Seismic response of large underground structures in liquefiable soils subjected to horizontal and vertical earthquake excitations

The seismic performance of large underground structures in soils, e.g., subway and highway tunnels, has been an important topic followed the damages of such structures in recent large earthquakes. The dynamic behavior of a subway station in saturated sandy deposit was investigated using the fully coupled dynamic Finite Element code DYNA Swandyne-II. A generalized plasticity model that can simulate both cyclic liquefaction and pressure dependency of soils was incorporated in the program to model the sandy deposit. A Mindlin beam element was also included and used to model the underground structure. The effects of vertical earthquake motions and the buried depth of the underground structure were analyzed. It is found that the effects of vertical motions depended on the characteristics of the excitations. It is also found that the increase in buried depth improved the safety of the underground structure against earthquake damage. A mitigation method against the floatation of underground structures using injection grouting was also studied and it was proved to be effective.