Washuzan, as cradlespot of the Seto Inland Sea National Park, is located at about the center of the park. Because Honshu-Shikoku Bridge Kojima-sakaide route is planned to pass the west end of Washuzan, it is necessary to provide fully for the protection of natural and social environments.
The Shimotsui-Seto Bridge (center span, 940 meters), a long span bridge for combined highway-rai1way service, is scheduled for direct connection to this area. For that reason, there is little freedom as to the alignments of highway and railway, and the road width and construction gauge on the suspension bridge, as shown in Fig. 2, are planned to make up the inner section of a tunnel. In the usual method of construction, therefore, the tunnel profile would become an exceptionally large unit. This was judged as involving great engineering difficulties, in view of the geological and constructional conditions of the project site. Therefore, this area had been scheduled for construction by the opencut method. However, many opinions calling for this area to be built in tunnel form were advanced by the Environment Agency.
In response to this, the Honshu-Shikoku Bridge Authority, after making studies of various tunneling methods while asking for the opinions of students and experts on tunnel engineering or rock mechanics, selected the single-tunnel type and double -deck twin-tunnel type, shown in Fig. 3, 4, as feasible tunnel construction plans. Further going through detailed studies of stability and constructional needs, the Authority reached general conclusions. Therefore, we present this paper introducing certain aspects of the Washuzan Tunnel project.
Washuzan is a hilly mass with a summit of 137 meters above sea level, extending about 1.5 km in the east-west and about 0.6 km in the north-south directions. The Washuzan Tunnel is planned to pass the west end of Washuzan in the north-south directions. The topography here gives a maximum overburden of about 30 meters, and a total tunnel length of about 200 meters.
(Figure in full paper)
The geology of Washuzan, as shown in Fig. 5, is composed mainly of biotite granite. The north slope retains rock textures but is advanced in weathering. Some way inward of the hill, on the south Slope, there is a layer of weathered diabase about 10 meters thick, almost parallel to the slope, The Washuzan granite relatively abounds in fissures. These fissures are relatively erect, their. dominant strikes being 45–90 degrees to the tunnel axis. Because the rock masses occur in blocks, it seems necessary to grasp their behavior properly. The groundwater level is low, and is unlikely to particularly affect construction work.
(Figure in full paper)
The single deck plan calls for constructing the highway and railway by a single profile as shown in Fig. 3. The plan features the fact that the tunnel has an arch of relatively long span such as seen in the excavation of a large cavity for underground power-house.
