The effect of altering the slurry circulation system on TBM tunnelling in weathered Kowloon granite

Abstract

Twin running tunnels were driven by slurry TBM through weathered Kowloon granite in Hong Kong for MTR Contract SCL 1103. The first drive encountered sub-vertical bands of unusually sticky completely decomposed granite (clayey cdg), as well as the normal granitic saprolite, mixed ground and rock. The TBM was reassembled in a new shield skin for the second drive, and this provided the opportunity to modify the slurry circulation and the operation of the TBM and STP. Plots of Specific Energy and Field Penetration Index against ring number were used to help justify the modifications. The charts helped to identify the extent of the clayey cdg and the adverse effect on the tunnelling. Following the completion of the tunnelling, various combinations of TBM parameters were charted, to find the combination that best correlated with the face conditions and provide a basis for assessing the beneficial effect of the change to the slurry circulation. It was found that plotting Specific Energy against Field Penetration Index provided a reasonable basis for identifying the face conditions in weathered granite. By comparing the data from the two drives, the charts showed that the change in the slurry circulation had a major beneficial effect on the tunnelling in the clayey cdg and in the mixed ground but had little effect on the tunnelling in the saprolite and the rock. The changes to the TBM and STP resulted in the second drive being completed 3.2 months more quickly than the first. In addition to saving time and materials, there were large savings in the electrical energy used, and a significant reduction in the number of interventions under compressed air. The assessment is part of a study on the use of the data generated during TBM tunnelling as a basis for improving the tunnelling.