The undrained seismic response of the Daikai Station

Abstract

The Daikai Station failed during the 1995 Kobe earthquake. Detailed analyses of the seismic response of the station had been conducted under the assumption of a drained response of the ground; that is, no excess pore pressure accumulate during the cycles of loading. Such assumption is supported by the location of the water table around the station, which was depressed, compared to locations far from the station, because the underground structure worked as a drain. The question that arises is whether such effect was beneficial or detrimental. In other words, what would have been the undrained response of the station during the earthquake? To that effect, a hyperbolic elastoplastic constitutive model has been developed to simulate the drained and undrained seismic response of geomaterials under small to moderate strains. The model captures the coupling between the shear and volumetric response of the ground, which produces compression volumetric strains during drained cyclic loading or positive excess pore pressures accumulation during undrained cyclic loading. The hysteretic behavior is controlled by variable scaling factors that simulate softening or hardening during cyclic loading. The dependence of the shear modulus for very small strains on the effective mean stress is considered. Accurate performance of the model is observed when comparing simulations with results from cyclic simple shear laboratory tests for drained and undrained loading, and from a centrifuge test on saturated sand with excess pore pressures generation. Model capabilities to evaluate soil-structure interaction are verified by comparing results of a centrifuge seismic test of a tunnel placed in sand. The model is used to evaluate the response of the Daikai station and shows the importance of using appropriate constitutive models and loading conditions (drained or undrained) to approximate the seismic behavior of underground structures. Important differences are seen in the tunnel distortions, depending on the drainage condition.