Effect of Ground Slope on the Dynamic Response of Piles Group in Liquefiable Soil based on 3D Numerical Simulation

1.Introduction

During the past earthquakes, liquefaction and the resulting deformations have caused significant damage to the deep foundations of bridges, ports, offshore structures and buildings that these damages have been more severe in mildly sloping grounds due to lateral spreading-induced liquefaction. Lateral spreading-induced soil liquefaction, has imposed significant damage to the deep foundations of bridges, ports, offshore structures and buildings. The behavior of piles in liquefied soil has been investigated by various researchers using field observations, large-scale (1-g) shake table tests, centrifuge tests as well as numerical simulations. Despite various experimental, numerical and field studies by previous researchers, there is also no comprehensive approach to assessing the effects of lateral spreading on pile groups. On the other hand, numerical simulations are an economical tool for investigating and a means of representing the seismic performance of the pile groups at sites with liquefaction-induced lateral spreading.
The main purpose of this study is to evaluate the effect of various pile groups (e.g., 1×1, 2×2 and 3×3) on reducing the potential for liquefaction during earthquake are investigated parametrically, applying three-dimensional finite element (FE) simulations using OpenSees software. To examine the ground inclination angle and array of pile groups' effects, different models have been subjected to the El Centro earthquake (1940). This study evaluates the effect of each of these factors on soil acceleration, lateral displacement, excess pore pressure and piles bending moment. The numerical model has been verified and calibrated in the literature through analysis of a well-documented large-scale (1-g) shake-table test.

2.Numerical Simulations

To gain insight into the effect of ground inclination angle on various pile groups in 10-m-thick mildly inclined liquefiable soil above the bedrock (Figure 5). The physical and mechanical properties of the soil layers and the pile respectively, are presented in Tables (1) and (2). Also, all the models in this study have been subjected to the El Centro earthquake (1940) (shown in Figure 9) with 0.15 g scaled peak ground acceleration.

3.Results and discussions

To investigate the effect of the ground inclination angle on the generation and dissipation of pore water pressure, the time history of excess pore water pressure for various pile groups in depths of 6 m is shown in Figure (18). According to Figure (18), as the ground inclination angle increases, the excess pore water pressure disappears sooner, which is due to the increasing effect of the dilatancy phenomenon.
Figure (20) shows time history of the pile head lateral displacement for different ground of the slopes (a_f = 0° to 6°) in the pile group 1×1- 2×2 - 3×3. According to Figure (20) in the initial seconds of the excitation due to the lack of soil liquefaction, the ground of the slope has a little effect on the lateral displacement of the piles but with the occurrence of liquefaction and reduction of soil shear strength, it is observed that with increasing slope, the lateral displacement of piles increases severely. It is also observed that in the horizontal model, the maximum lateral displacement of the pile head occurs in about 2 seconds, which corresponds to the maximum acceleration time of the El Centro earthquake. While in mildly sloping ground due to the increase of lateral pressure from the soil, the maximum deformation occurs at the end of the earthquake and causes permanent displacement in the piles.

4.Conclusions

The main important conclusions drawn from present study are as follows:

1. Based on the results, with increasing the ground slope angle in a specified point of the soil, less pore water pressure is produced and the dissipation of pore water pressure starts earlier, but the variations of pore water pressure increased.
2. In mildly sloping ground, the amount of pore water pressure in downslope is less than upslope ground and also with increasing ground slope angle, the rate fluctuations of excess pore water pressure increased in the down-slope of the pile group. This behavior is due to the high displacement downslope soil relative to upslope soil in the pile group.
3. In mildly sloping ground, despite the reduction of pore water pressure, lateral displacement of piles and soil severely increased. The reason for this contradiction due to the direction of static shear stress is parallel to the direction of soil slope. This shear stress is due to the weight of the soil mass, which increases the displacement of piles and soil in mildly sloping ground.

At the last, it should be noted that, the ground slope angle is a very effective parameter in the lateral and vertical displacement of piles that should be considered in design code.