In urban cities, underground tunnels and stations for rapid transit systems are commonly buried underground. Compared with cantilever above-grade structures, underground stations have performed relatively well in past earthquakes. This may be attributed to the unique service conditions of underground stations- lateral support by surrounding soils. Station designers need to consider the interaction with surrounding soils for earthquake loading.
Seismic design methods for underground stations are different from that of cantilever above-grade structures. Shear distortion is typically believed to be the major concern for underground structures. Three methods are discussed here: the dynamic earth pressure methods, the free-field racking deformation method and the soil-structure interaction methods. The advantages and disadvantages of these methods are compared and summarized.
Two models were employed in designing the Canada Line Stations in Vancouver, British Columbia, Canada. Both of the models originated from the dynamic earth pressure methods with the consideration of shear racking deformation. The first model assumes that dynamic earth pressure is transferred to the base by a moment frame consisting of walls and slabs. The whole structure is assumed to sit at the base. This model was for structures built in complicated soil condition, such as a station half-buried in the bedrock and half buried in the soft soil. In the second model, free-field racking deformation is used to check major structural members and joints first. Then the slabs and walls are designed to sustain the dynamic and static soil pressures. Moment redistribution at joints is considered to minimize the wall and slab thickness. All soil pressures are assumed to be transferred to the other side by slabs and resisted by the passive pressure. The concept of this model is to produce a flexible structure to move together with seismic shear wave. This model can produce more economical design than the first model. Both models were used in designing different stations.
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