LIQUEFACTION-INDUCED DOWNDRAG ON PILES AND DRILLED SHAFTS
Noel Vijayaruban, Vijayathasan
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The first part of the study presents an analytical method based on the neutral plane concept, for handling multiple liquefiable layers to account for liquefaction-induced downdrag in piles and drilled shafts. It assumes that the pile settlement equals the soil settlement at the neutral plane and that the load transfer during liquefaction within the liquefied layer is nearly zero. This method is illustrated with a case history from the 2010 Maule 8.8 Magnitude Earthquake in Chile. The study uses four representative locations along the bridge, where analysis of borehole records identified four distinct liquefiable zones within and below the pier embedment depth. The drag force, downdrag, and neutral plane depth were computed. The SPT N-indices available from the post-quake borehole records are applied to determine the potential axial load distribution before and after the liquefaction event. It was determined that the liquefiable zones (close to and deeper than the pier toe depth) contributed more to the downdrag or were the cause of the catastrophic failures of a few pier foundations at the site. This is due to loss of strength and volumetric compression below the pile toe depth as a result of the earthquake event.The second part of the study involves the numerical evaluation of drag force and downdrag and validation of the assumptions made in the above analytical procedure. Numerical models for the soil and pile were developed in the OpenSees finite element software and subjected to an earthquake excitation. The changes in effective stress and soil settlement as a result of the earthquake excitation and dissipation of excess pore pressure were incorporated in the pile model to develop axial load distribution and estimate downdrag. It is found that the settlements of pile and soil are same at the neutral plane location and that the load transfer during liquefaction within the liquefied layer is near zero. An appreciable soil settlement within the pile foundation due to liquefaction and strong shaking makes the entire pile foundation subject to drag force. This situation brings down the neutral plane close to toe level. The liquefaction-induced downdrag seems to be limited.