## 2 Evaluation Methods on Rock Stability
### Quantitative Analysis Method
#### 2.2.3 Numerical Simulation Analysis Method  
Numerical simulation analysis is a method used to obtain the stress distribution and SR stability by abstracting the engineering prototype and using numerical analysis software. Due to the intricacies of engineering rock mass, the accuracy of the mathematical method is not sufficient and the physical test simulation method is expensive and not easy to realize, so the numerical simulation analysis method has become a significant tool to solve the rock and soil problems under complex geological conditions, which has the advantages of a short period, low cost, simplicity, etc. [31]. At present, the numerical simulation analysis method is more widely used than the mathematical method. There are two common methods of numerical simulation analysis. The first one is the continuum mechanics method, which generally assumes that rock mass is an isotropic continuum. This method includes the finite element method, boundary element method, finite difference method, etc. [PERSON] used the finite element software Abaqus to research the variation in soil and support stress in the bifurcation section of the cavern group under different construction sequence conditions and put forward a reasonable construction scheme for the bifurcation section [32]. The finite difference method was used by [PERSON] et al. to evaluate the excavation scheme and stability of SR of a 3D cross-cavern group of a hydropower station [33]. The second common numerical simulation analysis method is the discontinuous mechanics method, because the rock mass of an underground cavern contains discontinuities such as cracks, joints and faults, which lead to discontinuity and anisotropy of rock mass. The discontinuous mechanics method is closer to practical engineering. Discontinuous mechanics methods include the discrete element method, block element method, discontinuous deformation analysis (DDA) method, displacement discontinuity method [34], etc. [PERSON] et al. used the discrete element method to study the excavation influence of an underground cavern group of a high in situ stress hydropower station and found that the maximum displacement of the cavern group caused by excavation unloading mostly concentrated near the spandler [35]. The theory of discontinuous deformation analysis of a block system was proposed by Dr. [PERSON], after the key block theory (KBT) was proposed by Professor [PERSON], for rock mass stability analysis to simulate large deformation of a bulk system by taking Professor [PERSON]'s ideas one step further. Dr. [PERSON] published a paper on this method in 1984, but its complete expression was given in his doctoral dissertation, which was completed in 1988 [36]. The DDA model is a kind of discontinuous mechanics model, where the deformation between element interfaces can be discontinuous and the shape of elements can be an arbitrary combination of convex, concave or polygon. The contact between elements does not necessarily require contact between corner points, which is significantly different from the finite element method. At the same time, DDA can reflect the features of large deformation and large displacement of the block system, which is similar to the calculation results of discrete elements, but the mechanical principle is completely different. The basic principle of discrete elements is [PERSON]'s second law; compared with this, the DDA method is based on a more rigorous mechanical foundation in dealing with the mechanical behavior of a block system [37; 38]. [PERSON] et al. used DDA to research the influence of regional in situ stress levels, anchorage, rock mass structural conditions, and structural plane strength parameters of a cavern on the deformation of the SR of a cavern [39]; [PERSON] et al.