Numerical and experimental investigation of the hydrogeological evolution around a borehole due to stress redistribution in fractured rock

•Hydrogeological evolution of a borehole is estimated from hydraulic tests and numerical simulations.•Fractures with increasing apertures are more abundant after borehole-induced stress redistribution.•Transmissivity increases after borehole installation and diameter expansion. The area around a borehole is often damaged by the drilling impact and stress redistribution during the installation of a borehole. Numerous studies have examined the damage caused by stress redistribution around a borehole by analyzing spalling at a borehole; however, the hydrogeological evolution of existing fractures around a borehole due to stress redistribution has not been investigated thoroughly, despite its potential influence on uncertainty during fractured rock hydraulic testing. This study numerically and experimentally evaluated the influence of stress redistribution on the hydraulic properties of fractures around a borehole to determine the changes in fracture apertures and permeability during the installation of a 3-inch-diameter vertical borehole, BDZ-1, at the Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel, and after expanding its diameter to 4 in. To confirm the results of the numerical experiments, geophysical logging and hydraulic tests were conducted before and after expanding the diameter of BDZ-1. The numerical results revealed the number of fractures with increased aperture after diameter expansion of borehole was much larger than the number of fractures with decreased aperture due to stress redistribution. Moreover, the transmissivity of the borehole vicinity also increased after diameter expansion. The results of the field tests supported those of the numerical tests, whereby changes in fracture apertures were observed and the transmissivity estimated from the hydraulic tests increased after expanding the borehole diameter. This study indicates that the vicinity of a deposition hole in a subsurface repository for radioactive waste may become a conduit for nuclides despite of little drilling impact due to the increased transmissivity around the hole from borehole-induced stress redistribution and fracture aperture change.