A shake table investigation of dynamic behavior of pile supported bridges in liquefiable soil deposits

Bridges are a part of vital infrastructure, which should operate even after a disaster to keep emergency services running. There have been numerous bridge failures during major past earthquakes due to liquefaction. Among other categories of failures, mid span collapse(without the failure of abutments) of pile supported bridges founded in liquefiable deposits are still observed even in most recent earthquakes. This mechanism of collapse is attributed to the effects related to the differential elongation of natural period of the individual piers during liquefaction. A shake table investigation has been carried out in this study to verify mechanisms behind midspan collapse of pile supported bridges in liquefiable deposits. In this investigation, a typical pile supported bridge is scaled down, and its foundations pass through the liquefiable loose sandy soil and rest in a dense gravel layer. White noise motions of increasing acceleration magnitude have been applied to initiate progressive liquefaction and to characterize the dynamic features of the bridge. It has been found that as the liquefaction of the soil sets in, the natural frequency of individual bridge support is reduced, with the highest reduction occurring near the central spans. As a result, there is differential lateral displacement and bending moment demand on the piles. It has also been observed that for the central pile, the maximum bending moment in the pile will occur at a higher elevation, as compared to that of the interface of soils of varied stiffness, unlike the abutment piles. The practical implications of this research are also highlighted.