Centrifuge Model Study of Impact on Existing
Undercrossing Induced by Deep Excavation
Z.W.Ning, X.Y.Xie, F.Z.Liu, X.R.Liu
Dept. of Geotechnical Engineering, Tongji University
The 5th China-Japan Joint Seminar for the Graduate Students in Civil Engineering, Nov. 7th-10th, 2008
Contents
 Introduction
 Centrifuge Test Facility
 Test Set-up & Procedures
 Result Analysis
 Conclusions
Contents
 Introduction
 Centrifuge Test Facility
 Test Set-up & Procedures
 Result Analysis
 Conclusions
Shanghai
Rail
Planning
Shanghai
RailTransit
Transit Planning
Yuanshen Road
Ventilation Shaft
Line 9
研究背景与主要研究内容
Project Introduction
Excavation
20m
Excavation
25m
Undercrossing
Excavation: 35m in depth
Undercrossing: 34m in width, 6 lanes.
Distance between the excavation and the undercrossing:
3m.
Undercrossing
Project Introduction
diaphragm wall
10 m
Length: 59m
Thickness: 1.2m
10 m
9 levels of inner struts
Barrier Piles-wall
40m
Length: 40m
Width: 40m
Piles-wall
Diameter of piles: 850mm
In order to protect the undercrossing, a barrier piles-wall made up
ofφ850mm mixing piles was constructed between the excavation and the
undercrossing. The depth of the piles-wall is 40m and extends 10m
laterally from the boundary of the excavation on each side.
Principle of Centrifuge Modeling
Centrifuge modeling is a useful tool to study the geotechnical
problems because of its ability to reproduce the same stress level
in a small-scale model.
Contents
 Introduction
 Centrifuge Test Facility
 Test Set-up & Procedures
 Result Analysis
 Conclusions
Excavation Simulation Approaches on Centrifuge
Conduct by on-board robot or
special device in-flight
Effective but Complicate
Simulation
Approaches
Drain off heavy liquid in-flight
instead of soil excavation
Weakness：the horizontal and
vertical stress of soil can not be
correctly simulated at the same
time.
Remove soil and construct struts
by hand when stopping the
centrifuge
Weakness：It does not
reproduce the real stress
path of soil correctly.
TLJ－150 Centrifuge in Tongji University
Centrifuge
Control Room
Capacity:
150 g-tons
Maximum g:
200 g
Effective r:
3m
In-flight Robot
Strong Box
Excavation and Struts Installation Simulation Device
In present study, a new device to
simulate the installation of
multi-level inner struts in-flight
was developed and the drainingfluid method was used at the
same time.
All the tests stated here were
carried out at Tongji Centrifuge
Laboratory in 100g acceleration
filed.
The operations of the device can
be conducted through a uniform
control platform in the control
room.
Demonstration of Simulation Concept
Locked
S oil
Locked
F luid
S trut
H ydraulic-lock
D iaph ragm w all
Locked
S trut
H ydraulic-lock
S teel rig id p late
All the struts can move freely in horizontal direction at the
beginning. When the surface of the replacing liquid fell beneath
struts at a certain level, the position of those struts would be
fixed by activating the hydraulic-control locks on them.
Details of Simulation Device
Steel strut
The strut is made of steel bar
with a load cell inside to
measure its inner force.
Hydraulic-control lock
The hydraulic-control lock is
installed at the end of each
strut to lock or free it.
Hydraulic-control valve
A hydraulic-control valve is
used to accurately drain the
liquid step by step to any
level according to the real
excavation procedures.
Contents
 Introduction
 Centrifuge Test Facility
 Test Set-up & Procedures
 Result Analysis
 Conclusions
Simplifications & Assumptions
Strong Box
Steel Plate
 Only half part of the prototype was modelled due to the symmetric layout of the real project.
 The ground profile was simplified into 3 layers: Clay, Silt, Fine Sand (top-down)
 The struts were reduced into 5 rows by two columns. Due to the limit free space in the
strongbox.
To make sure an equivalent lateral rigidity of the whole retaining structure to the prototype,
compensation were made both in the thickness of the diaphragm wall and the radius of the
struts.
Tests Design
In present study, four 1:100 scale centrifuge model tests were performed in a
strongbox with internal dimension of: 900mm×700mm×700mm.
Test No.
Soil
excavation
Barrier
Piles-wall
Depth of
Piles-wall
Width of
Piles-wall
Test 1
Yes
Yes
40m
40m
Test 2
Yes
Yes
20m
30m
Test 3
Yes
No
-
-
Test 4
No
No
-
-
Test1, Test2 and Test3 differed in parameters of barrier piles-wall.
Test 4 was an extra test with no excavation.
Instrumentation
6 LVDTs to measure the settlement of the
undercrossing in two directions.
10 load-cells to measure the struts’ stress.
5 eddy current sensors to measure the
displacements of the struts at 5 different
levels(-2m, -9m, -16m, -23m ,-30m).
Model &Instrumentation Arrangement
Model Scale：1:100
30
LVDT1
170
70
1
D redg e level
S truts
S teel rig id p late
250
S tru ts
U ndercrossing
Undercrossing
all
arieer p iles-w
B
Barrier
Piles-wall
30 70
LVDT3 LVDT2
170
80
300
LVDT4
100
1
LVDT5
50
70
90
70
LVDT6
Diaphragm
D iap h rag mwall
w all
LVDT2
70
Undercrossing
LVDT1
LVDT3
100
300
D iaph ragm w allwall
Diaphragm
R o o m fo r arran g em en t o f accesso rial d ev ices
330
U n d ercro ssin g
Barrier
Piles-wall
B arrier
p iles-w all
170
170
160
70 20
340
250
20
2 30
120
160
340
250
65
120
U n it: m m
2
L eg en d ：
L iq u id
U nit:m m
Plan view
L egend:
L iquid
C lay
Cross-section 1-1
S ilt
F ine S and
Model Preparation
Density
(kN/m3)
Water
content(%)
Thickness
(mm)
Clay
16.7
50.8
330
Silt
18.1
34.5
250
Sand
18.8
27.4
100
Clay
Silt
Fine Sand
The ground model consisted of three layers :
Clay, Silt, and Find Sand.
The clay and silt were saturated at first and then
were consolidated in the centrifuge under 100g
for 2-3 hours until at least 90% degree of
consolidation was achieved.
The Na2O.3SiO2 solution with a density of 1.36g/ml was
employed in these tests providing a lateral pressure close
to the soil.
Model Preparation
Diaphragm Wall
Undercrossing
Aluminum plates with various thicknesses were used to model the
diaphragm wall, undercrossing structure and the barrier piles-wall
according to the flexural stiffness of the prototype.
Model Preparation
1
3
2
4
Data Acquisition & Facility Operation
1
2
3
4
Contents
 Introduction
 Centrifuge Test Facility
 Test Set-up & Procedures
 Result Analysis
 Conclusions
Acceleration Time-history Curve
100g
The duration for each test keeping on 100g was around 20
minutes corresponding to approximately 5-month excavation
period in real project.
Undercrossing settlement time-history curves
45
0
40
-5
Settlement (mm)
-10
30
-15
25
20
-20
15
-25
10
Excavation Depth(m)
35
-30
5
-35
0
10:10:34
10:13:26
10:16:19
10:19:12
10:22:05
10:24:58
10:27:50
10:30:43
10:33:36
10:36:29
-5
-40
L-1
L-2
L-3
Time
L-4
L-5
L-6
Depth indicator
The typical settlement time-history curves of the undercrossing observed in Test 1
Settlement of test 1 and field monitoring in cross-section 1-1
excavation finished
10
1
Settlement (mm)
5
0
-5 0
5
10
15
20
25
30
35
40
-10
-15
-20
-25
1
-30
Distance from the boundary of excavation (m)
Test 1
Field monitoring
Test 1(mm)
Field monitoring(mm)
Distance from the excavation
3m
20m
37m
3m
37m
The first level of struts finished
0.3
-2.4
0.3
0.0
0.0
The second level of struts finished
0.9
-2.2
-1.0
0.8
1.4
The third level of struts finished
-6.5
-3.1
-2.8
-11.3
-2.8
The fourth level of struts finished
-10.0
-3.9
-3.0
-10.9
-1.7
The fifth level of struts finished
-12.7
-5.8
-4.0
-11.8
-1.6
Excavation finished
-14.6
-5.2
-4.2
-11.3
-0.9
Settlement of test 1, test2 and test3 in cross-section 1-1
Excavation finished
10
1
Settlement (mm)
0
-10
0
5
10
15
20
25
30
35
40
-20
-30
-40
-50
1
-60
Distance from the boundary of excavation(m)
Test1
Test2
Test3
Test1 (mm)
Test2 (mm)
Test3(mm)
Distance from the excavation
3m
20m
37m
3m
20m
37m
3m
20m
37m
The first level of struts finished
0.3
-2.4
0.3
0.5
-0.3
-0.1
-4.4
-0.1
0.2
The second level of struts finished
0.9
-2.2
-1.0
0.4
-5.1
-2.0
-7.9
-6.5
-2.0
The third level of struts finished
-6.5
-3.1
-2.8
-11.4
-12.2
-3.5
-13.7
-13.1
-3.5
The fourth level of struts finished
-10.0
-3.9
-3.0
-17.8
-15.9
-4.1
-19.9
-17.0
-4.2
The fifth level of struts finished
-12.7
-5.8
-4.0
-22.3
-18.0
-5.0
-28.6
-20.2
-4.8
Excavation finished
-14.6
-5.2
-4.2
-21.5
-15.2
-4.4
-33.4
-24.9
-4.3
Settlement of test 1 and field monitoring in cross-section 2-2
excavation finished
10
Settlement (mm)
0
-10 0
5
10
15
20
25
30
35
-20
2
2
-30
-40
-50
-60
-70 Range of Excavation
-80
Distance from the central-axis of excavation (m)
Test1
Field monitoring
Test1(mm)
Field monitoring(mm)
Distance from the central-axis
of excavation
0m
9m
16m
23m
0m
9m
24m
The first level of struts finished
0.3
-5.3
-5.6
-0.5
0.0
0.0
0.0
The second level of struts finished
0.9
-9.7
-4.1
-1.9
0.8
0.8
2.9
The third level of struts finished
-6.5
-23.3
-15.1
-3.2
-11.3
-9.0
-2.7
The fourth level of struts finished
-10.0
-28.3
-20.5
-6.1
-10.9
-8.7
-0.9
The fifth level of struts finished
-12.7
-31.3
-23.8
-7.8
-11.8
-11.7
-1.5
Excavation finished
-14.6
-37.8
-27.7
-10.1
-11.3
-10.3
-2.0
Settlement of test 1, test2 and test3 in cross-section 2-2
Excavation finished
10
Settlement (mm)
0
-10 0
5
10
15
20
25
30
35
-20
2
2
-30
-40
-50
-60
-70
Range of Excavation
-80
Distance from the central-axis of excavation (m)
Test1
Test2
Test3
Test1(mm)
Test2(mm)
Test3(mm)
Distance from the central-axis
of excavation
0m
9m
16m
23m
0m
9m
16m
23m
0m
9m
16m
23m
The first level of supports finished
0.3
-5.3
-5.6
-0.5
0.5
-0.6
-0.6
-0.5
-4.4
-10.0
-7.7
-0.5
The second level of supports finished
0.9
-9.7
-4.1
-2.0
0.4
-3.3
-2.1
-1.9
-7.9
-15.0
-12.2
-2.0
The third level of supports finished
-6.5
-23.3
-15.1
-3.1
-11.4
-22.1
-16.0
-3.1
-13.7
-21.3
-17.2
-3.1
The fourth level of supports finished
-10.0
-28.3
-20.5
-6.0
-17.8
-30.3
-22.0
-6.9
-19.9
-27.6
-23.4
-7.9
The fifth level of supports finished
-12.7
-31.3
-23.8
-8.2
-22.4
-36.8
-26.1
-9.2
-28.6
-36.9
-31.4
-10.2
Excavation finished
-14.6
-37.8
-27.7
-9.9
-21.5
-37.3
-30.0
-11.1
-33.4
-41.6
-35.1
-11.0
Contents
 Introduction
 Centrifuge Test Facility
 Test Set-up & Procedures
 Result Analysis
 Conclusions
Conclusions
 The newly developed device was proved to be workable under 100g
high acceleration field. It provide an effective way to accurately simulate
the excavation and struts installation procedures in-flight in centrifuge
modeling test.
 The impact of the excavation on adjacent undercrossing was
unneglectable with an impact range of about 3 times the width of the
excavation along the direction parallel to the undercrossing. The impact
weakened rapidly as the distance from the excavation increases.
 Barrier piles-wall was effective in protecting the undercrossing. The
longer the piles-wall, the less settlement occurred. However, there was
no significant difference of undercrossing settlement between tests with
and without piles-wall beyond the excavation range. It is suggested that
the width of piles-wall could be controlled within 2 times the width of the
excavation.
Dicussions
 The Na2O.3SiO2 solution we chose to replace the soil cause a larger
bottom heave than the real case due to its relative low pressure in
vertical direction. Thus, the observed settlement in our tests were larger
than the real case as well.
 Because liquid can not simulate the horizontal and vertical stress of soil
at the same time. We are trying to find the relationship between the
liquid density and the test results by numerical simulation in our further
research.
 The interface between model and strongbox should be carefully treated.
In our tests, the displacement of soil model near the boundary was
affected by the interface friction.
Thank You
ありがとうございます
The 5th China-Japan Joint Seminar for the Graduate Students in Civil Engineering, Nov. 7th-10th, 2008