# Impact of Rock Mass Strength Anisotropy with Depth on Slope Stability Under Excavation Disturbance
## 5 Evaluation of Transitional Rock Mass Strength Anisotropy with Depth on Slope Stability  
It is obvious that the disturbance of the rock mass, depending on the excavation method, causes a decreasing degree of disturbance from the surface to the depth. Therefore, even if the rock mass is completely homogeneous and isotropic with its jointing properties, the strength of the rock mass becomes anisotropic after disturbing the rock mass, which causes an increase in the overall strength from the surface to the depth. All the stability assessments given above were performed by using circular failure surfaces without taking into account the anisotropic mass strength behavior under the excavation disturbance. Therefore, the question of whether circular failure analysis is sufficient to determine the minimum factor of safety and the potential failure surface needs to be addressed. For this aim, the derived equation in this study (AP6) was taken into consideration to repeat the stability analyses considering the combined failure surface by a combination of circular failure surfaces and the surface of the anisotropic strength of the excavated rock mass. For this aim, a plane parallel to the surface of the general slope angle that includes the anisotropic strength in the stability analyses was defined (Figure 14). In this case, stability analyses were performed by using Janbu [24] due to the non-circular failure surface of the combined type of failure surfaces. The analyses by using the Janbu [24] method were performed with an increase in the distance (X\({}_{\rm A}\)) from the surface to the toe of the general slope for the linear plane parallel to the general slope surface, i.e., parallel to the distribution of the disturbance zone. The results were then presented as XA versus FOS graphs, as shown in Figures 15 and 16. In the analyses conducted for potential failure surfaces passing through points (1) and (2), the factors of safety gradually decreased as the X\({}_{\rm A}\) distance of the plane defined to account for the anisotropic strength of rock mass increased (Figures 15 and 16). The lowest factor of safety at point (1) was determined as a potential circular failure surface when  
Figure 14: Hypothetical slope model used to evaluate the anisotropic rock mass strength in slope stability analysis. the defined plane was sufficiently far away for no intersection with a potential circular failure surface. On the other hand, this result may not be interpreted as just circular analyses being sufficient for stability evaluation because the lowest factor of safety was obtained for a combined potential failure surface when point (2) was considered. However, very little difference was obtained when it is compared with the factor of safety against circular failure. Figure 15: For potential failure surfaces passing through point (1), (**a**) the X\({}_{\text{A}}\) distance of the plane defined to account for the anisotropic strength of the rock mass increases versus the FOS graph, and (**b**) the critical failure surface determined for the lowest factor of safety. Although the strength of the rock mass decreases toward the excavation surface of the slope profile, the instantaneous internal friction angle increases due to the nonlinear failure envelope as the normal stress becomes lower. For shallow failure surfaces, the factor of safety also increases because the instantaneous internal friction angle is significantly higher than the average slope of the profile. However, shallow sliding surfaces are unlikely to occur, as the interlocking of rock blocks in jointed rock masses typically prevents such failures. Slope stability is fundamentally defined as the equilibrium between the horizontal and vertical components of the weight of the rock mass along the failure surface projection. Achieving the critical failure surface in jointed rock masses necessitates a specific rock mass weight.