The authors have analysed c. 20 km of tunnelling using a double-shielded, hard-rock tunnel-boring machine (TBM). The study is interesting because the tunnelling encountered a variety of rock types, providing a comparison of the field performance.

In Table 2, the diameter of the cutter head is given as 6325 mm, with 47 discs at 75 mm spacing. A 75 mm spacing would require 42 equally spaced discs. For calculation purposes it is common to reduce the effective number of discs to allow for the increasing angle and closer spacing of the discs in the area of the gauge. Although not...

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