Abstract: This paper is to investigate mesoscopic features of coal seam and rock mass in the deep field, which includes the mineral, configuration, and structure of the fractured surface of specimens. The effect of confining pressure on creep damage mechanism and its regulation were also studied. Under short period creep loading, the observed mesoscopic features were classified into cleavage fracture-joint fractures, intergranular fracture, and intergranular-transgranular fractures under confining pressures of 0, 10, and 20 MPa, respectively. Under the confining pressure of 0 MPa (i.e., the uniaxial creep test), the failure fracture was caused by the simple stress acting on coal seam and rock masses directly. As a result, its cleavage fracture and joint fissure had distinct structure, clear orientation, and fresh interior without inclusions. Under the confining pressure of 10 MPa, the interface of intergranular fracture was found to be relatively wide, and the interior was not clear and filled with inclusions. It indicated that the grain boundary was easily broken under the higher confining pressure, and the mineral structure was changed by adjusting both the tangential movement among grains and the grain rotation under the action of torque. Under the confining pressure of 20 MPa, since the grain boundary was restricted and grain itself was difficult to be destroyed, the ultimate destruction of the fracture along the grain boundary fracture mainly showed the arc rotation. Besides, the siginificant feature of transgranular fractures was characterized by the shear displacement. Under strong restrictions, intergranular fracture structure was not clean and filled with little new inclusion caused by the earlier period of confining pressure. In contrast, the transgranular fracture was tiny, fresh, and pure without inclusions.

Key words: fracture, mesoscopic damage mechanism, confining pressure, cleavage fracture, intergranular fracture, transgranular fracture