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  The Project Which Uses an Innovative Construction Method to Expand an Operating Subway Tunnel Built by the Caisson Method under the Soft Ground

  This project involves renovating Minami-sunamachi station on the Tokyo Metro Tozai Line, by converting a single island platform with two tracks into two island platforms with three tracks to eliminate train delays and alleviate congestion during rush hours. Moreover, station facilities, escalator and elevator devices are also relocated to improve passenger flow. This Station was constructed by the caisson method in what was a canal at the time of construction; even now, the box elements have been surrounded with quite soft ground (N = 0-1). Thus, countermeasures against displacement of the ground and box elements must be considered for each process of construction work which is implemented by the cut-and-cover method. So, RC diaphragm walls are installed as retaining wall which compose permanent sidewalls, and a ground improvement is performed below the bottom of the excavation to build the new lower slabs beneath the existing box elements. This paper reports on the construction of the new lower slabs directly beneath the existing box elements by high-flow non-shrinkage concrete.

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