Your exclusive customer link to the digital world of tunnelling
EPB Shields in operation
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.
The screw conveyor for removing the muck and controlling the support pressure is characteristic of all Earth Pressure Balance Shields.
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.
Advance rate and material conveying rate regulate the support pressure at the tunnel face.
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.
Operating principle at a glance:
Cutting knives and disc cutters remove the soil.
Complete filling of the excavation chamber with a plastic soil paste creates active support pressure.
The controlled discharge with the screw conveyor transports the muck to logistics systems further back.
Hydraulic thrust cylinders in the shield or a jacking frame in the launch shaft push the machine or the entire pipe string forward.
Segmental lining or pipe jacking.
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.
EPB Machine with outer cutting wheel and center cutting wheel for excavation diameters of more than 10 meters in highly abrasive ground.
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.
Herrenknecht EPB Shields successfully accomplished breakthrough in more than 500 projects worldwide.
Technological innovations for special requirements
The particular advantage of EPB Shields is their great flexibility. Various developments by Herrenknecht have vastly increased the range of possible applications of the method. In permeable geologies, additional slurry pumps behind the screw conveyor enable the support pressure to be maintained.
In non-cohesive soils with a high groundwater content, the slurryfier box developed by Herrenknecht can be retrofitted. This special development connects the outlet of the screw conveyor with a slurry circuit for further hydraulic pumping.
The allrounder principle EPB
The concept of earth pressure support makes EPB technology an all-rounder for soft, cohesive soils of all kinds. In addition, the geological scope of application can be enhanced using soil conditioning and the technological innovations from Herrenknecht. This applies to both small excavation diameters in pipe jacking and large and even very large diameters with segmental lining.
Still got questions?
We are ready!
Steffen Dubé
President and General Manager Herrenknecht Tunnelling Systems USA Inc.
Gerhard Goisser
Commercial Manager Herrenknecht Tunnelling Systems USA, Inc.