# Impact of Rock Mass Strength Anisotropy with Depth on Slope Stability Under Excavation Disturbance
## 4 Slope Stability Evaluations of a Hypothetical Slope Model  
As is well known, the relationship between the factor of safety (FOS), slope angle (\(\beta\)), and slope height (H) needs to be investigated to ensure a safe, applicable, and economical slope design. Even if the characterization of a rock mass has been well studied and the necessary input parameters have been obtained with high quality, the strength reduction due to the disturbance effect should be reliably included in the analysis, especially for deep slope excavations. However, as mentioned above, there is no widely accepted approach for the use of disturbance effect in the design of slope excavations. Therefore, the effects of the six approaches discussed on the stability of the hypothetical slope models were investigated using a slope stability analysis for circular-type mass failure. As commonly observed in Turkiye and similar geological regions, Neogene-aged formations, such as lignite coal and borate mines, may be overlain by jointed rock masses such as limestone, marl, and mudstone (Figure 4). By considering the actual in situ view given in Figure 4, in order to reflect the moderate and weak strength jointed rock masses in the realistic hypothetical slope model, the presence of a 50 m thick mudstone rock mass overlying a limestone rock mass with a 50 m thickness was considered. On the other hand, another advantage of the use of these two rock masses with different rock mass strengths is that they are exposed to different degrees of disturbance depending on their excavatabilities together with the excavation method used. The assumed geomechanical properties of the selected limestone and mudstone rock masses are summarized in Table 1. Additionally,  
Figure 3: Representative graphical presentations of the approaches (**a**) AP1, (**b**) AP2, (**c**) AP3, (**d**) AP4, (**e**) AP5 and (**f**) AP6 depending on normalized depth to UDL. to minimize the influence of independent variables on slope stability assessment, only a drained slope case was considered in this study. As shown in Table 1, while the excavation of overburden could be performed by blasting the limestone rock mass, mechanical excavation with excavators could be sufficient for the mudstone rock mass. By considering this fact, the disturbance factor beyond the digging limit was selected as 0.9 and 0.7 for the limestone and mudstone rock masses, respectively. The evaluation of disturbance zones from the surface to the undisturbed rock mass condition, based on the four approaches, was considered in this study. For this purpose, a bench angle of 50\({}^{\circ}\) and a bench height of 10 m was used, while the overall slope angle was 31.4\({}^{\circ}\) for five grouped benches of the two rock masses. A counter-upslope bench width on the boundary between the two rock masses was selected as 20 m, resulting in an overall slope angle of the hypothetical slope model of 28.5\({}^{\circ}\) (Figure 5). In the hypothetical slope model presented in Figure 2, two critical points were defined for use in the slope stability analysis. The first critical point was located at the toe of the overall slope profile on the lower limestone bench, while the second critical point was at the toe of the lower mudstone bench (see Figure 5). In other words, the stability of the hypothetical slope profile was evaluated by considering potential failure circles that pass through only these two points. While the circular type analyses were performed in accordance with the simplified [PERSON] [23] method, the overall strength of two rock masses was defined according to [PERSON] et al. [4]. The slope was assumed to be in drained conditions, with no seismic events considered to minimize variable influences. \begin{table}
\begin{tabular}{c c c c c} \hline
**Rock Mass** & **UCS (MPa)** & **GSI** & **m\({}_{\mathbf{i}}\)** & **\(\boldsymbol{\gamma}\) (kN/m\({}^{3}\))** \\ \hline Limestone & 75 & 45 & 10 & 25 \\ Mudstone & 5 & 30 & 7 & 20 \\ \hline \end{tabular}
\end{table}