Length: 2100 m; Width: 1300 m; Area: 150.25 ha; Volume: 4.8x107m3.
Damages
Since the landslide originally occurred in 1888, the area has been
suffering numerous episodes of landslide disaster. Recently, landslides
have been induced by the snowmelt and inflicted damages such as sediment
discharge into Aga River and cracks along the roadways.
Geology, Mechanism of Failure and Type of Movement
The Takisaka Landslide is divided into two blocks along Seki Creek:
the northern block and the southern block. Geology of the site consists
of pre-Tertiary granitic rocks (basement complex) and includes from bottom
to top: lithified to nonlithified arkosic sandstone; green tuff; mudstone;
and overlying terrace deposits consisting of sands and gravels and colluvium.
The factors associated with the cause of sliding area: 1) the green tuff
weathered into a plastic clay: 2) existence of a fault fracture zone to
depth. Because of these factors, this landslide is one of the largest bedrock
slides in Japan where the depth to the slide plane is in excess of 100
m. Because of the increased slide movement during the snowmet, it has been
concluded that ground water inflow by snowmelt and river bank erosion from
the increased Aga River flow are driving forces of the movement.
Mitigation Measures
The mitigation works started in 1958. By 1982, partial surface drainage
control works and drainage well works had been completed. Since 1983, in
order to reduce the flow of ground water from the faults, a series of connected
large-diameter drainage wells were constructed. As a result, the average
annual movement of the landslide was reduced from 1 m prior to 1985 to
about 10 cm after 1985. However, portions of the slide resumed activity
during the snowmelt season of 1994, and subsquently drainage tunnel works
have been implemented.
