Bishamon Landslide

Location

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_.

Damages

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.