C. F. Leung

Behavior of Pile Groups Subject to Excavation-Induced Soil Movement in Very Soft Clay

A series of centrifuge model tests was conducted to investigate the behavior of pile groups of various sizes and configurations behind a retaining wall in very soft clay. With a 1.2-m excavation in front of the wall, which may simulate the initial stage of an excavation prior to strutting, the test results reveal that the induced bending moment on an individual pile in a free-head pile group is always smaller than that on a corresponding single pile located at the same distance behind the wall. This is attributed to the shadowing and reinforcing effects of other piles within the group. The degree of shadowing experienced by a pile depends on its relative position in the pile group. With a capped-head pile group, the individual piles are forced to interact in unison though subjected to different magnitudes of soil movement. Thus, despite being subjected to a larger soil movement, the induced bending moment on the front piles is moderated by the rear piles through the pile cap. A finite element program developed at the National University of Singapore is employed to back-analyze the centrifuge test data. The program gives a reasonably good prediction of the induced pile bending moments provided an appropriate modification factor is applied for the free-field soil movement and the amount of restraint provided by the pile cap is properly accounted for. The modification factor applied to the free-field soil movement accounts the reinforcing effect of the piles on the soil movement.

Severe Damage of a Pile Group due to Slope Failure

AbstractA pile group consisting of four cast-in-place concrete piles was instrumented to measure the induced bending moment along the piles due to excavation of an adjacent slope. The green field lateral soil movement profiles and the lateral pile deflection profiles were monitored by in-soil and in-pile inclinometers, respectively. The unexpected early arrival of a rain storm prior to the year-end monsoon caused the failure of the slope, which resulted in the severe damage of this pile group located at the slope crest. This paper examines the prefailure and postfailure behavior of the pile group and it is demonstrated that considering the uncracked and cracked bending stiffness of the piles is vital when evaluating the progressive damage of the pile group due to slope failure.