Simple and secure: The dismantled ground as supporting medium.
In all unstable soils support pressure must be built up at the tunnel face in order to equalize the earth and ground water pressure acting on the cutting wheel and thus prevent the tunnel face from collapsing. In the EPB process the earth material itself, which has been excavated by the cutting wheel, is used to support the tunnel face. The soil at the tunnel face is loosened by the cutters on the rotating cutting wheel and conveyed through the openings in the cutting wheel into the excavation chamber, where it mixes with the plastic soil already there.
The force of the support pressure is transmitted from the thrust cylinders via the pressure bulkhead onto the soil, thus preventing uncontrolled penetration of the soil from the tunnel face into the excavation chamber. A state of equilibrium is reached when the soil in the excavation chamber cannot be compacted any further by the native earth and water pressure.
The screw conveyor reduces the support pressure from overpressure to normal atmospheric pressure. The support pressure is controlled by the speed of the screw conveyor and the cross-section of the opening of the upper screw conveyor driver. In order to serve as a support medium the soil loosened at the tunnel face should have the following properties: good plastic deformability, a liquid to soft consistency, extrudability, low internal friction and low water permeability.
Ideal soils for this purpose are cohesive soils with a high clay, loam or silt content. By conditioning, that is additional preparation of the soil, the range of use of EPB technology can be substantially increased: by injecting foam and/or polymer into the excavation chamber, non-cohesive soils or fine-grained sand or even coarse-grained gravel can be made artificially workable and thus capable of being excavated by EPB technology.
