Seismic responses of a large underground structure in liquefied soils by FEM numerical modelling

A large underground subway structure could be severely damaged in a strong earthquake, such as the seismic damages of Daikai subway station in the 1995 Kobe earthquake. After the 1995 Kobe earthquake, the effects of earthquakes on underground structure became a hot topic. According to the seismic damage characteristics of the pipelines buried in the liquefiable soils, a large underground structure may be damaged ever more severely by the liquefaction. To examine the dynamic properties of saturated liquefiable soil, an existed constitutive model is revised and implanted into the commercial FEM software. Then the nonlinear seismic responses of large underground subway structures built in liquefiable soils are analyzed by a numerical modeling. The results of the comprehensive numerical analysis indicate that the existing subway station has a significant effect on the liquefaction of the nearby soils that are likely to be liquefied. The around sand soils under the suggested depth 20 m may be also liquefied under the influence of the large subway station, which should be non-liquefiable sand in many related codes of China. The subway station floats up as soon as the nearby soils are liquefied, and the soils accordingly flow from the lateral foundation to the bottom foundation of the subway station. It is also found that the floating of the subway station is completely out of sync in the main vibration stage of the inputted ground motions and the rising stage of the pore pressure, and it also lags behind in the main vibration stage for a considerable time and begins to stabilize only when the vibration weakens sharply and is close to zero. Generally, the dynamic softening soils under the subway station also have great effect on the seismic response of the large subway station.