Ohashi Tunnel Work is to build 430m-long 12.65m in diameter twin tunnels, which constitute the southernmost section of Yamate Tunnel. The Yamate Tunnel, a 10km long highway tunnel on the Central Circular Route owned and operated by Metropolitan Expressway Company Ltd, has been constructed under the heavily developed Tokyo subsurface area. A 12.94m diameter slurry type TBM was used for this project.
Due to area’s surface configuration, subsurface structures, and the highway alignment, the tunneling work has the following technical difficulties to be overcome.
(1) The route of the tunnels was designed to be vertically parallel all the way (430m). The minimum distance of the tunnels is 1.45m. Construction schedule required the upper tunnel construction first.
(2) The TBM had to drive directly under the concrete culvert of subway and the foundations of highway viaduct at the 123.5m-radius sharp-curved section. The minimum clearance of the TBM and the structures is 3.1m. As a result, Finite Element Analyses as well as preliminary ground measurement for ground deformation were conducted prior to the TBM works to estimate the effect of the tunnel excavation. The result indicated that no significant impact was expected. During the tunneling work, real time monitoring data of the existing structures, including subway tunnels, highway foundations, and the upper tunnel, were transmitted to the site office in order to respond to any contingencies, if necessary.
(3) The 2100-ton TBM had to be lifted down vertically by 16m and to be made 180-degree U-turn at the arrival shaft. Fourteen 400-ton hydraulic jacks were set on the 3m high I-girders, which were mounted on the shaft wall. The stroke of each hydraulic jack was synchronized within a few millimeters to hold the TBM horizontally during the lift-down work. Ball sliders, boxes of ball bearings, were laid beneath the TBM cradle to reduce the friction.
Keywords: 12.65m in diameter twin tunnel, Slurry type TBM, Steel segment, Ductile segment.