Maximum Safety in Heterogeneous Geologies.

The machine is used as a mixshield where gravelly geological conditions indicate an unstable tunnel face or mixed geological conditions. At the tunnel face the soil is loosened all over by the cutting wheel (1) rotating in the bentonite suspension. The soil then mixes with the bentonite suspension.The area of the shield in which the cutting wheel rotates is known as the excavation chamber (2) and is separated by the pressure bulkhead (3) from the section of the shield under atmospheric pressure.

The bentonite suspension supplied by the feed line (4) is applied in the excavation chamber via an air bubble (5) at a pressure equaling the native soil and water pressure, thus preventing an uncontrolled penetration of the soil or a loss of stability at the tunnel face.The support pressure in the excavation chamber is not controlled directly by the suspension pressure but by a compressible air cushion (5). For this reason the excavation chamber behind the cutting wheel is separated from the pressure bulkhead by a so-called submerged wall (6). The area of the submerged wall and pressure bulkhead is known as the pressure/working chamber.

Whereas the front area of the tunnel face is completely filled with suspension, the suspension behind the submerged wall barely covers the machine's axles and is kept at the exact target pressure value by the compressed air cushion, which is precisely controlled by a compressed-air control system. Irregularities in the bentonite feeding circuit can thus be compensated considerably more effectively.

The loosened soil mixed with the suspension is pumped through the feeding circuit to the separation plant outside the tunnel. In order to prevent blockages to the feeding circuit and to ensure trouble-free operation of the discharge pumps, a sieve of largish stones and clumps of soil is placed in front of the suction pipe to block the access to the suction channel.

The tunnels are normally lined with steel reinforced lining segments (7), which are positioned under atmospheric pressure conditions by means of erectors (8) in the area of the shield behind the pressure bulkhead and then bolted in place. Mortar is continuously forced into the remaining gap between the lining segments' outer side and the excavation diameter through injection openings in the tailskin or openings directly in the segments.

For further information, please request our brochures or our EPB/mixshield animation from our
media service.

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