TIME-DEPENDENT STRENGTH MOBILIZATION OF CLAY SEAMS EMBEDDED IN PROGRESSIVE ROCK SLOPE FAILURE PLANES

许多岩质边坡的破坏是由于岩体沿嵌有薄粘土层的滑动面滑动而发生的。通常不可能在从滑移面上取下的未受扰动的薄粘土缝上进行三轴试验，因为试样的厚度不足以放置在三轴仪的基座上。对薄粘土试件进行剪切试验的唯一可行方法似乎是使用剪切箱试验，由于其剪切机制简单，传统上被认为是一种次要的试验方法。研究人员对实际岩质边坡破坏的粘土层试件进行了多次系列剪切箱试验，发现等体积剪切箱试验能给出合理的有效应力强度参数。他们从理论上推导了将剪切箱试验得到的强度转换为三轴试验得到的强度的方法。这一方法已成功地应用于多例岩质边坡破坏的分析，并得出了典型的强降雨引发岩质边坡破坏的机理。然而，应用于边坡破坏分析的强度参数，从案例研究的反分析中发现，即使在剪箱试验中实际观察到有效应力方面的一些内聚力的情况下，也存在零内聚力。最终发现，这种零黏聚力是在偶发性强降雨期间，经历岩质边坡递进破坏的粘土层被淹没的结果。从实际应用的角度来看，最重要的结论是，降雨不仅可以有效地诱导滑面上高水压引起的边坡破坏驱动力，而且可以暂时降低由淹没引起的粘土层的凝聚力。这意味着在估计岩质边坡破坏的可能安全系数时，不应依赖于粘土层的黏聚力。少

Many of the rock slope failures take place as the rock mass slide along the slip planes in which thin clay seams are embedded. It is not usually possible to carry the triaxial tests on the undisturbed thin clay seams taken from the slip planes because the thickness of the specimens is not large enough to set on the pedestal of the triaxial apparatus. The only possible way of carrying shear tests on the thin clay specimens seems to use the shear box test which traditionally considered as a minor testing method due to its simple mechanism of shear. The investigators have carried many series of shear box tests on the clay seam specimens taken from the actual rock slope failures and found that the constant volume shear box tests can give reasonable strength parameters in terms of effective stress. They have theoretically derived the method to convert the strength obtained from shear box tests into the strength obtained from triaxial tests. This was successfully utilized in the analyses of … More rock slope failures and resulted in the findings of mechanism of rock slope failures typically triggered by heavy intensive rainfall. The strength parameters to be applied to the slope failure analyses, however, were found from the back-analysis of case studies to be of zero cohesion even in the cases that some cohesion in terms of effective stress was actually observed ion the shear box tests. This zero-cohesion was eventually found to be resulted from the submersion of the clay seams undergoing the progressive rock slope failure during the period of occasional heavy rainfalls. The conclusion most important from the practical point of view is that the rainfall was found to be effective not only in inducing the driving force of slope failure by high water pressure in the slip planes but also in temporarily reducing the cohesion of the clay seams caused by submersion. This implies that we should not rely on the cohesion of the clay seams in estimating the possible factor of safety of rock slope failures. Less