## 4 Classification Standards of Bursting Liability Indices
### Applicability and Uniformity of Bursting Liability  
#### 4.1.1 Bursting Liabilities of Non-Coal Rocks Using Classification Criterions of Coals  
The bursting liability classification results of coals and non-coal rocks based on the classification standards in GB/T 25217.2-2010 [24] were obtained, and the percentages of different bursting liability types (heavy, weak, and none) were drawn in the pie charts, as shown in Figure 8. In Figure 8a-c, taking \(\sigma_{\rm c}=50\) MPa (the \(\sigma_{\rm c}\) maximum value of coals) as the split point, the non-coal rocks with \(\sigma_{\rm c}>50\) MPa and those with \(0<\sigma_{\rm c}<50\) MPa were discussed separately. The ratios of the three indicators' bursting liability classification results are presented in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Statistical Parameter** & **Mines** & _DT/ms_ & \(W_{\rm ET}\) & \(W_{\rm{\it off}}\) & \(B\) \\ \hline \multirow{2}{*}{Variance} & coal & 131544.10 & 16.48 & 13.56 & 81.92 \\ \cline{2-6}  & non-coal & 872103.17 & 8.44 & 33.01 & 77.47 \\ \hline \multirow{2}{*}{Standard deviation} & coal & 362.69 & 4.01 & 3.65 & 10.36 \\ \cline{2-6}  & non-coal & 933.86 & 2.90 & 5.75 & 8.80 \\ \hline \multirow{2}{*}{Median} & coal & 158.00 & 3.89 & 2.73 & 13.54 \\ \cline{2-6}  & non-coal & 86.03 & 4.22 & 2.84 & 17.50 \\ \hline \multirow{2}{*}{Maximum} & coal & 2943.00 & 25.80 & 22.00 & 52.31 \\ \cline{2-6}  & non-coal & 3977.00 & 25.89 & 34.18 & 55.00 \\ \hline \multirow{2}{*}{Mean} & coal & 261.89 & 5.19 & 4.07 & 16.42 \\ \cline{2-6}  & non-coal & 378.13 & 4.77 & 5.00 & 18.53 \\ \hline \multirow{2}{*}{Minimum} & coal & 0.09 & 0.15 & 0.66 & 2.19 \\ \cline{2-6}  & non-coal & 4.87 & 0.60 & 0.44 & 3.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistical parameters of the rockburst indices of coals and non-coal rocks. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rock Type** & **Index** & \multicolumn{3}{c}{**Rockburst Tendency Proportion**} & \multirow{2}{*}{**Approximate Ratio**} \\ \cline{3-4} \cline{6-6}  & & **Heavy** & **Weak** & **None** \\ \hline \multirow{3}{*}{Coal} & \(W_{\rm{\it ET}}\) & 42.8\% & 46.7\% & 10.5\% & 9.9\%2 \\  & \(W_{\rm{\it off}}\) & 32.8\% & 49.8\% & 17.4\% & 3.5\%2 \\  & \(D^{\rm{T}}\) & 29.9\% & 53.5\% & 16.6\% & 3.5\%2 \\ \hline \multirow{3}{*}{Non-coal rocks} & \(W_{\rm{\it ET}}\) & 44.7\% & 45.5\% & 9.8\% & 9.9\%2 \\  & \(W_{\rm{\it off}}\) & 31.1\% & 45.6\% & 23.3\% & 6.9\%5 \\ \cline{1-1}  & \(D^{\rm{T}}\) & 11\% & 31.5\% & 57.5\% & 1.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Rockburst tendency ratios of coals and non-coal rocks. Figure 8: Percentages of coal and non-coal rocks with different bursting liabilities: (**a**) elastic strain energy index \(W_{ET}\), (**b**) bursting energy index \(W_{cf}\), and (**c**) duration of dynamic fracture \(DT\). Overall, the \(W_{ET}\) bursting liability degrees (heavy, weak, and none) of coals and non-coal rocks, were 42.8%, 46.7%, 10.5%, and 44.7%, 45.5%, 9.8%, respectively, as shown in Table 3 and Figure 8a. The \(W_{cf}\) bursting liability degrees (heavy, weak, and none) of coal and non-coal rocks, were 32.8%, 49.8%, 17.4%, and 31.3%, 45.6%, 23.3%, respectively, as shown in Figure 8b. It can be clearly concluded that there are obvious similar distribution properties between \(W_{ET}\) and \(W_{cf}\) for coal and non-coal rocks. The percentage ratio among heavy, weak and no bursting liability for elastic strain energy index \(W_{ET}\), bursting energy index \(W_{cf}\) and dynamic fracture duration time \(DT\) of coals is approximately 9:9:2, 3:5:2, and 3:5:2, respectively, and those of non-coal rocks are 9:9:2, 6:9:5, 1:3:6, respectively, as shown in Table 1. For \(W_{ET}\), \(W_{cf}\) and DT of coals, the weak bursting liability occupies the largest proportion, followed by heavy bursting liability, the least is for no bursting liability.