Optimizing face pressure balance, soil conditioning with foam/polymers, and screw conveyor discharge to prevent surface settlement in urban tunneling.
In dense urban areas, the most critical challenge when driving metro and utility tunnels is keeping surface settlement close to zero while passing beneath buildings and historic structures. Earth Pressure Balance (EPB) tunnel boring machines are the most reliable technology developed to meet this challenge.
In the EPB principle, the ground excavated at the face by the cutterhead is taken directly into the pressure chamber (plenum). The soil paste inside the chamber is pressurised to match the hydrostatic and lithostatic pressure at the tunnel face.
\sigma_{face} = \gamma \cdot H + uHere \gamma is the unit weight of the soil, H the cover depth and u the groundwater pressure.
Foam Injection: Special surfactant foams sprayed from the cutterhead nozzles turn cohesionless sand and gravel into a flowable plastic paste.
Polymer Additives: In soils with high water content, they retain the water and allow controlled discharge through the screw conveyor.
Wear Reduction: They lower the wear rate of the disc cutters and face armour by up to 40%.
In EPB TBMs designed by ARM, screw conveyor torque and the hydraulic thrust cylinders are linked through real-time PLC telemetry. Eight-point diaphragm pressure sensors in the excavation chamber regulate the advance rate with millisecond feedback, eliminating the risk of ground collapse.