The Nicoll Highway Collapse

The Nicoll Highway Collapse
D. W.
D
W Hight
Hi ht
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
34m Diameter
Temporary Staging
Area (TSA) Shaft 35m deep
210m 2-cell cut &
cover tunnels
200m cut30m
& cover
- 33m deep
stacked 4-cell
scissors crossover
tunnel
3-level cut & cover
Station (NCH)
25m - 30m deep
3-level cut & cover
Station (BLV)
C824
Collapse site
M3 area
370m stacked 2-cell
cut & cover tunnel
550m 2-cell cut &
cover tunnel and
siding (BLS)
800m twin bored
tunnel
20m - 25m deep
C825
C824 - 2km of cut and cover construction
mostly in soft clay - up to 35m deep
Circle
Ci l Li
Line St
Stage 1
35m 2-cell cut and
cover tunnel
Reclamation dates
Beach Road
The Concourse
Plaza Hotel
NCH station
Golden Mile
Cover
Tower
Cut and
tunnels
1930-1940’s
1930-1940
G ld sMil
Golden
Mile
Merdeka
M
d k
Bridge
Complex
reclamation
Nicoll Highway
1970’s
TSA
shaft
reclamation
Courtesy of Richard Davies
Kallang
Basin
31700
ABH32
54+900
M3
ABH85
AC-2
NOR
RTHING (m)
ABH30
M2
MC3007
ABH83
54+812
ABH84
ABH82
AC-4
M3010
AC-3
31650
ABH34
ABH31
MC3008
AC-1
ABH35
ABH33
ABH29
ABH28
ABH81
MC2026
ABH27
M2064
M2065
MC2025
31600
31550
31600
31650
31700
31750
EASTING (m)
Pre- and post-tender site investigation
31800
SOUTH
100
ABH 31
Fill
Upper estuarine
ABH 32
M309
M304
NORTH
Fill
Upper estuarine
Upper
U
Upper
90
Marine
70
Clay
Clay
Upper F2
80
Marine
Upper F2
Lower
Lower
Marine
Marine
Clay
Clay
Base marine clay
Lower
o e
Top of OA
estuarine
Lower F2
Old
Old
Alluvium
Alluvium
Typical section in M3
0
04
0.4
Cone resistance, qc (MPa)
08
0.8
12
1.2
16
1.6
0
2
20
Undrained shear strength (kPa)
40
60
80
100
0
0
AC1
AC2
AC3
AC4
AC1
AC2
AC3
AC4
10
10
20
Depth (m)
Depth (m)
Nkt=12
20
30
30
40
40
Piezocone data for M3 area
Construction details and sequence
SOUTH
NORTH
Fill
Formation level
approx 33m bgl
Upper E
Upper
Temporary diaphragm
walls,0.8m
Marine Clay
Upper F2
gp
Driven kingpost
Permanent bored piles
supporting
rail boxes
10 levels of steel struts at
3m +3.5m vertical
centres
Lower
Marine Clay
Sacrificial JGP
Permanent JGP
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 10th level strut
Tidal sea
TSA Shaft
M3
Utility crossings
M2
Curved walls
Nicoll
Highway
24/3/04
Support beam
Shaft
Kingpost
M3
No waler
C channel stiffener
Jacking point
Non-splayed strut
Walers
M2
Splayed strut
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 and observations prior to collapse
Plate stiffener
C channel
stiffener
Replacement of plate stiffeners at strut
strut--waler connection
Strut bearing
directly on Dwall
C channel
connection
Excavation for the 10th level of struts, including removal of the sacrificial JGP
Observations on the morning of the collapse
North wall
M301
M302
334
333
M303
4
2
3
1
6
7
335
336
337
338
339
340
7
7
H
H
KP 181
1
H
KP 182
1
M306
M305
M304
5
M307
KP 183
5
M308
Order in which distortion to waler noted
5
M309
M310
Excavation in progress
Excavation to 10th complete
Location of walers, splays and Dwall gaps
S335 9
S335-9
Strut 335-9 south wall
S338 9
S338-9
Strut 338
338--9 north wall
Instrumentation and results of monitoring
M2/M3 plan at 9th level
M2
M3
GWV24
I 65
I 104
All struts at S335
instrumented for load
measurements
TSA Shaft
Excavation Front
Beyond S335 at the
start of Day Shift on
18th April 2004
5000
4500
SG3358 Total Force
Excavation Front
Approaches S335 at
the end of Day
y Shift on
17th April 2004
4000
3500
Excavation Front for
10th Level Advancing
from S338
3000
2500
2000
1500
Excavation Front for
10th Level Between
S336 and S335
1000
500
15th April
16th April
17th April
18th April
19th April
20th April
0
140
130
120
110
100
90
80
70
60
50
Hours before collapse
40
30
20
10
0
Measured strut load
d (kN)
SG3359 Total Force
Change in Measured Load at Strut 335
5000
336(N) & 337(N)
4500
Waler Buckling Observed
335(N)
Support Bracket
at 335(S)
Drops
Off
Waler
Buckling
Observed
Me
easured strut load (kN)
4000
3500
3000
2500
2000
Strut Load 335-9
Strut Load 335-8
1500
338(N) & 335(S)
1000
Waler Buckling Observed
500
0
6.00
7.00
8.00
9.00
10.00
11.00
12.00
Time on 20April 2004
13.00
14.00
15.00
16.00
Observed trends in 8th and 9th strut loads
Observed
8th
9th
9am
m
Excavation approaches
+ passes beyond S335
5
18
20
April 2004
3.30pm
In
nstallation off 9th
Load
The trends were consistent with there being
g
yielding of the 9th level strut-waler connection
when the excavation passed beneath but with
no further
f th significant
i ifi
t changes
h
iin lload
d iin either
ith
the 9th or 8th level struts until the collapse was
initiated
Horizontal displacement (mm)
Horizontal displacement (mm)
0
200
400
400
200
0
0
0
I 104
10 April
I 65
10 April
10
10
Behind South Wall
North Wall
20
30
30
40
40
50
50
De
epth (m)
De
epth (m)
20
Horizontal displacement (mm)
Horizontal displacement (mm)
0
200
400
400
200
0
0
0
I 104
10 April
15 April
I 65
10 April
16 April
10
10
Behind South Wall
North Wall
20
30
30
40
40
50
50
De
epth (m)
De
epth (m)
20
Horizontal displacement (mm)
Horizontal displacement (mm)
0
200
400
400
200
0
0
0
I 104
10 April
15 April
17 April
I 65
10 April
16 April
17 April
10
10
Behind South Wall
North Wall
20
30
30
40
40
50
50
De
epth (m)
De
epth (m)
20
Horizontal displacement (mm)
Horizontal displacement (mm)
0
200
400
400
200
0
0
0
I 104
10 April
15 April
17 April
20 April
I 65
10 April
16 April
17 April
20 April
10
10
Behind South Wall
North Wall
20
30
30
40
40
50
50
De
epth (m)
De
epth (m)
20
Maximum horizontal displacemen
nt, mm
500
I104 max
I65 max
400
South wall
300
200
N th wallll
North
100
0
5-Jul
9-Aug
13-Sep
18-Oct
2003
22-Nov
27-Dec
31-Jan
6-Mar
10-Apr
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 2
2.5
5 days under load and was over
8m from excavation front
• Both
B th S335-9S
S335 9S & S338
S338-9N
9N b
buckled
kl d within
ithi 10 minutes
i t
• Load in S335-8 and S335-9 was almost constant
b t
between
18 A
Aprilil and
d iinitiation
iti ti off 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
b i llength
bearing
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 strutstrut-waler connection
• Capacity based on BS5950:1990 = 2550 kN
• Average ultimate capacity based on physical
load tests = 4100 kN
• Based on mill tests, 95%
9 % off 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 MohrC
Coulomb
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
p’
The shortcomings of Method A
Method B
105
105
100
100
95
95
90
90
85
85
RL (m)
RL (m)
Method A
80
80
75
75
70
70
65
65
60
60
55
-0.050
55
-0.050
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.000
0.050
Wall Disp. (m)
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 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 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 78.3 for S8
Exc to RL 75.3 for S9
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 72.3 for S10
M3 - South
S th Wall
W ll Di
Displacement
l
t
Method A versus Method B
Method B
105
100
100
95
95
90
90
85
85
Bending Moment (kNm/m)
M3 - South Wall bending moments
Method A versus Method B
4000
3500
3000
2500
2000
1500
1000
500
0
-500
-1000
4000
3500
3000
2500
-1500
Bending Moment (kNm/m)
2000
55
1500
55
1000
60
500
60
0
65
-500
65
-1000
70
-1500
70
-2000
75
-2500
75
-2000
80
-2500
80
-3000
RL (m)
105
-3000
RL (m)
Method A
Method B
M th d A
Method
( )
2206
S1
S2
S3
S4
S5
S6
S7
S8
S9
Strut Load (kN/m)
Strut Load (kN/m) _
g
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
M3 – strut forces
Method A versus Method B
2383
S1
S2
S3
S4
S5
S6
S7
S8
S9
• 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 9th 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
• Structural
St 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
• C
Collapse
ll
was an iinevitable
it bl consequence off 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
• Structural
St 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
• C
Collapse
ll
was an iinevitable
it bl consequence off 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
5
Shear wave section
4
JGP thic
ckness (m)
Pressuremeter section
3
2
Design thickness
1
0
0
2
4
6
Thicknesses of JGP in Type K
8
Post-collapse
Post
collapse investigations
Ground conditions and
soil properties
P36
BN3
P42/P48
P40P41/P47
P33
P54
CN3
P45
DN3
31700
P52
AN2
BN2E
TSAN2
CN2
BN2W
DN2E
54+900
DN2W
EN2
CN1
NOR
RTHING (m)
BN1W BN1E
66KVNI
DN1E
DN1W
FN2
BN1Ea
M2
31650
54+812
FN1
AN1
M3
CL-1a
EN1
CL-2
CL-3g
CL
3g
66KVS
CFVN
CTSN
CPTNa
CPTN
DS1E
DS1W
BS1Ea
BS1Eb
SPTSa
BS1W AS1
CS1
TSAS2
TSAS2b
ES1
FS1
AS2
BS2E
CS2
BS2W
DS2E
DS2W
31600
ES2
31550
31600
FS2
AS3
BS3E
BS3W
31650
31700
31750
DS3E CS3
ES3
DS3W
EASTING (m)
BS4W
CS4
BS4E
BS4Ea
DS4W
DS4E
Post--collapse ground investigation
Post
31800
0
0.4
Cone resistance, qt (MPa)
0.8
1.2
1.6
0
2
0
0.4
Cone resistance, qt (MPa)
0.8
1.2
1.6
BN1W
BS2W
qt
u2
fs
10
qt
u2
fs
North
South
10
De
epth (m)
Depth (m)
D
2
0
20
30
20
30
40
0
0
0.4
0.8
Pore pressure, u2 (MPa)
0.02
0.04
Friction, f s (MPa)
1.2
0.06
0.08
40
0
0
04
0.4
08
0.8
Pore pressure, u2 (MPa)
0.02
0.04
Friction, f s (MPa)
12
1.2
0.06
CPT profiles north and south of collapse area
0.08
100
φ=28.5o
Ko=0.5
t (kP
Pa)
50
φ=34.2o
0
0
50
100
150
200
s' (kPa)
-50
P-8
P-9
P-24
P-26
φ=35o
-100
Kisojiban’s CAU tests on
Upper and Lower Marine Clay
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
P36
BN3
P42/P48
P33
50
55
60
65
70
75
80
85
90
95
P40
100 105
P41/P47
P54
CN3
P45
31700
P52
DN3
TSAN2
AN2
BN2E
CN2
BN2W
DN2E
ABH32
54+900
DN2W
EN2
NORTHIING (m)
AN1
M3
ABH34
ABH85
BN1W
66KVNI
CN1
WN2
BN1Ea BN1E WN6 WN4
WN8
WN3 WN1
ABH30 WN12WN10
WN7AWN5
DN1E
FN2
WN14
WN11 WN9
WN16
DN1W
WN13
WN18
WN15
WN20
WN17
WN22
WN19
WN24MC3007
WN21
EN1
WN26
M3010 CL-1a
WN23 ABH83
WN28
WN25
AC 3
AC-3
WN30A
WN27
CL-2
FN1
WN29
CL-3g
WN32
WN31
WN34
BS1Ea
WN33
WS1
ABH84
66KVS SPTSa BS1Eb
ABH31
WN37 WN35
WS3
WS7 WS5
WS11AWS9
WS12A
WN36
WS13A
WS11C
WS12C
WS6 WS4A WS2
WS15WS13C
WN38
WS8
WS10
WS14
BS1W
WS11
WS11B
WS17
WS12B
WS12
WS13B
WS13
DS1E WS19
AS1
WN39
WS16
WS21
ABH29
WN40
WS18
ABH82 WS23
WS20
WS25
WS22
CS1
WS27
WS24 DS1W
ABH28
WS29
WS26
WS34 WS31
WS28
WS30
ABH81
WS36
WS32 ES1
WS35A
WS33
MC2026
WS37
AS2
BS2E
WS38
FS1
CS2
BS2W
M2064
AC-2
AC-4
M2
31650
54+812
CFVN
CTSN
CPTNa
CPTN
ABH27
ABH33
TSAS2
TSAS2b
M2065
AS3
DS2W
BS3E
ES2
31550
MC3008
DS2E
MC2025
31600
ABH35
AC 1
AC-1
31600
FS2
BS3W
31650
31700
DS3E
31750
CS3
DS3W
ES3
EASTING (m)
BS4W
CS4
BS4E
BS4Ea
DS4W
DS4E
Evidence for a buried valley in the Old Alluvium
31800
110
100
ELEVATION ((mRL)
90
80
70
60
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
50
Section along north wall
110
100
ELEVATION (mR
E
RL)
90
80
70
60
50
Section along south wall
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
P36
BN3
86.07
77.02
P42/P48
P33
P40
86.39
86.30
86 90
P41/P47 86.90
86.84
P54
CN3
86.58
82.27
31700
P45
86.20
DN3
P52
85.98
81.91
BN2E
82.61
DN2E
81.94
54+900
ABH32
M3
BN1Ea BN1E
81.57 80.68
CN1
AN1
83.41
82.91
66KVNI
81.88 WN5WN4WN3WN2WN1
WN6
WN7A
82.16
WN8
81.3881.2582.31
WN10WN9
WN11
81.4281.43
81.38
WN12
DN1E
WN13
81.2581.05
WN14
WN15
80.9581.21
80.66
WN16
80.37
M304
WN17
81.86
WN18
M305
79.8680.72
WN19
81.86
WN20
80.81
WN21
80.31
80.88
WN22
80.94
M303
WN23
WN24 79.9179.93
M3010
WN25
MC3007
80.15
WN26 80.65
82.55
ABH83
80.95
WN27 80.36
M302
79.57
CL-3g
KP182 KP183
79.72
AC-3
83.39
M209 M210
KP181
CL 2
CL-2
M211
80.22
81.69
BS1Ea
M301
338
NORTH
HING (m)
83.13
M2
31650
54+812
WN38
CFVN
WN39
CTSN 81.97
WN40
WN29
WN30A
WN31 78.34
77.51
WN32
WN33 77.71
WN34 78.02
WN35 80.58
WN36 80.53
80.66
WN37
81.43
78.35
78.04
80.83
81.06
ABH82
83.19
82.17
74.60
66KVSSPTSa BS1Eb
WS1
74.72
71.52 M309
ABH84
WS3
ABH31
M310
71.09
M307 M308
WS2
WS8 WS7
WS4A 78.20
M212 & M306 WS10WS9
76.59
82.07
73.84
75.20
missing82.57
76.77
72.2872.7072.61
WS16WS15
AS1
WS17M213
WS18
81.3981.39
WS19
75.98
WS20
WS21
81.2881.45
WS22
ABH29
CS1
81.38
WS23
80.84
WS24
WS25
79.9381.48
WS26
80.03
81.23
DS1E
WS27
79.99
81.59
WS28 80.37
79.76
WS29
81.09
78.25
WS30
78.52
WS31
WS32 78.7879.57
79.23
79.10
79.25
ABH85
81.69
AC-2
AC-4
81.73
79.98
ABH35
AC-1
81.69
80.76
ABH33
80.51
TSAS2b
82.16
82.01
WS34
WS35A
ABH81
79.69
WS37
80.13
WS38
80.65
BS2E
81 26
81.26
82.28
ABH27
M2064
80.69
79.00
31600
MC2025
DS2E
81.70
80.68
BS3E
81.29
31550
31600
31650
EASTING (m)
31700
BS4Ea
80.96
-14 -12 -10 -8
-6
-4
-2
0
2
4
6
8
10
31750
31800
BS4E
80.33
12
DS4E
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 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 certainlyy occurred in the deep
p
organic clays as a result of stress relief, reducing
their strength further
The buried valley
CN3
DN3
TSAN2
BN2E
AN2
BN2W
CN2
DN2E
DN2W
ABH-34
AN1
ABH-32
EN2
CN1
GA
BN1W
MA
I-65
DN1W
AC-2
AC-4
IN
FN2
II-67
67
BN1E
BN1Ea
ABH-30
WN1
66KVN
S
DN1E
ABH-85
WN10
WN20
EN1
WN25 WN24
94.9WN28 WN27WN26I-102
FN1
WN34
WN29
WN31WN30
WN32
WN33
ABH-83
BS1Ea
CL-2
BS1Eb
BS1W
66KVS
I-104
ABH-84 SPTSa
WS10
WS1
ABH-31
ABH
31
I-66
ABH-33
AS1
WS20
WN39
CFVN
ABH-35
MC3008
AC-3
AC-1
WN38
CPTNa
KP183
KP182
KP181
WN36
WN35
WN37
CL-1A
M3010
MC3007
CL-3G
DS1E
ABH-82
WN40
I-103
WS27
ABH-28
CPTN
WS30
WS34
WS29
WS28
DS1W
WS31
WS36
ABH-81
WS35
WS33
WS32
WS37
WS38
SE
ES1
ABH-29
TSAS2
CS1
WS26
A
TSAS2B
AL
W
L
MC2026
BS2E
FS1
AS2
ABH-27
ABH
27
BS2W
I-64
M2065
CS2
M2064
DS2E
DS2W
BS3E
ABH-80
AS3
BS3W
ES2
FS2
CS3
DS3E
ABH-26
ES3
DS3W
BS4E
Post-collapse geometry and sequence of excavation
superimposed on buried valley
GA
CN1
BN1W
S
MA
I-65
N1W
66kV Cable
BN1E
BN1Ea
IN
DN1E
ABH-30
WN1
66KVN
I
AC
WN10
WN20
5 WN24
ABH-83
CL-1A
M3010
MC3007
KP183
KP182
KP181
AC-3
CL 3G
CL-3G
BS1Ea
CL-2
66KVS
I-104
ABH 84 SPTSa
ABH-84
WS10
BS1Eb
BS1W
WS1
ABH-31
AS1
WS20
DS1E
ABH-82
DS1W
ABH-29
ABH
29
CS1
WS26
LL
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 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
Trigger
Yielding of 9th
Observed
8th
Minor additional loading
<2mm yielding in 9th
9th
9am
m
Excavation approaches
+ passes beyond S335
5
18
20
April 2004
3.30pm
In
nstallation off 9th
Load
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 p
piles 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
• W
Wallll EI reduced
d
d tto allow
ll
ffor cracking,
ki
b
based
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
• 9th level strut capacity set and strut allowed to strain
soften 72 hours after excavation to 10th level.
Geotechnical analyses
North
Fill
Estuarine
South
+102.9
+98.58
+96.58
+102.9
+98.58
+97.07
+84.58
+81.58
+85.57
+82.07
Upper Marine Clay
Upper Marine Clay
F2
Fill
Estuarine
F2
Lower Marine Clay
F2
OA (Sand) - OA-CZ
OA (Clay/Silt) - OA-CZ
+71.00
+67.00
+63.00
Lower Marine Clay
+63.57
+61.57
+57.57
OA (Clay/Silt) - OA-SW-1
OA (Sand) - OA-CZ
OA (Clay/Silt) - OA-CZ
Stratigraphy assumed for ICFEP analyses
North wall
9th
South wall
100
100
95
95
90
90
measured
predicted
Elevation (m RL)
85
predicted
measured
80
80
75
75
70
70
65
65
60
60
Horizontal displacement (mm)
55
50
85
0
50
100
150
200
250
300
350
Horizontal displacement (mm)
55
450
-400
-350
-300
-250
-200
-150
-100
-50
0
50
North wall
10th
South wall
100
100
95
95
90
90
measured
predicted
Elevation (m RL)
85
measured
predicted
80
85
80
75
75
70
70
65
65
60
55
50
60
Horizontal displacement (mm)
0
50
100
150
200
250
300
350
Horizontal displacement (mm)
55
450
-400
-350
-300
-250
-200
-150
-100
-50
0
50
Predicted
ed cted ttrends
e ds in 7th, 8th a
and
d 9th st
strut
ut loads
oads
4500
4000
Prop forrce (kN/m
m)
3500
3000
2500
Strut 7
9
Strut 8
2000
Strut 9
8
1500
1000
7
500
0
280.00
281.00
282.00
283.00
(
284.00
285.00
)
Time (days)
6 hours
286.00
Observed trends in 8th and 9th strut loads
Observed
8th
9th
9am
m
Excavation approaches
+ passes beyond S335
5
18
20
April 2004
3.30pm
In
nstallation off 9th
Load
• 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 llevell strut reached
h d iits capacity
i d
during
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
l
level,
l b
butt a hi
hinge did nott form
f
in
i the
th wallll until
til th
the
sacrificial JGP layer had been removed
Key findings from geotechnical analyses
Trigger required to initiate collapse
Relative vertical displacement between
the kingposts and Dwall panels
Vertical diisplacement of Dw
V
wall (mm))
100
Up
Top of southern wall-nolimit
Top of southern wall-OA1
Top
p of southern wall-OA2
50
8th
9th
10th
0
0
5
10
15
20
25
30
-50
Down
-100
Excavation depth (m)
Predicted settlement of south wall during excavation
35
Verrtical disp
placementts (mm)
500
Run 4apfnolimit
400
Run 4apfnolimit_OA1
Run 4apfnolimit_OA2
300
200
100
0
-5
0
5
10
15
20
25
-100
North Wall
South Wall
-200
Distance (m)
Predicted vertical displacement of lower JGP layer
after excavation to 10th level
30
20
Strut 262
no backfill
Soutth wall
kin
ngpost
Typical relative
displacements of
strut between
walls and kingpost
Diffference in leve l between wall and kingpost (mm)
10
North wall
2
1
0
7A
2
3
7B
3
-10
6A
4
6A
-20
1
5
7A
7B
4
-30
5
• 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
KP180
KP182
KP181
335
336
9th level
Sacrificial JGP
Excavation front
Lower JGP
Kingposts in long section
337
KP183
338
339
KP184
340
Post-collapse
Post
collapse investigations
Structural steel physical tests
and numerical analyses.
The effect of relative vertical
displacement
12mm stiffener
plates
5000
Waler
Axial load ((kN)
4000
3000
2000
1000
0
0
10
20
30
Axial displacement (mm)
40
50
Comparison of tests on connections with plate and C channel
stiffeners
12mm stiffener
plates
5000
Waler
Axial load ((kN)
4000
3000
C channel
stiffener
Waler
2000
C channel stiffener has similar capacity
to double plate stiffener but becomes
brittle after an initial ductile response
1000
Unforced sway
0
0
10
20
30
Axial displacement (mm)
40
50
Comparison of tests on connections with plate and C channel
stiffeners
5000
Test result
Axial loa
ad (kN)
4000
3000
C channel
stiffener
Waler
2000
1000
0
0
10
20
30
Adjusted axial displacement (mm)
40
Calibration of FE model of strutstrut-waler connection
50
5000
Test result
FE prediction
Axial loa
ad (kN)
4000
3000
C channel
stiffener
Waler
2000
1000
0
0
10
20
30
Adjusted axial displacement (mm)
40
Calibration of FE model of strutstrut-waler connection
50
5000
Ductile plateau 10-15mm
Test result
FE prediction
Axial loa
ad (kN)
4000
3000
C channel
stiffener
Yield
Waler
2000
Ductile plateau allows failure to
develop at both ends
1000
0
0
10
20
30
Adjusted axial displacement (mm)
40
Calibration of FE model of strutstrut-waler connection
50
5000
8m strut
Axial load (kN)
4000
3000
2000
C channel
stiffener
Waler
1000
0
0
10
20
30
Axial displacement
p
((mm))
40
Effect of strut length (bending restraint) on brittleness of
connection
50
5000
8m strut
18m strut
Axia
al load (kN)
4000
3000
2000
C channel
stiffener
Reduced restraint increases brittleness
and
d reduces
d
d
ductile
til plateau
l t
8m strut – effective kingpost
18m strut – ineffective kingpost
1000
Waler
0
0
10
20
30
Axial displacement (mm)
40
Effect of strut length (bending restraint) on brittleness of
connection
50
Test on C channel stiffened connection by Nishimatsu
Test on C channel stiffened connection by Nishimatsu
5000
axial load only
Axial load (kN)
4000
3000
2000
C channel
stiffener
Waler
1000
0
0
10
20
30
Axial displacement
p
((mm))
40
Effect of relative vertical displacement
50
5000
axial load only
axial load+RVD
Axial load (kN)
4000
3000
2000
C channel
stiffener
Waler
RVD reduces ductile plateau,
increases brittleness, makes
stable situation unstable
1000
Forced sway
0
0
10
20
30
Axial displacement
p
((mm))
40
Effect of relative vertical displacement
50
P
Force, P
D til
Ductile
strain
Effect of brittleness of strut to waler connection
2500
Ductile
2000
Run 4apf2100drop
Run 4apf2100drop4
)
Run 4apf2100drop2
1500
(
Run 4apf2100
p
Prop force
e (kN/m)
Run 4apf2100drop3
p
p
1000
500
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Total axial strain (%)
1.4
1.6
1.8
2.0
Effect of brittleness of strut to waler connection
2500
500
R 4
Run
4apf2100
f2100
Run 4apf2100drop2
Ductile – no collapse
Run 4apf2100drop3
Run 4apf2100drop4
2000
Prop force
e (kN/m)
Run 4apf2100drop
1500
1000
500
21 days
6 hours 9 days
0
-15
-10
-5
0
5
10
15
20
Time since reaching excavation level of 72.3m RL (days)
25
P
Force, P
D til
Ductile
No collapse
hours to collapse
Several days
y to collapse
strain
• Th
There was relative
l ti vertical
ti l di
displacement
l
tb
between
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 p
plateau 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 required to initiate collapse
Post-collapse positions of the Dwall
panels
GA
CN1
BN1W
S
MA
I-65
N1W
66kV Cable
BN1E
BN1Ea
IN
DN1E
ABH-30
WN1
66KVN
I
AC
WN10
WN20
5 WN24
ABH-83
CL-1A
M3010
MC3007
KP183
KP182
KP181
AC-3
CL 3G
CL-3G
BS1Ea
CL-2
66KVS
I-104
ABH 84 SPTSa
ABH-84
WS10
BS1Eb
BS1W
WS1
ABH-31
AS1
WS20
DS1E
ABH-82
DS1W
ABH-29
ABH
29
CS1
WS26
LL
Post-collapse geometry superimposed on buried valley
31650
CL-2
WS14
WS17
WS12D
SPTSa
WS10
WS11D
CS1
31630
WS13E
BS1Ea
66KVS
WS9
WS8
WS
S11C
14.7
WS15
WS13C
12
2.5
WS16
WS1
12C
12.5
NORT
THING (m)
31640
WS13B 13.7
WS13 16
WS13A 26
WS12B 15..2
WS12 12
9
WS12A 19
1
WS11B 7.5
19.6
WS11 1
WS11A 13.5
TUNNEL
SOUTH WALL
WS11E
WS12E
WS11F
Post collapse SI boreholes
Magnetic logging boreholes
Unsuccessful magnetic logging boreholes
2005 boreholes
WS13F
31620
31670
31680
31690
31700
EASTING (m)
Coincidence between inability to advance boreholes and
missing Dwall panels
WS11E,11F,
12E,13F
WS11-13A-D
Obstruction created by missing Dwall panels
2
1
M212
1
Gap
3
1
M306
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.
3 Soil
3.
S il flflows th
through
h resulting
lti gap b
between
t
M307 and M308, rotating panel 308, etc.
Sequence of south wall panel movements
drag
d
drrag
drag
4
5
304
305
303
3
302
2
301
1
2a
3a
4a
306
Sequence of south and north wall panel movements
SOUTH
NORTH
Fill
Upper E
Upper
Marine Clay
Differing restraint
imposed by bored
piles at north
and south walls
Upper F2
Lower
Marine Clay
Sacrificial JGP
Permanent JGP
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 reading of the strain gauges
under-reading
Significance of the bored piles
North wall
M301
M302
334
333
M303
4
2
3
1
6
7
335
336
337
338
339
340
7
7
H
H
KP 181
1
H
KP 182
1
M306
M305
M304
5
M307
KP 183
5
M308
Order in which distortion to waler noted
5
M309
M310
Excavation in progress
Excavation to 10th complete
Location of walers, splays and Dwall gaps
GA
CN1
BN1W
S
MA
I-65
N1W
66kV Cable
BN1E
BN1Ea
IN
DN1E
ABH-30
WN1
66KVN
AC
WN10
WN20
5 WN24
ABH-83
CL-1A
M3010
MC3007
KP183
KP182
KP181
AC-3
CL 3G
CL-3G
BS1Ea
CL-2
66KVS
I-104
ABH 84 SPTSa
ABH-84
WS10
BS1Eb
BS1W
WS1
ABH-31
AS1
WS20
DS1E
ABH-82
DS1W
ABH-29
ABH
29
I
CS1
WS26
LL
Repositioned bored piles at 66kV crossing
• 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 movementt was possible
ibl
• Resulting RVD fed back into buried valley
Relative vertical displacement
KP180
KP182
KP181
335
335
KP184
336
336
9th level
Upper JGP
Excavation front
Lower JGP
Kingposts in long section
KP183
337
338
339
340
• 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
under-prediction
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 buried
the
b i d valley,
ll
where
h
fforces on th
the under-designed
d d i
d strutt 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 p
preventing
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 o
over
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 and the speed at which the collapse developed
data,
Overall conclusions
• The collapse was not caused by hydraulic base
heave 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
plan requiring
q
g more frequent
q
use of
• Curvature of walls in p
walings
• Presence of 66kVA crossing
• Need to adopt
p sacrificial JGP layer,
y , removal of which caused
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 removed
• The potential for brittle failure of C channel connections
mustt be
b recognised
i d
Lessons learnt
• Temporary and permanent works should be subject to
independent checks
• The effects of relative displacement between kingposts
g of strutted
and walls should be considered in the design
excavations
• Forced swayy failure and its consequences
q
should be
recognised as a potential mechanism in design
• Monitoring did not warn of the impending collapse