## 2 Stability Evaluation of Coal Mine Goaf Site
### Stability Evaluation System of Coal Mine Goaf Site  
The coal mine goaf site undergoes a highly intricate deformation process. To establish a scientific system to evaluate the stability of the coal mine goaf site, it is crucial to thoroughly account for the impact brought about by a diverse array of factors on said site. By referencing the works of [PERSON] [4], [PERSON] [22], and [PERSON] [23], this paper classifies the control variables affecting the stability of coal mine goaf into four categories: coal mine goaf parameters, engineering geology, hydrological conditions, and external disturbance. The construction of the stability evaluation system of the coal mine goaf site is shown in Figure 1, where the parameters of the goaf are: span (C1), mining depth-to-thickness ratio (C2), area (C3), and specification (C4) (ratio of span to height); engineering geology: rock mass structure (C5), rock quality index (C6), and geological structure (C7); hydrological conditions: underground visible water (C8), and groundwater body (C9); external effects: surrounding mining (C10), adjacent coal mine goaf conditions (C11), and engineering layout (C12).  
Of these, rock mass structure, geological structure, underground visible water, groundwater body, surrounding mining, adjacent coal mine goaf working conditions, and engineering layout are qualitative indicators which need to be quantified before the calculation of catastrophe progression method. The quantitative method is as follows.  
Figure 1: Evaluation index system of coal mine goaf site stability.  
Rock mass structure: 1 is a complete block structure, 2 is a layered structure, 3 is a cataclastic structure, and 4 is a loose structure.  
Geological structure: 1 is no fault and fold, 2 is the small influence of fault and fold, 3 is a partial cutting of the fault, 4 is a fault penetrating surrounding rock.  
Underground visible water: 1 is no water trace, 2 is visible water trace during rainfall, 3 is water trace during heavy rainfall, and 4 is water trace during rainy season.  
Groundwater body: 1 is no water body around the surrounding rock, 2 is the surrounding rock being little influenced by the water body, 3 is the surrounding rock being influenced by the water body, and 4 is the surrounding rock being greatly influenced by the water body.  
Surrounding mining: 1 is no other operation influence in the mining area, 2 is the general influence of other operations in the mining area, 3 is the great influence of other operations in the mining area, and 4 is the great influence of other operations in the mining area.  
Working conditions in the adjacent coal mine goal: 1 is no other mining area within the influence area; 2 is a small area and small number of mining areas within the influence area; 3 is a large area and a large number of mining areas within the influence area but the distribution is dispersed; and 4 is a large area and a large number of mining areas within the influence area and the distribution is centralized.  
Engineering layout: 1 is reasonable, 2 is relatively reasonable, 3 is partially reasonable, and 4 is extremely unreasonable.