The effect of variation in the strength of weathered Kowloon granite on slurry tunnel boring machine tunnelling

The compressive strength of weathered Kowloon granite was assessed by the current authors in 2023, using the data from twin running tunnels constructed by slurry tunnel boring machine (TBM) in weathered Kowloon granite and a sawtooth strength profile in the rock was identified. Further analysis of the data shows just how variable the strength of the rock was, up to 50 m below the rockhead. The TBM data showed that the force applied to the cutting tools was passively following the variation in the strength of the rock. The TBM operators were not maintaining sufficient force on the cutting tools to optimise the penetration rate, particularly in the relatively weaker and more weathered rock. This was due to various factors, including: the small proportion of the total thrust applied to a slurry TBM that reaches the cutting tools; the high degree of variation in the strength of the weathered rock; and the number of tasks required from the TBM operator. The TBM operator was unable to focus exclusively on optimising the penetration rate. Based on information from TBM tunnelling in Kuala Lumpur, an automated system would provide superior performance to a human operator in weathered rock.