Conditions immediately following the Bishamon Landslide
- Hiyoshi-Cho, Hioki-Gun, Kagoshima Prefecture, (31_ 36'N; 130_ 23'E).
- Size of Slide
- Length: 250 m; Width: 300 m; Area: 6 ha; Volume: 1.0 X 10_m_.
- The landslide occurred on September 20, 1994, resulting in 2 deaths,
3 injuries, destruction of 2 residential structures, and affecting 436
m of roadway and 2.3 ha of cultivated field.
- Geology, Mechanism of Failure and Type of Movement
- The slide area is underlain by sedimentary rocks of the Mesozoic Period
(Cretaceous Epoch) with granitic intrusions located along the western slopes
of Yakudake. The upper portions of the slope are underlain by sedimentary
rocks and the lower portions are underlain by granite where the landslides
occur. The granitic rock is deeply weathered to argillite at about 40 m
deep, and has a particularly well-developed clay seam that exists parallel
to the granitic intrusion. Deeply and completely weathered granitic rocks
contain abundant clay to form an impervious zone. It is possible that pore
water pressure within the slope underlain by granitic rocks could remain
stable for a long period of time after the ground water level is raised
by rainfall. It has been concluded that the primary cause of the sliding
was a reduction in shear strength within the argillaceous granitic rocks
by increased residual pore water pressure. The failure occurred along the
weak beds (such as clay seams) where alteration is highest.
- Mitigation Measures
- Due to the large scale of the sliding mass, the mitigation measures
were oriented towards increasing the stability factor mainly by control
works combined with the selected constraint works where positive results
can be expected. Specifically, mitigation measures included soil removal
works at the head portion of the landslide and buttress fill along the
toe portion of the slide. Attempts were also made to control the sliding
force by ground water removal by installing drainage wells, and horizontal
gravity drains. Finally, restraint by piles were implemented to complete
the mitigation efforts.
Conditions following the completion of the mitigation measures
Cross section of the Bishamon Landslide