A challenge of an ultra-large section tunnel project in unconsolidated ground - design and construction of the Minoh toll road tunnel -

Abstract

Minoh toll road, part of the bypass for National Highway No. 423, is a northward extension of the Shin-Midosuji highway, which runs from central Osaka to the north of the city. In the future, it will be connected to the Second Meishin Expressway to be part of the broad network of highways. The Minoh toll road tunnel is a mountain tunnel about 5.6 km long that goes through the Hokusetsu Mountains (200-600 m in altitude) in the northern part of Osaka Prefecture. The southern sector of this tunnel discussed in this paper is approximately 2.2 km long. The geology around the southern entrance is unconsolidated ground characterized by gravel, sand and clay of the Osaka Group. Beyond this region, heavily fractured shale of the Tanba Zone is found. In addition to these adverse conditions, the entry and exit ramps must be constructed inside the tunnel because of the road location and alignment. This requires planning of a huge tunnel having an excavation section more than 300 m2. The challenging project is now being performed under adverse conditions Before starting the excavation of the main tunnel, a working tunnel (excavation section 41 m2, 347 m long) was parallelly driven to the north. In the Osaka Group and the heavily fractured zone of the Tanba Zone, the face was unstable, causing a large face extrusion, failure and significant deformations of the ground. This behavior made the tunneling very difficult. Considering the construction of the working tunnel, and the records of the main tunnel excavated toward the south from the crossing between working tunnel and main tunnel (about 350 m to the north from the south portal), the design and construction of the ultra-large section tunnel in unconsolidated ground were reviewed in detail. The sector with an excavation section exceeding 300 m2 was initially designed with the multiple side drift method. However, as a result of comparative study with other methods such as side drift and top heading, the side drift method was finally selected. Additional stability is achieved with a 800 mm-diameter pipe roof (slurry) in the largest section sector and for the preceding sector having a 260 m2 section, long forepiles and glass fiber reinforced plastic tubes were driven. This paper deals with the design and construction of the ultra-large section tunnel in the adverse geological conditions. It refers to the verification of the optimized design and establishment of the work management criteria, backed up with the numerical analysis and with the feedback of work records and measurement data obtained in the working tunnel and main tunnel. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.