## 5 Discussion  
When the analysis results are summarized in this study, for underground powerhouses, there exist differences at various considerations, such as geological structure, rock structure, cavern scale, cavern size, construction technique and quality requirement, etc. The differences could lead to the relative dispersion of statistical results, and the goodness of linear fitting is not the best. Therefore, the rationality of statistical results and the accuracy of analysis results are analyzed and discussed here. In terms of research methods, the existing powerhouse excavation design is mainly carried out from theoretical analysis, numerical simulation and local excavation test. Theoretical analysis and numerical simulation are convenient for the prediction and control of excavation disturbance, and they can show research results qualitatively and intuitively. However, it is too simplistic to reflect the real and detailed excavation results. For the excavation design of a large underground powerhouse, a field excavation test is the most ideal and real method, but it is impossible to judge the excavation stability of the next powerhouse by building one powerhouse firstly. The local excavation test can only reflect the excavation response of some areas. From a macro point of view, the excavation design analyses and recommendations summarized in this paper can be regarded as the summary of the actual excavation tests of large-scale factory buildings under various conditions, including 51 actual excavation tests. Considering the detailed and practical conditions, the results are authentic. At the same time, it also takes various special factors into account, such as soft rock, hard rock, karst and so on. The results are detailed and comprehensive. For the main influencing factors, construction technique and quality requirement are basically the same because the NATM has been used in most statistical engineering, and the blasting methods are basically the same. Geological structure and rock structure mainly affect the quality of surrounding rock, the influence degree of which can be represented by classification of surrounding rock. Figure 28 shows the distribution of surrounding rock classifications of the fifty underground powerhouses of Figure 1. It can be found that the distribution of surrounding rock types is similar to a normal distribution. Nearly 35-40 underground powerhouses have II-III surrounding rock, accounting for 81%. From this, the underground powerhouse is mainly composed of II-III surrounding rock, and the \begin{table}
\begin{tabular}{c c} \hline \hline
**The Stress Perpendicular to the Axis of** & **Recommended Blasting Excavation** \\
**Powerhouse (MPa)** & **Technology Firstly** \\ \hline \(<\)10 & Contour presplitting, Middle-drift, Expanding \\ \(>\)(10–12) & Middle-drift, Expanding \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The recommended blasting technology according to in situ stress condition. others are I, IV, V. Various underground powerhouses have little difference in surrounding rock classification. Thus, the results are very representative from the view of geological conditions. The influence of cavern scale and size on powerhouse excavation lies in the stress disturbance during the forming process of typical excavation section (see Figure 2). According to the existing statistics, the width of a typical section is between 20-30 m and the height is between 40-90 m, but the design of the height-width ratio is not affected by in situ stress level, which is around 2.3-2.5 (see Figure 29). Therefore, the shape of the stress redistribution field of each powerhouse is basically the same, and the influence of the cavern size and height width ratio on the results is small. Besides, in terms of excavation sequence, the stress changes induced by the previous excavation behavior are also one factor to determine the next excavation procedure, which can be monitored with a string of sensors [77].