Excavation-induced hydraulic conductivity reduction around a tunnel – Part 1: Guideline for estimate of ground water inflow rate

Field observations indicate that current engineering practice does not consistently estimate ground water flows into unlined rock excavated tunnels due to various factors that analytical solutions do not take into account. These factors include significant geological features, groundwater drawdown, inadequate estimates of hydraulic conductivity from packer tests, and stress-induced rock-mass permeability reduction in the vicinity of tunnel (lining-like zone). A key variable that is not properly accommodated in current practice, is the hydro-mechanical interaction within the joints in the surrounding rock mass. The significance of this variable is discussed in the 1st part of the paper which presents an analytical solution assessing ground water inflow rate into a tunnel using a mathematical derivation that takes into account the excavation-induced rock permeability reduction in the vicinity of a tunnel based on hydro-mechanical coupling effect. In the 2nd part of the paper, results from numerical analysis are presented to verify the proposed analytical solution for estimating ground water inflow rate into a tunnel. Further studies are currently underway to identify other key variables and their impact on the behavior of unlined tunnels and hydrological flow regime in the surrounding fractured rock mass using a distinct element method program which can fully consider hydro-mechanical coupled behavior of joints.