Load transfer mechanisms between underground structure and surrounding ground:: Evaluation of the failure of the Daikai Station

The Daikai Station, a cut and cover structure in the subway system in Kobe, collapsed during the Hyogoken-Nambu earthquake of January 17, 1995 in Japan. The Daikai Station is the first well-documented underground structure not crossing an active fault that has completely collapsed during an earthquake without liquefaction of the surrounding soil. What makes this case even more interesting is that tunnel sections adjacent to the station, with similar structural characteristics and analogous soil conditions, did not collapse. Dynamic numerical analyses have been conducted to investigate the load transfer mechanisms between the underground structure and the surrounding soil and to identify the causes for different behavior of similar sections of the station subjected to the same seismic loading. A hysteretic nonlinear soil model has been used for the analysis. The model captures well the soil's shear modulus degradation and the increase of damping with strain. The results from the analyses show that, for a given earthquake, there are two key factors that determine the response of an underground structure: the relative stiffness between the structure and the degraded surrounding ground, and the frictional characteristics of the interface. A stiff structure has small deformations; because the adjacent soil movement is restricted by the structure, the shear modulus degradation of the soil is limited which contributes to reduce further deformation of the soil and thus decreases the displacement demand on the structure. A strong interface is capable of transmitting larger shear to the structure but in turn increases the confinement of the soil surrounding the structure which limits the soil's shear modulus degradation. The model predicts larger deformations in the section that collapsed because this section had a smaller stiffness, and thus triggered drifts in critical structural elements which were larger than at other sections of the station which remained stable.