Ishikura Landslide

Location

Imafuku-Cho, Matsuura City, Nagasaki Prefecture, (33_ 20'N; 129_ 47'E).SIZE OF SLIDE:

Size of Slide

Length: 500 m; Width: 350 m; Area: 22 ha; Volume: 3.0 x 10_m_.

Damages

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.