(i) Close mode
It refers to mechanically operated TBM using bulkhead or slurry to balance earth pressure and groundwater. There is no manual access to the face of excavation.
(ii) Open mode
It refers free air hand-dug tunnel or compressed air handing tunneling with manual access to face of excavation.
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(iii) Mixed mode
It is similar to close mode except that it allows access to the face of TBM for manual removal of obstructions.
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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