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Mixshields are the specialists for safe and economical tunnelling in heterogeneous ground.
Mixshields in operation
The Mixshield technology developed by Herrenknecht is a further development of conventional slurry support technology. The support pressure is precisely adjusted using an automatically controlled air cushion. This means that heterogeneous geologies and high water pressures of up to more than 15 bar can also be safely managed, even with very large excavation diameters.
Greater safety thanks to precise support pressure control
In Mixshield tunnelling, a largely impermeable membrane is created at the interface between the ground and the bentonite suspension. It is only this that enables the required support pressure to be built up while at the same time it seals the tunnel face against groundwater. For exact and precise support pressure control at the tunnel face, a bulkhead separates the slurry-filled chamber into a front excavation chamber and a rear intermediate chamber, the working chamber. In the intermediate chamber, the support pressure is generated by a pressurized air bubble and transferred to the bentonite suspension. Communication between the two chambers takes place via a bulkhead opening in the invert area.
Sealing against high water pressures
Thanks to the closed, hydraulic slurry circuit, Mixshields can be used safely even at high water pressures of over 15 bar. The prerequisite is an appropriate seal between the atmospheric part of the machine and the ground ahead of it. This is why Mixshields are equipped with a multiple seal system. The drive is designed as a system with individual sealing chambers with a pressurized sealing compound. At the end of the machine, the annular gap between the atmospheric tunnel and the outside of the segments is protected against water and soil ingress by several rows of steel brushes and by filling the gaps with sealing compound.
Claw crusher (left) and roll crusher with mixing blades (right) crush stones and blocks or clods of clay during the advance.
Safe preparation for conveying
Mixshield drives in non-cohesive, heterogeneous soils often involve larger rocks or blocks of stone. Such boulders are crushed down to a conveyable size for smooth hydraulic removal. With Mixshields, a claw crusher placed in front of the grid is often used to crush the stones to a conveyable size. If fine-grained, cohesive soils are to be expected along the alignment, a roll crusher in front of the intake is used as an alternative. With the help of the laterally arranged mixing blades, the roll crusher conveys the material into the intake and crushes larger clods of clay.
Operating principle at a glance:
Cutting knives and disc cutters remove the soil at the tunnel face.
Hydraulic support using slurry suspension with a controlled pressurized air cushion.
Hydraulic conveying of the muck via a closed slurry circuit.
Hydraulic thrust cylinders in the shield push the machine forward.
Watertight segmental lining.
Optimize material flow, even with clogging
Clogging on the cutting wheel and in the excavation chamber can lead to increased wear and tear and negatively affect the advance rate. Herrenknecht has therefore developed various special solutions to optimize the material flow. These include:
- Cutting wheels with an open center area for optimal passage of the muck
- Increased flow rate of the slurry in areas sensitive to clogging
- Separate flushing systems for the cutting wheel center, excavation chamber and working chamber with swiveling jets in the invert area
The design of the excavation chamber additionally supports the flow of material. Measures such as smooth surfaces of the cutting edge and bulkhead, partial cladding of the cone in the shield area, reduced bulkhead openings or optional back buckets have proven themselves in practice.
Optimized access for maintenance and tool change
During the advance, the cutting wheel, cutting tools and other components must be checked, maintained or cleaned and obstacles removed. To this end, the slurry level in the excavation and working chamber is partially lowered using compressed air. The maintenance team enters the pressurized area via the personnel airlock. To allow the compressed air team to enter and exit, the pressure in the personnel airlock is gradually increased and vice versa. For chamber pressures above 3.6 bar, professional divers are preferably used for hyperbaric interventions. To ensure smooth project execution, Herrenknecht works closely with appropriate specialist companies.
Furthermore, the bulkhead opening can be closed pressure-tight using a bulkhead gate. This also makes it possible to completely lower the slurry level in the working chamber for work without overpressure in the invert area.
Accessible cutting wheel arms simplify tool changes underground for large machine diameters.
Easy tool change thanks to special solution
A special development from Herrenknecht eliminates time-consuming hyperbaric interventions to change cutting tools with large diameters and high pressures. The main arms of the accessible cutting wheel are designed to be accessible under atmospheric conditions. The worn cutting tools can be changed from the inside via a tool lock located at the rear, without lowering the slurry level or lengthy interruptions to tunnelling.
Mixshield drives in the large diameter range
Especially for multi-lane road tunnels, the Mixshield and accessible cutting wheel provide a scalable solution for the realization of crossings under waterways. In addition, in the back-up area the concrete elements for the carriageway level are installed already and further preparations made for later work to finish the interior. As a large part of the tunnel lining is already carried out during the drive, construction time can be reduced and the tunnel put into operation quickly.
Safe, flexible and economical
Mixshields from Herrenknecht are tunnelling specialists for complex ground conditions, especially with high water pressure and very large diameters. Thanks to tunnel face support via an automatically controlled air cushion, even sudden pressure and volume fluctuations can be compensated for very precisely. This means maximum tunnelling safety for even the most complex tunnel projects and minimizes subsidence on the surface.
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Steffen Dubé
President and General Manager Herrenknecht Tunnelling Systems USA Inc.
Gerhard Goisser
Commercial Manager Herrenknecht Tunnelling Systems USA, Inc.