-10

-15

-20

-25

-30

0 50 100Su (kPa)

GL -1.8 m

GL -4.35 m

GL -7.65 m

GL -10.95 m

GL -13.45 m

SM

ML-CL

SM-SP t = 19.7 kN/m3

CL t = 18.8 kN/m3

GL -24.0 m

B = 25.8 m

t = 20.3 kN/m3

t = 19.2 kN/m3

(0.7 m

thickness)

5.1 m 5.1 m 5.1 m5.1 m5.4 m

Center post

GL -1.0 m

GL -3.55 m

GL -6.85m

GL -10.15m

H350x350x12x19

H400x400x13x21

H400x400x13x21

H300x300x10x15

GL 0.0 m

CKoU-AC

CKoU-AE

CKoU-DSS

For analysis

Wall

10

Finite element model

10 m

140 m

44

.7 m

13

.45

m3

1.2

5 m

25.8 m 57.1 m47.1 m 10 m

Hinges

11

Wall deflection and soil heave at final stage

-200 0 200 400

Wall deflection, mm

-24

-20

-16

-12

-8

-4

0

De

pth

, m

0 4 8 12 16 20 24 28Distance to wall, m

0

100

200

300

400

500

Hea

ve,

mm

98% SMstage max

99% SMstage max

SMstage max = 1.00

0 2000 4000

Wall deflection, mm

-24

-20

-16

-12

-8

-4

0

Dep

th,

m

0 4 8 12 16 20 24 28

Distance to wall, m

0

2000

4000

6000

8000

Hea

ve,

mm

98% SRmax

99% SRmax

SRmax = 1.84937

EP support system E support system

-1000 -600 -200 -100 0 200 600 1000

M kNm/m

-10000

-14000

0

-1000

N, kN/m

100

-500

500

100010000

14000

1234

5c

BL of 3rd

and 4th

strut layers

4

55

BL of wall

1 2 3 45

12

Interaction diagram

EP support system

BL of 1st strut layer

2 3

4

5

Excavation

stage

1st strut layer

BL of 2nd

strut layer

3

4

5

2nd strut layer

3rd strut layer4th strut layerwall

Struts start to yield before wall

13

Incremental displacement plot at SRmax as using EP support system

Plastic hinges

14

Plastic point plot at SRmax as using EP support system

32 m

15

Incremental displacement plot at SRmax as using E support system

16

Plastic point plot at SRmax as using E support system

35 m

3.2 Taipei Shi-Pai Case

17

0 20 40 60Su (kPa)

-10

-15

-20

-5

Fill

ML

CL

CL

GL -15.4 m

t = 18 kN/m3

t = 18 kN/m3

t = 17.7 kN/m3

6.15 m

B = 12.3 m

6.15 m

Wall

Center post

GL -1.9 m

GL -4.2 m

GL -6.5 m

GL -9.3 m

GL 0.0 m

t = 18 kN/m3

GL -1.4 m

GL -3.7 m

GL -6.0m

H300x300x10x15

H300x300x10x15

H400x400x13x21

CKoU-AC

CKoU-AE

CKoU-DSS

For analysis

(0.5 m

thickness)

0 1000 2000

Wall deflection, mm

-16

-12

-8

-4

0

Dep

th,

m

0 2 4 6 8 10 12 14

Distance to wall, m

0

1000

2000

3000

4000

5000

Hea

ve,

mm

98% SRmax

99% SRmax

SRmax = 1.54807

0 1200 2400

Wall deflection, mm

-16

-12

-8

-4

0

De

pth

, m

0 2 4 6 8 10 12 14Distance to wall, m

0

500

1000

1500

2000

2500

Hea

ve,

mm

98% SMstage max

99% SMstage max

SMstage max = 0.6734

18



19


Plastic hinges

20


22 m

21


22


26 m

3.3 Hangzhou Case

23

0 20 40 60 80 100

su, kPa

40

30

20

10

0

Dept

h, m

Field Vane (V01 undisturbed)

Field Vane (V07 undisturbed)

Chen et al., 2013

FILLCL

OL

OL

OL

OL

OL

CL

GL -15.7 m

GL -0.0 mf 609, d 12

f 609, d 16

f 609, d 16

11.9 m

Diaphragm

wall (0.8 m

thickness)

B = 21.2 m

9.3 m

Bored pile (f 800)

GL -46.0 m

Center post

2x(280x82x7.5x12.5)

GL -33.0 m

1

2

3

f 609, d 164

Not

installed

Left

wall

Right wall

0 400 800

Wall deflection, mm

-40

-32

-24

-16

-8

0

De

pth

, m

0 4 8 12 16 20 24Distance to wall, m

0

200

400

600

800

Hea

ve,

mm

98% SMstage max

99% SMstage max

SMstage max = 0.6652

104

0 1042x10

43x10

4

Wall deflection, mm

-40

-32

-24

-16

-8

0

Dep

th,

m

0 4 8 12 16 20 24

Distance to wall, m

010

4

2x104

3x104

4x104

5x104

Hea

ve,

mm

98% SRmax

99% SRmax

SRmax = 3.07975

24



25


Plastic hinges

26


33 m

Failure surface

(Chen et al., 2013)

27


28


50 m Failure surface

(Chen et al., 2013)

3.4 Nicoll Highway Case

29

0 20 40 60 80 100

su, kPa

50

40

30

20

10

0

De

pth

, m

Whittle and Davies (2006)

Fluvial deposit

Fluvial deposit

GL -0.0 m

GL -44.3 m

Fill

Estuarine

Upper marine

clay

Lower

marine clay

Estuarine

Old alluvium

10.05 m

Diaphragm wall (0.8 m thickness)

B = 20.1 m

10.05 m

GL -42.3 m

H-350

H-400 (G50)

2H-400 (G50)

2H-400

2HR-400

H-414 (G50)

2H-400 (G50)

2H-414 (G50)

2H-400 (G50)

GL -30.6 m

Center post

1

2

3

4

5

6

7

8

9

Jet grout

piles

0 3x103

6x103

Wall deflection, mm

-48

-40

-32

-24

-16

-8

0

De

pth

, m

0 4 8 12 16 20 24Distance to wall, m

0

103

2x103

3x103

4x103

5x103

Hea

ve,

mm

98% SR max

99% SR max

SR max = 1.13033

0 5x103

104

Wall deflection, mm

-48

-40

-32

-24

-16

-8

0

Dep

th,

m

0 4 8 12 16 20 24

Distance to wall, m

0

4x103

8x103

1x104

Hea

ve,

mm

98% SRmax

99% SRmax

SRmax = 5.26420

30



31


Plastic hinges

32


45 m

33


34


50 m

35

FS by various methods

Case history

FEM Conventional method

Using EP support

system

Using E

support

system

Terzaghi’s

method

Bjerrum

& Eide’s

method

Slip

circle

method

Taipei Rebar

Broadway case1.00 1.849 1.088 0.952 0.809

Taipei Shi-Pai

case

SMstage = 0.673

(at final excavation

stage)

1.548 0.901 0.792 0.732

Hangzhou case

SMstage = 0.665

(at final excavation

stage)

3.080 1.071 0.983 1.043

Nicoll highway

case1.13 5.264 >1.342 >1.750 >1.222

4.6 Failure Mechanisms of Excavations in Clay

36

Overall failure of excavations

Failure surface

Center post

Overlapped area of failure surfaces

Broken wall

Center post

Failure surface

Overlapped area of failure surfaces

Broken wall

37

Overall failure of excavations

4. Conclusions

38

The FEM using the EP support system gives more reasonable results than that using the E support system

based on case studies

Terzaghi’s method, Bejerrum & Eide’s method, and slip circle method could give reasonable results of FS at these

cases

There are two possible failure mechanisms of deep excavations in clay

1

2

3

39

THANK YOU

Documents

FAILURE MECHANISM OF DEEP EXCAVATIONS IN … · 1. Introduction Collapses of deep excavations in urban area 3 Taipei Shi-Pai case Taipei Rebar Broadway case