Abstract
In this paper, authors describe the characteristics of typical fractures in a loosened rock mass of weathered granite, northern Yamanashi Prefecture, and indicate a landslide primary deformation process leading to loosening of the rock. The loosened rock mass is divided into 1) tension crack zone, 2) conjugate crack zone, and 3) extruding deformation zone, from the head to the toe part. The stress condition and deformation process in the loosened rock mass can be estimated from characteristics and orientations of the cracks. There is a topographic step at the top part of each loosened rock mass. The displacement ratios (100X/L：X is displacement of loosened rock mass, L is length of loosened rock mass) calculated by the topographic steps are 0.8-4.0 %.
The critical displacement ratio of juvenile landslides in the weathered granite of this area is about 5 %. Compared with the previous authors' studies, the value is close to the critical displacement ratio of landslides in the Neogene sedimentary soft rock.
Topographic development pathway leading to landslide from the loosened rock mass can be indicated by displacement ratio and slope angle. The topographic development pathway of weathered granite is different from Neogene sedimentary rock and crystalline schist's topographic development pathway.
