What are the differences between EPB shields and hydro-shield systems?

The EPB (Earth Pressure Balance) shields and the hydroshields (slurry shield) are closed tunnel boring machines used in soft and saturated soils, but they are distinguished by the support fluid they use, the chamber pressure they reach and the evacuation logistics of the excavated material.

Appearance	EPB shield	Hydroshield
Supporting fluid	Floor conditioned with foam / polymers forming a pressurized paste.	Bentonite sludge-water that circulates and is recycled at the surface.
Typical chamber pressure	Up to ≈ 4 bar (suitable for moderately saturated silts, clays and sands).	Up to 10-12 bar (suitable under deep aquifers or sea beds).
Debris management	Screw conveyor that dehydrates the material in the mouth-tunnel.	Pumping of the detritus as suspension to the separation plant.
Consumption of additives	Foam + polymer (less sludge volume).	Bentonite + flocculants (higher volume, better support).
Risk of overpressure	Low, due to the limited viscosity of the paste.	Requires continuous monitoring to avoid leaks or fractures.
Typical environment	Urban tunnels in cohesive or mixed soils with moderate phreatic.	Submarine outfalls, crossings under rivers and very permeable sands with high phreatic.

Conclusion: the EPB is the best choice when the soil can be blended with additives to form a plastic plug to equalize the pressure; the hydro shield is used in high water pressure scenarios, where the bentonite sludge transmits pressure uniformly and seals seepage, although it requires more complex sludge pumping and separation logistics.