In loose or highly weathered rock slopes, the circular shear failure mechanism occurs in large scale. Most rock slope stability investigations ignore the influence of geometric parameters, i.e., failure surface entry point distance (le). This study presents failure surface determination based on constant element stress-based method. The failure surfaces and their sliding mechanism are analyzed first by using FLAC^3D (Fast Lagrangian Analysis of Continua in three-dimension). A real case of instability corresponds to a loose rock is presented and its stability is determined by numerical analysis. To investigate the effectiveness of present method the influence of shear strength (c, φ) and geometric parameters (α, β) on the safety factor (FS) is also analyzed by using Geo5, FLAC^3D and ABAQUS computer codes. Then, Geo5 and Statistical Package for the Social Sciences (SPSS) softwares were used to determine the effect of dimensionless parameter lambda (λ) on the failure surface entry point distance (le) and length of failure arc (L). The results show that shear failure may occur in non-homogeneous slope. Shear failure has exit and entry points situated either on the surface or near the toe of slope. The length of failure arc (L) is affected by the le and λ. Moreover, it can be acknowledged that the relation of both L and le with shear strength parameters (c, φ) is logarithmic, respectively. So, based on the rock strength parameters, L and le are predictable by using the equations derived from non-linear regression.

在松散或高度风化的岩石斜坡上，圆形剪切破坏机制会大规模发生。大多数岩石边坡稳定性研究都忽略了几何参数的影响，即破坏面入口点距离（le）。这项研究提出了基于基于恒定元素应力的方法确定破坏面。首先使用FLAC ^3D（^三维连续体快速拉格朗日分析）分析失效表面及其滑动机理。给出了一个与岩石松动相对应的真实情况，并通过数值分析确定了其稳定性。为了研究本方法的有效性，剪切强度（c，φ）和几何参数（α）的影响。，还使用Geo5，FLAC ^3D和ABAQUS计算机代码分析了安全系数（FS）上的β）。然后，使用Geo5和社会科学统计软件包（SPSS）软件确定无量纲参数lambda（λ）对破坏面入口点距离（le）和破坏弧长（L）的影响。结果表明，在非均质边坡中可能发生剪切破坏。剪切破坏的出口和入口点位于斜坡的表面或附近。失效弧的长度（L）受le和λ的影响。而且，可以承认两者之间的关系具有抗剪强度参数（c，φ）的L和le分别是对数的。因此，基于岩石强度参数，可以使用非线性回归方程来预测L和le。