On the slope of joint structure at Sounkyo Gorge in Hokkaido, toppling of a long columnar of welded tuff has occurred frequently and the road has hazard of rock fall. In this study, the occurrence mechanism of the toppling was considered theoretically on the basis of the observational results of the fracturing processes.
At first, it was proved from the analyses on the thermal deformation of a rock columnar that a small and a large stone fell alternately in the opened joint on the back side of the columnar in response to the seasonal change in temperature gradient and that the inclination of the columnar increased gradually with the lapse of time.
Secondly, it was clarified from the investigation on the hook-shaped crack propagations at the root of the columnar that toppling was caused when the rotational center of the inclined columnar moved from the outside to the inside just over the centroid in the processes of compressive fracture of the outside part. Additionally, the long duration of acoustic emission activity just before toppling has supported the validity of the theoretical considerations.
