What is the purpose of the Bentonite slurry used in some TBM operations?

Bentonite slurry serves several critical purposes in certain TBM operations, most notably in Slurry TBMs. Primarily, it provides face support. The slurry is pumped into the excavation chamber to balance the earth and water pressures acting on the tunnel face, preventing collapse or instability. It also forms a filter cake. As the slurry permeates the soil or rock around the excavation face, it deposits a thin layer of bentonite particles, creating a relatively impermeable membrane that reduces groundwater inflow. Bentonite facilitates material transport. The excavated soil or rock is mixed with the slurry, which then transports the mixture hydraulically to a separation plant on the surface. The separation plant removes the solids from the slurry, and the cleaned slurry is recirculated back to the TBM. Bentonite reduces friction. It acts as a lubricant, reducing friction between the cutterhead and the surrounding ground, which lowers the required torque and energy consumption. It also aids in sealing. It helps seal the gaps between the tunnel lining segments, preventing water inflow and improving the overall water-tightness of the tunnel. The term 'Bentonite slurry' is a mixture of bentonite clay and water. 'Slurry TBMs' are tunnel boring machines that use a slurry to support the excavation face. 'Filter cake' is the layer of bentonite particles deposited on the excavation face.