Earth pressure balance, or EPB, is a mechanized tunneling method in which the excavated material is used to support the tunnel face whilst it is being plasticised using foam/slurry & other additives to make it transportable and impermeable. The excavated material is transported into the tunnel boring machine (TBM) via a screw conveyor arrangement which allows the pressure at the face of the TBM to remain balanced without the use of slurry.
EPB Tunnel Boring Machines are mainly designed for soft soils containing water under pressure. The EPB TBM can handle loose sedimentary deposits with large boulders and high water table.
The excavated material is transported by a screw conveyor from the excavation chamber to the belt conveyor. The combination of the output of the screw conveyor and the advance rate of the TBM ensures that the support pressure in the excavation chamber can be exactly controlled. Continuously monitoring of the state of balance in the excavation chamber by means of earth pressure sensors, allows the operator to control and synchronize all tunneling parameters. It allows high advance rates and reduces the risk of changes at the surface.
To improve the ground conditions for EPB tunnelling several conditioning materials can be injected through the cutter head and the bulk head. By changing the ground conditions tunnelling in soils containing gravel, sand or water or in unstable geology is also possible
The cutter head of the machine will be designed specially for the ground conditions of the project. Depending on the requirements the head will be equipped with disc cutters or rippers, or a combination of both. All cutter heads will have an optimized opening ratio.
The CREG EPB TBMs have an articulated shield. The articulated joint between front and tail shield is equipped with seals allowing angular movement between the shields and prevents the ingress of water.
The screw conveyor removes the muck from the excavation chamber.
For difficult geology with big pebbles the TBM can be equipped with ribbon type screw conveyor instead of standard screw conveyor.