DAMAGE TO DAIKAI SUBWAY STATION

A detailed Reconnaissance survey was conducted at the Daikai subway station which is the first subway structure that completely collapsed due to the earthquake. The station is a box frame structure with columns at the center, measuring 17 m wide and 7.17 m high in the exterior dimensions, and 120 m long. A complete collapse occurred in more than half of the center columns, which resulted in the failure and collapse of the ceiling slab and subsidence of subsoils over the station by more than 2.5 m at maximum. Many diagonal cracks were also observed on the walls in the transverse direction.Judging from the damage pattern, a strong horizontal force was imposed on the structure from the surrounding subsoils below the ground surface. The relative displacement of the surrounding soils between the ceiling level and base level during the earthquake and the inertia force of the overburden soil transferred to the ceiling slab are supposed to cause the damage. Evidence of damage due to vertical force was not clearly observed.