What is the main operational principle behind a Slurry TBM's face support system?

The main operational principle behind a Slurry TBM's face support system is to maintain face stability by applying a controlled slurry pressure to the excavation face, counteracting the earth and water pressures. This prevents collapse or excessive deformation of the ground ahead of the TBM. A slurry, typically a mixture of bentonite clay and water or other specialized fluids, is pumped into the excavation chamber at the TBM face. The slurry permeates into the surrounding soil, forming a filter cake on the excavation face. This filter cake acts as a membrane, preventing groundwater inflow and stabilizing the soil particles. The pressure of the slurry is carefully controlled to match or slightly exceed the in-situ earth and water pressures. This pressure balance ensures that the soil remains stable and prevents the tunnel face from collapsing. The excavated material is mixed with the slurry and transported hydraulically through pipelines to a separation plant on the surface. The separation plant removes the soil particles from the slurry, and the cleaned slurry is then recirculated back to the TBM. The slurry pressure is continuously monitored and adjusted to maintain face stability as the TBM advances. The key to success is maintaining a consistent slurry pressure and ensuring the formation of a stable filter cake. The term 'face stability' refers to the ability of the excavated tunnel face to remain stable and prevent collapse. 'Bentonite clay' is a type of clay that swells when mixed with water, forming a viscous slurry. The 'filter cake' is a layer of slurry solids that forms on the excavation face, reducing permeability and providing support.