Water inflow into excavations in fractured rock—a three-dimensional hydro-mechanical numerical study

Water inflow into rock excavations is a complex three-dimensional (3-D) problem influenced by a number of processes, such as stress–permeability coupling, groundwater degassing, bubble trapping, temperature effects or turbulence effects. Results from inflow experiments in the field show that when going from a slim borehole to a larger diameter hole, the inflow into the larger hole is often less than predicted; the explanation for this is not yet fully known. A single process or a combination of processes may be responsible for reduction of inflow into the larger diameter holes. In this study, a coupled hydro-mechanical, 3-D discrete element analysis has been conducted with the objective of gaining a better understanding of the influence of effective stress redistribution induced by excavation and related inflow into a cylindrical opening in a fractured rock mass. Matrix and fracture data from the Äspö Hard Rock Laboratory in Sweden have been used as input for the model. Several aspects related to fracture inflow into underground excavations have been studied (hydro-mechanical coupling, uncertainty in rock mass characteristics, relevance of non-linear fracture behaviour and influence of the excavation diameter). Results of this study show that stress–permeability coupling is one of the causes for the usually less than expected inflow into larger diameter holes.