## 4 Excavation Design Based on SSR and Stress Unloading Effect
### Excavation Procedure Design Based on the SSR
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Project Name** & **SSR** & **Project Name** & **SSR** & **Project Name** & **SSR** \\ \hline Jinping I & 2.0–3.8 & Laxiwa & 4 & Pushhue & 8.3 \\ Houziyan & 2.0–4.0 & Shuangjiangkou & 4 & Xliuoudu (left) & 9.5–20.2 \\ Lianghekou & 2.0–4.6 & Jiangya & 4.1 & Wujiangdu (Expansion) & 10 \\ Ertan & 2.5–8.7 & Suotengying & 4.1 & Tongbai & 10.9 \\ Baihetan & 2.7–4.7 & Dagangshan & 4.2–7.0 & Three gorges (right) & 11.1 \\ Pubugou & 2.8–22 & Malutang II & 4.4–6.0 & Yantan (2) & 11.4 \\ Jinping II & 2.8–8.0 & longtan & 5.0–10.0 & Yele & 11.4 \\ Guandi & 3.2–7.3 & Goupitan & 5 & Jiangkou & 12.2 \\ Liyang & 3.2–16.3 & Shismahing & 5 & Dongfeng & 14 \\ Wudongde & 3.3 & Guangzhou I & 5.8 & Shuibuya & 16 \\ Xilongchi & 3.3–5.0 & Zhouning & 6.1–7.9 & Baise & 17 \\ Pengshui & 3.3–6.0 & Yangfanggou & 6.2 & Sanbanci & 17.2 \\ Xiaowan & 3.5 & Xiangjiaba & 6.2–9.8 & Xiaolangdi & 20 \\ Dachaoshan & 3.6–6.5 & Huizhou & 6.3 & Daguangba & 20 \\ Lubuge & 3.7 & Yangjiang & 6.7 & Nuozhadu & 20.1–30.2 \\ Changheba & 3.8–7.5 & Yixing & 6.8 & Mianhuatan & 23 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of SSR in forty-eight underground powerhouses. Figure 20: Statistics of excavation procedures at the top arch zone. (**a**) The distribution of excavation procedures based on SSR. (**b**) The relationship between H, S, H/S and SSR. The recommended value is given by the concentration range in the corresponding graph. For excavation procedures in Figure 23, the MEP and CPMB methods are distributed across the entire range of the strength-stress ratio, mainly in the range of SSR < 7.0. The CPHB method is mainly distributed in high SSR (SSR > 7.0). The SSR has obvious influence on the determination of stratification height and protective cover thickness. In Figure 24, as SSR rises, the stratification height presents a certain increasing trend. For low SSR (2 < SSR < 4), the excavation height is generally concentrated on about 3.0-7.0 m, and the value of 3.0-5.0 m is mainly the refined excavation method in the recent years under the high in situ stress power station, which aims to further weaken the unloading effect of high in situ stress. When the SSR is relatively high (>7.0), the suitable stratification height is 5.0-8.0 m, which means that the excavation height can be increased appropriately as SSR > 7.0. For the MEP method in the high side wall area, the protective cover thickness increases with the decrease of SSR. When the SSR is 2-4, the protective cover thickness is about 3.0-5.0 m. When the SSR is high (>7), the protective cover thickness can be reduced to 1.0-3.5 m, as shown in Figure 25.  
To sum up, the influence of SSR on the selection of the excavation procedure was investigated and recommended, as shown in Table 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**SSR**} & **Rock Anchor Beam Zone** & **High Side Wall Zone** \\ \cline{2-4}  & **Protective Cover** & **Stratification Height** & **Protective Cover** \\  & **Thickness** & **Thickness** & **Thickness** \\ \hline
2.0–4.0 & 3.5–6.0 & 3.0–7.0 & 3.0–5.0 \\
4.0–7.0 & 2.5–5.0 & 4.5–8.0 & 1.5–5.0 \\ >7.0 & 1.0–5.0 & 5.0–8.0 & 1.0–3.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Recommendations for excavation procedure design based on SSR (m). Figure 21: The distribution of excavation procedures based on SSR. Figure 22: The dominant distribution of protective cover thickness under different SSR at the rock anchor beam zone.