# Experimental Investigation on Mechanics and
## 3 Results and Discussion  
### Mechanical Properties of Coal during Triaxial Compression  
The stress-permeability-strain evolution of tectonic coal under triaxial compression is depicted in Figure 3a. According to the coal deformation trend, the entire compression process can be divided into four stages: the compaction stage (I), the linear elastic stage (II), the yielding stage (III), and the post-destruction stage (IV) [31]. During the compaction stage, the primary pores and fractures in coal are gradually compressed with the increase of deviatoric stress. As a result, the axial strain and volumetric strain increase and the permeability of coal decreases accordingly. However, the radial strain has remained approximately constant mainly due to the confinement of confining pressure. During the linear elastic stage, the primary pores and fractures in coal continue to close. The radial strain has a slight increase, and the axial strain and volumetric strain increase linearly with the increase of deviatoric stress. Moreover, the coal permeability almost drops to its minimum because of no new pores and fractures are generated [32]. During the yielding stage, the coal expands to the radial direction obviously; however, the increase rate of axial strain and volumetric strain begin to drop slowly. At the same time, the coal permeability starts to rebound due to more and more new fractures and cracks are produced with the increase of deviatoric stress. During the post-destruction stage,the deviatoric stress decreases with the increase of the axial strain, radial strain, and volumetric strain. The coal permeability increases furtherly with forming more and more macro-cracks.  
Figure 3: Stress-permeability-strain curves of coal under triaxial compression. (**a**) tectonic coal; (**b**) intact coal.  
Figure 3b shows the stress-permeability-strain evolution of intact coal under triaxial compression which also can be divided into four stages. However, there are several obvious differences between two groups of stress-permeability-strain curves. Firstly, the radial strain of intact coal increases continuously from the beginning of compression, while that of tectonic coal has an apparent increase from the yielding stage. Secondly, the volumetric strain of intact coal is going down and even negative during the post-destruction stage while that of tectonic coal increases during the whole process, which means the intact coal volume has an expansion at last. Thirdly, the peak stress (\(\Delta\sigma_{c}\)) of intact coal is nearly twice as much as that of tectonic coal and the permeability of intact coal is smaller than that of tectonic coal. Eventually, the deviatoric stress of intact coal during the post-destruction stage has a sharp drop while that of tectonic coal decreases slowly which shows an apparent strain softening behavior.  
In order to compare the mechanics of tectonic coal and intact coal under triaxial compression, the compaction stress (\(\Delta\sigma_{a}\)), damage stress (\(\Delta\sigma_{b}\)), peak stress, residual stress (\(\Delta\sigma_{d}\)), and the Young's modulus (\(E\)) are collected furtherly, as shown in Table 2. It can be found that the compaction stress and residual stress of tectonic coal and intact coal are almost the same, even though their damage stress or peak stress are quite different. Meanwhile, the Young's modulus of tectonic coal is only 74% that of intact coal.