Study on evaluation of stability of slope holding discontinuous plane causing slope failure

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According to our field survey, slope failure frequently occurred along a discontinuous plane during tiny rainfall. The discontinuous planes resulting in slope failures were classified as following : 1) fractured clay in landslide area, 2) schist developed in slate, 3) thin glossy black soil adhered to joint or schist in metamorphic rock, 4) boundary plane between weakly and strongly weathered granite rocks, and 5) clay-seam interpenetrated in granite rock. During two years from 2003 to 2004, we investigated 37 sites of slope disasters occurred in Yamaguchi prefecture and then measured shear strength mobilized on the discontinuous plane in the site or the laboratory using an ultra simplified in-situ shear test apparatus. Also the direct box shear tests were performed on two types of Masado that were sampled in Ube and Iwakuni, to demonstrate the validity of the in-situ test. These test results suggested that the strength parameter obtained by the in-situ test should be used in the stability analysis of cut slopes having the discontinuous planes. Secondly, weathered slopes of the Kanmon group were surveyed on six risk factors such as height of slope, rainfall, folding etc. A fault was regarded as one of the discontinuous planes. Slopes that have the fault on the top and two more factors are prone to collapsing. The results should be effective to determine the inclinations of cut slopes. Thirdly, the tree root was taken up as an important factor related to surface failure. The direct box shear test was performed on Ube Masado mixed with tree roots. The test results showed that an increase of the apparent cohesion remarkably exhibited rather than that of the internal friction angle under condition of low overburden pressure. Finally, if the discontinuous planes are seen inside a slope, it means that possibility causing slope failure is high.