Although fewer slurry pressure balance machines have been used in China, CREG have experience in the use of these machines and have designs to suit differing conditions.
Slurry machines are usually used in granular non-cohesive soils. There are normally two basic potential reasons for choosing a slurry machine in preference to an EPB.
- The excavated materials has no fine material, which means that maintaining a face pressure along the screw conveyor is difficult – or impossible.
- The tunnel is deep and the ground water pressure is so high that the screw would need to be excessively long to hold a face pressure.
It is normally a combination of one or both of these reasons, that a slurry machine process is preferred to an EPB machine.
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 first slurry machines developed during the 1960’s and early 1970’s used this method.
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 method that CREG use in their designs of slurry tunnelling machine.
The reason for choosing this system is
- Pressure control of the face can be maintained within tighter tolerances – normally ±0.1bar.
- Pressure regulation of gases is easier and more accurate that pressure regulation of fluids
- This system is independent of the slurry flow and allows a larger flow around the circuit without it affecting the accuracy of face pressure control. This means that the discharge slurry has lower density which reduces the risk of blockages and allows more efficient operation of the STP
- An active face pressure can be maintained, controlled and adjusted even when the pumping system is switched off.