Numerical Simulation Of Seismic Response Of Liquefiable Site And Bridge Pile Foundations

Sandy soil would have occurred liquefaction under earthquake. Ground deformation induced by liquefaction is a hot and difficult issue in the geotechnical field. Ground deformation induced by liquefaction is the main reason for pile damage in the bridge foundation. Understanding the deformation and failure mechanism of pile foundation under liquefaction process can prevent and reduce the damage of bridges caused by liquefaction.There exist multi-layer saturated liquefiable sandy soils in the site of the being-constructed Shuhe bridge, with the depth of sand layer exceeding 60m. It is necessary to evaluate the liquefaction potential of the deep liquefiable sandy soil for the construction of pile foundations of critical bridge project. Hence, undisturbed sand samples were taken at the depth of over 20m in a borehole, which was justified as severe liquefaction according to Seismic Code of Highway Bridges in China, and dynamic triaxial test was conducted for the sand samples. The liquefaction potential of sandy soil was estimated in combination of seismic response analysis of site soils. In addition, based on PL-Finn constitutive model, the liquefaction process was analyzed for the site centered around the abovementioned borehole, by using FLAC3D, to simulate the area of the liquefied soil and large soil displacement due to liquefaction. The numerical simulations indicate that PL-Finn model can better demonstrate the variation of pore water pressure, liquefied zone distribution as well as the large displacement of site soils due to liquefaction. The results by both approaches indicated that the liquefaction for the sandy soils with depth exceeding 20m was not observed. However, much attention needs to be paid for the effect of the large displacement of site soils due to the seismic liquefaction on the bridge pile foundation.To study the impact of soil liquefaction on the pile, the model of pile-soil system of the bridge is developed by using FLAC3D. Numerical simulations show that the liquefied zone with the pile would decrease slightly in comparation the case without the pile. Considering the deformation increase and stiffness reduction of soil by liquefaction, the control section for flexural analysis of the pier-pile system is not at the bottom of the pier, but at the bottom of the liquefied area. The strength checks of pile shaft indicate that the moment demand of the pile under the design earthquake leve is smaller than the flexural strength.