# Full Life Cycle Evaluation of Stability Pile in High Slope with Multi-Layer Weak Interlayers
## 6 Discussion
### Discussion of the Pile Length (\(L_{\mathrm{p}}\))  
In the initial state, \(F\)s > 1.35 only when the pile was longer than 22 m (Model 1) or 20 m (Model 2), which suggests that slope stability requires the pile to be longer than a certain value. However, the \(F\)s of the slope of \(L_{\mathrm{p}}\)s of 20 m, 22 m, and 24 m in Model 2 all exceeded 1.35 (Figure 9), while the \(L_{\mathrm{p}}\)s were shorter than the buried depth (24.3 m) of the weak interlayer. The composite texture and interaction of the piles and the rock and soil mass significantly enhanced the capability of resisting sliding, so it was unnecessary for the pile to run through all of the weak interlayers. It is worth noting that the increase of \(F\) is not linear with a lengthening of the pile. In our study, \(F\) hardly increased and maintained a fixed value once the pile was longer than the critical length of 28 m in Model 1. The critical length was 30 m in Model 2. Similar characteristics have been reported in [PERSON] et al. (20) and [PERSON] (20). Nonlinear increases in \(F\) suggest that the reinforcement effect is limited by a critical pile length, and excessively long piles lead to wasted material and unnecessary costs. The high compressive stress and the high shear stress areas are always observed at the same depth under the 1.35 state; their values are shown in Table 3. It was found that the maximum principal stress and the maximum shear stress in Mode 1 were always lower than those in Model 2, with a higher \(F\) in Model 1 (Figure 9), which may reflect the influence of the distribution of geological materials. The more complex formation and poor mechanical parameters in Model 2 reduced the \(F\) of the same length, leading to greater stress on the pile. Under the limit equilibrium state, the pile should be long enough to cut off the plastic zone (26 m in Model 1, 30 m in Model 2) to prevent sliding together with the slope. In addition, the longer \(L_{\mathrm{p}}\) in Model 2 further proved the influence of the mechanical parameters of geological materials on pile design. Aboveall, the determination of \(L_{\mathrm{p}}\) should consider the dimensions of the slope (20) and the three states of the pile. A \(L_{\mathrm{p}}\) of 30 m might be an optimal estimate for this slope with multi-layer weak interlayers. Discussion of the Pile Diameter (\(D\)) and the Ratio of On-Center Spacing to Pile Diameter (\(S_{\mathrm{p}}\)/\(D\))  
On the one hand, the slight increase in \(F\) with an increase in \(D\) showed less sensitivity of \(F\) to \(D\), similar to the situation in sandy slopes (20). On the other hand, with a lower \(F\) in the initial state and relatively higher changes of the maximum compressive stress, the maximum shear stress occurs; this was the case for the maximum displacement of the pile head in the 1.35 state of Model 2, which is a common feature in evaluations of \(D\) and \(S_{\mathrm{p}}\)/\(D\) \begin{table}
\begin{tabular}{l l c c c c c c c c c} \hline \hline  & & & & & & & **Pile Length (m)** & & & & \\  & **20** & **22** & **24** & **26** & **28** & **30** & **32** & **34** & **35** & **40** \\ \hline Maximum principal & Model 1 & / & 17.96 & 10.12 & 13.82 & 14.38 & 14.14 & 14.54 & 14.57 & 14.76 & 15.20 \\ stress (MPa) & Model 2 & 17.85 & 18.86 & 18.87 & 19.27 & 19.33 & 17.63 & 17.58 & 16.86 & 17.63 & 18.27 \\ Maximum shear & Model 1 & / & 8.15 & 5.52 & 6.79 & 7.10 & 7.01 & 7.21 & 7.20 & 7.21 & 7.45 \\ stress (MPa) & Model 2 & 8.46 & 9.00 & 9.01 & 9.17 & 9.20 & 9.05 & 9.04 & 8.70 & 9.06 & 9.38 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The magnitude of the maximum principal stress and the maximum shear stress of various pile lengths in Model 1 and Model 2 in the 1.35 state. As discussed in Sections 6.2 and 6.3, the reason for this was the complex distribution of geological materials and their poor mechanical parameters, which resulted in a complex stress field of the slope with multi-layer weak interlayers.