## 4 Excavation Design Based on SSR and Stress Unloading Effect
### Blasting Excavation and Stress Transient Unloading Effect  
As previously stated, when in situ stress is high enough, transient unloading effect of high in situ stress is considered to be one of the key factors on excavation disturbance. Specifically, for the drill-blasting method, the forming time of new excavation contour in the blasting excavation process is about several milliseconds to tens of milliseconds, so the stress unloading accompanied by blasting excavation is a transient process. It will inevitably produce dynamic stress in surrounding rock, leading to dynamic damage or cracking of rock mass, and ultimately affect the surrounding rock stability [14]. Obviously, the method of treating rock excavation unloading as a transient process has advantages on excavation design optimization. Therefore, considering the control of stress transient unloading effect under high in situ stress, the supplementary suggestions for blasting excavation design are summarized.  
At present, research shows that deformation and vibration induced by blasting excavation are obviously affected by stress transient unloading. In addition, the deformation curve is of step-style shape as the excavation contour forms [33]. The vibration induced  
Figure 23: The distribution of excavation procedures based on SSR.  
Figure 24: The dominant distribution of stratification height under different SSR at the high side wall zone.  
Figure 25: The dominant distribution of protective cover thickness under different SSR at the high side wall zone.  
by stress transient unloading is proportionate to in situ stress level, which can be more than 50% of the total vibration in cutting blasting [74; 75; 76]. During the cavern excavation in Baihetan, Jinping II, Ertan, etc., a large number of rock-cracking phenomena, such as spalling rib, slabbing, deep cracks parallel to the excavation contour and so on, occurred due to the strong unloading effect and drastic adjustment of the surrounding rock stress field, as shown in Figure 26.  
For the high side wall zone, due to the large-section and larger excavation volume, the transient unloading effect of high in situ stress is particularly obvious. The vibration induced by middle-drift or expanding excavation is cut off by the reasonable use of presplit blasting. Thus, the MEP method in the high side wall zone can be evolved into five types [14], as shown in Figure 27 and Table 5.  
The methods above are the combinations of presplitting blasting and smooth blasting, which mainly reduce the influence of transient unloading effect on side wall and rock anchor beam, and ensure the shaping effect and excavation quality of surrounding rock. Furthermore, [PERSON] et al. [14] pointed out the crack forming difficulty of different blasting methods before bench excavation. It can be found that the clamping stress of presplitting blasting directly in AA (see Figure 27) is 126.3% of in situ stress in the far field while that of middle-drift firstly followed by presplitting is 86%. The improvement effect of middle-drift is emphasized on the difficulty of excavation under high in situ stress, therefore, the suggestions on blasting excavation technology are shown in Table 6. Under lower in situ stress (<10 MPa), presplit blasting, middle-drift excavation and expanding excavation can be selected as the blasting methods while presplit blasting is not recommended to be used firstly under higher ground stress.  
Besides, the excavation disturbance induced by transient unloading is not only related to in situ stress level, but also to the loading size and loading area. The dynamic unloading effect of initial in situ stress can be reduced by controlling the maximum single blasting charge, reducing the hole diameter and detonating firstly in low stress position, etc.