920
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 PHYSICAL MODEL
The experiment of this study was conducted using shaking table
device of the earthquake research center at Sharif University of
Technology (SUT).
In order to hold the physical model, a rigid box was used
which had a length of 3.5 m, width of 1 m and height of 1.5 m.
Figure 1 shows the schematic cross section and plan view of the
physical model along with the layout of transducers. As seen,
the soil profile consists of three distinct layers including a non-
liquefiable crust with a thickness of 25 cm and relative density
of about 60%, that is made of sand and clay (10% by weight of
sand); a 1m thick middle liquefiable layer consisting of loose
sand with relative density of about 15% and a lower non-
liquefiable dense sand layer having 25 cm thickness and relative
density of about 80%. All the soil layers have a slope of 7%.
The sand used in physical model is standard Firoozkuh silica
sand (No. 161) which has a uniform grain size distribution and
is widely used in Iran for geotechnical physical modeling.
Model piles of this study were initially designed as steel piles in
prototype scale according to recommendations by JRA 2002
since representing a stiff pile comparing to concrete ones.
Subsequently, mechanical and geometrical properties of the
piles were calculated in model scale using similitude laws
proposed by Iai et al. (2005). In this regard, the geometrical
scale was selected as λ=8 (prototype/model). All model piles
were made of aluminum pipes. Material properties of the model
piles are summarized in Table 1.
As sketched in Figure 1, various types of transducers were
employed in different parts of the model including
accelerometers and pore pressure transducers in the free field
(far from the piles) to measure soil accelerations and excess
pore water pressures; pore pressure transducers close to the
piles to monitor build-up and dissipation of the excess pore
pressures in the near field (close to the piles); displacement
transducers (LVDTs) attached to the pile cap and also in free
field to record pile and soil lateral displacements and finally
strain gauges pasted along the piles to record bending moments.
Base excitation was applied parallel to the model slope. The
excitation was a sinusoidal acceleration record having
amplitude of 0.3g and frequency of 3 Hz whose duration was 12
sec consisting of two rising and falling parts, each of duration of
about 1.0 sec at the beginning and end of shaking.
Table 1. Mechanical and geometrical properties of pile foundations.
Material
Height
(m)
Outer/inner
diameter (cm)
I (cm4)
EI
(kN.m2)
Aluminum
1.25
5.2/4.7
5.904
4.054
3 SUMMARY OF EXPERIMENTAL RESULTS
In this section a summary of the main measured data during the
shaking table test is briefly presented and discussed.
3.1. Soil acceleration in free field
Sample soil acceleration time histories in the free field part of
the model (soil far from the piles) are shown in Figure 2. As can
be observed in this figure, the amplitude of acceleration records
in liquefiable layer decreased dramatically at the beginning
stages of shaking as the soil underwent liquefaction.
3.2. Excess pore water pressure records
Representative excess pore pressure time histories recorded in
free field area are shown in Figure 3. The trends show that the
soil liquefied after about 3 cycles of shaking since the middle
layer composed of very loose sand. Drainage of excess pore
pressures or consolidation of the liquefied sand initiated from
the lower depths (PWP1) and followed by pore pressure
reduction in upper elevations (PWP2).
Figure 1. Plan view and cross section of the physical model
.
-0.4
-0.2
0
0.2
0.4
Acceleration(g)
ACC4 (surface)
-0.4
-0.2
0
0.2
0.4
Acceleration(g)
ACC3 (65cmdepth)
-0.4
-0.2
0
0.2
0.4
Acceleration(g)
ACC2 (95cmdepth)
-0.4
-0.2
0
0.2
0.4
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Acceleration(g)
Time (sec)
ACC1 (Base)
Figure 2. Sample acceleration time histories of soil in the free field.
-2
0
2
4
6
8
10
12
14
Excess pore
pressure (kPa)
PWP2
-2
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 3
Excess pore
pressure (kPa)
PWP1
4
Time (sec)
Figure 3. Sample excess pore water pressure records in free
field.
3.3. Soil and pile group lateral displacement records
Figure 4, summarizes displacement records of the pile cap and
soil at the free field. As seen, the soil started to move downward
right after being liquefied. Unlike the free field soil
displacement which kept increasing until the end of shaking,
