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Why do Slurry Shield TBMs have difficulty when tunneling in clay?

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In Slurry Shield TBMs, the slurry forms a filter cake on the face of excavation which has the following purposes:

(i) It provides the surface for slurry pressure to act on.

(ii) It acts as a seal against the y of groundwater into the tunnel.

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(iii) In case the TBM breaks down, this filter cake serves as a sealing membrane at the tunnel face which allows man-entry into the excavation and working chamber upon provision go compressed air.

Slurry Shield TBMs are widely used for non-cohesive soils ranging from fine-grained sand to coarse-grained gravel. It is less suitable when operating in clay because:

(i) Most slurry separation plant could not separate clay from slurry. As such, the cost of frequent replacement of bentonite slurry is substantially increased.

(ii) Clayey materials to clog and cause blockages in slurry system leading to sudden pressure surges.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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