Based on the self-developed shear-seepage coupling test device for coal rock

a series of direct shear tests was performed on the unfilled joints and joints filled with gypsum

debris

and clay. The 3D optical scanning technology was also employed to examine the morphology and aperture evolution of joints during shearing. The results show that peak shear stress and normal displacement of joints exhibit same regularity

which decreases as filling material varies from gypsum

debris to clay

showing that the nature of filling materials contributes significantly to the shear properties of joints. Unfilled joint is evidently worn after shearing and its JRC decreases most significantly due to direct contacting between the upper and lower surfaces. The JRC of joints filled with gypsum and debris changes relatively little

while it remains nearly the same for joints filled with clay. The materials failure and surface abrasion are mainly controlled by the aperture evolution and local stress concentration. With shear displacement increasing

stress concentrates in the region aperture decreases

where materials failure and abrasion of unfilled joints occurred.