Parallel running tunnels were driven by a 7.46 m-diameter slurry tunnel boring machine (TBM) through weathered Kowloon Granite in Hong Kong for MTR Contract SCL 1103. The final part of the tunnelling was in a full face of Grade III or Grade II rock, up to 50 m below the rockhead. Using the data obtained during the tunnelling, the average strength of the rock was calculated for every ring of advance. The distribution of the rock strength calculated from the two drives was similar, and comparable with the data from the ground investigation. The strength assessed from the TBM data showed a notably sawtooth profile along each drive, with large changes in strength over relatively short distances. The change in strength between each advance averaged 13 MPa, and locally reduced by 80% over 9 m. The changes in strength indicate that the rock had been weakened locally by weathering, consistent with the early stages of the formation of a corestone weathering profile. The focus of the weathering was on a set of inferred joints spaced at an average of about 17 m that represented a small proportion of the joints encountered during tunnelling.