Yotei Tunnel is the tunnel in the Hokkaido Shinkansen (bullet train line from Shin-Hakodate-Hokuto to Sapporo) running between Niseko town and Kutchan town (Abuta District, Hokkaido) and 9,750 m in length.
Yotei Tunnel is surrounded by Yotei Volcano on the east side and Niseko Volcanic Group on the northwest side. The maximum overburden is approximately 57 meters and the excavating ground is mainly composed of debris-avalanche deposits which means volcanic deposits produced by the orogenic movements and the eruptive activity from Yotei Volcano. This unconsolidated and sandy or silty layer sometimes contains high compressive strength boulders formed by andesite over 1 meter in diameter. Furthermore, from the additional survey, we estimate that the groundwater level for the entire length almost reaches the ground surface. Therefore, with NATM under high hydraulic pressure and unconsolidated ground conditions, there are risks of tunnel collapse and drought exerted by a lot of spring water from the tunnel cutting face. As a measure for these risks, a casting support tunneling system using TBM (SENS) is adopted for Yotei Tunnel because there are tunneling excavation cases under unconsolidated ground conditions. However, it is important to discuss the countermeasures for the high hydraulic pressure and high compressive strength boulders because there are few cases under those conditions.
This paper discusses the geological survey around Yotei Tunnel and the examination of construction methods for high hydraulic pressure and boulders. Afterward the research introduces the present work in the section of Yotei Tunnel in Sapporo side (Yotei Tunnel Hirafu Section).