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Open shield type TBM refers to those providing lateral support only. They can be further classified into single shield and double shield.
Closed shield type TBM refers to those providing lateral support and frontal support. Some common TBM method under this category includes compressed air TBM, slurry shield TBM, earth pressure balance machine and mixed confinement shield.
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.