DESIGN AND CONSTRUCTION OF LARGE-SCALE OPEN-CUT TUNNEL FOR THE METROPOLITAN EXPRESSWAY

About 30 kilometres north of Tokyo's central district of Nihonbashi is a part of Saitama Prefecture that includes Urawa City, Omiya City, and Yono City. There, work is progressing on the construction of a business district called the Saitama New Urban Center. This complex project includes the construction of the underground OE26(1) section of the Saitama Omiya Line of the Metropolitan Expressway. This line will link the New Urban Center with surrounding roads. Located under the Saitama New Urban Center district, the OE26(1) was constructed by the open-cut tunnel method accompanied by large-scale excavation up to a depth of 34 m, a width of 34 m, and a section length of 100 m. Excavation was executed using an earth-retaining wall constructed by the cast-in-place pile wall method known as the soil mixing wall(SMW). The 850-mm diameter SMW is 53.5 to 57.0 m long, making it the longest wall of its kind in Japan. Inside the tunnel, the box-shaped rigid-frame tunnel section changes from a double-deck shape to a parallel structure, resulting in a non-uniform cross section with a level difference of 10 m between the eastbound and westbound lanes. An earth-retaining wall structure that employed large-scale support was introduced to handle this difference. To prevent the excavation bed from heaving, the groundwater level was lowered using the deep well method. In-situ pumping and recharging tests were performed to prepare the recharging work done to return the pumped water to the aquifer. This report describes the design and execution of construction work on this large-scale excavation tunnel for the Metropolitan Expressway. (a) For the covering entry of this conference, please see ITRD abstract no. E204151.