Conditions immediately following the Ishikura Landslide
- Imafuku-Cho, Matsuura City, Nagasaki Prefecture, (33_ 20'N; 129_ 47'E).SIZE
- Size of Slide
- Length: 500 m; Width: 350 m; Area: 22 ha; Volume: 3.0 x 10_m_.
- Tension cracks were observed prior to the major failure of July 4,
1990. The heavy rainfall preceding the sliding (235.4 mm from July 1-2)
rapidly raised the ground water level to 4 to 5 m, which in turn lubricated
the slide plane along a zone of weakness and induced the failure. The damages
included breach of one reservoir, toppling of one check dam, numerous damages
to roadways, disturbance of 1042 m of irrigation channels, 5.8 ha of rice
paddies, 1.6 ha of cultivated field, and 1.4 ha of forest.
- Geology, Mechanism of Failure and Type of Movement
- The majority of the slide area is underlain by Upper Tertiary (Early
to Middle Miocene) rocks, and by Lower Tertiary rocks on the upslope areas.
These Tertiary rocks are in turn overlain by basaltic volcanic rocks forming
a cap rock structure. The slide plane was found to be near the bedding
plane between sandstone and coal-bearing shale. The angle of the slide
plane ranges between 3 and 5 degrees.
- Mitigation Measures
- After the initial sliding, numerous tension cracks developed within
the body of the slide mass. The open tension cracks promoted easier infiltration
of rain water. Due to this increase in pore water pressure the possibility
of renewed activity within the slide mass increased significantly. Consequently,
a 3-dimensional dewatering system combined with drainage wells and drainage
tunnels were constructed. Additionally, soil removal works were implemented
at the head area, and steel pile works and anchor works were implemented
at the toe area.
Cross section of the Ishikura Landslide