Abstract

This paper presents the results of numerical analysis carried out using PLAXIS 3D on behaviour of rock-socketed pile subjected to independent and combined loading. The numerical procedure adopted in the analysis was validated by comparing the load test results reported in the literature. After validation of the numerical analyses, the parametric analysis were performed on the rock-socketed pile subjected to independent loading and combined loading for different rock conditions, soil cover depths and socketing lengths. From the results of parametric study, it is found that the vertical and lateral load capacities under combined loading are not significantly affected, when the soil cover depth is high. However, if the soil cover depth is low, then the behaviour of rock-socketed pile under combined loading is found to be significantly different, compared to its behaviour under independent loading. It is also seen that the rock conditions and socketing length have profound effect on pile behaviour under combined loading.

1. Introduction

In the present construction activities, large diameter bored piles are being used to carry heavy loads from super structures. These heavy loads are essentially to be taken to bed rock level and socketed into rock, when top soil is weak. In the piling specifications, socketing in solid/hard rock is usually specified for termination criteria. But, in many cases, the depth at which hard rock strata is available is very large such that the termination of pile upto that level becomes uneconomical. Therefore, the capacity of pile needs to be increased by utilizing the shaft resistance provided by the soft rocks. Many researchers [1–9] have studied the load transfer process in rock-socketed piles through model and field tests and theoretical analysis.

Many designers prefer to design drilled shafts to take load in side shear only rather than combined side shear and end bearing because the amount of movement required to mobilize side shear is relatively small, while that required to mobilize end bearing is relatively large [10]. Both empirical and analytical methods have been used to predict the ultimate unit side shear of rock sockets [5]. Empirical methods are generally based on full-scale load tests in which the ultimate unit side shear is back-calculated from instrumentation. The ultimate unit side shear (fmax) is then related to the unconfined compressive strength of the soil/rock (qu) using empirical constants, usually denoted α, β and c as

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