Determination of coal-rock interface strength by laboratory direct shear tests under constant normal load

Surface boreholes were drilled in Southern West Virginia to obtain the specimens containing natural coal-rock interface. The shear behavior of the natural coal-rock interface was studied by performing direct shear tests under constant normal load conditions. The test results indicate that: (1) The average cohesion and friction angle of the coal-rock interfaces are 0.786 MPa and 29.7 degrees, respectively; (2) the normal stiffness of the coal-rock interface ranges from 0.57 to 4.44 MPa/mm, while the shear stiffness varies from 1.14 to 3.47 MPa/mm. Additionally, analysis on the shear behavior of the coal-rock interface and the corresponding AE signals during shearing indicates that AE can be used to evaluate the shear behavior of the coal-rock interface. Furthermore, it has been found that when normal loads are smaller than 3 kN (equivalent normal stress <1.5 MPa), the shear behavior of coal-rock interface strongly depends on the surface roughness of coal-rock interface; as the normal load is increased to or larger than 4 kN (equivalent normal stress >2.0 MPa), however, the shear behavior of coal-rock interface are more influenced by the strength differences between the two halves rather than the surface roughness of the coal-rock interface.