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Herrenknecht - Model EPB Shield -Tunnelling Machine
In soft, cohesive soils, tunnel boring machines with earth pressure balance support are preferred. With so-called Earth Pressure Balance Shields (EPB), the loosened and conditioned soil acts as a plastic support medium to support the tunnel face. When the excavation chamber is completely filled, this prevents uncontrolled penetration of the soil into the machine and creates the conditions for fast and safe tunnelling. In addition, soil conditioning and technical innovations from Herrenknecht can be used to further expand the geological range of applications. This applies both to small excavation diameters in pipe jacking and to large and very large diameters in shield tunnelling.
- Geology Soft ground Soft ground (clay, silt, loam, low water permeability)
- Diameter: 1.7–16m
Stability through support pressure
A rotating cutting wheel equipped with tools presses against the tunnel face, loosening and conditioning the soil. The soil enters the excavation chamber through openings in the cutting wheel, where it mixes with the existing chamber filling of loosened and conditioned soil. Mixing arms on the cutting wheel and bulkhead knead the mass to the desired consistency. The pressure of the thrust cylinders is transferred via the bulkhead to the loosened and conditioned soil in the excavation chamber. When the external earth and water pressure is equal to the pressure of the supporting chamber filling, the necessary balance has been achieved to prevent heave or settlement on the surface.
Screw conveyor speed and advance rate regulate the pressure conditions
A screw conveyor transports the excavated material from the bottom of the excavation chamber onto a belt conveyor. The interaction between the screw conveyor speed and the advance rate ensures precise control of the support pressure. Earth pressure sensors in the excavation chamber continuously monitor the earth pressure. This allows the machine operator to optimally adjust all tunnelling parameters, even in changing geological conditions, to achieve the balance described above.
Wider range of applications thanks to good soil conditioning
Not all soils have ideal properties for EPB tunnelling in their natural state. However, the range of applications of the method can be vastly increased by soil conditioning. This involves changing the plasticity, consistency and water permeability of the geology by injecting various conditioning agents such as water, bentonite or foam. EPB Shields can thus achieve good advance rates even in heterogeneous soils containing gravel, sand or water or in unstable rock.
In stable geologies, EPB Shields can also be operated in so-called open mode. Here, active face support is not required and the excavation chamber is not filled. The screw is only used to convey the material from the excavation chamber onto the belt conveyor.
Adaptive tailskin sealing system
Under the protection of the tailskin, the individual segments are erected to form a ring by the erector. At the end of the tailskin, several rows of steel brushes are arranged all around, the spaces between which are continuously filled with sealing compound, thus creating a seal between the atmospheric tunnel and the earth and water pressure in the annular gap.
Herrenknecht`s adaptive tailskin sealing system optimizes sealing compound consumption in tunnel boring machines through automation. It requires only one parameter – the sealing compound consumption in kilograms per square meter of segment surface – and then calculates all necessary settings. The system monitors sealant consumption in real time, detects deviations and enables corrections to be made, significantly increasing safety and efficiency. A manual mode offers additional flexibility for specific challenges. The even distribution of the sealing compound optimizes the seal against external influences such as earth pressure and groundwater. This reduces the amount of sealing compound used by up to 30% and increases the service life of the tailskin seal.
All advantages at a glance:
- High, consistent advance rates possible in cohesive soils with a high clay or silt content
- Geological range of applications can be expanded by soil conditioning
- No additional support medium required
- Special solutions available for heterogeneous ground conditions