Panoramic view of the Washiodake Landslide
- Shitonouji, Emukae-Cho, Kitamatsuura-Gun, Nagasaki Prefecture, (38_
17'N; 129_ 40'E).
- Size of Slide
- Length: 1000 m; Width: 500 m; Maximum thickness: 90 m; Area: 42.8 ha.
- The landslide was suspected to have occurred around 1945. In the spring
of 1950, tension cracks along the forest roads and up to the ridge area
were observed, and appeared to be expanding. An investigation was initiated,
and by the summer of 1950, the tension cracks along the head area were
confirmed. Today, the main scarp is about 15 m high. Additional investigations
have been conducted as the slide movement continues, and the complete outline
of the slide mass was established by 1995. Associated with the sediment
movement of the toe area to Emukae River, movement of the slide mass also
resulted in deformation of the railroad alignment, and damages such as
shifting to fields and rice paddies. Cracks in residential structures were
also reported. The landslide area encompasses 33 residential structures,
one public hall, 500 m of rail, one rail station, 11.2 ha of cultivated
fields, 17.3 ha of forests, and 500 m of class II Emukae River. Furthermore,
if a massive failure were to occur it would result in substantial damages
to the downstream areas of the Emukae River.
- Geology, Mechanism of Failure and Type of Movement
- The slide area is underlain by Tertiary sedimentary rocks consisting
mainly of sandstone and mudstone, and structurally form a dip-slope condition.
Near the summit, these sediments are overlain by basalt of Pliocene age.
The Tertiary sediments repeat a rhythmic cycle consisting of a coarse to
medium grained sandstone, sandy mudstone, and siltstone to shale with a
coal seam between the clay beds derived from tuff at the top of each cycle.
These coal seams act as conduits for main ground water flow. The type of
landslide above is called Hokusho-Type, and is a translational rock slide.
At the site, it is characterized by basaltic cap rocks that cover the ridge
area (also the source of ground water). Since the coal seams along the
monocline structure act as a slide plane, the overlying sandstone and mudstone
beds fail as rock blocks.
- Mitigation Measures
- The mitigation measures implemented between 1952 and 1989 consisted
mainly of control works to reduce the annual movement to about 6 cm/yr.
However, due to heavy rains in 1990 and 1992, the rate of movement has
increased. As an emergency measure, drainage well works have been implemented
along with partial restraint works.
Cross section of the Washiodake Landslide