The cutting wheel of a TBM in most cases is a bit larger than the TBM itself. There is only a small difference (a few centimetres on a TBM with a diameter of 10m or more), but this is significant. It means that for a slurry shield, bentonite can flow from the tunnel face back to the grout used to fill up the tail void. However, it is also possible that the grout can flow from the tail void to the face. It is important to quantify the flow of both the bentonite and the grout as these determine the quality of the tail void grouting and in non-cohesive soils these flows have to be controlled to avoid erosion of the soils around the TBM. Furthermore these flows determine the pressure distribution on the shield that is important in both the TBM design and for the calculation of settlement. A lack of knowledge of pressure distribution on a TBM has led to TBM deformation during tunnelling (Barends, 2005).
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