Effects of normal stress on water flow through a single roughwalled rock joint

Water ingress to underground openings is one of the major concerns in mining and tunnelling, because, rock joints provide natural seepage paths for groundwater flow. To study the water flow through deformable rock joints, triaxial tests with water flow were conducted under various confining stress levels (0.7-5.0 MPa). Two tests were performed on fully mated fracture specimens, and three tests were performed on a non-mated specimen where the fractured surfaces were displaced by 2.0 mm at the start of the test. The results show that the volumetric flow rate decreases with the increase in the normal stress, but the decreasing rate of the flow rate tends to be smaller as the normal confining pressure increases. Similar tests were conducted on non-mated specimen to simulate typically mismatched rock joints. The actual values of flow rates of the three tests conducted on nonmated specimen were different from each other, but the trends in the data measured with the increase of the confining stress were similar. Together with mechanical aperture closure, the variation of hydraulic aperture were analysed based on cubic law, which shows similar changing trend with flow rate as confining pressure increases from 0.7 to 5.0 MPa. However, the closure of the mechanical aperture is much larger than corresponding hydraulic aperture variation due to rough fracture surface, approximately 10 times that of the hydraulic aperture in this experimental investigation.