Speaker
Description
Water inflow into subsurface excavations in crystalline fractured rock is a complex problem, largely due to hydromechanical coupled processes associated with excavation. Field experiments can exhibit scale effects, where inflow into excavations can be less than predicted based on observations from smaller diameter holes. This work uses a recent study examining flow through a single fracture subject to normal loading, which indicates a stress dependent reduction in transmissivity. This relationship is implemented within a small scale DFN model representing a deposition tunnel/hole. The approach includes spatial variations in stress induced by excavation, allowing transmissivity to evolve as a function of normal stress acting on individual fractures. Stress normal to individual fractures is calculated and transmissivity corrections are applied based on the local stress sate. These corrections depend both on fracture orientation and the in-situ stress field. A range of realisations are used to evaluate the ability of this approach to predicting inflow into deposition tunnels accounting for the excavation influence zone. In addition, the impact of internal transmissivity heterogeneity is investigated compared against the conventional assumption of uniform in-plane transmissivity. The results aim to clarify whether stress dependent transmissivity improves inflow predictions when upscaling from boreholes to larger excavations.