Length: 430 m; Width: 250 m; Depth: 30 to 40 m; Area: 11 ha; Volume:
20 x 10_m_.
Damages
Intermittent rains from June through August, 1993 (totaling 919 mm),
triggered the landslide causing complete destruction of one, half destruction
of two and partial destruction of seven residential structures; cracks
on the roadways at nine locations; cracks on stone masonry and retaining
walls at five locations; and a ground failure behind a residential structure.
Geology, Mechanism of Failure and Type of Movement
The Otoshi Landslide is located along the northwestern slopes of Takao
Mountain (elevation: 544.1 m). The mountain is underlain by alternating
beds of sandstone and mudstone of Upper Tertiary (Miocene) age. The average
strike of bedding in the region is NNW to SSE, and dipping at about 15
degrees NEE into the slope. The region is also underlain by an old, very
large scale landslide. The Otoshi Landslide represents reactivation of
old landslide deposits. The slide planes of the current landslide are located
along the interface between the old slide deposits and the bedrock (sandstone
and mudstone). The failure propagated headword as the lower slope failed.
Mitigation Measures
As emergency measures, horizontal gravity drains were installed for
dewatering the shallow ground water, and drainage wells were constructed
for dewatering the deep ground water. Caisson piles were constructed due
to the presence of a large thrust (about 3,500 tf/m). Steel piles were
also installed to secure the passive pressure zone of the deep foundations.
