The Nicoll Highway Collapse

The Nicoll Highway Collapse
D W Hi ht
D. W. Hight
T.O. Henderson + A.R. Pickles
S. Marchand

Content
• Background and construction sequence
• Observations up to the point of collapse, including
monitoring
• The collapse
• Post-collapse investigations
• Design errors
• Jet grout layers
• 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
• Lessons to be learnt

C825
34m Diameter
Temporary Staging
Area (TSA) Shaft -
35m deep
210m 2-cell cut &
cover tunnels
200m cut 30m & cover - 33m deep
stacked 4-cell
scissors crossover
3-level cut & cover tunnel
Station (NCH) 25m - 30m deep
370m stacked 2-cell
cut & cover tunnel
20m - 25m deep
C824 - 2km of cut and cover construction
mostly in soft clay - up to 35m deep
C824
Collapse site
M3 area
Circle Ci Circle l Li Line St Stage 1
800m twin bored
tunnel
3-level cut & cover
Station (BLV)
35m 2-cell cut and
cover tunnel
550m 2-cell cut &
cover tunnel and
siding (BLS)

Plaza Hotel
The Concourse
NCH station
Courtesy of Richard Davies
Reclamation dates
Golden Mile
Tower
Cut and Cover
tunnels
Beach Road
1930-1940’s
1930-1940 s
reclamation
GGolden ld Mil Mile
Complex
1970’s
reclamation
Nicoll Highway
TSA
shaft
MMerdeka d k
Bridge
Kallang
Basin

NORRTHING
(m)
ABH27
31700
31650
31600
MC2025
54+812
ABH81
ABH28
M2
MC3007
ABH82
ABH83
ABH29
54+900
ABH30
M3010
ABH32
M3
ABH84
AC-3
ABH31
MC2026
ABH85
AC-2
ABH34
AC-4
AC-1
MC3008
31550 31600 31650 31700 31750 31800
EASTING (m)
ABH33
M2064 M2065
Pre- and post-tender site investigation
ABH35

100
90
80
70
SOUTH NORTH
ABH 31 ABH 32
Fill
Upper estuarine
UUpper
Marine
Clay
Upper F2
M309
M304
Fill
Upper estuarine
Upper
Marine
Clay
Upper F2
Lower
Lower
Marine
Marine
Clay Clay
Base marine clay
Old
Alluvium
Top of OA
Typical section in M3
Lower o e
estuarine
Lower F2
Old
Alluvium

Depth (m)
Cone resistance, qc (MPa)
0 04 0.4 08 0.8 12 1.2 16 1.6 2
0
10
20
AC1
AC2
AC3
AC4
30 30
40 40
Depth (m)
Piezocone data for M3 area
Undrained shear strength (kPa)
0
0
20 40 60 80 100
10
20
AC1
AC2
AC3
AC4
Nkt=12

Construction details and sequence

Formation level
approx 33m bgl
Temporary diaphragm
walls,0.8m
Driven kingpost gp
SOUTH NORTH
Permanent bored piles
supporting
rail boxes Sacrificial JGP
10 levels of steel struts at
3m +3.5m vertical
centres
Permanent JGP
Fill
Upper E
Upper
Marine Clay
Upper F2
Lower
Marine Clay
Lower E
Lower F2
OA SW2 (N=35)
OA SW1 (N (N=72) 72)
OA (CZ)

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

M3
M2
TSA Shaft
Utility crossings
Nicoll
Highway
Curved walls
Tidal sea
24/3/04

No waler
C channel stiffener
Walers
M3
Shaft
Jacking point
Non-splayed strut
Splayed strut
Kingpost
Support beam
M2

Two Struts Bearing
Direct on Single panel
Single g Strut with Splays y
Bearing on Waler for Single
panel
Struts on Walers - No Splays
Gaps in Diaphragm Wall for 66kV Crossing

South side 13 March 04

Events Events and and observations observations prior to collapse

Plate stiffener
C channel
stiffener
Replacement of plate stiffeners at strut strut-waler waler connection

C channel
connection
Strut bearing
directly on Dwall

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

Observations on the morning of the collapse

M301
333
334
M302
M303
North wall
M304
M305
4 2 3
1
6 7
335 336 337 338 339 340
H H H
KP 181 KP 182
KP 183
1 5 5 5 7 7
M306 M307 M308 M309 M310
1 Order in which distortion to waler noted
Excavation to 10 th complete
Excavation in progress
Location of walers, splays and Dwall gaps

S335 S335-99
Strut 335-9 south wall

Strut 338 338-9 9 north wall
S338 S338-99

Instrumentation and results of monitoring

M2
GWV24
I 65
M2/M3 plan at 9 th level
I 104
M3
All struts at S335
instrumented for load
measurements
TSA Shaft

SG3358 Total Force
SG3359 Total Force
Excavation Front for
10th Level Advancing
from S338
140
15th April
130
120
Excavation Front
Approaches S335 at
the end of Day y Shift on
17th April 2004
Excavation Front
Beyond S335 at the
start of Day Shift on
18th April 2004
5000
4500
4000
3500
3000
2500
2000
strut loadd
(kN)
1500
10th Level Between 1000easured
Excavation Front for
110
S336 and S335 M
16th April
100
90
17th April
80
70
60
18th April
Hours before collapse
50
40
19th April
30
20
10
500
20th April
0
0

(kN)
Meeasured
strut
load
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
Change in Measured Load at Strut 335
336(N) & 337(N)
Waler Buckling Observed 335(N)
Support Bracket Waler at 335(S) Buckling Drops Observed Off
Strut Load 335-9
Strut Load 335-8
338(N) & 335(S)
Waler Buckling Observed
0
6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00
Time on 20April 2004

Load
Observed trends in 8 th and 9 th strut loads
Innstallation
of f 9th
8th
9th
Excavation approaches
+ passes beyond S335
Observed
5 18 20
April 2004
9amm
3.30pm

The trends were consistent with there being g
yielding of the 9th level strut-waler connection
when the excavation passed beneath but with
no ffurther th significant i ifi t changes h iin lload d iin either ith
the 9th or 8th level struts until the collapse was
initiated

Deepth
(m)
0
Horizontal displacement (mm)
0 200 400
I 104
10 April
Horizontal displacement (mm)
400 200 0
I 65
10 April
10 10
20
30
Behind South Wall North Wall
40 40
50 50
0
20
Deepth
(m)
30

Deepth
(m)
0
Horizontal displacement (mm)
0 200 400
I 104
10 April
15 April
Horizontal displacement (mm)
400 200 0
I 65
10 April
16 April
10 10
20
30
Behind South Wall North Wall
40 40
50 50
0
20
Deepth
(m)
30

Deepth
(m)
0
Horizontal displacement (mm)
0 200 400
I 104
10 April
15 April
17 April
Horizontal displacement (mm)
400 200 0
I 65
10 April
16 April
17 April
10 10
20
30
Behind South Wall North Wall
40 40
50 50
0
20
Deepth
(m)
30

Deepth
(m)
0
Horizontal displacement (mm)
0 200 400
I 104
10 April
15 April
17 April
20 April
Horizontal displacement (mm)
400 200 0
I 65
10 April
16 April
17 April
20 April
10 10
20
30
Behind South Wall North Wall
40 40
50 50
0
20
Deepth
(m)
30

displacemennt,
mm
Maximum
horizontal
500
400
300
200
100
I104 max
I65 max
South wall
NNorth th wall ll
0
5-Jul 9-Aug 13-Sep 18-Oct 22-Nov 27-Dec 31-Jan 6-Mar 10-Apr
2003
2004
Comparison of inclinometer readings I65 and I104

• S338-9 stood for 8 days under load and was 20m
from excavation front
• S335 S335-9 9 stood for 22.5 5 days under load and was over
8m from excavation front
• BBoth th S335-9S S335 9S & S338 S338-9N 9N bbuckled kl d within ithi 10 minutes i t
• Load in S335-8 and S335-9 was almost constant
bbetween t 18 AApril il and d iinitiation iti ti of f collapse ll
• All C-channel connections failed downwards at both
ends
• The south wall was pushing the north wall back
Key observations

The collapse

3.33pm

3.33pm

3.34pm

3.34pm

3.34pm

3.41pm

3.46pm

Post-collapse Post collapse investigations
Design errors

Errors
• Misinterpretation of BS5950 with regard to stiff
bbearing i llength h
• Omission of splays
Effects
• Design capacity of strut-waler strut waler connection was 50% of
required design capacity where splays were omitted
Errors in structural design of strut strut-waler waler connection

• Capacity based on BS5950:1990 = 2550 kN
• Average ultimate capacity based on physical
load tests = 4100 kN
• Based on mill tests, 9 95% % of f 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 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)
• Method A is an effective stress analysis of an undrained
problem bl
• Assumes isotropic elastic behaviour and a Mohr-
CCoulomb ffailure
criterion
• As a result mean effective stress p’ p is constant until yyield
• Method A was being applied to marine clays which were
of low over-consolidation over consolidation or even under-consolidated
under consolidated
because of recent reclamation
• Method B is a total stress analysis
Methods A and B

t
Cu from Method A
Cu from Method B
The shortcomings of Method A
p’

RL (m)
Method A Method B
105 105
100
95
90
85
80
75
70
65
60
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
Exc to RL 81.6 for S7
Exc to RL 72.3 for S10
Exc to RL 78.3 for S8 Exc to RL 75.3 for S9
RL (m)
100
95
90
85
80
75
70
65
60
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
Exc to RL 81.6 for S7
Exc to RL 72.3 for S10
Exc to RL 78.3 for S8 Exc to RL 75.3 for S9
M3 - SSouth th Wall W ll Di Displacement l t
Method A versus Method B

RL (m)
105
100
95
90
85
80
75
70
65
60
55
-3000
-2500
-2000
-1500
-1000
-500
Method A Method B
0
500
1000
1500
Bending Moment (kNm/m)
2000
2500
3000
3500
RL (m)
4000
105
100
95
90
85
80
75
70
65
60
55
-3000
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
Bending Moment (kNm/m)
M3 - South Wall bending moments
Method A versus Method B
2000
2500
3000
3500
4000

Strut Load (kN/m) _
MMethod th dA A
g ( )
3000
2800
2600
2400
2206
2200
S1
2000
S2
1800
S3
1600
S4
1400
1200
S5
1000
S6
800
S7
600 S8
400
S9
200
0
Strut Load (kN/m)
3000
2800
2600
2400
2200
2000
1800
1600
Method B
M3 – strut forces
Method A versus Method B
2383
1400
S3
1200
S4
1000
S5
800
S6
600 S7
400
S8
200
0
S9
S1
S2

• Method A over-estimates the undrained shear
strength of normally and lightly overconsolidated
clays l
• Its use led to a 50% under-estimate of wall
displacements and of bending moments and an
under-estimate of the 9 th level strut force of 10%
• The larger than predicted displacements mobilised
the capacity p y of the JGP layers y at an earlier stage g
than predicted
Method A

• St Structural t l design d i errors
• Removal of splays p y at some strut locations
• Introduction of C-channel waler connection detail
• Use of Method A in soil-structure interaction analysis
• CCollapse ll was an iinevitable it bl consequence of f th the
design errors which led to the applied loads on the
struts increasing with time and equalling the capacity
of the strut-waler connection
COI view on the principal causes of
the collapse

• St Structural t l design d i errors
• Removal of splays p y at some strut locations
• Introduction of C-channel waler connection detail
• Use of Method A in soil-structure interaction analysis
• CCollapse ll was an iinevitable it bl consequence of f th the
design errors which led to the applied loads on the
struts increasing with time and equalling the capacity
of the strut-waler connection
COI view on the principal causes of
the collapse

Post-collapse Post collapse investigations
Jet grout

Excavation of sacrificial JGP in Type H

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

Shear wave velocity measurements in JGP at Type K

JGP thicckness
(m)
5
4
3
2
1
0
Shear wave section
Design thickness
Pressuremeter section
0 2 4 6 8
Thicknesses of JGP in Type K

Post-collapse Post collapse investigations
Ground conditions and
soil properties

NORRTHING
(m)
31700
31650
CFVN
CTSN
CPTNa
CPTN
54+812
FN2
FN1
EN2
M2
EN1
FS1
ES1
DN3
DN2E
DN2W
DN1E
DN1W
CL-3g CL 3g
CN3
DS1E
DS1W
CN2
BN3
BN2E
BN2W
P33
P36
P42/P48
P40P41/P47
P45
P52
AN2
54+900
M3
AN1
CN1 BN1W BN1E
BN1Ea
66KVNI
CL-2
CS1
CS2
CL-1a
BS1Ea
BS1Eb
66KVS
SPTSa
BS1W AS1
31600
ES2
DS2E
DS2W
BS3E
AS3
31550 31600
FS2
31650
BS3W
31700 31750 31800
DS3E
CS3
ES3
BS2W
BS2E
DS3W
EASTING (m)
BS4W
BS4E
CS4
BS4Ea
DS4W
DS4E
AS2
P54
TSAN2
TSAS2
TSAS2b
Post Post-collapse collapse ground investigation

Depth D (m)
Cone resistance, qt (MPa)
0
0
0.4 0.8 1.2 1.6 2
10
20
30
BN1W
q t
u2
f s
Cone resistance, qt (MPa)
0
0
0.4 0.8 1.2 1.6 2
BS2W
North 10
South
40
0 0.4 0.81.2 40
Pore pressure, u 0 04 08 12
2 (MPa)
0 0.4 0.8 1.2
Pore pressure, u2 (MPa)
0 0.02 0.04 0.06 0.08
Friction, f s (MPa)
Deepth
(m)
20
30
q t
u2
f s
0 0.02 0.04 0.06 0.08
Friction, f s (MPa)
CPT profiles north and south of collapse area

t (kPPa)
100
50
0
-50
-100
φ=34.2 o
φ=28.5 o
K o =0.5
0 50 100 150 200
s' (kPa)
P-8
P-9
P-24
P-26
φ=35 o
Kisojiban’s CAU tests on
Upper and Lower Marine Clay

NORTHIING
(m)
ABH27
31700
31650
31600
50 55 60 65 70 75 80 85 90 95 100 105
CFVN
CTSN
CPTNa
CPTN
MC2025
DN3
DN2E
CN3
CN2
BN2E
BN2W
54+900
BN3
P33
P45
P36
P40 P41/P47
P52
P42/P48
AN2
P54
TSAN2
Tender SI boreholes
Tender SI CPTs
Detailed design SI
Detailed design SI CPTs
Post collapse SI
Post collapse SI CPTs
Boreholes for bored piles
Magnetic logging boreholes
DN2W
ABH32
AN1
EN2
M3
BN1W
CN1
66KVNI
BN1Ea BN1E WN4 WN2
WN8
WN6
ABH30 WN10
WN3 WN1
FN2
DN1E
WN12 WN7AWN5
WN14
DN1W
WN11
WN9
WN16
WN18 WN13
WN20 WN15
M2
WN22
WN17
WN19
WN24
EN1
MC3007 WN21
WN26 WN23 ABH83
M3010 CL-1a
WN28 WN25
WN30A WN27
AC AC-3 3
FN1 WN32 WN29 CL-3g
CL-2
WN34 WN31
WN33
BS1Ea
ABH84 66KVS SPTSa
BS1Eb WS1
WN37 WN35
ABH31 WS3
WS9 WS7 WS5
WN36
WS13A WS12A WS11A
WN38
WS15WS13C
WS12C WS11C
WS14 WS10 WS8 WS6
DS1E WS17
BS1WWS4A
WS2
WN39
WS13B
WS19
WS13 WS12B WS12 WS11B WS11
WS16
AS1
WS21
WN40
ABH82 WS23 ABH29 WS18
WS25 WS20
WS22
CS1
ABH28
WS27 WS24
WS29
DS1W
WS34
WS26
WS31 WS28
ABH81
WS36 WS30
WS35A
WS32 ES1
WS33
MC2026
WS37
AS2
WS38
BS2E
FS1
CS2
BS2W
ABH34
ABH85
AC-2
AC-4
AC AC-1 1
MC3008
ABH33
TSAS2
TSAS2b
M2064 M2065
54+812
ES2
DS2E
DS2W
FS2
31550 31600 31650 31700 31750 31800
ES3
DS3E
EASTING (m)
BS3E
BS3W
BS4W
BS4E
BS4Ea
DS4E
Evidence for a buried valley in the Old Alluvium
DS3W
DS4W
CS3
CS4
AS3
ABH35

ELEVATION (mRL) (
110
100
90
80
70
60
50
Section along north wall
Bottom of F2 Clay in Marine Clay
Detailed design site investigation
Bottom of F2 Clay in Marine Clay
Post collapse site investigations
Top of OLD ALLUVIUM
Post collapse site investigations

ELEVATION E
(mRRL)
110
100
90
80
70
60
50
Section along south wall

NORTHHING
(m)
ABH27
80.69
31700
31650
31600
CFVN
CTSN 81.97
82.17
MC2025
81.70
54+812
DN3
81.91
DN2E
CN3
82.27
54+900
BN2E
82.61
BN3
77.02
P33
86.39
P45
P36
86.07
P52
85.98
P42/P48
P40 P41/P47
86 86.90 90
86.30 86.84
P54
81.94
M3
ABH32
83.41
AN1
82.91
ABH81
80.13
BN1E 66KVNI
CN1 BN1Ea
81.88
WN1
WN3WN2
81.57 80.68
WN4
WN7A WN6WN5
WN11 WN10WN9WN8
82.3182.16
83.13
81.3881.25
WN13WN12
81.38 81.4281.43
DN1E
WN15 WN14 80.9581.21
81.2581.05
WN16
80.66
81.86
WN17
80.37
WN18
80.72
M304
WN19
79.86
M305
WN20
81.86
WN21
80.81
80.31
WN22 80.88
M2
WN23 80.94
M303
WN24 79.93
WN25 79.91
M3010
WN26 80.65 MC3007 80.15
WN27 80.95 ABH83
82.55
80.36
KP182
KP183
WN29 79.57
M302
CL-3g
WN30A 79.72
83.39 M209
KP181 AC-3
WN31
M210
KP181
WN31
M210
78.34
CL 2
WN32 77.51 80.22
CL-2 M211
WN33 77.71
M301 BS1Ea 81.69
WN34 78.02
78.04
74.60
WN35 80.58
66KVS
WN36 80.53
SPTSa
BS1Eb
WS1
80.66
ABH84 74.72 WS3
WN37
71.09 M307 71.52
81.43
M308 WS7
M309 M310 ABH31
WS10WS9WS8
WS4A WS2
M212 & M306
78.20
76.59
78.35
WN38
WS16WS15
missing82.57
72.2872.7072.6173.84
82.07 76.77 75.20
WN39 80.83
WS18 WS17
WS19 81.39 M213
AS1
81.39
WS21
WS20
75.98
WN40 81.06
WS22
81.2881.45
ABH82 WS23 ABH29 81.38 CS1
WS24
80.84
83.19
WS25
81.48
WS26
79.93
WS27
80.03
79.99 DS1E 82.01
WS28 80.37
81.23
81.59
WS29 79.76
WS34
WS30 78.25
81.09
WS31 78.52
WS35A WS32 79.57
79.23
78.78
79.10
79.69
79.25
WS37
WS38 80.65
81 81.26 26
BS2E
M2064
79.00
DS2E
80.68
86.20
338
86.58
ABH85
81.69
AC-2
81.73
ABH33
80.51
AC-4
79.98
AC-1
80.76
TSAS2b
82.16
Tender SI boreholes
Tender SI CPTs
Detailed design SI boreholes
Detailed design SI CPTs
Post collapse SI boreholes
Post collapse SI CPTs
Boreholes for bored piles
Magnetic logging boreholes
31550 31600 31650 EASTING (m) 31700 31750 31800
-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12
Distortion to upper F2 layer caused by the collapse
DS4E
79.24
82.28
BS3E
81.29
BS4Ea
80.96
BS4E
80.33
ABH35
81.69

Buried valley in the Old Alluvium

Buried valley in the Old Alluvium

Coincidence between buried valley and
distortion 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 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 side
and fluvial clays on the north side
• Gas exsolution almost certainly y occurred in the deep p
organic clays as a result of stress relief, reducing
their strength further
The buried valley

CN3
DN3
GA
ABH-32
ABH-34
ABH-85
EN2
CN2
AN2
TSAN2
WN1
BN2W
BN1W
DN2W
66KVN
BN2E
I-67
AN1
CN1
DN2E
AS MAIN
94.9
MC3007
MC3008
M3010
ABH 31
ABH-30
AC-2
AC-4
AC-3
AC-1
ABH-83
FN2
EN1
FN1
WN1
WN10
WN20
WN30
WN36
WS1
66KVS
WN26
WN28
WN29
WN33
WN34
WN35
WN24
WN25
WN27
WN31
WN32
DN1W
BN1W
BS1W
I-65
I-102
BN1Ea
BN1E
66KVN
CL-1A
CL-2
CL-3G
KP181
KP182
KP183
I67
DN1E
BS1Ea
BS1Eb
ABH-35
SEA WALL
ABH-33
ABH-31
ABH-28
ABH-27
ABH-29
ABH-81
ABH-82
ABH-84
MC2026
FS1
ES1
DS1E
CS1
TSAS2
WS34
WS30
WS20
WS10
WN40
WS38
66KVS
CPTN
CFVN
CPTNa
SPTSa
WS35
WS36
WS37
WN39
WN38
WN37
TSAS2B
WS32 WS31
WS29
WS28
WS26
WS27
WS33
DS1W
BS1W
AS2
AS1
I-104
I-103
I-66
BS2E
M2064
ABH-26
ABH 27
ABH-80
FS2
ES2
DS2E
CS2
DS3E
CS3
AS3
DS3W
BS2W
DS2W
BS3W
I-64 M2065
BS3E
ES3
BS4E
DS3W
Post-collapse geometry and sequence of excavation
superimposed on buried valley

DN1E
N1W
WN24
5
MC3007
CL CL-3G 3G
ABH-82
WS26
GAS AS MAIN
WN20
ABH-83
WS20
DS1E
DS1W
ABH-29
CN1
ABH-30
CL-2
CS1
LL ABH 29 CS1
M3010
BN1Ea
BN1W
WN10
I-65
KP181
BN1E
66KVN
CL-1A
KP182
66KVS
I-104
ABH ABH-84 84 SPTSa
WS10
66kV Cable
KP183
BS1Ea
BS1Eb
BS1W
AS1
AC-3
WS1
ABH-31
Sequence of excavation in relation to buried valley
WN1
I
AC

• 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 after
crossing the buried valley
Significance of crossing the buried valley without
collapse

• 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
8m 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 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 load
increases in the monitoring
Potential triggers

Load
Innstallation
of f 9th
8th
9th
Yielding
of 9th
Excavation approaches
+ passes beyond S335
Observed
Minor additional loading

Post-collapse Post collapse investigations
Back analyses of the collapse

• Analyses by Dr Felix Schroeder and Dr Zeljko
Cabarkapa p using g Imperial p College g Finite Element
Program (ICFEP)
• 2D section through M307 (I104) and M302 (I65)
• Bored piles p are not modelled ( (enhanced JGP) )
• Upper and Lower Marine Clays, F2 and lower Estuarine
Clay modelled using Modified Cam Clay
• Coupled p consolidation
• Fill and OA sand modelled using Mohr Coulomb. OA-CZ
clay/silt modelled as Tresca
Geotechnical analyses

• WWall ll EI reduced d d tto allow ll ffor cracking, ki bbased d on
reinforcement layout
• Bending moment capacities set according to
reinforcement layout and ultimate strengths of steel
and a d co concrete c ete as supp supplied ed
• JGP treated as brittle material
• 9 th level strut capacity set and strut allowed to strain
soften 72 hours after excavation to 10 th level.
Geotechnical analyses

North South
+102.9
+102.9
Fill
Estuarine
+98.58
+96.58
+98.58
+97.07
Fill
Estuarine
Upper Marine Clay Upper Marine Clay
F2
Lower Marine Clay
F2
OA (Sand) - OA-CZ
OA (Clay/Silt) - OA-CZ
+84.58
+81.58
+71.00
+67.00
+63.00
+85.57
+82.07
+63.57
+61.57
+57.57
F2
Lower Marine Clay
OA (Clay/Silt) - OA-SW-1
OA (Sand) - OA-CZ
OA (Clay/Silt) - OA-CZ
Stratigraphy assumed for ICFEP analyses

Elevation
(m RL)
100
95
90
85
80
75
70
North wall 9th South wall
predicted
measured
predicted
measured
65 65
60
Horizontal displacement (mm) Horizontal displacement (mm)
55
50 0 50 100 150 200 250 300 350
55
450 -400 -350 -300 -250 -200 -150 -100 -50 0 50
100
95
90
85
80
75
70
60

Elevation
(m RL)
100
95
90
85
80
75
70
65
60
North wall 10th South wall
predicted
measured
predicted
measured
Horizontal displacement (mm) Horizontal displacement (mm)
55
50 0 50 100 150 200 250 300 350
55
450 -400 -350 -300 -250 -200 -150 -100 -50 0 50
100
95
90
85
80
75
70
65
60

Prop forrce
(kN/mm)
4500
4000
3500
3000
2000
1500
Predicted trends in 7th , 8th and 9th ed cted t e ds ,8 a d 9 strut st ut loads oads
2500 Strut 7
1000
500
0
280.00 281.00 282.00 283.00 284.00 285.00 286.00
( )
9
8
7
Time (days)
6 hours
Strut 8
Strut 9

Load
Observed trends in 8 th and 9 th strut loads
Innstallation
of f 9th
8th
9th
Excavation approaches
+ passes beyond S335
Observed
5 18 20
April 2004
9amm
3.30pm

• 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 to
pass its peak strength during excavation to the 9th level
• Th The 9th 9 h llevel l strut reached h d iits capacity i dduring i excavation i
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
llevel, l bbut t a hi hinge did not t fform in i th the wall ll until til th the
sacrificial JGP layer had been removed
Key findings from geotechnical analyses

Trigger required required to initiate collapse
Relative vertical displacement between
the kingposts and Dwall panels

Vertical V diisplacement
of Dwwall
(mm) )
100
50
Up
Top of southern wall-nolimit
Top of southern wall-OA1
Top p of southern wall-OA2
0
0 5 10 15 20 25 30 35
-50
-100
Down
Excavation depth (m)
Predicted settlement of south wall during excavation
8th
9th
10th

Verrtical
dispplacementts
(mm)
500
400
300
200
100
Run 4apfnolimit
Run 4apfnolimit_OA1
Run 4apfnolimit_OA2
0
-5 0 5 10 15 20 25 30
-100
-200
North Wall
Distance (m)
South Wall
Predicted vertical displacement of lower JGP layer
after excavation to 10 th level

Typical relative
displacements of
strut between
between
walls and kingpost
(mm)
and kingpost
l between wall
Diffference
in leve
20
10
0
-10
-20
-30
North wall
1
7A
2
7B
3
6A
5
Strut 262
no backfill
kinngpost
2
Soutth
wall
4 5
3
6A
4
1
7A
7B

• Calibrated 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 displacement

9 th level
KP180
Sacrificial JGP
Excavation front
Kingposts in long section
KP181
335
336
Lower JGP
KP182
337
338
KP183
339
340
KP184

Post-collapse Post collapse investigations
Structural steel physical tests
and numerical analyses.
The effect of relative vertical
displacement

Axial load (kN) (
5000
4000
3000
2000
1000
12mm stiffener
plates
Waler
0
0 10 20 30 40 50
Axial displacement (mm)
Comparison of tests on connections with plate and C channel
stiffeners

Axial load (kN) (
5000
4000
3000
2000
1000
C channel stiffener has similar capacity
to double plate stiffener but becomes
brittle after an initial ductile response
Unforced sway
12mm stiffener
plates
Waler
C channel
stiffener
Waler
0
0 10 20 30 40 50
Axial displacement (mm)
Comparison of tests on connections with plate and C channel
stiffeners

Axial loaad
(kN)
5000
4000
3000
2000
1000
Test result
C channel
stiffener
Waler
0
0 10 20 30 40 50
Adjusted axial displacement (mm)
Calibration of FE model of strut strut-waler waler connection

Axial loaad
(kN)
5000
4000
3000
2000
1000
Test result
FE prediction
C channel
stiffener
Waler
0
0 10 20 30 40 50
Adjusted axial displacement (mm)
Calibration of FE model of strut strut-waler waler connection

Axial loaad
(kN)
5000
4000
3000
2000
1000
Ductile plateau 10-15mm
Yield
Ductile plateau allows failure to
develop at both ends
Test result
FE prediction
C channel
stiffener
Waler
0
0 10 20 30 40 50
Adjusted axial displacement (mm)
Calibration of FE model of strut strut-waler waler connection

Axial load
(kN)
5000
4000
3000
2000
1000
8m strut
C channel
stiffener
Waler
0
0 10 20 30 40 50
Axial displacement p (mm) ( )
Effect of strut length (bending restraint) on brittleness of
connection

Axiaal
load (kN)
5000
4000
3000
2000
1000
Reduced restraint increases brittleness
and d reduces d dductile til plateau l t
8m strut – effective kingpost
18m strut – ineffective kingpost
8m strut
18m strut
C channel
stiffener
Waler
0
0 10 20 30 40 50
Axial displacement (mm)
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

Axial load
(kN)
5000
4000
3000
2000
1000
axial load only
C channel
stiffener
Waler
0
0 10 20 30 40 50
Axial displacement p (mm) ( )
Effect of relative vertical displacement

Axial load
(kN)
5000
4000
3000
2000
1000
RVD reduces ductile plateau,
increases brittleness, makes
stable situation unstable
Forced sway
axial load only
axial load+RVD
C channel
stiffener
Waler
0
0 10 20 30 40 50
Axial displacement p (mm) ( )
Effect of relative vertical displacement

Force, P
P
DDuctile til
strain

Prop
forcee
(kN/m)
p ( )
Effect of brittleness of strut to waler connection
2500
2000
1500
1000
500
Ductile
Run 4apf2100drop
Run 4apf2100drop4
Run 4apf2100drop3
p p
Run 4apf2100drop2
Run 4apf2100
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Total axial strain (%)

Prop
forcee
(kN/m)
Effect of brittleness of strut to waler connection
RRun 44apf2100 f2100
Run 4apf2100drop2
Run 4apf2100drop3
Run 4apf2100drop4
Run 4apf2100drop
2500 500
2000
1500
1000
500
0
6 hours 9 days
Ductile – no collapse
21 days
-15 -10 -5 0 5 10 15 20 25
Time since reaching excavation level of 72.3m RL (days)

Force, P
P
hours to collapse
DDuctile til
No collapse
Several days y
to collapse
strain

• Th There was relative l ti vertical ti l di displacement l t bbetween t th the
diaphragm wall, which settled when the sacrificial
JGP was removed, and the kingpost, gp which rose, i.e.
relative vertical displacement between the ends and
centre of the strut (RVD)
• The strut-waler connection was ductile-brittle. The
ductile plateau p explains p why y both ends could fail
• The brittleness of the connection determines the time
taken for the collapse to develop
Trigger required to initiate collapse

• RVD reduces the length of the ductile plateau and
increases the brittleness
• RVD can make k a stable t bl situation it ti unstable t bl
• RVD can shorten the time to collapse
• Why downward failure at both ends?
• Why collapse after crossing the buried valley?
Trigger required to initiate collapse

• Free
• Fixed
• Forced sway
Downward failure at both ends

Trigger Trigger required required to initiate collapse
Post-collapse positions of the Dwall
panels

DN1E
N1W
WN24
5
MC3007
CL CL-3G 3G
ABH-82
WS26
GAS AS MAIN
WN20
ABH-83
WS20
DS1E
DS1W
ABH-29
CN1
ABH-30
CL-2
CS1
LL ABH 29 CS1
M3010
BN1Ea
BN1W
WN10
I-65
KP181
BN1E
66KVN
CL-1A
KP182
66KVS
I-104
ABH ABH-84 84 SPTSa
66kV Cable
KP183
BS1Ea
BS1Eb
BS1W
Post-collapse geometry superimposed on buried valley
WS10
AS1
AC-3
WS1
ABH-31
WN1
I
AC

NORTTHING
(m)
31650
31640
31630
31620
CL-2
WS16
CS1
WS15
WS14
WS13B 13. 7
WS13 16
WS13A 26
WS12B 15. .2
WS12 12
WS12A 199
WS11B 17.5
WS11 19.6 1
WS11A 13.5
WS13C
122.5
WS112C
12.5
S11C
14.7
WS17 WS
WS13E
WS13F
WS12D
WS11D
WS11E
WS12E
WS11F
WS10
66KVS
WS9
SPTSa
WS8
BS1Ea
Post collapse SI boreholes
Magnetic logging boreholes
Unsuccessful magnetic logging boreholes
2005 boreholes
31670 31680 31690 31700
EASTING (m)
TUNNEL
SOUTH WALL
Coincidence between inability to advance boreholes and
missing Dwall panels

WS11E,11F,
12E,13F
WS11-13A-D
Obstruction created by missing Dwall panels

1
M212
1
1
Gap M306
2
1. Panels M306 and M212, each side of the
gap, and panel 213 fail by toe kick-in and
rotate back. back
2. Soil flows through resulting gap between
M306 and M307, rotating panel 307.
33. SSoil il fl flows th through h resulting lti gap bbetween t
M307 and M308, rotating panel 308, etc.
Sequence of south wall panel movements
3

2
301
drag
3
302
dr rag
4
303
drag d
1 2a 3a 4a
306
Sequence of south and north wall panel movements
5
304
305

Differing restraint
imposed by bored
piles at north
and south walls
SOUTH NORTH
Sacrificial JGP
Permanent JGP
Fill
Upper E
Upper
Marine Clay
Upper F2
Lower
Marine Clay
Lower E
Lower F2
OA SW2 (N=35)
OA SW1 (N (N=72) 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
south side and prevented failure as the buried valley
was crossed
• Loads carried by the bored piles contributed to the
under under-reading reading of the strain gauges
Significance of the bored piles

M301
333
334
M302
M303
North wall
M304
M305
4 2 3
1
6 7
335 336 337 338 339 340
H H H
KP 181 KP 182
KP 183
1 5 5 5 7 7
M306 M307 M308 M309 M310
1 Order in which distortion to waler noted
Excavation to 10 th complete
Excavation in progress
Location of walers, splays and Dwall gaps

DN1E
N1W
WN24
5
MC3007
CL CL-3G 3G
ABH-82
WS26
GAS AS MAIN
WN20
ABH-83
WS20
DS1E
DS1W
ABH-29
CN1
ABH-30
CL-2
CS1
LL ABH 29 CS1
M3010
BN1Ea
BN1W
WN10
I-65
KP181
BN1E
66KVN
CL-1A
KP182
66KVS
I-104
ABH ABH-84 84 SPTSa
WS10
66kV Cable
KP183
BS1Ea
BS1Eb
BS1W
AS1
AC-3
WS1
ABH-31
Repositioned bored piles at 66kV crossing
WN1
I
AC

• Upward displacement of KP 180 and 181
accentuated by toe displacement of M306 and M212,
where bored piles had been re-positioned and
additional dditi l ttoe movement t was possible ibl
• Resulting RVD fed back into buried valley
Relative vertical displacement

9 th level
KP180
Upper JGP
Excavation front
Kingposts in long section
KP181
335
335
Lower JGP
KP182
KP183
336
336 337
338 339
340
KP184

• 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
increase in brittleness
• RVD makes a stable situation with overstress
unstable bl
• RVD results in downward failure of connection
• RVD was the trigger for the failure
Failure 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 omitted
• The collapse initiated some time after the excavation crossed
th the bburied i d valley, ll where h fforces on th the under-designed d d i d strut- t t
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 p y a significant g role in ppreventing g the collapse p as the
valley was crossed
• 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 g the wall toe to kick-in and cause additional uplift p of
the local kingposts

Overall conclusions
• This additional upward displacement of the kingposts relative to
the wall fed back into the system, introducing forced sway
failure
• Downward movement of the walls has been predicted by
analysis; the potential for relative upward movement of the
kingposts has been confirmed by surveys
• Forced sway s a failure fail re red reduced ced the strain oover er which hich 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 data, and the speed at which the collapse developed

Overall conclusions
• The collapse was not caused by hydraulic base
heave and and was not related to poor workmanship
• Wall rotation, , which had been linked with inadequate q
penetration of the wall into the OA, was not the cause
of the collapse
• Several factors had to act in combination to cause
the 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
and d across th the excavation ti resulting lti iin an asymmetric t i section ti
• Curvature of walls in plan p requiring q g more frequent q use of
walings
• 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

• JGP is a brittle material
Lessons learnt
• 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 removed
• The potential for brittle failure of C channel connections
must tb be recognised
i d

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 g of strutted
excavations
• Forced sway y failure and its consequences q should be
recognised as a potential mechanism in design
• Monitoring did not warn of the impending collapse