Q. Differences between open shield and closed shield?

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. 

Compressed air TBM is suitable for cohesive soils under water table (e.g. ground with low permeability with no major discontinuities). Slurry shield TBM is suitable for soft ground and soft rock under water table and also for ground for high permeability. Earth pressure balance machine is suitable for soft ground and soft rock under water table. It is not recommended for very abrasive and hard ground.