Length: 570 m; Maximum Width: 400 m; Depth: 50 m; Area: 14.6 ha.
Damages
DAMAGES: Records indicate this slide has undergone movement since 1889.
Recent large scale landslide damages occurred on June 8, 1992 due to torrential
rains associated with the early summer rainy spell. A 400 m section of
the Prefecture Highway was closed down into a one way road, and a bridge
abutment was destroyed at the toe of the landslide. Six residential structures
within the slide show some tilt, and numerous cracks formed along the local
roadway that made it difficult to drive upon.
Geology, Mechanism of Failure and Type of Movement
The Nishitani region is situated on the southwestern portion of the
Kii Mountains along steep slopes. Numerous landslides, including the subject
landslide, are concentrated along the southern side of the Honmiya Fault.
The steep slopes are underlain by Late Tertiary alternating beds of sandstone
and shale. The slide is undergoing creep type movement and is characterized
by very slow movement. The Nishitani Landslide originated at the ridge
slope, and based on the disturbed micro-topography along the ridge and
activity status of the slide, has been broadly classified into three zones:
1) old landslide area near the summit; 2) current landslide area including
secondary scarps; and 3) areas that have been severely eroded by the Nishitani
River. The depths to the slide plane range to about 45 m from the top to
the mid-portion, and to about 30 to 40 m from the mid-portion to the bottom.
The slide plane is classified as a "keel-shape slide plane".
Although the movement is slow, the slide is currently active. A strong
good correlation exists between the level of activity and rainfall rates.
There appears to be abundant paths and avenues for ground water migration
into the deep portions of the mountain. It has been suggested that the
primary causes for slide movement include fault fractures and dip slope
conditions, and erosion of the toe of the slide by the river and lowering
of the river bed. The influence of increased deep ground water by rainfall
is suggested to be the primary cause.
Mitigation Measures
Because it was clear that the main cause of the 1992 disaster was increased
ground water associated with rainfall, the mitigation works were oriented
towards ground water removal, and have been continuously implemented to
date.
