Full control in complex geological conditions.
In all unstable soils a support pressure must be built up at the tunnel face to counteract the native earth and ground water pressure and prevent an uncontrolled penetration of soil or a loss of stability at the tunnel face.
In slurry-supported shields either a bentonite suspension (a mixture of swelling clay minerals and water) or water is used to support the unstable tunnel face during tunnelling. The process is used widely in non-cohesive soil conditions ranging from fine-grained sand to coarse-grained gravel.
At the interface between soil and suspension a so-called filter cake is formed, which has the necessary cohesion for excavation. At the same time, it seals the soil to prevent ingress of ground water. Moreover, if the drilling machine breaks down, this filter cake forms a sealing membrane at the tunnel face which enables entry into the excavation or working chamber after compressed air has been applied.
In mixshield technology the support pressure in the excavation chamber is not directly controlled by suspension pressure but by a compressible air cushion between the pressure bulkhead and the submerged wall. Fully automated control loops, which permanently monitor the pressure conditions at the tunnel face and build up a corresponding balancing pressure via the support media, ensure precise pressure control. This enables sudden pressure irregularities in difficult heterogeneous geological conditions to be very effectively equalized, thus ensuring that slight overburdens between the machine and the surface do not lead to ground settlement or heaves.
In slurry shield technology the support pressure is controlled directly by the suspension pressure, that is by the inflow and outflow of the bentonite suspension pumped round the circuit. This technology is limited to use in geological conditions where excessive and sudden pressure irregularities are not to be expected.
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