ABSTRACT
Simultaneously backfill grouting limits the movement due to self-weight and thrust forces generated by the TBM, and the displacement differences between two measurement points, by filling voids left by the TBM between the excavation profile and extrados of the segments, so the system becomes monolithic. On Snowy 2.0, preventing any deviations from construction tolerances of the pre-cast lined tunnel set by the British tunnelling society was a challenge when building a large diameter tunnel in a sharp negative slope using a bi-component backfill grouting and a conventional single shield tunnelling boring machine. Even when injecting grout quite above the theoretical volume per advance, that flowed to the cutting wheel before the gel time formation set by the design specification, and voids were identified during proof drilling, especially in the tunnel crown, even after the secondary grouting had been done. The control of these tolerances constitutes a major challenge in the use of the mechanised tunnelling method. This paper presents the development of the solution on Snowy 2.0 for the Main Access Tunnel through the installation of two auxiliary grouting lines and a manometer, for grouting from the back part of the shield in the tunnel crown, whilst ensuring a grout pressure is maintained in between the acceptable range limits, both above and below the tunnel spring line. After starting the grouting injection from the lines independent from the TBM, the rings built met the tightest construction tolerances set for the Main Access Tunnel.
