# Evaluation Method for Rock Burst Hazards in Strip Filling of Working Faces in Deep Coal Mines
## 3 Impact Hazard Evaluation Method  
### Indicators and Membership Functions Determination  
#### 3.1.1 Stress State of Coal  
1. Coal body The causes for rock bursts are mainly related to the stress distribution, strength, and inherent impact tendency of the coal body [26; 27]. To quantitatively analyze the influence law of sensitivity factors on the impact risk of the filling working face, the membership function \(U(x)\) of the impact risk of the filling working face is established as follows: \[U(x)=f(\sigma,[\sigma_{\rm c}],W_{\rm ET})\] (1) where \(\sigma\) is the coal seam abutment pressure; \([\sigma_{\rm c}]\) is the uniaxial compressive strength of the coal; and \(W_{\rm ET}\) is the elastic energy index of the coal seam. The impact risk is approximately linear with the stress level of the coal seam, and the ratio of abutment pressure to coal strength is one of the key factors [28; 29]. Under the condition of a given coal strength, the greater the abutment pressure, the more load is transferred from the overlying strata to the coal, and the higher the risk of coal impact instability. The ratio of coal stress to its uniaxial compressive strength is recorded as \(I_{\rm c1}\), which is  
\[I_{\rm c1}=\frac{\sigma}{[\sigma_{\rm c}]} \tag{2}\]  
The necessary condition for the coal impact instability is that the abutment pressure exceeds the uniaxial compressive strength of coal [30]. According to the engineering practice experience, if the coal body stress is smaller than the uniaxial compressive strength of 1.5 times the coal seam, then a rock burst does not occur in the coal body; when the coal stress is equal to 1.5-2.0 times the uniaxial compressive strength of the coal seam, there is a weak impact risk; if the coal stress is between 2.0 and 2.5 times of the uniaxial compressive strength of the coal seam, there is a medium impact risk; and when the coal stress is greater than 2.5 times the uniaxial compressive strength of the coal seam, the coal seam has a strong impact risk. The membership function \(\varphi_{\text{L}_{\text{cl}}}\) of coal stress ratio \(l_{\text{cl}}\) to local impact risk is established [31], as shown in Equation (3):  
\[\varphi_{l_{\text{cl}}}= \begin{cases}0.167l_{\text{cl}}&I_{\text{cl}}<1.5\\ 0.5l_{\text{cl}}-0.5&1.5\leq l_{\text{cl}}<2.5\\ 1&I_{\text{cl}}\geq 2.5\end{cases}  \tag{3}\]  
2. Strip coal pillar  
The impact risk of the strip working face is related to the stress concentration coefficient of the strip coal pillar. In the process of roadway excavation, a large number of large-diameter boreholes are constructed on the inside of the strip coal pillar, which destroys the integrity and causes plastic damage to the shallow, which reduces its effective bearing capacity. Therefore, the abutment pressure is transferred to the deeper parts. In the middle of the strip coal pillar with a certain width, the coal is in a three-way stress state, and its bearing capacity is much greater than that of the shallow surrounding rock. Generally, this part of the coal exists in an elastic area. The coal in the elastic area maintains its structural integrity and is the main load-bearing body for the overlying strata. The width of the elastic area determines the overall bearing capacity of the strip coal pillar. The larger the elastic area, the stronger the bearing capacity of the strip coal pillar and the stronger the ability to gather the elastic deformation energy. Conversely, the smaller the elastic area, the weaker the bearing capacity of the strip coal pillar and the more likely the overall impact instability of the working face [32]. The size of the elastic area is affected by many factors, such as working face width, size and depth of the pressure relief boreholes, burial depth, coal strength, and coal seam thickness. Before the coal mining on both sides, the working face is subjected to the static load transmitted by the overlying strata, and the abutment pressure is evenly distributed.