# Experimental Investigation on Mechanics and
## 3 Results and Discussion
### Seepage Lagging and Recovery Effect during Triaxial Compression  
As mentioned above, during the yielding stage of triaxial compression, the coal produces new fractures and cracks leading to the increase of coal permeability. However, the rebound time of coal permeability (\(t_{2}\)) is later than the initial time of yielding stage (\(t_{1}\)), namely, the seepage lagging effect, as shown in Figure 4. The time difference of \(t_{2}\) and \(t_{1}\) is defined as the seepage lagging time (\(\Delta t\)). As we can see from Table 3, the seepage lagging time of intact coal is much larger than that of tectonic coal. Therefore, when the tectonic coal is subjected to the damage stress, its permeability increases immediately. For the intact coal subjected to the damage stress, its permeability increases after a period of time. It is mainly because that the primary fractures of tectonic coal are developed, and the new produced cracks are easy to connect the primary fractures and pass through the coal body. The new produced fractures of intact coal must reach a certain degree to pass through the coal body due to the limited primary fractures in coal. The permeabilities of both tectonic coal and intact coal show an evolutionary trend of decreasing first and then recovery. However, their critical permeabilities of each stage are different, as shown in Figure 4 and Table 4. The primary permeability (\(k_{0}\)), minimum permeability (\(k_{1}\)) and residual permeability (\(k_{2}\)) of tectonic coal and intact coal are \(3.01\times 10^{-16}\) m\({}^{2}\), \(0.75\times 10^{-16}\) m\({}^{2}\), and \(1.47\times 10^{-16}\) m\({}^{2}\) and \(0.80\times 10^{-16}\) m\({}^{2}\), \(0.21\times 10^{-16}\) m\({}^{2}\), and \(1.96\times 10^{-16}\) m\({}^{2}\), respectively. On the one hand, for the tectonic coal, the minimum permeability is only 24.9% of the primary permeability, and the residual permeability is only 48.8% of the primary permeability. For the intact coal, the minimum permeability is 26.3% of the primary permeability while the residual permeability is 245.0% of the primary permeability. On the other hand, the primary permeability and minimum permeability of tectonic coal are 3.76 times \begin{table}
\begin{tabular}{c c c c c c} \hline
**Coal Sample** & \(\Delta\sigma_{a}\) **(MPa)** & \(\Delta\sigma_{b}\) **(MPa)** & \(\Delta\sigma_{c}\) **(MPa)** & \(\Delta\sigma_{d}\) **(MPa)** & \(E\) **(GPa)** \\ \hline Tectonic coal & 4.22 & 18.46 & 20.96 & 16.64 & 1.58 \\ Intact coal & 4.17 & 37.59 & 39.92 & 16.00 & 2.14 \\ \hline \end{tabular}
\end{table} 
Table 2: Critical stresses of tectonic and intact coal during each stage. \begin{table}
\begin{tabular}{c c c c} \hline
**Coal Sample** & \(t_{1}\) **(s)** & \(t_{2}\) **(s)** & \(\Delta t\) **(s)** \\ \hline Tectonic coal & 339.0 & 343.4 & 4.4 \\ Intact coal & 522.0 & 575.0 & 53.0 \\ \hline \end{tabular}
\end{table} 
Table 3: Critical time of tectonic and intact coal during each stage. and 3.57 times that of intact coal, respectively. However, the residual permeability of tectonic is only 75.0% that of intact coal. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Coal Sample** & \(k_{0}\) (10\({}^{-16}\) m\({}^{2}\)) & \(k_{1}\) (10\({}^{-16}\) m\({}^{2}\)) & \(k_{2}\) (10\({}^{-16}\) m\({}^{2}\)) & \(k_{1}\)/\(k_{0}\) (\%) & \(k_{2}\)/\(k_{0}\) (\%) \\ \hline Tectonic coal & 3.01 & 0.75 & 1.47 & 24.9 & 48.8 \\ Intact coal & 0.80 & 0.21 & 1.96 & 26.3 & 245.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Critical permeabilities of tectonic and intact coal during each stage. Figure 4: Stress-permeability-time curves of coal under triaxial compression. (**a**) tectonic coal and (**b**) intact coal. The permeability recovery value after tectonic coal failure is very limited, even lower than that of intact coal, although its primary permeability is much higher than that of intact coal. On the contrary, the permeability recovery value after intact coal failure is more than twice of its primary permeability.