At August 27, 1999, at Nakutaki Area in Yamaguchi City, Japan, a rock slope of an old granite quarry collapsed and some of the 3,300 m3 rubble covered a road along the slope. A laser displacement measurement was employed to monitor an unstable rock mass left on the slope, to secure repair works under it. Although accumulated displacements around 50 mm were recorded, the rock mass did not fall down. The rates of the measured displacements showed no accelerative increase in spite of the large accumulated displacements, and the fact is consistent with the low activity of acoustic emission monitored at the same time. Thus, it could be concluded that measurement of displacement like this without having so high accuracy could be also effective to monitor an unstable rock mass, if continuous measurement is conducted.
Effective monitoring methods to predict rock slope failure are sought, in particular in Japan, after a rock mass of 11,000 m3 failed into a road tunnel and killed 20 passengers, on February 10, 1996, in the Toyohama tunnel, Hokkaido, Japan (Ishijima & Roegiers 1996, Ishijima & Fujii 1997). We consider that laser displacement measurement, among other methods, is promising for monitoring slope stability, because continuous monitoring is possible without targets set on a steep and dangerous slope. On August 27, 1999, a rock slope 40 km from Yamaguchi University failed and some of the 3,300 m3 rubble covered a road along the slope over a length of 28 m. Fortunately no persons were injured, because a small failure was observed on August 21, six days before the main failure, and the road was therefore closed at the time. However, an unstable rock mass was left on the failed slope, and works under the unstable rock mass were needed to reopen the road. Thus, to provide security during the works, laser displacement measurement was employed to monitor the unstable rock mass, with AE (acoustic emission) monitoring.
The main failure occurred on the slope shown in the right-hand part of Figure I. The failed slope measured 40 m wide and 60 m high, and the volume of the failed rocks was estimated to be 3,300 m3. Most of the failed rocks were piled below the slope as shown in Figure I; however, some of them reached the road 70 m below and along the slope over a length of 28 m, as shown in Figure 2. This site used to be an old quarry of Cretaceous granite, and some parts of the slope shape appear to have been made by the quarrying. Four joint sets are dominant in the Cretaceous granite of this area. Two of the four are almost vertical tectonic joints coinciding with lineaments in the surroundings, and these joints separated the rock mass forming the shape into vertical slabs. The other two are slightly inclined sheeting joints parallel to current topography.