Download Seismic Responses of Liquefiable Sandy Ground with Silt Layers

Journal of Applied Science and Engineering, Vol. 16, No. 1, pp. 9-14 (2013)
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Seismic Responses of Liquefiable Sandy Ground with
Silt Layers
H. T. Chen1*, B. C. Ridla2, R. M. Simatupang2 and C. J. Lee1
1
Department of Civil Engineering, National Central University,
Taoyuan, Taiwan 320, R.O.C.
2
Department of Civil Engineering, National Central University, Taiwan and University of Brawijaya, Indonesia
Abstract
This paper presents the numerical simulation results of liquefable sand-silt stratum with silt
intralayers under strong earthquakes. The numerical simulation results showed that the existence of
silt intralayers in a sandy soil stratum will reduce the ground settlement and the excessive pore water
pressure above the silt layer will also become smaller than that in the regular sand stratum. However,
the pore water pressure beneath the silt layer will become higher due to the impermeable character of
silt layer. Although the existence of more silt layers decreases the ground settlement furthermore, the
pore water pressure will have slower dissipation.
Key Words: Liquefaction, Sandy Stratum, Silt Intralayers, Effective Stress Analysis
1. Introduction
2. Method of Analysis
Liquefaction is a phenomenon that the structural and
the geotechnical engineers concern most as it can result
in serious damage to the ground and the building such as
sand boiling, lateral spreading, excessive settlement, tilting and overturning of structures. For a long time, many
liquefaction-related studies mainly treated the ground as
sandy ground; however, in reality there may be layers of
silt or clay embedded in the sandy ground. In some earthquakes the failure of ground did not occur during the
earthquake but after the earthquake stopped. The investigations on such a phenomenon showed that it may be due
to the existence of a silt layer in the sandy ground where
a water film develops at the bottom of the silt layer with
high pore water pressure [1]. This indicates that the
sandy soil stratum with silt intralayers may become unstable even after the main shake, causing the sliding of
slope. The purpose of this study is to investigate numerically the behavior of liquefable sand-silt stratum with
many layers of silt under strong earthquakes.
For the numerical simulation the three-dimensional
nonlinear effective stress finite element method was
adopted [2]. This method was developed on the basis of
Biot theory for porous media. The nonlinear soil behavior was modeled using the Cap model with MohrCoulomb type failure line and the pore pressure model
consistent with the Cap model was adopted [3]. The
lateral boundaries can be modeled as either roller-type
boundaries or absorbing boundaries, while the bottom
bedrock is always fixed.
This method adopts the U-W form of equation of
motion [4] as follows:
*Corresponding author. E-mail: [email protected]
(1)
where u is the displacement of soil particle and w is the
displacement of water relative to soil particle. The vector {J} is made up of 1’s and 0’s to account for the de-
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H. T. Chen et al.
sired direction of input motion. &&
f is the input motion
specified at the bedrock of soil stratum.
3. Verification and Validation
In this study the validation and verification of numerical simulation was first conducted by using the results of centrifuge tests on three models [5]. Although
the validation was made for all three models [6], here
only the comparisons for two models are presented.
Shown in Figure 1 are the two finite element models
which were constructed in accordance with the models
used in the centrifuge test. The model in Figure 1a denoted as Sand model corresponds to the sandy stratum
which was divided into 11 layers with the top and the
bottom layers having the thickness of 1.2 m and the remaining layers with thickness of 2.4 m for each layer.
Figure 1b shows the model denoted as Sand-Silt 1 model
where the silt layer of 1.6 m thick was placed at the depth
of 5.6 m from the surface and the model was divided into
13 different layers.
The input motions measured at the base of shaking
table on the centrifuge platform was used as the input
motion. Figures 2 and 3 show the comparison for the surface settlement and excessive pore water pressure development, respectively. It can be seen that the simulation
results show the same trend as the experimental results
and the agreement is acceptable.
4. Numerical Results and Discussions
4.1 Modal Description
Shown in Figure 4 are the five models adopted in this
study. Sand model consisted of sand only. For Silt 1
model a silt layer of 2 m thick was placed at the depth of
8 m from the surface. Two silt layers of 2 m thick for each
were placed at the depth of 8 m and 20 m, respectively,
from the surface for Silt 2 model. Silt 3 model was the
Figure 1. Finite element models: (a) Sand model, (b) Sand-Silt 1 mode1.
Figure 2. Comparison for surface settlement: (a) Sand model, (b) Sand-Silt 1 model.
Seismic Responses of Liquefiable Sandy Ground with Silt Layers
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Figure 3. Comparison of excess pore water pressure development: (a) Sand model, (b) Sand-Silt 1 model.
Figure 4. Finite element models: (a) Sand model, (b) Silt 1 model, (c) Silt 2 model, (d) Silt 3 model, (e) Silt 4 model.
one where two silt layers of 2 m thick for each were
placed at the depth of 8 m and 14 m from the surface, respectively. For Silt 4 model, three silt layers of 2 m thick
for each were placed at the depth of 8 m, 14 m and 20 m
from the surface, respectively. All the models had dimensions of 26 m ´ 26 m ´ 30 m (length ´ width ´ depth)
and were divided into 15 layers with element size of 2 m
´ 2 m ´ 2 m. Detailed properties of the models can be
seen in the thesis by Simatupang [6].
A real earthquake motion recorded in 1999 ChiChi
earthquake at Chiayi station (Chiayi input motion) was
used for this 3D simulation study. Before the simulation,
from the selected earthquake the maximum acceleration
of all components was selected and normalized to 0.2 g;
thereafter, the same scaling factor was applied to the motions of the other two directions. These three scaled component of motions were then used as the input motions
for the simulation.
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4.2 Discussions
Figure 5 shows the time history of settlement on the
surface for Sand, Silt 1, Silt 2, Silt 3, and Silt 4 models
subjected to Chiayi input motion. The largest settlement
occurs in the Sand model, which is around 0.87 m. The
maximum settlements of Silt 1, Silt 2, Silt 3, and Silt 4
models are 0.55 m, 0.37 m, 0.34 m, and 0.22 m, respectively. Silt 1, Silt 2, and Silt 3 models have smaller settlement than the Sand model due to the existence of silt
layer near the surface. Silt 4 model has the smallest settlement from all models due to the existence of three silt
layers.
Shown in Figure 6 are the excess pore water pressure
ratios at different depths for Sand, Silt 1, Silt 2, Silt 3,
and Silt 4 models subjected to Chiayi input motion. In
this figure, it can be seen that the effect of silt layer in the
sandy soil stratum is significant. At each depth, the behavior of EPWP was different. All five models liquefy at
the depth of 1 m where Silt 1, Silt 2, Silt 3 and Silt 4
models have lower EPWP ratio and faster dissipation
than Sand model. But at the depth of 5 m, only the Sand
model liquefies, while the Silt 1, Silt 2, Silt 3 and Silt 4
models show almost the same development of EPWP
without liquefaction. At the depths of 7 m and 9 m,
which are inside the silt layer of Silt 1, Silt 2, Silt 3, and
Silt 4 models, the development of EPWP is slower than
that of Sand model before liquefaction and after the liquefaction occurs, the trend reverses. At this depth,
there is a water film beneath a less permeable soil layers
and it takes longer time to dissipate the EPWP. At the
depths of 17 m, 21 m and 29 m, liquefaction does not occur for all five models; the development of EPWP for
Sand model and Silt 1 model is almost the same, meaning that the EPWP is not affected by the existence the silt
layer in Silt 1 model while a slight increase is observed
for the Silt 2, Silt 3, and Silt 4 models at later time.
Figure 7 depicts the initial effective stress and the
EPWP profiles for all five models at several selected
time. All five models show the similar behavior in the
Figure 5. Time history of settlement for 5 models (Chiayi input motions).
Figure 6. Time history of EPWP ratio at different depths for 5 models (Chiayi input motions).
Seismic Responses of Liquefiable Sandy Ground with Silt Layers
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Figure 7. EPWP profile at different time for 5 models (Chiayi input motions).
development of EPWP up to 5 seconds. At 15 seconds,
for Silt 1 model, a jump in the EPWP occurs between the
top and bottom of silt layer and for Silt 2, Silt 3, and Silt 4
models the jump occur between the top and bottom of
each silt layer but there is no jump for Sand model. The
variation of EPWP between the three silt layers of Silt 4
model is that the value of EPWP decreases to a value
smaller than that of Sand, Silt 1, Silt 2, and Silt 3 models
at the top of lower silt layer. The above phenomenon becomes less pronounced as the EPWP keeps increasing
from 15 seconds to 40 seconds. At 40 seconds, EPWP in
Silt 4 model is higher than that in Sand, Silt 1, Silt 2, and
Silt 3 models. For the dissipation of EPWP, it starts from
the bottom of soil stratum and proceeds upward. The
Sand model shows fastest dissipation. Silt 1, Silt 2, Silt 3,
and Silt 4 models show that the dissipation of EPWP is
slow beneath the silt layer. As a result, Silt 4 model has
the lowest dissipation rate, while the dissipation rate for
Silt 1 model is the same as that of Sand model for the
depth larger than 21 m and the dissipation rate for Silt 3
model is faster than that of Silt 2 for the depth larger than
19 m. After liquefaction (50 seconds to 80 seconds), the
trend of EPWP profiles of Silt 2, Silt 3, and Silt 4 is
similar to the post-liquefaction scheme predicted in [1].
5. Conclusion
The existence of silt intralayer in a sandy soil stratum will reduce the ground settlement. The excessive
pore water pressure above the silt layer will also become
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smaller than that in the regular sand stratum. However,
the pore water pressure beneath the silt layer will become
higher due to the impermeable character of silt layer.
This can be dangerous especially when it is happened in
the slope ground, because the water film will be produced during the motion and will remain even after the
motion stops, leading to sliding or lateral movement of
ground. Although the existence of more silt layers decreases the ground settlement furthermore, the pore water pressure will have slower dissipation.
References
[1] Kokusho, T. and Kojima, T., “Water Film in Liquefied
Sand and Its Effect on Lateral Spread,” Journal of
Geotechnical and Geoenvironmental Engineering,
ASCE, Vol. 125, No. 10, pp. 817-826 (1999).
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Foundation Bridge, Dissertation, Doctor of Philosophy, Department of Civil Engineering, National Central University, Jhongli, Taiwan (2000). (in Chinese)
[3] Pacheco, M. P., Altschaeffl, A. G. and Chameau, J. L.,
“Pore Pressure Prediction in Finite Element Analysis,”
International Journal for Numerical Methods in Engineering, Vol. 13, pp. 477-491 (1989).
[4] Zienkiewicz, O. C. and Shiomi, T., “Dynamic Behavior of Saturated Porous Media; the Generalized Biot
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Vol. 8, pp. 71-96 (1984).
[5] Lee, C. J., Wei, Y. C., Lien, H. C. and Chen, H. T.,
“Centrifuge Modeling on the Seismic Responses of
Sandy Deposit with a Thin Silt Seam,” 8th International Conference on Urban Earthquake Engineering,
Tokyo Institute of Technology, Tokyo, Japan (2011).
[6] Simatupang, R., A Numerical Investigation on Stone
Columns as a Countermeasure for Liquefaction of
Sandy Soil Stratum with Interlayers of Silt, Master
Thesis, Department of Civil Engineering, National
Central University, Jhongli, Taiwan (2011).
Manuscript Received: Nov. 12, 2012
Accepted: Jan. 20, 2013