変質火山岩に由来する過圧密粘土地盤における切土斜面の崩壊

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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 cutting slope of weathered and fissured overconsolidated clays derived from volcanic rocks. In this paper the origin and 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, electric prospecting, electric logging, drilling survey, groundwater tracing and interpretations of aerial photograph and of 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 were formed by the hydrothermal alteration and the weathering of volcanic rocks. The slip surface observed was located in the dipping joints with the seam contained montmorillonite between clayey upper tuff and clayey lower tuff. The condition of rainfall was 300mm in the effective accumulated amount of rainfall and 26mm in the amount of hourly rainfall preceding the time of activation of the landslide, and filly satisfied the rainfall level 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 ridgy terrain. Physical properties of upper clay (clayey upper tuff) are typically G_s=2.70, w=82%, w_L=103%, w_p=69%, clay fraction (<2μm)=35% and bulk density=1.40 (t/m^3). Its mechanical properties are c'=0.5th/m^2 andφ'=34°in the residual strength, overconsolidation ratio=3.0, residual factor R=0 and brittleness index I_B=1.00〜0.64. Stability analyses using the conventional slice-method based on the effective stress gave the decrease in calculated factor of safety owing to the cutting of hilling of hillside and the saturation of ground.
一般社団法人日本応用地質学会の論文
1981-12-30

変質火山岩に由来する過圧密粘土地盤における切土斜面の崩壊

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