A study towards collapse of pile supported bridge foundations in seismically liquefiable soils

Abstract

The behaviour of pile supported bridges in case of liquefaction during the earthquakes is not completely understood as can be seen from the failure of bridges during recent major earthquakes. It has been a recurring observation in most of the failure of pile supported bridges that the middle spans resting on the middle piers collapse, while the abutments and the piers close to them remain stable. Therefore, this study was carried out to investigate the mechanisms behind such midspan collapse of pile supported bridges in liquefiable soil deposits. Firstly, this thesis reports the simplified analytical expression developed to explain the midspan collapse of these bridges. It has been found that for the simply supported bridges, where each of the pier acts independently of the other, the natural period of the piers elongates in case of liquefaction due to increase in the unsupported length of the pile. Due to this process, the central piers of the bridge have higher natural period as compared to the adjacent ones, which in turn induces higher lateral displacement demand on the former. This phenomenon perpetuates differential lateral displacement for the adjacent piers. Hence, if enough seating length is not provided, the span may get unseated. Further, it has also been shown through the detailed case studies of collapse of around six bridges in various different earthquakes across the globe that this failure due to effects related to elongation of natural period of the piers can also make a bridge susceptible to fail in case of liquefaction, along with the other failure mechanisms. Further, it has been observed through the shake table tests that the natural frequency of the various pile supported piers of the bridge reduces during the course of liquefaction, with the central pier attaining the lowest natural frequency among all. Due to the increase in the flexibility of central pile owing to liquefaction, the maximum bending moment is observed at a shallower depth of pile, rather than at the interface of liquefied-nonliquefied soil. However, for the abutment piles, where the effect of lateral spreading is more as compared to any other piers, it has been found that the maximum bending moment is located at a section at the interface of liquefied-nonliquefied soil. Therefore, the design of piles for various bridge supports should be designed appropriately.