## 1 Introduction
For the stress transient unloading effect, numerous research results show that the redistribution of surrounding rock stress caused by stress dynamic unloading has an important influence on the formation and development of the excavation-disturbed area [20; 21; 22]. [PERSON] et al. [23] proposed that the damage zone is influenced by both the magnitude of in situ stress and waveform of the dynamic disturbance. It will be larger with the increase of in situ stress degree [24]. Furthermore, through the mechanical analysis of the excavation load release process, the transient dynamic effect of in situ stress under medium and high in situ stress was proposed and demonstrated by the construction process, especially the coupling process of different construction methods and high-level in situ stress dynamic redistribution [4]. Simultaneously, the unloading effect under high in situ stress was considered as the main inducement of a rock loosening zone [25; 26; 27]. The combined action of explosion and stress transient release was also confirmed by the analysis of excavation vibration [28]. From this, the excavation control technology based on the transient unloading effect control was established through the excavation procedure optimization, contour blasting methods comparison and active prevention methods of rock burst in deep-buried caverns, etc. [14]. Based on the above-mentioned theoretical research, the non-simultaneous excavation of adjacent caverns was suggested to minimize the excavation impact of adjacent caverns through the numerical simulation and engineering practice [29; 30; 31]. The reasonable arrangement of excavation time and space distance in each area of the powerhouse is very effective to reduce the excavation damage and it was suggested to reduce the excavation volume of each step as much as possible. [PERSON] et al. [32] illustrated that more energy release caused by large-scale integral excavation could aggravate the possibility of rock-burst. Furthermore, the step-style deformation curve is the most common in powerhouse excavation, which shows the sudden adjustment of surrounding rock stress with the excavation sequence [33]. Correspondingly, [PERSON] et al. [34] calculated the creep deformation curve and the corresponding optimal supporting time point. To make the construction procedure adjustment more scientific, the dynamic feedback and planning method was proposed and applied [35]. At present, blasting technology is relatively mature. Both presplitting blasting and smooth blasting can obtain a smooth excavation profile, meanwhile presplitting blasting can form pre-cracking to make isolation and crack resistance [36]. [PERSON] et al. [15] found that middle-drift excavation beforehand would make the crack formation easier, which  
Figure 2: Four excavation zones of the main powerhouse. is proved in powerhouses, such as Ertan, Guandi, Jinping I and II, etc. In addition, the reserved protective cover was more adapted to the actual construction for vibration control. Meanwhile, the deep presplitting followed by thin stratification excavation gradually becomes the main choice of hydropower station under high in situ stress, such as Xilioudu, Baihetan, Changheba, etc. [37; 38]. Furthermore, the fine construction was emphasized, including the selection of drilling equipment, drilling accuracy and blasting parameters, [PERSON] and [PERSON] [39] achieved a good excavation effect in the Xiangjliaba powerhouse where the influence depth caused by blasting excavation and stress unloading was a new record of 0.7-0.9 m.  
To a certain extent, previous studies have shown that the engineering analogy method can provide guidance for construction procedure design today. [PERSON] [40] summarized the excavation characteristics of nine typical underground powerhouses in China and provided the procedure suggestions from construction access road, machinery and vibration control. [PERSON] et al.