Right from the very early stages in the design phase of metro projects around the world, design consultants have included a specification clause stipulating that the tunnel boring machines must be capable of boring in closed mode and be able to maintain face pressure. This of course is to mitigate the risk of ground settlement associated with open-face machines when they encounter unstable geology. However, the clause is also related to preventing depletion of the water table on many projects. After studying this clause in the tender documents, contractors invariably start to assess the advantages and disadvantages of slurry machines versus EPB machines. The problem is that neither slurry nor EPB machines perform particularly well when boring in rock.
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