Slurry TBM at High Hydrostatic Pressure: Air-Cushion Balance and Slurry Plant Sizing

In a slurry shield, the air cushion is the control element that makes high hydrostatic pressure manageable. Rather than letting the feed line dictate support pressure, the compressed air pocket absorbs discharge pulses from the screw conveyor and holds the face pressure within a narrow band. Under a deep sub-river or estuary crossing, that damping matters most: a small over-pressure can fracture the cover, while under-pressure invites a blow-in and uncontrolled loss of slurry.

Mixed-face conditions are where the air cushion earns its place. Where gravel and sand meet weathered rock, permeability changes metre by metre, so support pressure must be adjusted without stopping the drive. Engineers should expect fluctuating torque, uneven cutter wear on the rock interface, and the need for tailored bentonite or polymer conditioning. Abrasive gravel also drives disc cutter consumption, so keeping interchangeable cutter hubs and spare assemblies on site protects the programme.

Slurry treatment capacity is routinely undersized because it is calculated on average advance rate instead of peak instantaneous spoil yield. Desanding, cycloning and fines removal must handle the maximum cutting rate in gravel, plus the water balance for a closed loop where nothing can be discharged into a river. For Middle East metro and utility drives in desert mixed ground, that means planning plant capacity, spare slurry pumps, seals and screen panels early — the components OUSUN supplies factory-direct from Deyang.