| Tunnel construction costs depend on the type of bedrock. When hard rock exists between layers of rock that are not hard、 drilling excavation must be combined with blasting、 which raises the construction cost. Preliminary geological surveys、 such as geophysical exploration or boring、 are necessary to determine the distribution of rock by type. When tunnels for national highways are constructed in Hokkaido、 Japan、 horizontal boring is executed to examine such distribution before tunnel excavation. However、 the geological information from horizontal boring is sometimes insufficient to determine the rock types throughout a section of tunnel、 and the presence of rock of types not found by horizontal boring can greatly hinder excavation. In this study、 rock in cores from horizontal boring was classified into typical types、 and the relationship between the unconfined compressive strength and magnetic susceptibility of each rock type was examined. Observation of cores and measurement of compressive strength test confirmed different strengths in rock from the horizontal bore core. Hard rock tended to have low magnetic susceptibility、 whereas soft rock tended to have high magnetic susceptibility. As one move from soft rock to hard rock、 or vice versa、 the magnetic susceptibility changes at about 1 or 2 meters from the boundary between the rock types. This suggests that immediately beyond locations where the magnetic susceptibility decreases、 hard rock probably exists. |
