Effect of Earthquake Induced Lateral Soil Movement On Piles in A Sloping Ground
Effect of Earthquake Induced Lateral Soil Movement On Piles in A Sloping Ground
Effect of Earthquake Induced Lateral Soil Movement On Piles in A Sloping Ground
ABSTRACT
The performance of piles in liquefying ground under earthquake loading is a complex problem due to the effects
of a progressive build-up of pore water pressures in the saturated soils. The loss of soil strength and stiffness due
to liquefaction may develop large bending moments and shear forces in piles, possibly leading to pile damage.
The significance of liquefaction-related damage to pile foundations has been clearly demonstrated by the major
earthquakes that have occurred during past years. The present investigation is to find out the effect of earthquake
induced lateral soil movement on piles in sloping ground. The present study was carried out by numerically. In
this, 1995 Kobe earthquake data (Japan) is used. Parametric study has been done on the same model by varying
slope in the soil layers and L/D ratio of the pile. The dynamic analysis was carried out for slope angle of 1V:1.5H
in with L/D=16, L/D=25 & L/D=33. In each case, bending moment and displacement variation with depth of the
pile is noticed. Based on the study, it is concluded that for a constant slope and constant depth of liquefiable layer,
lateral displacement and bending moment is significantly increased for L/D=16 when compared to higher L/D
ratio’s of 25 and 33. However, further increase in L/D ratio is not having any significant effect in the lateral
displacement.
these approximations depends on the modeller’s ability to Table 1: Soil Properties (Adopted from Abdoum
portray what is happening in the field. Often the problem and Dobry, 2002)
being modelled is complex and has to be simplified to obtain Nevada Sand Slightly Cemented
Description
a solution. Two of the major factors which have a vast (Dr=40%) Sand
impact on both the real and model piles are; (1) the Density (kN/m3) 25 28
constitutive properties of the soils and (2) the soil– structure Φ 32 35
interaction at the interface over the structural surface. Ψ 2 5
k (m/s) 6.05 × 10-5 3.197 × 10-5
Constitutive Models µ 0.25 0.3
Finite element method has become more popular as a soil E (kN/m2) 38000 49000
response prediction tool. This has led to increased pressure
(flexural rigidity) and axial stiffness EA (where E is the
on researchers to develop more comprehensive descriptions
Young’s modulus, I is moment of inertia and A is the cross
for soil behaviour, which in turn leads to more complex
sectional area of the pile) are the input values. The values
constitutive relationship. Prevost and Popescu state that
of pile properties are presented in Table 2.
for a constitutive model to be satisfactory it must be able
to: (1) Define the material behaviour for all stress and strain Table 2: Pile Property (Adopted from Abdoum and
paths; (2) identify model parameters by means of standard Dobry, 2002)
material tests; and (3) Physically represent the material Material Concrete
response to changes in applied stress or strain. Previous Diameter (m) 0.6
EA (kN) 3.56 × 105
studies have explored constitutive models and found that
EI (kNm2) 8000
the use of isotropic models such as elasto-plastic Mohr–
Coulomb and Drucker-Prager models are sufficiently Boundary Conditions
accurate. In the past, linear elastic constitutive models have An absorbent boundary is aimed to absorb the increments
been commonly used in developing pile design methods. of stresses on the boundaries caused by dynamic loading,
that otherwise would be reflected inside the soil body. In
3. PLANE STRAIN ANALYSIS USING PLAXIS 2D
PLAXIS absorbent boundaries are generated by selecting
Description of Approach the standard absorbent boundaries. For plane strain models,
For this study, the model tests are analyzed using a plane the standard absorbent boundaries are generated at the left,
strain finite element approach. Plane strain analysis is the the right and the bottom boundary. The applied load is the
most straightforward of the finite element approaches product of the input value and the corresponding load
described above, and allows good representation of the pile multiplier.
group configuration and geometry, without being unduly
Calculations
complicated. In this study, a two dimensional finite element
program PLAXIS 2D has been used to model the soil layers The calculation involves two phases. The first one is a
and the pile using the concept of plane strain condition. plastic calculation in which the pile is activated and the
The model created is shown in Figure 1. The analyses are soil above the slope is deactivated. The prescribed
conducted with three layers of sand, represented by Mohr- displacement is activated while defining the calculation
Coulomb model. The Mohr-Coulomb model is used for the phases. The second is a dynamic analysis in which the
earthquake is simulated. In order to analysis the effect of
the earthquake in detail the displacements are reset to zero
at the beginning of this phase. Earthquakes are usually
0.04
0.03
0.02
Acceleration (m/sec )
2
0.01
0
0 2 4 6 8 10 12 14 16
-0.01
Time (sec.)
governed by an associated flow rule. The values of soil -0.04
applied by means of prescribed horizontal displacements. passive failure of the top nonliquefiable layer against the
In the present analysis, the prescribed horizontal pile, while the bending moments near the bottom increased
displacement is given as 0.01m. In the present earthquake monotonically and never decreased, as the bottom
analysis, the Kobe earthquake acceleration data (Figure 2) nonliquefiable layer did not fail.
is used. A time interval of 25 sec is used for the calculation. The shapes of the bending moment profiles indicate
Then parametric study is done on the model by providing that the deformed shape of the pile had a double curvature
a slope of 1V:1.5H in the first layer alone. Bending moment caused by the top and bottom soil layers loading the pile in
and displacement variation of the pile is obtained for all opposite directions. The double curvature shape indicates
the cases. Again the same procedure is followed for L/D that the nonliquefied shallow layer pushed the pile laterally
ratio 16, 25 and 33. resisting this bending action. Both the passive failure of
the top layer and the moment concentrations at the top and
4. RESULTS AND DISCUSSION
bottom boundaries of the liquefied layer indicated by the
For validation, the bending moment for a pile obtained figures are consistent with the experience from earthquake
using FEM model is compared with the results of a similar case histories. Another interesting aspect is that the bending
study which is done by Abdoun and Dobry (2002), using moments vary linearly within the liquefied layer, suggesting
centrifuge modelling. A model with three layers of soil and that they are essentially controlled by the loading of the
a pile is created. Slightly cemented sand is used in top and top and bottom layers, with the pressure of the liquefied
bottom layers. Middle layer is Nevada sand (Dr=40%) soil being negligible.
which is the liquefiable layer. The model created for the
analysis, is validated by Abdoun and Dobry (2002). Effect of L/D Ratio on Lateral Pile Behaviour Under
Earthquake
Figure 5 shows the variation of displacement with the L/D
ratio when a slope of 1V:3H is provided in the first layer
(slightly cemented sand) alone. Displacement reduces with
increase in L/D ratio. When L/D ratio is increased from 16
to 33, the decrease in displacement is by 70.98%. In the
case of L/D=16, it can be seen that the displacement is
high when compared to other cases. This is because only
bottom 2m of the pile is in the slightly cemented sand and
hence the anchorage of the pile in the non-liquefiable layer
Fig. 3: Deformed Mesh is less when compared to the other cases.
The results obtained using FEM modelling is compared
with the results given by Abdoun and Dobry (2002). Figure
3 shows the deformed mesh. Graphs are plotted with
bending moment on x-axis and depth of the pile on y-axis.
Figure 4 shows the variation of bending moment with the
depth of the pile. The results are captured after 25sec of
the application of earthquake loading, which are
corresponding to maximum values.
the cases of L/D=16 and L/D=25 is near to the boundary the pile when slope is introduced in the first layer alone
between top slightly cemented sand layer and the liquefiable (slightly cemented sand). The maximum bending moment
Nevada sand layer whereas in the case of L/D=33, the point reduces with increase in L/D ratio. In all the cases,
where this shift occurs is at the boundary between maximum bending moment occur for the slope 1V:1.5H.
liquefiable Nevada sand layer and bottom slightly cemented When L/D ratio is increased from 16 to 33, the bending
sand layer. moment has got reduced by 277.3% for the slope 1V:3H.
When L/D ratio is increased from 16 to 33, the bending
moment has got reduced by 263.19% for the slope 1V:1.5H.
Fig. 6: Depth of the Pile Vs Bending Moment Fig. 8: Max. Bending Moment Vs Depth of the Pile