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AVN machines in operation
Herrenknecht AVN or Slurry Machines with a cone crusher are all-rounders for safe tunnelling in the diameter range of 0.4 to around 4 meters. These microtunnelling experts show their full potential particularly in non-accessible machine sizes. The slurry-supported excavation concept makes it possible to use these machines in all kinds of ground conditions, ranging from silt to clay to incohesive soils, and further to gravel and rock. Depending on the diameter, this is possible both in pipe jacking and for segmental lining.
Flexible in every aspect
AVN Machines belong to the category of closed, full face excavation machines with a hydraulic slurry circuit. The soil to be excavated is removed using a cutterhead adapted to the respective geology. This makes it possible to use the machines in almost all geological conditions. In soft soils and mixed geologies, standard or mixed ground cutterheads are used, while a rock cutterhead with disc cutters is used for tunnelling in stable rock formations. A cone-shaped crusher inside the excavation chamber crumbles stones and other obstructions to a conveyable grain size while tunnelling and advancing. Afterwards, the material falls through openings similar to a strainer in front of the suction port and is then removed through the slurry line together with the suspension. The excavation diameter can be enlarged using an upsize kit and a modified cutterhead. This means that the AVN Machine can be used to tunnel different diameters and for different types of pipes.
Smooth transport of excavated soil due to variable water nozzle options
In cohesive soils, in particular, keeping the crusher cone clean and avoiding clogging is a special challenge. Based upon years of practical project experience, Herrenknecht has developed a range of water nozzle options, which can cope with different geological requirements. Additional water can be injected to clean the system using high pressure nozzles integrated into the cone crusher. These jets cut loam or clay in the excavation chamber avoiding clogging. The medium pressure nozzles use the standard slurry ports. Changing the size of the nozzles can adapt the suspension pressure and optimize the material flow. A newly developed, optional camera system in the excavation chamber provides the operator with an additional tool to visually monitor wear and tear and verify the situation inside machines with a diameter of more than 1.2 meters.
Pipe jacking over great distances
AVN Machines are often used in pipe jacking operations. The maximum drive length depends on several factors such as skin friction, available jacking force, or the nominal diameter of the jacking pipes. Automatic lubrication systems reduce the friction between pipe skin and soil by injecting bentonite into the annular gap. In the range of diameters that allow man-entry it is possible to use additional hydraulic intermediate jacking stations. These are installed in the pipe string at intervals which are defined specifically for each project. By dividing the pipe string into individual sections, the necessary thrust force is reduced and distributed respectively. This means that the distance between launch and reception shaft can be more than 1,000 meters, depending on the project conditions and the number of intermediate jacking stations.
Operating principle at a glance:
Cutting knives and disc cutters remove the soil.
Hydraulic support using slurry suspension
Hydraulic conveyance of excavated material through a closed slurry circuit
A jacking frame in the launch shaft or hydraulic thrust cylinders in the shield push the machine forward.
Pipe jacking or segmental lining
Precisely around the curve
Depending on the project requirements, a range of surveying technologies is available to control the AVN Machines. The U.N.S. (Universal Navigation System) developed by Herrenknecht can be used in all diameter ranges. Various gyro systems and electronic water leveling systems allow for exact positioning. The interaction between articulation joint and steering cylinders in the front section of the shield keeps the AVN Machine precisely on course even in three-dimensional curves and tight radii. The U.N.S. is a modular guidance system which can be used universally. It can be combined with both customer-specific systems, for instance for measuring pipe joint gaps, as well as additional Herrenknecht modules. This includes the optional FCE (Fuzzy Control Expansion) system which adds automatic control of and to microtunnelling machines and minimizes deviations from the planned alignment. Furthermore, systems with a laser theodolite are used successfully in the accessible diameter range.
The benefit of a door
In some tunnelling operations obstructions such as sheet pile walls, steel girders or large boulders are expected to occur from the very beginning. In these cases, AVN Machines equipped with an additional door behind the cutterhead are often recommended. This Herrenknecht development provides direct access to the tunnel face. If needed, workers can safely enter the pressurized front through an air lock to remove boulders, for example. Tunnelling operations through rock formations can, in particular, be extended to longer drive lengths, since worn cutting discs can be exchanged via back-loading mounting systems.
Tunnelling without reception shaft
So-called blind-hole tunnels are a special challenge, especially in non-accessible microtunnelling diameters. Blind-hole tunnels end directly in the ground, making conventional recovery of the machine impossible. Herrenknecht has developed special cutterheads with a fold-away mechanism for this very special application. When the required drive length has been reached, the microtunnelling machine can be pulled back to the launch shaft with its cutterhead folded away. The jacking pipes remain in the soil. A typical project application of blind-hole tunnels is for pipe arches. Steel pipes are installed side by side in the ground to form a pipe arch. Protected by this arch, the actual tunnel diameter is then excavated using mining techniques. Crossings beneath railway lines can be excavated and metro stations can be built safely. A further application of blind-hole tunnels is the installation of house connections or tunnelling operations in which the reception shaft has not yet been completed for logistical reasons.
AVND Machines: Maximum safety for larger diameters
AVN technology has been expanded to offer extra safety in non-homogeneous, less stable geological conditions and for larger diameters. AVND Machines are equipped with an additional pressure chamber behind the excavation chamber. Similar to Mixshields, the support pressure is controlled using a communicating pipe and an automatically controlled air cushion inside the pressure chamber. This allows for even more precise control at a heterogeneous tunnel face and reduces the pressure peaks in the excavation chamber. This makes AVND Machines specialists for tunnelling operations sensitive to settlement even under high groundwater pressure and with shallow overburdens.
Because of their variety of applications, AVN Machines from Herrenknecht have successfully proven their value in more than 1,000 projects worldwide. This mature machine technology enables safe, fast, and efficient tunnelling in even the most difficult projects in pipe jacking or segmental lining.
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