Large-scale improvement work of subway stations in soft ground

Tokyo Metro Tozai Line runs through the center of Tokyo, from the western to eastern part of it. It was opened for service in 1969. Because of recent population explosion along the railway line, ridership during morning rush hours has risen at least 200%, resulting in chronic train delays. Moreover, passengers on the platform are exposed to increased risk of contact with the cars or falling. To reduce the congestion and ensure safety and improve service on the line, various efforts are carried out, such as improvement of stations. The structures of two stations have been improved with large-scale ground improvement works and rigid diaphragm walls, because the stations are located in very soft ground. This paper describes outline of the works, construction situation and results of influence on surroundings, such as heave of existing structure and deformation of railway track.