1787
Technical Committee 204 /
Comité technique 204
0. 0195
0. 02
0. 0205
0. 021
0
10
20
30
40
Rel at i ve
di spl acement / m
The di st ance f r om l ef t sl ope f oot / m
Figure8. Re-springback displacement of the bottom of E section
foundation trench after waterway excavation
3 THE DEFORMATION CHARACTERISTICS OF IMT
FOUNDATION SOIL BASED ON CENTRIFUGE
MODEL TEST
3.1Design and preparation of the model
Before the experiment, with detailed surveying data numerical
analysis was performed to simulate loading conditions using
modulus from stress path tests. Due to the limition of the
centrifugal box size and the fact that the slope excavation of the
model upper foundation trench is larger than the actual
condition, so immersed tube tunnel foundation was simulated
which was influenced by the difference between the slope
excavation of the foundation trench and actual condition in
centrifugal model test. The model used in the centrifugal
simulation test was verified by comparison analysis. The
centrifuge type is TLJ - 3 geotechnical centrifuge. Its maximum
capacity is 60
g ·
T, the effective radius is 2.0 m and the
maximum centrifugal acceleration is 200 g. The model box size
is 70
36
50
㎝
3
. The model scale was selected to be 100g
based on the box’s headroom size and the prototype scope of
simulated model. The preparation of the model foundation soil
is controlled by the nature of the prototype foundation soil (See
table 2) and centrifugal similar rate. Every foundation soil
thickness is determined by scaling down the thickness of the
section in corresponding condition according to the geometric
scale. The dry density of cushion layer and coarse grained soil
and the strength parameters of the other soil must be in
aggrement with prototype.
IMT model is the key structure in the test. This test adopts
full section simulation. The high and width of the prototype
IMT is 11.5 m and 40 m. Due to the limitation of test model
height, the height of IMT model must be controlled in 5cm. The
width of IMT section is reduced in proportion accordingly. The
organic glass is used as IMT model materials. The height and
internal dimension of model are controlled by the axial stiffness
EA, bending rigidity EI and pressure stress from immersed tube
to foundation of the prototype. Upper backfill on IMT is
simulated using the steel plate of the same weight. So the height
of the model is effectively reduced. Since the steel plate has
almost no deformation, the deformation of the upper plate can
be directly measured to obtain the sedimentation of IMT in
construction operation.
In natural ground segment of IMT, B section is deep
excavation section; C section is thick clay section and E section
need to excavate a channel after the construction, it involves
excavation unloading problems after the backfill back silting.
Therefore typical cross sectional B, C and E of standard
conditions were simulated in the test. The test of C section with
1 m cushion layer and the most adverse conditions was
conductedto compare with standard conditions experimental
results.
3.2The arrangement of the measuring points in the model
Earth pressure cell and pore water pressure cell were buried in
the foundation soil (Soil pressure box main layout in the
boundary of different soil layer) and laser shift sensor were
mounted in the top of model box in order to monitor vertical
stress distribution of foundation soil, stress distribution of
basement, development process of pore water pressure and
vertical rebound recompression displacement of basement
during the test procedure. As shown in Figure 9(S1-S9 are earth
pressure cell; P1and P2 are pore water pressure cell; W1 and
W2 are laser displacement sensor).
High sensitivity strain gauges were mounted on the bottom
and sides of immersed tube in order to measure the strain of
immersed tube during the test process. All above tests data was
automatically collected by DDS data acquisition system.
Figure9. The layout of pressure cell
3.3 Research results analysis
The stress distribution curve of IMT base groove bottom surface
of B section after construction is shown in Figure 10. The stress
distribution curve with time of IMT base groove bottom surface
of B section within 3 years after the completion of the
construction backfill is shown in Figure 11.
Figure 10 indicates that test results are the same as the
numerical results. The stress distribution of foundation trench
bottom has a saddle-shaped distribution. The basal stress of
IMT’ carriageway is relatively small. While the largest basal
stress value comes from both sides of IMT, and the second
largest basal stress comes from the partition wall. The
difference between the maximum and minimum stress values is
not more than 38 kPa.
Figure 11 indicates that stresses level off after initial
increasesThe stress increment is relatively little, which is about
10kPa.
‐250000
‐200000
‐150000
‐100000
0
10
20
30
40
50
St r ess/ Pa
The di st ance f r oml ef t sl ope f oot / m
Numerical calculation curve
test curve
Figure10. Stress distribution of the bottom of B section foundation
trench
‐210000
‐190000
‐170000
‐150000
‐130000
0
200 400 600 800 1000 1200
St r ess/ Pa
The t i mes af t er const r uct i on/ d
S1
S2
S3
S4
S5
S6
Figure11. Stress-time distribution of the bottom of B section foundation
rench t
The stress distribution curve of IMT base groove bottom
surface of E section after backfilling and channel excavation is
shown in Figure 12.
In Figure 12, the stress distribution of foundation trench
bottom has a saddle-shaped distribution. The basal stress of
IMT’ carriageway is relatively small, both sides of IMT have
maximum basal stress value, and the basal stress of partition
wall have the second largest basal stress. The difference
