Statistical Determinations of Steady-State, Groundwater Inflow in Rock Tunnels for the Radial Flow Condition

This paper presents improvements to the current state of the practice in estimating steady-state groundwater inflows into rock tunnels. This analysis applied statistical methods to packer test data obtained from exploratory borings during hydrogeological/geotechnical site investigations from the Chattahoochee, Nancy Creek, and Milwaukee tunnels. The current state of the practice for estimating groundwater inflows in rock tunnels is to apply a semi-empirical procedure developed and published by Dr. Ronald Heuer in 1995. Dr. Heuer identified two fundamental boundary flow conditions that can exist for a rock tunnel based on the work of Goodman. These flow conditions control the steady state groundwater inflow into a given tunnel. The conditions are referred to as the radial flow condition and vertical recharge flow condition. For the radial flow condition, a statistical analysis presented in this paper indicates that insufficient amount of testing could lead to underestimations of the inflow quantities. Packer test data plotted in histograms were observed in all cases to be log-normally distributed for the radial flow condition. Underestimation in the inflow could occur if the total number of packer tests was small. Specifically, the extreme high-end permeability fraction of the distribution could be under-represented or absent. Modeling using Monte Carlo simulations was observed to be an effective tool for incorporating high-end permeability data in groundwater inflow estimates.