In the site of Iidabashi Station of the Metropolitan Oedo Subway Line, a staircase incline 40m long, linking the station premises with the surface, was constructed by a tunneling method other than the cut-and-cover technique, because it is located under a road with heavy traffic and numerous underground structures. The tunnel, accommodating stairs and elevators, has a great incline of 30 degrees. It was therefore impossible to perform the pipe jacking or shield tunneling, and consequently the mountain tunneling method (NATM) was adopted. In view of the unconsolidated ground, high groundwater level, and small earth covering of 8 to 15 m, chemical grouting, vertical reinforcement bolting and forepoling implemented as auxiliary methods. The large egg-shaped section was of about 70 m~2, that corresponds to a two-lane motorway tunnel, was divided into four portions for ensuring face stability. Under the stringent control by measurement of displacements, the construction was completed without exerting adverse impacts on the surroundings.
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