## 3 Test Results
### Particle breakage and Distribution Law after Each Loading and Unloading Seepage Test  
After each loading and unloading seepage test, the broken coal samples were screened and weighed using sieves with the pore sizes of 1, 2.8, 4.6, 6.8, 10.2, 15.1 and 18.2 mm, respectively. Afterwards, the mass and mass percentage of the coal samples with different particle sizes were recorded and calculated, as shown in Figure 5. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Test Status** & **Fitting Equation of Permeability** & **Correlation Coefficients (\(R^{2}\))** \\ \hline First loading & \(K=403.2-115.4\) ln(\(\sigma_{1}+0.1\)) & 0.995 \\ First unloading & \(K=237.7-49.8\) ln(\(\sigma_{1}+2.7\)) & 0.999 \\ Second loading & \(K=180.2-32.4\) ln(\(\sigma_{1}+0.3\)) & 0.995 \\ Second unloading & \(K=173.2-27.5\) ln(\(\sigma_{1}+3.1\)) & 0.999 \\ Third loading & \(K=132.0-13.1\) ln(\(\sigma_{1}-0.2\)) & 0.990 \\ Third unloading & \(K=121.5-8.5\) ln(\(\sigma_{1}+1.6\)) & 0.998 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The fitting equations of permeability and effective stress on broken coal samples under cyclic loading and unloading conditions. Figure 5: Mass and mass percentage of coal samples with different particle sizes after each loading and unloading seepage test: (**a**) mass percentage of coal samples with different particle sizes after each loading and unloading seepage test, (**b**) mass and mass percentage of coal samples with different particle sizes after the first loading and unloading seepage test, (**c**) mass and mass percentage of coal samples with different particle sizes after the second loading and unloading seepage test, (**d**) mass and mass percentage of coal samples with different particle sizes after the third loading and unloading seepage test. As shown in Figure 5, after the first loading and unloading seepage test, the broken coal sample with a particle size of 15.1-18.2 mm was crushed to form a mixture with multiple particle sizes due to its having been crushed during the test stage; the crushing amount was 59.74%, which was calculated by dividing the crushing mass of the particles after the test by the original mass of the particles. In the mixture, the proportion of the original particle size mass was the largest, reaching 40.26%, followed by the particle size (10.2-15.1 mm) mass produced by crushing, which was 19.60%, and the smaller particle size mass produced by crushing accounted for a smaller proportion and the difference was not significant, which was approximately between 6% and 10%. After the second loading and unloading seepage test, the particles with a particle size of 15.1-18.2 mm in the mixture were further crushed. The crushing amount was 9.21%, the mass proportion of particle size of 15.1-18.2 mm decreased from 40.26% to 31.05%. However, the proportion of other particles in the mixture increased. Among them, the mass proportion of the particle size of 10.2-15.1 mm increased the most, from 19.60% to 25.46%, and the increase amount reached 5.86%. The increase amount of the other smaller, crushed particles was small and the difference was not large; about 0.5% to 1%. After the third loading and unloading seepage test, the particles with a particle size of 15.1-18.2 mm in the mixture were crushed again, with a crushing amount of 4.59%, and the mass proportion of which decreased from 31.05% to 26.46%. The mass proportion of other smaller particles in the mixture increased and the increase amount was small, between 0.5% and 1%. To sum up, the larger the particle size of a coal sample was, the greater the breakage probability and crushing amount of the particle will be after each loading and unloading seepage test. The major reasons for this are as follows: the larger particles probably contained more defects and their strength was generally lower than that of the smaller particles. Under the same stress, large particle damage probability and breakage amount were obviously greater.