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

根据现场调查，在降雨较少的情况下，边坡破坏多发生在不连续面上。导致边坡失稳的不连续面可分为：1)滑坡区破碎粘土，2)板岩中发育的片岩，3)变质岩中粘结节理或片岩的薄光滑黑土，4)弱风化与强风化花岗岩之间的分界面，5)花岗岩中互穿的粘土层。在2003~2004年的两年间，我们对山口县发生的37个边坡灾害现场进行了调查，并在现场或实验室用超简化的原位抗剪试验仪测定了不连续面上的抗剪强度。为了验证现场试验的有效性，还对在Ube和Iwakuni抽样的两种Masado进行了直接箱剪试验。这些试验结果表明，现场试验得到的强度参数可用于含不连续面路堑边坡的稳定性分析。其次，对坎门组风化边坡进行了边坡高度、降雨、褶皱等6个危险因素的调查，将断层作为不连续面之一。顶部有断层和另外两个因素的斜坡很容易坍塌。计算结果对确定路堑边坡的倾角具有一定的参考价值。第三，认为树根是地表破坏的重要因素。进行了宇部Masado与树根混合后的直箱剪切试验。试验结果表明，在低覆岩压力条件下，表观粘聚力的增加明显，而不是内摩擦角的增加。最后，如果在斜坡内部看到不连续的平面，这意味着导致斜坡破坏的可能性很高。

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.