Analysis of the Transmissivity Development During the Successive Stages of Pre-Grouting of a Tunnel in Hard Rock

Paper in proceeding, 2009

The effect of pre-excavation grouting is in general very difficult to evaluate. The final ingress of water to a tunnel can only be measured after the excavation is completed. The ingress to the tunnel if no grouting were performed is not known. An effort to stepwise evaluate the sealing effect from pre-grouting was carried out for a 20 m section the TASS tunnel at 450 m depth in the Äspö Hard Rock Laboratory in Sweden. Pressure build-up tests were performed in an observational borehole in the tunnel front before and after each grouting round. Inflow tests were performed in the grouting boreholes of each grouting round. Finally, the inflow into the corresponding section of the completed tunnel was measured. The transmissivity was evaluated at all stages of the multi-step grouting and after excavation of the tunnel section. Evaluations of these tests show transmissivity reduction of the grouted rock mass caused by the sealing effect of each grouting event, with the dominant transmissivity decrease produced by the grouting of the first round in the regular fan.