## 2 Overviews of In Situ Stress and Fifty-One Underground Powerhouses in China
It is pretty obvious that in situ stress levels of hydropower powerhouses are gradually increasing with construction development. In addition, most of the hydropower powerhouses are located in the southwest region, such as Ertan, Xiaowan and Houziyan, etc. All the powerhouses above are arranged in deep rock, and basically adopt the design mode that the main powerhouse, main transformer room and tailrace gate chamber are arranged in parallel, the same as shown in Figure 1b. Specifically, it is obvious that in situ stress levels of powerhouses gradually increase with the construction starting time basically except for Ertan. In addition, hydropower powerhouses under high in situ stress are mainly located in the southwest region of China. In the recent decades, under the construction demand of numerous underground powerhouses mentioned above, a series of construction technology and excavation procedures have gradually occurred. Facing the new problems such as high in situ stress, etc., excavation technology has been optimized as the engineering construction increases afterwards. Figure 7 presents the improvement history of excavation technology for underground powerhouse in China. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Project Name**} & **Installation Capacity (MW)?** & **Main Powerhouse Size (Length \(\times\) Width \(\times\) Height/m & **Lithology** & **Rock Classification** & **In Situ Stress Level (MPa)** \\ \hline Malutang II & \(3\times 100\) & \(97.4\times 19.6\times 41.1\) & Gneiss & I & 9.0\(\sim\)12.5 \\ Three gonges (right) & \(6\times 700\) & \(311.3\times 32.6\times 87.3\) & granite & I, II & 11.2\(\sim\)12.3 \\ Tongbai & \(4\times 300\) & \(182.7\times 24.5\times 62.3\) & Granite & II, III & 10.9\(\sim\)12.2 \\ Xiangjiaba & \(4\times 800\) & \(255.4\times 33.4\times 85.2\) & Sandstone & II & 8.2\(\sim\)12.2 \\ Wudongde & \(12\times 850\) & \(330.0\times 32.5\times 89.8\) & Limestone, & II, III & 6.0\(\sim\)12.0 \\ Xilongchi & \(4\times 300\) & \(149.3\times 21.8\times 49.0\) & Limestone & III & 12.0 \\ Goupitan & \(5\times 600\) & \(230.5\times 27.0\times 75.3\) & Limestone, & II, III & 12.0 \\ Shisanling & \(4\times 200\) & \(145.0\times 23.0\times 46.6\) & Claystone & II, III & 9.5\(\sim\)11.7 \\ Pengshui & \(5\times 350\) & \(252.0\times 30.0\times 68.5\) & Limestone, & II, III & 8.5\(\sim\)11.0 \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \end{tabular}
\end{table} 
Table 1: Cont. In Figure 7, there are mainly four stages for excavation technology development in China [64]. Since the construction of the Gutianxi I station, the backward technology of blasting construction in China has gradually developed. In addition, smooth blasting, presplitting blasting and buffer blasting are gradually used in underground engineering. With the introduction of NATM [65; 66], the excavation method of hand drill, free blasting and scaffolding support was replaced by the technology of combined bolting and shotcrete. In 1985, the rock anchored beam technology was introduced into the Lubuge hydropower station in Norway [67], and further improved and developed in China [52]. In this stage, the construction principle of multi-sequences and cross-sequences was summarized. After opening to the outside world, blasting excavation technology and procedure design have been developed rapidly. Therefore, a series of systematic excavation methods has been formed in China, including various typical excavation and support technologies as shown in Figure 7[10]. The drill-blasting method has always been the main technical method for powerhouse excavation and the relevant excavation method system has been basically formed [5; 6; 7]. However, there is no good basis for parameter setting of excavation procedures in the face of so complex conditions [41; 42]. Thus, under high in situ stress, it is quite necessary to  
Figure 6: Development history of underground powerhouse in China.