Groundwater Inflow into Fractured Rock Tunnels

Abstract: Groundwater inflow is a challenge in construction of tunnels in fractured bedrocks since it affects the safety function of tunnels and leads to potential problems in the surrounding environment, such as subsidence, dropdown of the groundwater table. Quantification of groundwater inflow into the tunnel is also important for design of grouting in the construction of the tunnel. Modelling groundwater flow in fractured bedrocks currently remains a challenge. Commonly used groundwater models are based on continuum assumptions and they do not consider realistic structures of discrete fractures, which leads to high potential uncertainty in prediction of tunnel groundwater inflow. This thesis focuses on prediction of tunnel groundwater inflow, using a discreet fracture-matrix (DFM) model. The DFM model is evaluated and compared with the conventional continuum model based on Darcy’s law. This DFM model considers, in particular, multi-scale heterogeneity, e.g. fracture networks and variable fracture aperture structures. Applying this DFM model, the impact of variable fracture aperture structures on tunnel inflow is investigated through stochastic analysis. The results show that under the same boundary conditions, the traditional continuum model overestimates the inflow compared to the DFM model. The difference in equivalent permeability is 2 to 3 orders of magnitude. The sensitivity analysis shows that the discreet fracture model is sensitive to the variability of fracture aperture. The estimated equivalent permeability values by discreet fracture modelling is in the order of 5×10-6 to 1×10-7 m/s for a fracture density of 1.2 fractures per square meter. This study demonstrates that the DFM represents the more realistic features of fractured rock structures, which is useful and can be used to predict groundwater inflow in fractured rock tunnels.