Length: 2000 m; Width: 500-1100 m; Average Depth: 50 m; Area: 150 ha;
Volume: 7.5x107m3.
Damages
The landslide activity can be divided into two parts. The Early Period
failure occurred between 25,000 to 46,000 years ago, and the Late Period
Failure was speculated to be occurred prior to the year 1311. The Late
Period Failure was at the same depth as the Early Period and the creep
type sliding has been continued. The triangulation survey station located
at the head portion of the slide dropped 3.3 m between 1907 and 1983. Although
the rate of deformation is extremely slow, cracks were visible along the
roadway and structures and minor repair works have been performed repeatedly.
Geology, Mechanism of Failure and Causes of Failure
Geology consists mainly of Miocene mudstone beds with interbedded sandstone
of less than 10 m thick. For the Upper Tertiary formation, it is relatively
well lithified, and slump structures have developed within the formation.
Bodies of porphyrite intruded along the bedding planes of the mudstone
beds at the upper portion of the landslide zone. The sliding had developed
joint and consists of jointed shale or fractured shale. The surficial material
consists of debris flow deposits containing mostly of gravelly soil. The
mudstone beds dip 10 to 30 degree approximately parallel to the topography
forming a dip slope condition. Occasional presence of severely fractured
beds which are interbedded within the mudstone forming a weak zone and
weakening of the rock by the development of joint system from the tectonic
movement is one of the primary causes of the Early Period landslides. The
reason for the continuation of the current activity which was suspected
to be originated during the Late Period movement is assumed the increased
pore water pressure from snowmelt and snow load.
Mitigation Measures
The mitigation plan includes the installation of drainage wells over
the slope. Currently, the plan has been implemented starting from the upper
portion of the slide.
