The KITANOMINE tunnel (2928 m, Japan) is a long bored tunnel, which is notable for embracing a 550 m long watertight section. The north part of the tunnel is surrounded by squeezing mudstone, which became heavily multiple fractured and then clay-converted as the tunnel advanced. The deterioration was so severe that the tunnel convergence and the crown settlement reached a non-negligible level. According to this, scrutinizing the responses of tunnel supports to every tunnel excavation, the specification of the tunnel supports has been repeatedly upgraded. A variety of tunnel support patterns were consequently applied: earlier enclosure with supports; profile change (to be more circular); large side-pile installation. They successfully contributed to reducing the ground deformation. This paper presents the modifications of the tunnel support pattern according to the deterioration of the ground conditions and the following reductions in ground deformation around the tunnel.

1 Introduction

The Asahikawa-Tokachi Road is planned as a 120 km long regional highway linking Asahikawa City with Shimukappu Village. Connecting the highway with the Hokkaido Expressway and Doto Expressway, it will contribute to helping make a broad transportation network and vitalize inter-regional linkages between people and goods in Hokkaido. The Furano road (8.3 km), a part of the Asahikawa-Tokachi Road, was antecedently launched in 2002 because Furano city had suffered from chronic traffic congestion due to the transportation of agricultural products and the combined traffic from tourists and local residents. The Kitanomine Tunnel (tentative name) is a 2928 m long bored tunnel, a part of the Furano Road, the excavation of which started in 2009. The locations of Asahikawa-Tokachi Road, Furano road and the Kitanomine tunnel are shown in Figure 1.

The tunnel construction site and the surrounding area is rich in water resources and famous for natural scenic attractions. Accordingly, a lot of tourists have visited the Furano city from all over the world. Thus, in order to reduce the environment impact on the natural resources, a watertight section was allocated so that the groundwater level around the tunnel would be quickly recovered after the tunnel lining. The cross section area is 74.9 m2 for the non-watertight section, while 98.5 m2 for the watertight section. The excavated tunnel diameter is 12.6 m, 13.0 m in the same order.

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