## 3 Test Results and Theoretical Analysis
### Analysis of Pre-Peak Loading and Unloading Results  
The damage and softening degree of water on coal samples mainly depends on the mineral composition, particle composition, initial water content, stress state and other factors of coal samples [32]. It can be seen from Figure 3 that the difference of the initial water content makes the coal samples show different deformation properties during the whole deformation and failure process. The strength of the coal sample is larger in dry state, the deformation is larger in the whole failure process, the pre-peak deformation value is almost linear elastic, and the duration is longer. The water affects the self-properties of the coal sample, moisture enters into the pore and crack, wets the particles on the free surface, weakens the relationship between the particles, weakens the cohesion and internal friction coefficient of the coal sample, and causes the strength of the coal sample to decrease obviously. With the increase of water content, the deviatoric stress gradually decreases, from 0% to 0.15% of water content, the ultimate strength from 57.12 to 53.96 MPa, which decreases by 5.84%. Compared with the strength of the dry coal sample, when the water content of coal sample is 0.32%, the ultimate strength is 51.88 MPa, which decreases by 9.68%. When the moisture content of coal sample is 0.65%, the ultimate strength is 49.62 MPa, which decreases by 13.86%. It can be seen that the initial moisture content has a significant impact on the mechanical damage of the coal sample until the final fracture failure. Combined with the test results, the Mohr stress circle of each coal sample under different water content conditions is shown in Figure 4. It can be seen that with the increase of water content, the center of the Mohr stress circle moves to the left, and the ultimate strength and peak strain decrease gradually. Figure 3: Deviation stress–strain-permeability curves of each sample under different water content conditions. (**a**) water content 0% (**b**) water content 0.15% (**c**) water content 0.32% (**d**) water content 0.65%In the pre-peak loading process, the specimen exhibits compression deformation in the axial direction and expansion deformation in the radial direction. According to the test results in Figure 4, the deformation parameters under different stress states are listed in Table 1. When the axial stress is loaded to 18.6 MPa, the axial, radial and volumetric strain of the specimens are different under different water content conditions. When the axial stress continues to be loaded to 25.85 MPa, the absolute values of the axial, radial and volumetric strain of the samples increase gradually under different water content conditions, but due to the anisotropy of the raw coal sample, the deformation difference is large. When the sample reaches its ultimate strength, the axial, radial and volumetric strain of coal samples decrease obviously with the increase of water content. Compared with the dry coal sample, the axial strain of the coal samples with water contents of 0.15%, 0.32% and 0.65% decreased by 4.98%, 11.59% and 17.99% respectively, the radial strain decreased by 40.4%, 14.46% and 17.43% respectively, and the volumetric strain decreased by 52.54%, 8.66% and 18.65%, respectively. It can be seen that with the increase of water content, the deformation amount of coal samples in all directions tends to decrease under the same stress. According to the test results in Table 1, the curves of peak intensity and peak axial strain of coal samples with water content under different water content conditions were drawn, and the relationships between peak strength, peak axial strain and water content were established.