On the 24th June 1978 a landslide occurred at Shiraki, Okuchi-shi, located in the north of Kagoshimaken, Japan; destroying two inhabitants and two houses and causing the damage in a road and a paddy field.The landslide occurred during the improvement-work of road and at the time of heavy rainfall, in the cuttingslope of weathered and fissured overconsolidated clays derived from volcanic rocks. In this paper the originand mechanism of the landslide are described and analysed from the standpoint of geotechnical engineering.In order to investigate strata, geological structure and ground water condition in the landslide area, electricprospecting, electric logging, drilling survey, groundwater tracing and interpretations of aerial photograph andof topographic map were performed. The dependence of the landslide on rainfall also was investigated.Physical and mechanical properties of clays were cleared by means of laboratory soil tests.
The strata consisting of lava, upper tuff and lower tuff with a great may of joints and fissures wereformed by the hydrothermal alteration and the weathering of volcanic rocks. The slip surface observed waslocated in the dipping joints with the seam contained montmorillonite between clayey upper tuff and clayeylower tuff.
The condition of rainfall was 300 mm in the effective accumulated amount of rainfall and 26 mm in theamount of hourly rainfall preceding the time of activation of the landslide, and fully satisfied the rainfalllevel of threshold value of slope-movements in the southern Kyushu.
This landslide is classified as the rockslide of weathered materials derived from volcanic rocks in a ridgyterrain.
Physical properties of upper clay (clayey upper tuff) are typically Gs=2.70, ω=82%, ωL=103%, ωP=69%, clay fraction (<2μm) =35% and bulk density=1.40 (t/m
3). Its mechanical properties are c'=0.5 tf/m
2and φ=34°C in the peak strength, φ=14°C in the residual strength, overconsolidation ratio=3.0, residual factorR=0 and brittleness index IB=1.00-0.64.
Stability analyses using the conventional slice-method based on the effective stress gave the decrease incalculated factor of safety owing to the cutting of hillside and the saturation of ground.
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