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.