CENTRIFUGE STUDIES ON EARTHQUAKE RESPONSE OF PILE FOUNDATIONS IN CLAY

Abstract

Pile foundations are widely used for both inland and offshore structures, especially under adverse soil conditions. Of particular interest is the case of bridges which are structurally simple, yet highly response-sensitive to soil-structure interaction effects. The performance of these structural systems under seismic excitations has been the subject of considerable attention, particularly after the numerous failures of pile-supported bridges during recent major earthquakes. In this thesis, dynamic responses of pile foundations on soft ground have been studied. A series of dynamic centrifuge tests were performed on the model piles placed in a soft kaolin bed in order to study the interference of the vibration of piles on the free-field ground motion. This is almost an initial attempt to predict dynamic centrifuge modeling for pile foundations in clay. The experimental investigations have been carried out at Geotechnical Centrifuge Laboratory of Centre for Soft Soil Engineering of National University of Singapore under a joint venture of Indian Institute of Technology, Roorkee and National University of Singapore. The models in the laminar shaking box were consolidated in 50g gravitational field, created by spinning the centrifuge arm, to achieve prototype stress levels in soil. A series of past earthquake time histories had been fired to the model in high g and output signals were processed using DAISYLAB version 3.0. The investigation revealed the soil-pile interaction effects during earthquakes obtaining response at different positions in the soil and model piles. The investigation was further extended to study the effect of super structural load on the response at the pile top. The vibration was found to be deamplified at the pile top in presence of single pile acted as reinforcing bars in soil with negligible imposed load. However, for all practical cases, the pile foundation resting on clayey layers found to amplify response of super structure under earthquake excitation. For repetitive excitations, Young's modulus of soil decreases increasing damping ratio due to early yielding and results in no clear peak in the response spectrum at the surface and mid depth of the soil model.