1478
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
By comparing Figures 9 and 12, it can be concluded that the
peak quantities of graph inCase II have reached to values upper
than unity and also the increase in R
u
in this case happens a
little faster. This can be rlated to the effect of upper clay layer in
Case II which prevents the water to flow outside the ground due
to lower permeability. As a result, when a layer of clay is
present, liquefaction happens more rapidly and may extend to
deeper ground as well as shallower depths.
4.2
Pile bending moments
At the beginning of loading, it is observed that the maximum
value of bending moment occurs at shallower depths and as the
time passes, the place of maximum bending moment moves
downward due to reduction in soil strength after liquefaction.
Figure 15.Maximum bending moments in pile at base excitation
frequency of 2 Hz for Case I with clay layer thickness of 8 m (No
medium sand layer exists).
By comparing Figures 13 and 14, it is concluded that before
liquefaction occurs or at early times of liquefaction, maximum
bending moment for Case II is approximately 100% more than
that of Case I, but it has to be said that this large bending
moment is a resultoflow shear strength of clay sothat the pile
acts like a cantilever beam through this depth and therefore the
maximum bending moment increases.
Figure 16. Maximum bending moments in pile at base excitation
frequency of 5 Hz for Case I .
5 CONCLUSION
Figure 13. Maximum bending moments in pile at base excitation
frequency of 2 Hz for Case I
From Figures13 and 14, it is observed that after liquefaction
occurs, pile bending moment is increased up to 37% in Case II
in comparison to Case I. In Case II, if the first layer of 8 m is
completely constituted of clay, the bending moment is increased
45% in comparison to Case I. This shows that as it was
deducted in the previous section, clay layer can affect the
deeper sandy layer of soil to liquefy more intensely and as a
consequence of decrease in soil shear strength, the soil moves
more freely and exerts more pressure to pile. So in this case,
kinematic forces increase and cause the pile bending moments
to grow significantly.
- With increase of input motion frequency, liquefaction does not
happen at deeper layers in comparison to lower frequencies. As
a result,maximum pile bending moments after liquefaction are
decreased in this situation.
- As input motion frequency decreases, liquefaction occurs a
little faster.
- If a layer of clay is present over the sandy layer, due to low
permeability of clay, water drains at a lower late. So pore
pressure is built up under the clay layer and causes the sand to
liquefy more intensely.
- At presense of clay, pile bending moments after liquefaction
can increase up to 45% in comparison to Case I with no clay
layer.
6 REFERENCES
Elgamal A.,Yang Zh., Parra E., Ragheb A. 2003. Modeling of cyclic
mobility in saturated cohesionless soils.
International Journal of
Plasticity
19, 883-905.
Wilson D.W., 1998. Soil-Pile-Superstructure interaction in liquefying
sand and soft clay.
PhD dissertation
. University of California.
Ishihara K. 1997. Terzaghi oration: geotechnical aspects of the 1995
Kobe earthquake.
Proceedings of ICSMFE
. Hamburg. 2047-2073.
Figure 14. Maximum bending moments in pile at base excitation
frequency of 2 Hz for Case II with clay layer thickness of 4 m.
Yao Sh., Kobayashi K., Yoshida N., Matsuo H. 2004. Interactive
behavior of soil-pile-superstructure system in transient state to
liquefaction by means of larg shake table tests.
Soil dynamics and
Earthquake Engineering
24. 397-409.
When frequency is increased, it is shown in figure 16 that
pile bending moments decrease significantly and also the place
of maximum bending moment after liquefaction has moved
upward due to less liquefaction of deeper soil at lower
frequencies.
Rahmani A., Pak A. 2011. Dynamic behavior of pile foundations under
cyclic loading in liquefiable soils.
Computers and Geotechnics
40.
114-126.
Zienkiewicz O.C., Chan A.H.C., Pastor M., Schrefler B.A., Shiomi T.
1999. Computational geomechanics with special reference to
earthquake engineering,
John Wiley & Sons
.
Cheng Zh., Jeremic B., 2009, Numerical modeling and simulation of
pile in liquefiable soil,
Soil Dynamics and Earthquake Engineering
29. 1405-1416.
Ishihara K. 1996. Soil behavior in earthquake geotechnics.
Oxford
University Press Inc
. New York. 247-281.
