The Nicoll Highway CollapseThe Nicoll Highway Collapse

D W Hi htD. W. HightT.O. Henderson + A.R. Pickles

S. Marchand

ContentContent• Background and construction sequence• Observations up to the point of collapse, including

monitoringmonitoring• The collapse• Post-collapse investigations• Design errors• Jet grout layers• Ground conditions and the buried valley• Ground conditions and the buried valley• The need for a trigger• Back analyses of the collapse• Relative vertical displacement and forced sway• The bored piles• Collapse mechanism and its trigger• Collapse mechanism and its trigger• Lessons to be learnt

C824210m 2-cell cut &

cover tunnels

34m Diameter Temporary Staging Area (TSA) Shaft -

35m deep

3-level cut & cover Station (BLV)

550m 2-cell cut & cover tunnel and

siding (BLS)

C824

Collapse siteM3 area

3-level cut & cover Station (NCH)

200m cut & cover stacked 4-cell

scissors crossover tunnel

25m - 30m deep

30m - 33m deep

35m 2-cell cut and cover tunnel

C825

M3 area370m stacked 2-cell cut & cover tunnel

20m - 25m deep

800m twin bored tunnel

C824 - 2km of cut and cover construction

mostly in soft clay - up to 35m deep

Ci l Li St 1Ci l Li St 1Circle Line Stage 1Circle Line Stage 1

Reclamation dates

G ld Mil

The Concourse

Plaza HotelM d k

Beach Road

1930-1940’sGolden Mile

Tower

Golden Mile Complex

Plaza Hotel

NCH station Merdeka Bridge

1930-1940 sreclamation

Nicoll Highway

Cut and Cover tunnels

1970’sreclamation

TSAshaft

Kallang

reclamation

Kallang Basin

Courtesy of Richard Davies

31700

54+900 ABH32

31650

RTH

ING

(m)

M354+900

M2

54+812

M3010MC3007

MC3008

ABH30

ABH34

ABH35

ABH83

ABH85

AC-1

AC-2

AC-3

AC-4

NO

R

MC2026

ABH27

ABH28ABH29

ABH31 ABH33

ABH81

ABH82

ABH84

31550 31600 31650 31700 31750 31800

EASTING (m)

31600

M2064 M2065

MC2025

ABH27

EASTING (m)

Pre- and post-tender site investigation

SOUTH NORTHABH 31 ABH 32

M304M309100 Fill

U

Fill

Upper estuarineUpper estuarine

Upper

M304M309

90

UpperMarine

Clay

MarineClay

Upper F2

80

Upper F2 Upper F2

LowerMarine

LowerMarine

70

Clay Clay

Base marine clay LowerTop of OA

o eestuarine

Lower F2

OldOld

Alluvium

Typical section in M3Alluvium

Alluvium

0 0 4 0 8 1 2 1 6 2Cone resistance, qc (MPa)

0 20 40 60 80 100Undrained shear strength (kPa)

0 0.4 0.8 1.2 1.6 20

AC1AC2AC3AC4

0 20 40 60 80 1000

AC1AC2AC3AC4

10

AC4

10

AC4

Nkt=12

20

Dep

th (m

)

20

Dep

th (m

)30 30

40 40

Piezocone data for M3 area

Construction details and sequenceConstruction details and sequence

Fill

SOUTH NORTH

Upper

Upper E

Fill

Formation levelapprox 33m bgl

Marine Clay

Upper F2

Temporary diaphragmwalls,0.8m

Driven kingpost

Sacrificial JGP

LowerMarine Clay

gp

Permanent bored pilessupportingrail boxes Sacrificial JGP

Permanent JGPLower E

10 levels of steel struts at 3m +3.5m vertical

centres

Lower F2

OA SW2 (N=35)

OA SW1 (N 72)OA SW1 (N=72)

OA (CZ)

General Excavation sequence for M3 – up to level for removal of sacrificial JGP and installation of 10th level strut

Tidal sea

TSA Shaft

Utility crossingsM3

Curved wallsM2

Nicoll Highway

24/3/04

Support beam

ShaftKingpost

M3

C channel stiffener

No waler

C channel stiffener

Jacking point

M2Walers

Non-splayed strut

Splayed strut

M2Walers

Two Struts Bearing Direct on Single panel

Single Strut with Splays Struts on Walers - No Splays

g yBearing on Waler for Single

panel

Gaps in Diaphragm Wall for 66kV Crossing

South side 13 March 04South side 13 March 04

Events and observations prior to collapseEvents and observations prior to collapseEvents and observations prior to collapseEvents and observations prior to collapse

Plate stiffener

C channelstiffener

Replacement of plate stiffeners at strutReplacement of plate stiffeners at strut--waler connectionwaler connection

Strut bearing

C channel

Strut bearing directly on Dwall

C channel

connection

Excavation for the 10th level of struts, including removal of the sacrificial JGPExcavation for the 10 level of struts, including removal of the sacrificial JGP

Observations on the morning of the collapse

North wall

14 2 6 73

M305M304M303M302M301

335 336 338 339 340334333 337

H H HKP 182KP 181 KP 183

1 5 5 5 7 7

M306 M307 M308 M309 M310

1 Order in which distortion to waler noted

Excavation to 10th complete

Excavation in progress

Location of walers, splays and Dwall gaps

S335 9S335-9

Strut 335-9 south wall

S338 9S338-9

Strut 338Strut 338--9 north wall9 north wall

Instrumentation and results of monitoringInstrumentation and results of monitoringInstrumentation and results of monitoringInstrumentation and results of monitoring

M2/M3 plan at 9M2/M3 plan at 9thth levellevel

M3

TSA Shaft

M2

TSA Shaft

I 65

All struts at S335 instrumented for load

measurementsGWV24

I 104I 104

5000Excavation Front

Beyond S335 at the

4000

4500SG3358 Total ForceSG3359 Total Force Excavation Front

Approaches S335 at the end of Day Shift on

start of Day Shift on 18th April 2004

3000

3500

y17th April 2004

Excavation Front for 10th Level Advancing

from S338

d (k

N)

2000

2500

stru

t loa

d

1000

1500

Excavation Front for 10th Level Between easu

red

0

500

20th April19th April18th April17th April16th April15th April

S336 and S335 M

0102030405060708090100110120130140

Hours before collapse

Change in Measured Load at Strut 335

5000

4000

4500

5000

336(N) & 337(N)

Waler Buckling Observed

Support Bracket at 335(S) Drops Off335(N)

Waler Buckling Observed

3000

3500

4000

(kN

)

2000

2500

3000

trut l

oad

1500

2000Strut Load 335-9

Strut Load 335-8

338(N) & 335(S)easu

red

s

500

1000 338(N) & 335(S)

Waler Buckling ObservedMe

06.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00

Time on 20April 2004

Observed trends in 8th and 9th strut loads

Loadf9

th

Observed

nsta

llatio

n of

Observed8th

9th

Excavation approaches

In

m m

9th

+ passes beyond S335

5 18 20

9am

3.30

p

April 2004

The trends were consistent with there being gyielding of the 9th level strut-waler connection when the excavation passed beneath but with

f th i ifi t h i l d i ithno further significant changes in load in either the 9th or 8th level struts until the collapse was initiatedinitiated

0 200 400

Horizontal displacement (mm)

0I 104

400 200 0

Horizontal displacement (mm)

0I 65

10

I 10410 April

10

I 6510 April

10 10

Behind South Wall North Wall

20

epth

(m)

20

epth

(m)

30

De

30

De

40 40

50 50

0 200 400

Horizontal displacement (mm)

0I 104

400 200 0

Horizontal displacement (mm)

0I 65

10

I 10410 April15 April

10

I 6510 April16 April

10 10

Behind South Wall North Wall

20

epth

(m)

20

epth

(m)

30

De

30

De

40 40

50 50

0 200 400

Horizontal displacement (mm)

0I 104

400 200 0

Horizontal displacement (mm)

0I 65

10

I 10410 April15 April17 April

10

I 6510 April16 April17 April

10 10

Behind South Wall North Wall

20

epth

(m)

20

epth

(m)

30

De

30

De

40 40

50 50

0 200 400

Horizontal displacement (mm)

0I 104

400 200 0

Horizontal displacement (mm)

0I 65

10

I 10410 April15 April17 April20 April

10

I 6510 April16 April17 April20 April

10 10

Behind South Wall North Wall

20

epth

(m)

20

epth

(m)

30

De

30

De

40 40

50 50

500

400

nt, m

m

I104 maxI65 max

300

disp

lace

men

South wall

200

m h

oriz

onta

l

N th ll

100Max

imu North wall

5-Jul 9-Aug 13-Sep 18-Oct 22-Nov 27-Dec 31-Jan 6-Mar 10-Apr0

Comparison of inclinometer readings I65 and I10420042003

• S338-9 stood for 8 days under load and was 20m• S338-9 stood for 8 days under load and was 20m from excavation front

• S335 9 stood for 2 5 days under load and was over• S335-9 stood for 2.5 days under load and was over 8m from excavation front

B th S335 9S & S338 9N b kl d ithi 10 i t• Both S335-9S & S338-9N buckled within 10 minutes

• Load in S335-8 and S335-9 was almost constant b t 18 A il d i iti ti f llbetween 18 April and initiation of collapse

• All C-channel connections failed downwards at both ends

• The south wall was pushing the north wall back

Key observations

The south wall was pushing the north wall back

The collapseThe collapse

3.33pm

3.33pm

3.34pm

3.34pm

3.34pm

3.41pm

3.46pm

Post-collapse investigationsPost collapse investigations

Design errors

Errors• Misinterpretation of BS5950 with regard to stiff

b i l hbearing length• Omission of splays

Effects• Design capacity of strut-waler connection was 50% ofDesign capacity of strut waler connection was 50% of

required design capacity where splays were omitted

Errors in structural design of strutErrors in structural design of strut--waler connectionwaler connection

• Capacity based on BS5950:1990 = 2550 kN• Capacity based on BS5950:1990 = 2550 kN

• Average ultimate capacity based on physicalAverage ultimate capacity based on physical load tests = 4100 kN

9 % f• Based on mill tests, 95% of connections had capacity of 3800 kN- 4400kN

• Predicted 9th level strut load in 2D analyses which ignored bored piles was close to ultimate capacity therefore collapse wasultimate capacity therefore collapse was considered by the COI to be inevitable

Inevitability of collapse

• Method A and Method B refer to two alternative ways of modelling undrained soil behaviour in Plaxis (Pickles, 2002)2002)

• Method A is an effective stress analysis of an undrained blproblem

• Assumes isotropic elastic behaviour and a Mohr-C fCoulomb failure criterion

• As a result mean effective stress p’ is constant until yieldp y

• Method A was being applied to marine clays which were of low over-consolidation or even under-consolidatedof low over consolidation or even under consolidated because of recent reclamation

• Method B is a total stress analysis

Methods A and BMethod B is a total stress analysis

t

Cu from Method A

Cu from Method B

p’

The shortcomings of Method A

105 105

Method A Method B

90

95

100

90

95

100

75

80

85

RL

(m)

75

80

85

RL

(m)

60

65

70

60

65

70

55-0.050 0.000 0.050 0.100 0.150 0.200 0.250 0.300

Wall Disp. (m)

Exc to RL 100.9 for S1 Exc to RL 98.1 for S2 Exc to RL 94.6 for S3

Exc to RL 91.1 for S4 Exc to RL 87.6 for S5 Exc to RL 84.6 for S6

55-0.050 0.000 0.050 0.100 0.150 0.200 0.250 0.300

Wall Disp. (m)

Exc to RL 100.9 for S1 Exc to RL 98.1 for S2 Exc to RL 94.6 for S3

Exc to RL 91.1 for S4 Exc to RL 87.6 for S5 Exc to RL 84.6 for S6

M3 S th W ll Di l t

Exc to RL 81.6 for S7 Exc to RL 78.3 for S8 Exc to RL 75.3 for S9

Exc to RL 72.3 for S10

Exc to RL 81.6 for S7 Exc to RL 78.3 for S8 Exc to RL 75.3 for S9

Exc to RL 72.3 for S10

M3 - South Wall DisplacementMethod A versus Method B

95

100

105

100

105

Method A Method B

75

80

85

90

95

RL

(m)

75

80

85

90

95

RL

(m)

55

60

65

70

75

55

60

65

70

75

-300

0

-250

0

-200

0

-150

0

-100

0

-500 0

500

1000

1500

2000

2500

3000

3500

4000

Bending Moment (kNm/m) -300

0

-250

0

-200

0

-150

0

-100

0

-500 0

500

1000

1500

2000

2500

3000

3500

4000

Bending Moment (kNm/m)

M3 - South Wall bending momentsMethod A versus Method BMethod A versus Method B

M th d A Method Bg ( )

220622002400260028003000

_ S1

2383

200022002400260028003000

Method A Method B

8001000120014001600180020002200

Stru

t Loa

d (k

N/m

)_ S1

S2

S3

S4

S5

S6

S7 800100012001400160018002000

Stru

t Loa

d (k

N/m

)

S1

S2

S3

S4

S5

S6

0200400600 S8

S9

0200400600 S7

S8

S9

M3 – strut forcesMethod A versus Method B

• Method A over-estimates the undrained shear strength of normally and lightly overconsolidated lclays

• Its use led to a 50% under-estimate of wall displacements and of bending moments and an under-estimate of the 9th level strut force of 10%

• The larger than predicted displacements mobilised the capacity of the JGP layers at an earlier stage p y y gthan predicted

Method AMethod A

St t l d i• Structural design errors

• Removal of splays at some strut locationsp y

• Introduction of C-channel waler connection detail

• Use of Method A in soil-structure interaction analysis

C ll i it bl f th• Collapse was an inevitable consequence of the design errors which led to the applied loads on the struts increasing with time and equalling the capacity of the strut-waler connectionof the strut-waler connection

COI view on the principal causes ofthe collapse

St t l d i• Structural design errors

• Removal of splays at some strut locationsp y

• Introduction of C-channel waler connection detail

• Use of Method A in soil-structure interaction analysis

C ll i it bl f th• Collapse was an inevitable consequence of the design errors which led to the applied loads on the struts increasing with time and equalling the capacity of the strut-waler connectionof the strut-waler connection

COI view on the principal causes ofthe collapse

Post-collapse investigationsPost collapse investigations

Jet grout

Excavation of sacrificial JGP in Type H

JGP quality in 100mm cores from borehole M1 in Type KJGP quality in 100mm cores from borehole M1 in Type K

Shear wave velocity measurements in JGP at Type K

5 Shear wave section

4

Pressuremeter section

3

ckne

ss (m

)

2

JGP

thic

Design thickness

1

0 2 4 6 8

0

Thicknesses of JGP in Type K

Post-collapse investigationsPost collapse investigations

Ground conditions andsoil propertiessoil properties

BN3

P33

P36

P40P41/P47P42/P48

31700

54+900 AN1

BN2E

CN3

DN2E

DN3AN2

BN2WCN2

TSAN2

P45P52

P54

31650

RTH

ING

(m)

M354+900

M2

54+812

66KVNI

AN1

BN1E

BN1Ea

BS1Ea

CL-1a

CL-2CL-3g

CN1DN1E

BN1W

DN1W

DN2W

EN1

EN2

FN1

FN2

NO

R

66KVSAS1

BS1EaBS1Eb

BS2E

CFVN

CL 3g

CS1CTSN

DS1ESPTSa

TSAS2b

AS2

BS1W

CPTNCPTNa

CS2

DS1W

ES1

FS1

TSAS2

31550 31600 31650 31700 31750 31800

EASTING (m)

31600BS3E

DS2E

DS3E

AS3

BS2W

BS3W

CS3

DS2W

DS3W

ES2

ES3

FS2

EASTING (m)BS4E

BS4Ea

DS4E

BS4W

CS4

DS4W

PostPost--collapse ground investigationcollapse ground investigation

0 0.4 0.8 1.2 1.6 2Cone resistance, qt (MPa)

00 0.4 0.8 1.2 1.6 2

Cone resistance, qt (MPa)

0

10

BN1Wqt

u2fs

10

BS2Wqt

u2fs

North South

20

Dep

th (m

)

20

epth

(m)

30

D

30

De

400 0.4 0.8 1.2 40

0 0 4 0 8 1 2Pore pressure, u2 (MPa)

0 0.02 0.04 0.06 0.08Friction, fs (MPa)

0 0.4 0.8 1.2Pore pressure, u2 (MPa)

0 0.02 0.04 0.06 0.08Friction, fs (MPa)

CPT profiles north and south of collapse area

100

φ=28.5o

50

Ko=0.5

φ=34.2o

0Pa)

0 50 100 150 200s' (kPa)

0

t (kP

-50

P-8P-9

-100

P-24 P-26

φ=35o

Kisojiban’s CAU tests onKisojiban’s CAU tests onUpper and Lower Marine ClayUpper and Lower Marine Clay

50 55 60 65 70 75 80 85 90 95 100 105

Tender SI boreholesTender SI CPTsDetailed design SIDetailed design SI CPTsPost collapse SIPost collapse SI CPTs

BN3

P33

P36

P40 P41/P47

P42/P48

31700

Post collapse SI CPTsBoreholes for bored pilesMagnetic logging boreholes

BN2E

CN3

DN2E

DN3

AN2

BN2WCN2

TSAN2

P45P52

P54

31650ING

(m)

M354+900

M2M3010MC3007

ABH30

ABH32

ABH34

ABH35ABH83

ABH85

AC 1

AC-2

AC 3

AC-4

66KVNI

AN1

BN1EBN1Ea

CL-1a

CN1

DN1E

BN1W

DN1W

DN2W

EN1

EN2

FN2WN1WN10

WN11WN12

WN13WN14

WN15WN16

WN17WN18

WN19

WN2

WN20

WN21WN22

WN23WN24

WN25WN26

WN28

WN3

WN30A

WN4WN5

WN6WN7A

WN8

WN9

31650

NO

RTH

I

54+812 MC3008

ABH28

ABH29

ABH31 ABH33

ABH81

ABH82

ABH84

AC-1AC-3

66KVS

AS1

BS1EaBS1Eb

CFVN

CL-2CL-3g

CS1CTSN

DS1E

SPTSa

TSAS2b

BS1W

CPTNCPTNa

DS1W

ES1

FN1

TSAS2

WN27WN29

WN30A

WN31WN32

WN33WN34

WN35WN36WN37

WN38WN39

WN40

WS1

WS10WS14WS15WS16

WS17

WS18WS19

WS2

WS20WS21

WS22WS23

WS24WS25

WS26WS27

WS28WS29

WS3

WS30WS31

WS32

WS34WS36

WS4A

WS5WS6

WS7WS8

WS9

WS11WS11A

WS11BWS11C

WS12WS12A

WS12BWS12C

WS13WS13A

WS13BWS13C

31550 31600 31650 31700 31750 31800

31600

M2064 M2065

MC2025

MC2026

ABH27

BS2E

BS3E

DS2E

AS2

AS3

BS2W

BS3W

CS2

DS2W

ES1

ES2

FS1

FS2

WS32WS33WS35A

WS37WS38

31550 31600 31650 31700 31750 31800

EASTING (m)BS4E

BS4Ea

DS3E

BS4W

CS3

CS4

DS3WES3

Evidence for a buried valley in the Old AlluviumDS4E

DS4W

100

110

90

(mR

L)

70

80

ELEV

ATI

ON

(

Bottom of F2 Clay in Marine ClayDetailed design site investigation

50

60

Detailed design site investigationBottom of F2 Clay in Marine ClayPost collapse site investigationsTop of OLD ALLUVIUMPost collapse site investigations

Section along north wall

100

110

90

100

RL)

70

80

ELEV

ATI

ON

(mR

60

E

50

Section along south wall

Tender SI boreholesTender SI CPTsDetailed design SI boreholesDetailed design SI CPTs

BN377.02

P33

P36

P42/P48

86.07

86 90

31700

Detailed design SI CPTsPost collapse SI boreholesPost collapse SI CPTsBoreholes for bored pilesMagnetic logging boreholes

BN2E

CN3

DN2E

DN3

82.61

82.27

81.91

P40 P41/P47

P45P52

P54

86.3986.30 86.84

86.90

86.2085.98

86.58

31650HIN

G (m

)

M354+900

M2KP181 KP182 KP183

WN1

WN10WN11WN12WN13WN14WN15WN16WN17WN18WN19

WN2

WN20WN21WN22WN23WN24

WN25WN26WN27

WN29

WN3

WN30AWN31

WN4WN5WN6WN7AWN8WN9 82.16

81.2180.9580.6680.3780.7279.8681.8680.8180.3180.88

82.31

80.9479.9379.9180.1580.9580.3679.57

79.72

81.2581.3881.4381.4281.3881.0581.25

66KVNI

AN1

BN1EBN1Ea

CL 2CL-3g

CN1

DN1E

DN2E

81.88

82.91

80.6881.5783.13

81.86

81.94

M301082.55MC300780.65

ABH32

ABH35ABH83

ABH8583.41

81.6983.39

81.69

AC-1

AC-2

AC-3

AC-481.7379.98

338 M305M304

M303

M302

M210M20931650

NO

RTH 54+812

KP181WN31WN32

WN33WN34

WN35WN36WN37

WN38

WN39

WN40

WS1

WS10WS15WS16

WS17WS18WS19

WS2

WS20WS21WS22WS23WS24WS25WS26WS27WS28WS29

WS3

WS30WS31WS32

WS34WS35A

WS37WS38

WS4AWS7WS8WS9

78.3477.5177.71

78.0280.58

80.5380.66

81.4378.35

80.83

81.06

83.19

78.20

72.2881.3981.39

81.4581.2881.38

75.20

80.8481.4879.9380.0379.9981.5979.7678.2578.5279.57

76.59

78.7879.1079.25

79.2379.69

80.6581 26

76.7773.8472.6172.70

66KVS

AS1

BS1EaBS1Eb

BS2E

CFVN

CL-2

CS1CTSNDS1E

SPTSa

TSAS2b

71.09

75.98

74.6074.72

81.97

78.0480.22

82.0182.17 81.09

71.52

82.16

ABH29

ABH31 ABH33

ABH81

ABH82

ABH84

81.23

82.07 80.51

80.13

80.37

82.57

80.7681.69M301M211

M210M209

M310M309M308M307

M213

M212 & M306missing

31550 31600 31650 31700 31750 31800EASTING (m)

31600

81.26 BS2E

BS3E

DS2E

82.28

81.29

80.68

M206479.00

MC202581.70

ABH2780.69

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

BS4EBS4Ea

DS4E

80.3380.96

79.24

Distortion to upper F2 layer caused by the collapse

Buried valley in the Old Alluvium

Buried valley in the Old Alluvium

Coincidence between buried valley anddistortion to upper F2 layer

• There was a buried valley crossing the site of the collapse diagonally from south-west to north-east

• The presence and setting of the buried valley explain the asymmetric conditions and the different collapse on the north and south sideson the north and south sides

• The buried valley coincides with the major ground distortion on the south side and was clearly influential in the collapse

• Below the Lower Marine Clay the buried valley was infilled with estuarine organic clays on the south sideinfilled with estuarine organic clays on the south side and fluvial clays on the north side

• Gas exsolution almost certainly occurred in the deep y porganic clays as a result of stress relief, reducing their strength further

The buried valleyThe buried valley

CN3

DN3

GA

ABH-32ABH-34

ABH-85EN2

CN2AN2

TSAN2

WN1

BN2W

BN1W

DN2W

66KVN

BN2E

I-67

AN1

CN1

DN2E

AS MAIN

94.9MC3007 MC3008

M3010

ABH 31

ABH-30 AC-2AC-4

AC-3

AC-1

ABH-83

FN2

EN1

FN1

WN1

WN10

WN20

WN30

WN36 WS166KVS

WN26WN28

WN29

WN33WN34

WN35

WN24WN25

WN27

WN31WN32

DN1W

BN1W

BS1W

I-65

I-102

BN1Ea BN1E66KVN

CL-1A

CL-2

CL-3GKP181

KP182KP183

I 67

DN1E

BS1Ea

BS1Eb

ABH-35

SEA WALL

ABH-33ABH-31

ABH-28

ABH-27

ABH-29

ABH-81

ABH-82

ABH-84

MC2026

FS1ES1

DS1E

CS1 TSAS2

WS34 WS30

WS20WS10

WN40

WS38

66KVS

CPTN

CFVNCPTNa

SPTSa

WS35WS36

WS37

WN39

WN38

WN37

TSAS2B

WS32WS31

WS29WS28

WS26WS27

WS33

DS1W

BS1W

AS2

AS1

I-104

I-103

I-66

BS2E

M2064

ABH-26

ABH 27

ABH-80

FS2

ES2

DS2E

CS2

DS3ECS3

AS3

DS3W

BS2W

DS2W

BS3W

I-64 M2065

BS3E

ES3

BS4E

DS3W

Post-collapse geometry and sequence of excavationsuperimposed on buried valley

GAS WN1BN1W 66KVN ICN1AS MAIN ABH-30 AC

WN10

WN20N1WI-65

BN1Ea BN1E66KVN

DN1E66kV Cable

MC3007M3010

AC-3

ABH-83

WN20

WN245CL-1A

CL 3GKP181

KP182KP183

ABH-31ABH 84

WS166KVS

SPTSaBS1WI-104

CL-2

CL-3G

BS1Ea

BS1Eb

ABH-29

ABH-82

ABH-84

DS1E

CS1

WS20WS10

SPTSa

DS1W

AS1

LL

ABH 29 CS1WS26

DS1W

Sequence of excavation in relation to buried valley

• The buried valley was crossed without collapse developing

• Strut forces would have been a maximum in the buried valley and would have varied across the valley

• The collapse was not, therefore, inevitable

• An external influence (trigger) is required to explain the timing of the collapse and why it occurred afterthe timing of the collapse and why it occurred after crossing the buried valley

Significance of crossing the buried valley without Significance of crossing the buried valley without collapsecollapse

• S338-9 stood for 8 days under load and was 20m from excavation frontfrom excavation front

• S335-9 stood for 2.5 days under load and was over 8m from excavation front8m from excavation front

• Both S335-9S & S338-9N buckled within 10 minutes

• Load in S335-8 and S335-9 was almost constant between 18 April and initiation of collapse

• All C-channel connections failed downwards at both ends

• The south wall was pushing the north wall back

Key observations

• 3D effects cannot explain why the collapse was initiated at 9am – S338 was 20-24m from theinitiated at 9am S338 was 20 24m from the excavation face and had stood for 8 days without distress, S335 was 8-12m from the excavation face

• Time effects cannot explain why the collapse was initiated at 9am – there was no evidence of loadinitiated at 9am there was no evidence of load increases in the monitoring

Potential triggersPotential triggers

Load

g of

9th

er

f9th

Yiel

din

Observed

Trig

g

nsta

llatio

n of

Observed

Minor additional loading<2mm yielding in 9th

8th

9th

Excavation approaches

In

m m

9th

+ passes beyond S335

5 18 20

9am

3.30

p

April 2004

Post-collapse investigationsPost collapse investigations

Back analyses of the collapse

• Analyses by Dr Felix Schroeder and Dr Zeljko Cabarkapa using Imperial College Finite Element p g p gProgram (ICFEP)

• 2D section through M307 (I104) and M302 (I65)• 2D section through M307 (I104) and M302 (I65)

• Bored piles are not modelled (enhanced JGP)p ( )

• Upper and Lower Marine Clays, F2 and lower Estuarine Clay modelled using Modified Cam ClayClay modelled using Modified Cam Clay

• Coupled consolidationp

• Fill and OA sand modelled using Mohr Coulomb. OA-CZ clay/silt modelled as Tresca

Geotechnical analysesclay/silt modelled as Tresca

W ll EI d d t ll f ki b d• Wall EI reduced to allow for cracking, based on reinforcement layout

• Bending moment capacities set according to reinforcement layout and ultimate strengths of steel and concrete as supplieda d co c ete as supp ed

• JGP treated as brittle material

• 9th level strut capacity set and strut allowed to strain soften 72 hours after excavation to 10th level.

Geotechnical analyses

North South

F2 F2

Upper Marine Clay Upper Marine Clay

Estuarine EstuarineFill Fill

+102.9+98.58+96.58

+84.58+81.58

+102.9+98.58+97.07

+85.57+82.07

OA (Sand) - OA-CZ

OA (Sand) - OA-CZOA (Clay/Silt) - OA-SW-1

F2

Lower Marine ClayLower Marine Clay+71.00

+67.00+63.00 +63.57

+61.57+57.57

OA (Clay/Silt) - OA-CZ OA (Clay/Silt) - OA-CZ

Stratigraphy assumed for ICFEP analyses

North wall 9th South wall

95

100

95

100

85

90

85

90

measuredpredicted

80

85

80

85

predicted

n (m

RL)

measured

predicted

70

75

70

75

Ele

vatio

60

65 65

55

60

50 0 50 100 150 200 250 300 35055

60

450 -400 -350 -300 -250 -200 -150 -100 -50 0 50

Horizontal displacement (mm) Horizontal displacement (mm)

North wall 10th South wall

95

100

95

100

85

90

85

90

measuredpredicted

80

85

80

85

n (m

RL)

measuredpredicted

predicted

70

75

70

75

Ele

vatio

60

65 65

55

60

50 0 50 100 150 200 250 300 35055

60

450 -400 -350 -300 -250 -200 -150 -100 -50 0 50

Horizontal displacement (mm) Horizontal displacement (mm)

Predicted trends in 7th, 8th and 9th strut loadsed cted t e ds , 8 a d 9 st ut oads

4000

4500

3000

3500

4000

m)

1500

2000

2500 Strut 7Strut 8Strut 9

9

8rce

(kN

/m

500

1000

1500

7

Pro

p fo

r

0280.00 281.00 282.00 283.00 284.00 285.00 286.00

( )

6 hoursTime (days) 6 hours

Observed trends in 8th and 9th strut loads

Loadf9

th

Observed

nsta

llatio

n of

Observed8th

9th

Excavation approaches

In

m m

9th

+ passes beyond S335

5 18 20

9am

3.30

p

April 2004

• The analyses matched reasonably well the build up in• The analyses matched reasonably well the build up in horizontal wall movements, and the trends in the forces in the 7th, 8th and 9th level struts

• To match movements and forces at all stages it was necessary to model the jet grout as a brittle material

• The upper JGP was predicted to pass its peak strength during excavation to the 6th level and the lower JGP toduring excavation to the 6th level and the lower JGP to pass its peak strength during excavation to the 9th level

Th 9 h l l h d i i d i i• The 9th level strut reached its capacity during excavation to the 10th level

Key findings from geotechnical analyses

• The collapse had to be initiated by allowing the 9th level strut to strain soften – a ductile failure of the connection was not associated with a collapse

• The bending moment capacity of the south wall was reached on the first stage of excavation below the 9th l l b t hi did t f i th ll til thlevel, but a hinge did not form in the wall until the sacrificial JGP layer had been removed

Key findings from geotechnical analyses

Trigger required to initiate collapseTrigger required to initiate collapseTrigger required to initiate collapseTrigger required to initiate collapse

Relative vertical displacement between the kingposts and Dwall panels

)

100

Top of southern wall-nolimitTop of southern wall-OA1Top of southern wall-OA2 8thw

all (

mm

)

Up

0

50p

9th

10th

ent o

f Dw

-50

00 5 10 15 20 25 30 35

ispl

acem

-100

Verti

cal d

i

Down

Excavation depth (m)

V

Predicted settlement of south wall during excavation

500

300

400

Run 4apfnolimit

Run 4apfnolimit_OA1

Run 4apfnolimit_OA2

ts (m

m)

200

plac

emen

t

0

100

-5 0 5 10 15 20 25 30

rtica

l dis

p

-200

-100

North Wall South Wall

Ver

Distance (m)Distance (m)

Predicted vertical displacement of lower JGP layerafter excavation to 10th level

20

2

Strut 262no backfill

10

(mm

)

Nor

th w

all

th w

all

ngpo

st

Typical relative Typical relative displacements of displacements of

strut betweenstrut between0

and

kin

gpos

t

1

3

7A

Soutkin

strut between strut between walls and kingpostwalls and kingpost

l bet

wee

n w

all 2

3

3

7B

-10fe

renc

e in

leve

4

6A

-20

Diff

1

5

6A

7A

-30

4 57B

• Calibrated FE analyses predict downwardCalibrated FE analyses predict downward displacement of Dwall and upward displacement of kingpost when sacrificial JGP layer excavated

• Survey data supports upward vertical displacement of centre of strut relative to ends

Relative vertical displacementRelative vertical displacement

KP184KP183KP182KP181KP180

339340

335 336 337 338

Sacrificial JGP

9th level

Excavation front

Lower JGP

Kingposts in long section

Post-collapse investigationsPost collapse investigations

Structural steel physical testsand numerical analyses.

The effect of relative verticalThe effect of relative vertical displacement

5000

12mm stiffenerplates

Waler

4000

3000

(kN

)

2000Axia

l loa

d (

1000

0 10 20 30 40 500

Axial displacement (mm)

Comparison of tests on connections with plate and C channel stiffeners

5000

12mm stiffenerplates

Waler

4000

3000

(kN

)

C channel

2000Axia

l loa

d (

Waler

stiffener

1000

C channel stiffener has similar capacityto double plate stiffener but becomesbrittle after an initial ductile response

0 10 20 30 40 500

Unforced sway

Comparison of tests on connections with plate and C channel stiffeners

Axial displacement (mm)

5000Test result

4000

3000

ad (k

N)

C channelstiffener

2000Axia

l loa Waler

1000

0 10 20 30 40 500

Adjusted axial displacement (mm)

Calibration of FE model of strutCalibration of FE model of strut--waler connectionwaler connection

5000Test resultFE prediction

4000

FE prediction

3000

ad (k

N)

C channelstiffener

2000Axia

l loa Waler

1000

0 10 20 30 40 500

Adjusted axial displacement (mm)

Calibration of FE model of strutCalibration of FE model of strut--waler connectionwaler connection

5000Test resultFE prediction

Ductile plateau 10-15mm

4000

FE prediction

3000

ad (k

N)

C channelstiffener

2000Axia

l loa WalerYield

1000

Ductile plateau allows failure to develop at both ends

0 10 20 30 40 500

Adjusted axial displacement (mm)

Calibration of FE model of strutCalibration of FE model of strut--waler connectionwaler connection

50008m strut

4000

3000

oad

(kN

)

2000Axia

l l

Waler

C channelstiffener

1000

0 10 20 30 40 50Axial displacement (mm)

0

Effect of strut length (bending restraint) on brittleness of connection

p ( )

50008m strut18m strut

4000

3000

al lo

ad (k

N)

2000Axia

Waler

C channelstiffener

Reduced restraint increases brittlenessd d d til l t

1000and reduces ductile plateau8m strut – effective kingpost

18m strut – ineffective kingpost

0 10 20 30 40 50Axial displacement (mm)

0

Effect of strut length (bending restraint) on brittleness of connection

Test on C channel stiffened connection by Nishimatsu

Test on C channel stiffened connection by Nishimatsu

5000axial load only

4000

3000

oad

(kN

)

2000Axia

l l

Waler

C channelstiffener

1000

0 10 20 30 40 50Axial displacement (mm)

0

Effect of relative vertical displacement

p ( )

5000axial load onlyaxial load+RVD

4000

3000

oad

(kN

)

2000Axia

l l

Waler

C channelstiffener

1000RVD reduces ductile plateau, increases brittleness, makes

stable situation unstable

0 10 20 30 40 50Axial displacement (mm)

0

Forced sway

Effect of relative vertical displacement

p ( )

P

Force, P D til

P

Force, P Ductile

strain

Effect of brittleness of strut to waler connection

2500

Ductile2000

Run 4apf2100dropRun 4apf2100drop4Run 4apf2100drop3

Ductile

1000

1500

p(

)

p pRun 4apf2100drop2Run 4apf2100

e (k

N/m

)

500

rop

forc

e

00.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

P

Total axial strain (%)Total axial strain (%)

Effect of brittleness of strut to waler connection

2500R 4 f2100

2000

500Run 4apf2100Run 4apf2100drop2Run 4apf2100drop3Run 4apf2100drop4Run 4apf2100drop

Ductile – no collapse

1500

e (k

N/m

)

500

1000

rop

forc

e

0-15 -10 -5 0 5 10 15 20 25

6 hours 9 days 21 daysP

Time since reaching excavation level of 72.3m RL (days)

P

Force, P D til

P

Force, PNo collapse

Ductile

hours to collapse Several days to collapse

strain

hours to collapse y

Th l ti ti l di l t b t th• There was relative vertical displacement between the diaphragm wall, which settled when the sacrificial JGP was removed, and the kingpost, which rose, i.e. gprelative vertical displacement between the ends and centre of the strut (RVD)

• The strut-waler connection was ductile-brittle. The ductile plateau explains why both ends could failp p y

• The brittleness of the connection determines the time taken for the collapse to develop

Trigger required to initiate collapseTrigger required to initiate collapse

• RVD reduces the length of the ductile plateau and• RVD reduces the length of the ductile plateau and increases the brittleness

RVD k t bl it ti t bl• RVD can make a stable situation unstable

• RVD can shorten the time to collapse

• Why downward failure at both ends?Why downward failure at both ends?

• Why collapse after crossing the buried valley?

Trigger required to initiate collapseTrigger required to initiate collapse

• Free• Fixed• Forced sway

Downward failure at both endsDownward failure at both ends

Trigger required to initiate collapseTrigger required to initiate collapseTrigger required to initiate collapseTrigger required to initiate collapse

Post-collapse positions of the Dwall panels

GAS WN1BN1W 66KVN ICN1AS MAIN ABH-30 AC

WN10

WN20N1WI-65

BN1Ea BN1E66KVN

DN1E66kV Cable

MC3007M3010

AC-3

ABH-83

WN20

WN245CL-1A

CL 3GKP181

KP182KP183

ABH-31ABH 84

WS166KVS

SPTSaBS1WI-104

CL-2

CL-3G

BS1Ea

BS1Eb

ABH-29

ABH-82

ABH-84

DS1E

CS1

WS20WS10

SPTSa

DS1W

AS1

LL

ABH 29 CS1WS26

DS1W

Post-collapse geometry superimposed on buried valley

31650

CL-2

31640

66KVSBS1Ea

SPTSa19.6

13.5

7.5

9.27

TUNNELSOUTH WALL

31640

THIN

G (m

)

WS10

WS14WS15WS16

WS8WS9

WS1

1W

S11A

WS1

1BS1

1C

WS1

2W

S12A

WS1

2B12

C

WS1

3W

S13A

WS1

3B3C

11

14.7

12 1 915.

2.5

16 2613.

2.5

31630

NOR

T

CS1

WS17 WS

WS1

WS1 112

WS11EWS12E

WS11DWS12D

Post collapse SI boreholesMagnetic logging boreholesUnsuccessful magnetic logging boreholes2005 boreholes

WS11F

WS13F

WS13E

31670 31680 31690 31700

EASTING (m)

31620

Coincidence between inability to advance boreholes and missing Dwall panels

EASTING (m)

WS11E,11F,

12E,13FWS11-13A-D

Obstruction created by missing Dwall panels

2 3

M3061

Gap1

M306M212

1

M306Gap M306M212

1. Panels M306 and M212, each side of the gap, and panel 213 fail by toe kick-in and rotate backrotate back.

2. Soil flows through resulting gap between M306 and M307, rotating panel 307.

3 S il fl th h lti b t3. Soil flows through resulting gap between M307 and M308, rotating panel 308, etc.

Sequence of south wall panel movements

3044

5 305

rag

rag

drag

303

3

4dr

agdr

ag

drdr d

301

302

2

1 2a 3a 4a

306

1 2a 3a 4a

Sequence of south and north wall panel movements

Fill

SOUTH NORTH

Upper

Upper E

Fill

Marine Clay

Upper F2Differing restraintDiffering restraint

LowerMarine Clay

Sacrificial JGP

Differing restraintDiffering restraintimposed by bored imposed by bored

piles at northpiles at northand south wallsand south walls

Lower E

Sacrificial JGP

Permanent JGP

Lower F2

OA SW2 (N=35)

OA SW1 (N 72)OA SW1 (N=72)

OA (CZ)

• The bored piles had a major influence on the displacements of the Dwall panels during the collapse and on their post-collapse positions

• The bored piles restricted toe movements on the• The bored piles restricted toe movements on the south side and prevented failure as the buried valley was crossed

• Loads carried by the bored piles contributed to the under reading of the strain gaugesunder-reading of the strain gauges

Significance of the bored pilesSignificance of the bored piles

North wall

14 2 6 73

M305M304M303M302M301

335 336 338 339 340334333 337

H H HKP 182KP 181 KP 183

1 5 5 5 7 7

M306 M307 M308 M309 M310

1 Order in which distortion to waler noted

Excavation to 10th complete

Excavation in progress

Location of walers, splays and Dwall gaps

GAS WN1BN1W 66KVN ICN1AS MAIN ABH-30 AC

WN10

WN20N1WI-65

BN1Ea BN1E66KVN

DN1E66kV Cable

MC3007M3010

AC-3

ABH-83

WN20

WN245CL-1A

CL 3GKP181

KP182KP183

ABH-31ABH 84

WS166KVS

SPTSaBS1WI-104

CL-2

CL-3G

BS1Ea

BS1Eb

ABH-29

ABH-82

ABH-84

DS1E

CS1

WS20WS10

SPTSa

DS1W

AS1

LL

ABH 29 CS1WS26

DS1W

Repositioned bored piles at 66kV crossingRepositioned bored piles at 66kV crossing

• Upward displacement of KP 180 and 181• Upward displacement of KP 180 and 181 accentuated by toe displacement of M306 and M212, where bored piles had been re-positioned and dditi l t t ibladditional toe movement was possible

• Resulting RVD fed back into buried valley• Resulting RVD fed back into buried valley

Relative vertical displacementRelative vertical displacement

KP184KP183KP182KP181KP180

335

335337 338

336335

336 339340

Upper JGP

9th level

Excavation front

Lower JGP

Kingposts in long section

• C channel stiffened connection undergoes brittle failure

• Critical length of strut and of load in strut result in minimal lateral restraint to connection

• Restraint from rising kingpost results in downward force on connection

• RVD results in reduction in ductility of connection and• RVD results in reduction in ductility of connection and increase in brittleness

• RVD makes a stable situation with overstress blunstable

• RVD results in downward failure of connection

• RVD was the trigger for the failure

Failure mode of connection and RVDFailure mode of connection and RVD

Overall conclusions

• The use of Method A in the numerical analyses to model near normally consolidated soils is fundamentally incorrect

• Its use led to under prediction of wall displacements and• Its use led to under-prediction of wall displacements and bending moments and so to a reduction in the redundancy in the system. The JGP was strained beyond its peak and a plastic hinge formed in the wall as excavation of the sacrificial JGP was underway

Overall conclusions

• There were errors in the design of the strut-waler connection resulting in a design capacity that was 50% of the required capacity where splays were omittedcapacity where splays were omitted

• The collapse initiated some time after the excavation crossed th b i d ll h f th d d i d t tthe buried valley, where forces on the under-designed strut-waler connections would have been a maximum

• An additional perturbation or trigger was necessary to explain the timing of the collapse, the downward failure of the walers at both ends and the trends in the monitoring data

Overall conclusions

• The permanent bored piles in combination with the JGP played a significant role in preventing the collapse as the p y g p g pvalley was crossed

• The collapse was triggered when working in the vicinity of• The collapse was triggered when working in the vicinity of the 66kV cable crossing

• At this location, the permanent bored piles had been repositioned and the JGP layout had been modified, allowing the wall toe to kick-in and cause additional uplift of g pthe local kingposts

Overall conclusions

• This additional upward displacement of the kingposts relative to the wall fed back into the system, introducing forced sway failurefailure

• Downward movement of the walls has been predicted by analysis; the potential for relative upward movement of theanalysis; the potential for relative upward movement of the kingposts has been confirmed by surveys

Forced s a fail re red ced the strain o er hich the• Forced sway failure reduced the strain over which the connection remained ductile, increased the brittleness of the connection and allowed a stable situation to become unstable

• Forced sway failure can explain the timing of the collapse, the form of the observed distortions, the trends in the monitoring data and the speed at which the collapse developeddata, and the speed at which the collapse developed

Overall conclusions

• The collapse was not caused by hydraulic base heave and was not related to poor workmanshipheave and was not related to poor workmanship

• Wall rotation, which had been linked with inadequate , qpenetration of the wall into the OA, was not the cause of the collapse

• Several factors had to act in combination to cause the collapsethe collapse

Unforgiving site

• Deepest excavation in marine clay in Singapore –shortcomings in use of Method A not previously apparent because of depth dependence

• Ground conditions – buried valley in OA infilled with soft fluvial and organic clay, rapid variation in depth of marine clay along

d th ti lti i t i tiand across the excavation resulting in an asymmetric section

• Curvature of walls in plan requiring more frequent use of p q g qwalings

• Presence of 66kVA crossing• Presence of 66kVA crossing

• Need to adopt sacrificial JGP layer, removal of which caused p y ,step increase in 9th level strut load and step increase in wall settlement

Lessons learnt

• JGP is a brittle material

• The mass properties of JGP need to be more carefully evaluated

• Coring of JGP is not an adequate check

• The use of numerical modelling of soils in design should be carried out by specialists and its incompatibilities with current codes needs to be removedcurrent codes needs to be removed

• The potential for brittle failure of C channel connections t b i dmust be recognised

Lessons learnt

• Temporary and permanent works should be subject to independent checks

• The effects of relative displacement between kingposts and walls should be considered in the design of strutted gexcavations

• Forced sway failure and its consequences should be y qrecognised as a potential mechanism in design

• Monitoring did not warn of the impending collapseMonitoring did not warn of the impending collapse