LATERAL CAPACITY OF PILES IN LIQUEFIABLE SOILS

Abstract

The performance of piles in liquefying ground under earthquake loading is a complex phenomenon.. The process of generation of excess pore water pressure in saturated sand during earthquakes causes loss of shear strength of the ground. Many buildings and transportation facilities supported by deep foundations were damaged due to the loss of bearing capacity and excessive settlement during the 1964 Niigata and 1995 Kobe earthquakes. Studies of seismic loadings on pile foundations and liquefaction phenomenon have been performed extensively in the last four decades. However, the combined problem of .seismic behaviour of piles in liquefiable soil has received relatively less attention, especially the effect of liquefaction on lateral capacity. The present study deals with developing a simplified procedure to evaluate the lateral capacity of piles in liquefiable soils from commonly available field test data (SPT). The excess pore pressure generated due to liquefaction is determined using the method proposed by Seed, Martin and Lysmer. The lateral capacity is then determined using spring model (based on Winkler's hypothesis) incorporating the effect of excess pore pressure on soil stiffness. Layered soil system, ground water level, the earthquake magnitude and duration of earthquake shaking are accounted. Analysis for lateral load on pile foundation is carried out using `Finite Difference' scheme. A computer code in C++ has been developed for this purpose. Numerical results have been obtained for typical problems using the package and the effect of normal ground conditions on the pile behaviour is brought out. It has been observed that the lateral capacity of a single pile in liquefiable soil reduces drastically as compared to static case. Using Equivalent Cantilever Approach, design charts are developed to determine the pile head deflection for free and fixed head, fully embedded piles in homogenous soil deposits prone to liquefaction.