九州四万十帯切取り斜面の岩盤崩壊過程

Abstract

A mountainous rock cliff collapsed on to the National heighway of Route 327 in Miyazaki Prefecture, Japan, on 31 May 1990. Fortunately no human life was lost owing to the previous holding up of traffic. While the creep of the slope continued to be monitored, whole process of the failure was visually recorded. The dip of stratum is opposite to the slope, whose rock is schalstein of Paleogene Miocene.Covering the slope with mortar was going on after cutting the slope as improvement work of the highway.The relationship between the rupture time and the strain rate which was obtained both from extensometers and from the visual records showed that the creep process of the rock slope to the failure can be explained with Saito's theory (1968) on the tertiary creep, in which as time goes on, strain rate increases along the curve of the Saito's formula (1966) on secondary creep. Though the motion of the final stage for 20 seconds just before the climax of the failure has taken a similar process in general as the law in the Saito's tertiary creep theory, this final creep seemed to have slightly changed its mechanism. The creep accelerated almost continuously, but rock fall occurred intermittently, alternating active and inactive periods of rock fall. Its frequency had increased since early time, but the main portion of volumetric accumulation of fallen debris had focused on the climax time of the failure. After the climax, secondary failures of rather smaller scale continued to occur intermittently for several hours.