A notable depth-dependence of the effective permeability of fractured rock with a low matrix permeability, low swelling capacity, and limited fracture mineral infilling

Previous studies have suggested that in fractured low-permeability rock characterized by limited fracture mineral infilling and low swelling capacity, the effective hydraulic conductivity of the rock mass decreases with depth to a value similar to that of the matrix. This reduction is attributed to a decrease in the local flow-path connectivity within fractures as a result of normal-stress-dependent fracture closure; however, the depth dependence of the effective hydraulic conductivity has yet to be demonstrated empirically. We measured groundwater inflow during the excavation of shafts and tunnels at the Horonobe Underground Research Laboratory, Japan, in fractured siliceous mudstone at a depth of ~500 m, where previous borehole investigations had indicated a low flow-path connectivity within fractures. The tunnel, with an inner diameter of 4–5 m and total length of 208 m, intersected numerous fractures and faults. Nevertheless, the measured groundwater inflow was remarkably small (1.2–1.9 m3/day). Modelling this inflow indicated that such limited flow requires an effective hydraulic conductivity of ~1 × 10−10 m/s, similar to the hydraulic conductivity of the rock matrix. In contrast, an effective hydraulic conductivity of ~2 × 10−8 m/s has been estimated for the same formation at depths of ~350 m. This reduction in the effective hydraulic conductivity by two orders of magnitude with increasing depth can be explained by normal-stress-dependent fracture closure. The depth dependence of the effective hydraulic conductivity demonstrated here represents an important hydrogeological characteristic that should be considered when assessing the geological disposal of high-level radioactive waste.