## 3 Results and Discussion
### Design Review with Back-Analysis
#### 3.3.2 Sensitivity and Parameter Analysis  
A sensitivity analysis was conducted to evaluate the effects of variations in geotechnical parameters on displacement, shear stress, and the factor of safety. This analysis was conducted according to the approach suggested by [PERSON] and [PERSON] [39], who proposed that for n geotechnical parameters, all \(2n+1\) models should be analyzed. A range of \(\pm 25\%\) from the reference values was considered for different lithologies to assess their influence on stability (Table 8). In order to examine the effect of changing a parameter value as a percentage, the analysis was conducted while keeping all other parameters constant. The safety coefficient and its effect on displacement were examined. For sensitivity analysis, \(2\times(2\times 7+1)=30\) models were created using the HB body model in both moderately and completely weathered sandstone units, and their effects are shown as an average percentage (Figure 19). The results indicated that the displacement values in the completely weathered rock masses were primarily controlled by unit weight and Poisson's ratio, while GSI had a more significant effect in the moderately weathered units. As the weathering level of the rock mass decreases, the effect of the GSI value increases noticeably. It was determined that the effect of the GSI value on displacement was greater in the moderately weathered units than in the completely weathered units. It has been determined that shear stress is controlled by unit weight in completely weathered rocks. As the degree of moderate weathering decreases, the effect of the GSI value increases noticeably. Notably, defining a low GSI value in a moderately weathered rock mass reduces shear stresses considerably. When the parameters affecting the safety coefficient are examined, the GSI and the disturbance factor have a high impact, regardless of the weathering degree. The disturbance factor was recommended by [PERSON] [29] to be 0.7 for weak rock slopes subjected to mechanical excavation. Thus, no modifications were made to the disturbance factor, D, in the updated design. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Parameter** & **Symbol** & **Unit** & \begin{tabular}{c} **(\%-25)** \\ **Min.** \\ \end{tabular}  & **Ref.Val** & \begin{tabular}{c} **Max.** \\ **(\%+25)** \\ \end{tabular}  \\ \hline \multirow{4}{*}{ \begin{tabular}{c} Highly Weathered \\ to Completely \\ Weathered \\ Weathered Sandstone \\ W4-W5 \\ \end{tabular}  } & Weight per unit & \(\gamma\) & \(\text{kN}/\text{m}^{3}\) & 16 & 21 & 26 \\  & \begin{tabular}{c} of volume \\ Elasticity modulus \\ Poisson’s ratio \\ Uniaxial compressive strength \\ Material constant for intact rock \\ Geological strength index \\ Disturbance factor \\ \end{tabular}  & \ \begin{tabular}{c} \(\gamma\) \\ Elasticity modulus \\ Poisson’s ratio \\ Uniaxial compressive strength \\ Material constant for intact rock \\ Geological strength index \\ D \\ \end{tabular}  &
\begin{tabular}{c} \(\gamma\) \\ Elasticity modulus \\ - \\  
[PERSON] [40] defined the m\({}_{\rm i}\) value of claystone as 4 \(\pm\) 2 and the m\({}_{\rm i}\) value of sandstone as 17 \(\pm\) 3. In this study, based on the design, the Cakraz Formation was determined to be intercalated with completely weathered sandstone-claystone. Therefore, selecting 17 as the value of m\({}_{\rm i}\), which is accepted for completely weathered sandstones, cannot allow a full description of the sandstone-claystone weathering zone. Therefore, in the new design created at the completely weathered sandstone level, an m\({}_{\rm i}\) value of 13 was adopted to better represent the geological conditions. In this analysis, we examined the influence of parameter modification on displacement, stress, and factor-of-safety values. It was found that unit weight and Poisson's ratio exerted the most significant impact on the displacement value of a thoroughly weathered rock mass.