# Stability Index of Surrounding Rock during Deep Rock Excavation Considering Energy Release Speed
## 1 Introduction  
Under the constraints of the United Nations Framework Convention on climate change and carbon emission rights, controlling carbon emissions has become a precondition and consensus of development all over the world. Hence, the development of alternative clean energy sources, such as hydropower, is of great significance to the energy structure. However, areas abundant in hydropower energy often have a significant elevation drop; due to the influence of topography, large-scale underground rock mass excavation is required [1]. As the burial depth increases, tectonic movement of the crust and rock stratum generates a tectonic stress field, which, when combined with the gravitational stress field, can result in extremely high in situ stress. For instance, the diversion tunnel of the Jinping II hydropower station has a burial depth of up to 2500 m [2], with a measured maximum principal stress of 42 MPa and a predicted maximum principal stress of 72 MPa by deduction. The excavation of high-stress rock mass can easily induce dynamic damage to the surrounding rock and even induce engineering disasters, such as rockburst [3; 4]. The disasters induced by high-stress rock excavation, represented by rockburst, pose significant threats to the safety of workers and construction equipment [5; 6; 7]. Therefore, ensuring the stability of surrounding rock is crucial for the construction of deep engineering projects. Rockburst can be classified into two types: \"self-excitation\" and \"far-field stimulation\". \"Self-excitation\" refers to a type of rockburst that occurs when the stress state of surrounding rocks changes during excavation, resulting in a reduction in the energy storage limit of surrounding rocks. If the energy storage of surrounding rocks exceeds their energystorage limit, the stored energy will be released suddenly, resulting in a rockburst. \"Farfield stimulation\" refers to a type of rockburst induced by dynamic disturbance from a distance, such as blasting vibration, microseism, or other rockburst events. This type of disturbance may break the original equilibrium state of the surrounding rock and enables it to accumulate kinetic energy [8]. It is evident that the energy release of surrounding rock induced by excavation is closely related to the stability of the surrounding rock in both the near and far fields [9]. Numerous researchers investigated the influence of the energy release of surrounding rocks on their stability. [PERSON] et al. [10] were the first to discover that the sudden release of initial stress during rock excavation can cause over-relaxation of rock and generate stress waves in the rock mass. The analysis results of the dynamic response of surrounding rock under ideal conditions of excavation of a long tunnel by [PERSON] et al. [11] show that the unloading of initial stress can induce vibration in the surrounding rock. The faster the unloading rate of rock is, the greater the amplitude of the induced dynamic tensile stress field is. [PERSON] et al. [12] found that the unloading rate and dynamic disturbance have significant effects on the occurrence and scale of rockburst. The research of [PERSON] et al. [13] and [PERSON] et al. [14] also indicated that the unloading wave induced by the blasting excavation of rock mass under medium- or high-level in situ stress is a crucial factor that leads to the relaxation of surrounding rock. The higher the initial in situ stress, the greater the amplitude of the excavation load is and the stronger the vibration of surrounding rock induced by it. [PERSON] et al. [15] and [PERSON] [16] further studied the vibration induced by dynamic unloading of in situ stress and compared it with the vibration induced by a blasting load. The results show that while the in situ stress is at a medium or high level, the vibration induced by dynamic unloading may be larger than that induced by the blasting load and become the primary component of the vibration of surrounding rocks. From the perspective of energy, [PERSON] et al.