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Basic terms in mechanized tunnelling

________________ This glossary explains the most important technical terms and definitions from the world of mechanized tunnelling technology.


Motor-driven mixing arm in the bottom section of the shield in front of the suction pipe for stirring the excavated material mixed with bentonite suspension in Mixshields. It prevents material from settling and cohesive soil from becoming clogged in the shield bottom area in front of the grill and suction pipe.

Air lock

A lock through which people and/or materials can pass from the atmospheric area to the pressurized area of the TBM. Enables access to the tunnel face to control the cutting wheel, exchange tools or remove obstacles.

Annular gap

See annulus


Cavity between the surrounding soil and the pipe string or the tunnel lining. Is created by the slightly larger diameter of the cutting wheel or cutterhead compared to the pipe string or tunnel structure.


Tool mounting that makes it possible to change the disc cutters from a protected position behind the cutting wheel or cutterhead.


A highly expansive clay. Used in tunnelling technology as a bentonite suspension for slurry-supported methods. Used during tunnelling both as a transport medium for excavated material in the slurry circuit and as a support medium (thixotropic liquid) for the tunnel face.

Blind hole

A tunnel which ends in the ground as a blind alley.


Geological obstacles (mostly single large pieces of rock encountered in sandy or clayey soils).

Breathing air

Compressed air that has been filtered to be suitable for breathing. Required in man locks and for pressurizing maintenance personnel in the excavation chamber for maintenance works.


The bypass is a part of the feed and slurry circuit. By switching the bypass, the feed line is connected to the slurry line so that the flow in the slurry circuit can be maintained when the TBM is stopped without flushing the tunnel face. Thus, the number of repeated starting procedures for the slurry pumps is considerably reduced.

Chisel tools

Specially formed excavation tool on a longitudinal cutting head.


Grounds rarely have all characteristics required for the use as support medium in EPB-shield (EPB) operation mode and must therefore be conditioned. During the conditioning process, the natural ground is therefore conditioned to act as a plastic support medium. Depending on the particular geology, conditioning is possible by adding water, bentonite, polymer or foam.

Cone crusher

Cone-shaped part of a tunnel boring machine behind the cutterhead. Due to its special form, it crushes the excavated material into conveyable grain sizes during rotation of the cutterhead. Is mainly used with AVN Machines.

Control stand

Work station of the machine operator. Either aboveground in the control container (if remote controlled) or directly inside the machine.


In the large diameter range, this describes a tool carrier used in hard rock machines, equipped mainly with disc cutters and designed for full-face excavation. Takes up the excavated material during rotation and conveys it via muck chutes to the muck ring. In the small diameter range, this is a general description for the rotating head of a tunnel boring machine equipped with excavation tools. A difference is made between standard, heterogeneous ground and rock cutterheads.

Cutting knives

Special cutting tools equipped with a hard metal disks. The soft ground tools are tightened to the support and can be changed from the rear. Soft ground tools are used in heterogeneous ground.

Cutting wheel

Disc-shaped or spoke-type tool carrier, mainly with cutting knives, for the excavation of soft and heterogeneous ground and mixed soils.


Disc Cutter Rotation Monitoring. System for the real-time measurement of the rotation and temperature of disc cutters during tunnelling. Monitoring can identify tool wear and a need to exchange parts. Is integrated directly into the fastening set of individual disc cutters.


Mostly ring-shaped, center-free drive for the cutting wheel or cutterhead of the tunnel boring machine. A large, ring-shaped bearing with integral teeth equipped with several pinions on the periphery that are operated hydraulically or electrically via planetary gears.


Manipulator for positioning concrete segments during ring building.

Excavation chamber

Chamber and / or space directly behind the cutterhead or cutting wheel.


Universal excavation tool for partial face excavation. Depending on the condition of the soil, it can be equipped with an excavation shovel, a bucket toothhydraulic hammer.


The external diameter of a microtunnelling machine can be upsized by attaching the additional so-called extension kit; i.e. in general the microtunnelling machine can be used for jacking of pipes both of the respective standard diameter and of the next larger diameter.

Filter cake

An air-impermeable membrane that forms at the interface between soil and bentonite suspension in slurry-supported tunnelling methods. Seals the soil against infiltrating ground-water and makes it possible to control the balance between the pressure applied on the bentonite suspension and the earth and groundwater pressure.


The natural heat of the earth which can be exploited by means of drilling and which can be used to generate electric power or for power-heat coupling.

Guidance system

System allowing the position of the TBM to be determined. Depending on the diameter, either systems with a gyrocompass (interval measurement) or with laser technology (permanent measurement) are employed. 

In-situ casting

Single-leaf shaft construction method using transferable formwork to complete the final construction of the shaft during the shaft process. The transferable formwork is filled with concrete in-situ (in place on the construction site) and once the concrete has hardened it forms the finished shaft construction and often incorporates steel reinforcement. The voncrete hardening process means parallel shaft construction is restricted.


Determines the current roll angle of the laser station and transfers this data via the control unit to the computer of the plc system.

Intermediate jacking station

Cylindrical steel jackets with integrated hydraulic cylinders that are installed during pipe jacking at defined distances apart in the pipe string. These divide the entire pipe string into individual sections, thus reducing the required jacking force on the jacking frame in the launch shaft and enabling very long pipe jacking stretches.

Long distance tunnelling

Long distance tunnelling is mainly characterised by the employment of intermediate jacking stations and the possibility of the exchange of tools. In this respect, the pipe lubrication has an important function.

Main drive

The cutting wheel drive; usually ring-shaped, free centre drive for the cutting wheel and/or the cutterhead of the tunnel boring machine. A large, ring-shaped bearing with internal teeth equipped with numerous pinions on the periphery, which are driven hydraulically or electrically via planetary gears. The free centre makes this drive ideal for Mixshield technology.

Main jacking station

A jacking system, installed in the launch shaft which is often called jacking frame (pipe jacking).

Man lock

Air lock to allow acces to the excavation chamber under compressed air for cutting wheel inspection, cutter change and removal of obstacles. The man lock consists of two chambers (front chamber/main chamber).

Material lock

A material lock is provided on the TBM to allow tools and other materials to be passed into the pressurized chamber at the front of the TBM. In order to make the handling of heavy objects easier, the transfer lock is equipped with transport trolleys which travel on rails.


Machines with partial face excavation.


Tunnelling method during which an unmanned tunnel boring machine is controlled remotely in pipe jacking from a control container on the surface.

Muck conveyance

Technology for conveying of the extracted material with the help of muck pumps (piston pumps). It is comparable to the concrete pump technology and is employed with EPB machines.

Muck pump

Piston pump for the stroke by stroke conveyance of highly viscous slurries, for example, grout, bentonite, concrete or conditioned excavated material. Suitable for high slurry pressures.

Muck ring

Belt conveyor muck hopper that is positioned in the excavation chamber behind the cutterhead and transfers the excavated material to the belt conveyor.

Open shields

Tunnel boring machines without the option of active tunnel face support. They do not have a closed system for pressure balance at the tunnel face and are generally used in non-aquiferous and stable geology. The open shields include, among others, hard rock and Partial-face Excavation Machines.


Distance between the upper edge of the pipeline and the surface of the ground and/or the bottom of a body of water located above the pipeline. Often also called cover or overlay.


Difference between the excavation diameter and the diameter of the shield skin or pipe string.

Pipe arch

Tunnelling construction method for creating underpasses, for example. A “shield” consisting of several pipes installed next to one another acts as a support structure. Under its protection, large tunnel diameters can be excavated using mining techniques along short routes.

Pipe jacking

Tunnelling construction method for the creation of pipelines consisting of individual product or casing pipes. The tunnel boring machine and the pipe string behind it are advanced up to the target shaft with the help of a hydraulic jacking frame in the launch shaft.

Pipe lubrication

Procedure employed to reduce the skin friction between pipeline and surrounding earth, as well as to support the annulus by means of injected bentonite suspension.


Jacking pipes are lowered separately into the launch shaft and after being jacked with the help of the main jacking station, they form the pipeline.


Deflection of the targeted laser beam as a result of changing air density along the beam path.

Rotary coupling

Sealed connection between a stationary and a rotating component. Usually used for a number of fluids such as hydraulic oil for various components, bentonite suspension or foam. The largest rotary coupling on a tunnel boring machine is located in the transition area to the rotating cutting wheel.

Sea outfall

General term for the construction of pipelines from the coastline into the open sea.

Segmental lining

Tunnel lining method using individual precast concrete segments. The individual segments are transported through the completed part of the tunnel and assembled by the erector to form closed rings directly behind the TBM. The TBM is equipped with hydraulic thrust cylinders which push the shield forward from the last built tunnel ring.


Sinking of the ground surface due to loosening and disturbance of the natural layering around the void (stress redistribution).

Shield skin

Exterior steel jacket and basic construction of a TBM. Serves as protection against the surrounding earth and (ground)water. TBMs without a closed shield (Gripper TBMs) are only used in rock.


General term for the mechanized production of vertical shafts.

Slurry circuit

In the excavation chamber, the excavated ground is mixed with bentonite suspension which serves as transport medium in the hydraulic slurry system. Slurry pumps (centrifugal pumps) transport the suspension via the slurry line to the separation plant. The separated bentonite suspension is fed back into the circuit via the feed line.

Slurryfier box

Special development for extending the application area of Earth Pressure Balance Shields. Connects the outlet of the screw conveyor with the slurry circuit so that the excavated material can be conveyed hydraulically.

Soil conditioning

Describes the process of conditioning soils with additives like bentonite, surfactants, water and polymers to systematically change their properties, for example, the consistency. Is used in tunnel construction, especially in EPB technology.

Support pressure

Creation of an overpressure in the excavation chamber to compensate for the earth and/or (ground)water pressure.


Mixture of substances consisting of a liquid and the fine particles of solid substances suspended in it. Serves as support and transport medium and/or flushing liquid in mechanized tunnelling.


Rear section of the shield skin. Can be attached to the shield skin with joints or as a rigid structure.

Tailskin seal

The tailskin seal lies on the exterior contour of the segment or jacking pipe and seals annular gap between the inside of the tailskin and the outside of the tunnel lining.


The electronic target is used to control the position of the tunnel boring machine. It uses sensors to measure its position relative to the laser beam and passes values to the computer in the control panel. This enables the exact position of the machine to be checked at all times.


Tunnel boring machine

Tunnel face

Area where the material is excavated.


General term for the construction of tunnels.


General term for the fresh air supply in the tunnel (primary ventilation).

Working air

Compressed air supplied for the operation of pneumatic machines such as tools, winches, cranes and pumps.

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