Abstract
Torrential rains in 9th-11th, July, 1972 caused sellective occurrence of slope failure to the southeast-faced mountainslope in the Tanzawa Range. This paper discussed some facters making the difference of distribution of landslip occurrence in slope direction.
The surveyed area is divided into two geomorphic areas by nick points in mountain slope: the upstream area which is characterized by small relief and finely developed dissecting valleys and downstream area which is characterized by large relief and deep but small density of valleys. The slope failure in the upstream area occured selectively on the southeast faced mountainslope.
Most of slope failure occured in artifical forests of 10 to 15 years old. The amount of roots of the trees, total length of roots and maximum penetrating depth were measured in 16 points. The degrees of development of the root system are not so different in the forest age, but show clear difference in the direction of slope. The root systems of the trees develop well in the northwest slope and relatively poor, in the southeast slope.
In the southeast-faced mountain slope, the thickness of soil is not large and the hardness of the top of C horizon is small, which suggests the weak shear strength of soil in the southeast faced mountain slope.
