This slide is thought to have originated at least in the B.C. Although
this slide has maintained a relatively slow rate of movement in recent
years, activity has somewhat accelerated since 1988 with an average annual
movement of 30 cm/year, causing serious damage to the residential structures
and roadways.
Geology, Mechanism of Failure and Type of Movement
Geologically, the region is located within the Sanbagawa crystalline
schist zone which is sandwiched between the Median Tectonic Line and the
Mikabu Tectonic Line, and basic schist of the Sanbagawa Belt. Due to large
scale tectonic activity, the rocks are highly fractured and weathered.
The slide occurred on a dip slope where the average slope is 24 degrees.
The head of the slide is located at 950 m above sea level, and the toe
is located at 650 m above sea level near the Kashio river bed. The slide
is divided into two blocks; the Upper Block, and the Lower Block. The Upper
Block slide is structurally controlled, and occurred within the unweathered
muddy schist. The Lower Block slide occurred within deeply weathered muddy
schist with advanced argillation. The slide occurred due to pore water
pressures acting on the slide planes. The slide planes are located at depths
of 10 to 20 m deep for the Upper Block, and 20 to 30 m deep for the Lower
Block.
Mitigation Measures
Because of the size and high degree of slide activity, the initial
phases of the mitigation works were concentrated on groundwater removal
followed by planned restraint works.
