Slurry TBM provide face support by pressurizing slurry in excavation chamber to balance soil and water pressure on working face. The excavated muck is transported by a slurry circulation system. These machines are characterized by stabilizing working face in sandy to gravely ground with precise settlement control.
Precise pressure control There are two methods of ensuring that the face pressure is maintained with a slurry system. The first is by the use of differential pumping. I.e. the pumps used to supply and discharge the slurry from the machine are careful controlled so that the pressure between them is maintained at the face pressure. In these systems it is normal to use the variable speed of the discharge pump to control the desired flow, and the variable speed of the supply pump to determine the face pressure in the machine. The alternative method uses a pocket of air trapped in a chamber behind the bulkhead to apply a pressure to the slurry in the excavation chamber. This is the standard method CREG use in their design of slurry tunnel boring machines. This pressure control of the face can be maintained within tighter tolerances – normally ±0.1bar. Pressure regulation of gases is easier and more accurate than pressure regulation of fluids This system is independent of the slurry flow and allows a larger flow around the circuit without affecting the accuracy of face pressure control. An active face pressure can be maintained, controlled and adjusted even when the pumping system is switched off.