The closed shields -EPB o hydro shield- allow to excavate soft and saturated soils while keeping the face stabilized by pressure. Their work cycle combines excavation, immediate support and controlled evacuation of the material to advance safely under cities, rivers or coastal areas.
Assembly and launching
The TBM is assembled in an attack shaft or initial cavern. Once the starting zone is sealed, the cylinders push the shield against a reaction wall while the cutter wheel is activated.
2. Excavation and pressure balance
In EPB the chamber is filled with conditioned soil (foam / polymer); in hydro-shielding, bentonite sludge is used. The internal pressure is adjusted to equalize soil and water pressure, avoiding subsidence or swelling.
3. Material extraction
- EPB: auger doses the excavated "paste" towards the outlet.
- Hydroshield: pumps send the suspension to the separation plant to recover bentonite and evacuate detritus.
4. Laying of voussoirs
As the wheel advances, an erector installs a ring of concrete segments that forms the final lining of the tunnel. This takes place inside the shield, safe from outside pressure.
5. Glue injection and sealing
Cement or bentonite grout is pumped into the annular space between segments and soil, sealing seepage and preventing surface settlement.
6. Sludge transport and management
The dewatered detritus (EPB) or the separated suspension (hydroshield) is loaded onto belts or trucks for recycling or authorized deposit, minimizing the environmental impact.
7. Monitoring and guidance
Pressure, torque and thrust sensors send real-time data to the cab. A laser-inertial system maintains alignment with millimeter accuracy, ensuring that the tunnel meets the design grade.
Result: the closed shield combines pressurized excavation, immediate support and slurry management to advance safely, cleanly and in a controlled manner in environments where other methods would be unfeasible.
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