PREDICTING GROUNDWATER INFLOW INTO HARD-ROCK TUNNELS

Deep hard-rock tunnels are typically excavated below the water table through fractured rock aquifers. During construction, groundwater enters the tunnel both under transient conditions at the heading and under steady-state conditions along the length of tunnel. If the tunnel is long, the accumulated steady-state inflow can become large to the point of impacting construction. Equations for estimating inflow must be based on a geologically reasonable hydrogeologic model. Hydraulic conductivity is typically the most important variable in such equations because of its large range in fractured rock, and is best evaluated as a statistical distribution. For the Chattahoochee Tunnel, in Cobb County, Georgia, this distribution was obtained with over 300 packer tests performed in 50 boreholes along the alignment. The distribution of packer-test data may not accurately reflect hydraulic conductivity, however, due to the limits of the test method and the luck of the field investigation. These discrepancies may be overcome by using log-normal plots of the data to develop a permeability model that will be used in lieu of the data to calculate inflow.