Study of mechanical properties of raw coal under high stress with triaxial compression

According to the raw coal samples from B10 seam of -780 m elevation in Huainan coal mine, the MTS815.04 electro-hydraulic servo testing system is used to carry out the conventional triaxial compression test under high stress. The deformation, strength, parameters and failure characteristics of coal are studied. The results show that: (1) The deviatoric stress-axial strain curves of coal are mainly composed of stages of elastic, yield, post-peak brittle failure or strain softening segment. Among them, the elastic period is longer obviously, and the greater the confining pressure is, the steeper the curve is, the greater the elastic modulus is. But, the yield period is shorter. (2) The post peak brittle failure feature of coal is apparent under uniaxial loading or low confining pressure. As the confining pressure increases, the ductile features start to appear after peak. The higher the confining pressure is, the more obvious characteristics of ductility are. When the confining pressure reaches to 50 MPa, the post-peak axial strain almost presents plastic flow pattern. (3) With the increase of confining pressure, the peak axial strain increases in parabolic trend, and the peak lateral strain increases in linear trend. (4) The deviatoric stress-volume strain curve of coal shows dilatancy mechanism under low confining pressure. The lower confining pressure is, the more obvious the dilatancy features shows. In the high confining pressure, the curve from pre-peak to post-peak is always right extend, and presents a continuous shrinkage state. And with the increase of confining pressure, the peak volume strain presents a parabolic increase form, which reflects the shrinkage characteristics significantly. (5) With the increase of confining pressure, the strength of coal increases in linear trend. The strength parameters c, φ are 12.72 MPa, 24.12°, respectively. (6) The failure modes of coal samples are mainly given priority to shear failure, and the rupture angle is in the range of 23°-35°. With the increase of confining pressure, the rupture angle increases in parabolic trend. The change can be better explained by Mohr strength theory.