Abrasion resistant protective plate

Special (high alloy) steel plates mounted on the cutterhead that are designed to withstand high abrasion.


A horizontal entrance to an underground excavation site.

Advancing tail piece

The structure usually mounted on the TBM Back-Up in a conveyor muck removal system. The structure includes a tail pulley and protected working window mounted on hydraulic actuators so that idler assemblies can be installed while the belt is still moving.

Auger Boring Machine

A surface mounted pipe-jacking unit used to bore small diameter (2.0 m and under) drives. The machine can bore and line a variety of ground conditions based on the cutting head attachment. Muck is removed by means of a rotating auger.


Back-loading cutterhead

A cutterhead that allows cutters to be changed and inspected from behind or in front of the cutterhead. It is used as a safer alternative to the front loading cutterhead.

Back-up system

A TBM support system that trails the TBM and contains all the tunneling accessories and support equipment including the TBM power unit. It also allows for muck transport via conveyor systems or muck cars.

Belt splice stand

A mechanized unit used to add more belt to the continuous conveyor system as the tunnel gets longer. Can be placed either underground or on the surface.

Belt storage cassette

A storage system for the conveyor belt which pays out the belt as the TBM bores, allowing continuous TBM advance. Belt tension is maintained by tensioned rollers on the cassette, which retract to be closer as the conveyor extends.

Booster drive

A drive that allows power to be added to a conveyor belt at specific locations on the conveyor system as the tunnel advances.

Boring cycle

The cyclical process of a tunnel boring machine’s advance, which includes boring and re-gripping to the tunnel wall. The cycle varies slightly for each machine type.

Bucket lip

The replaceable leading edge or “scoop” portion of the muck buckets.


Conveyor system

A series of conveyors and associated equipment that is used to remove spoil (or muck) from the TBM heading to an outside location .

Conveyor cassette

See “Belt Storage Cassette”.

Core drill

See “Probe Drill”.

Curve idler

Self-adjusting rollers (or idlers) that guide the conveyor belt around curves. The pivoting structure allows for changing load conditions without reducing carrying capacity or unnecessarily increasing belt tension.


A disc shaped cutting tool that cuts rock when mounted to a TBM. The discs are installed in the cutterhead, and when the cutterhead rotates against a rock face, the cutters excavate rock.


The rotating circular structure or front section of a tunnel boring machine in which the cutters are installed and the spoils are picked up from the tunnel invert.


Digital guidance system

A guidance system that automatically tracks and displays the direction of the TBM and facilitates accurate steering by the TBM operator.

Double Shield TBM

A shielded machine designed for tunneling in weak or broken rock conditions. The machine can bore while simultaneously installing a lining from within its shield.

Drive train

All the components that generate power for the machine.

Dust scrubbers

A mechanism that removes dust particles from the air. Such dust particles are created from the cutting action of the cutterhead.


Earth Pressure Balance Machine

A shielded TBM designed specifically for operation in soft ground containing water under pressure.


Flexible ducting

Ventilation Tubes that are non rigid and can be packed into a cassette which pays out ventilation ducting as the TBM progresses.

Floating grippers

See “Gripper Shoes”.

Foam concrete

A porous type of concrete that is readily placable and can be pumped long distances at relatively high rates.

Front-loading cutterhead

A cutterhead that allows cutters to be changed only from in front of the TBM (between the machine and the rock face).


Gage cutters

The cutters located on the periphery of the cutterhead of a TBM or SBU.

Gripper shoes

Convex steel structures attached to both sides of a TBM that extend and anchor against the side of the tunnel walls to support the TBM while boring and react both the torque and thrust of the TBM..


Head drive

Drive that prevents conveyor belt overloading during start-up.

High voltage cable reels

A special cable reel that usually holds several hundred meters of high voltage cable. Such cable is paid out as the TBM advances.

Horizontal Directional Drill

A machine that sits on the surface and that bores underground in an arc profile. HDDs are normally used for crossings under rivers, roads, etc.

Housing seats

Surface of the cutter housing which takes the load of the cutter

Hydraulically actuated supports

Roof and side supports powered by hydraulics that stabilize the cutterhead during boring and keep it centered.



A roller that is part of a conveyor system and supports the conveyor belt on both the troughed side or return side of the belt.

Invert cleaning screw conveyor

A special auger system mounted in the cutterhead support area that is used to pass muck in the invert back into the cutterhead area to be picked up by the buckets. It is usually aided by hand shoveling



The width of a cut made by a single cutter.



The finished interior of the tunnel, often consisting of pre-cast concrete segments erected by the TBM or continuous poured concrete.


Main Beam TBM

An open type TBM designed for tunneling in hard, self supporting rock where shields are not required.

Main bearing

The bearing on which the cutterhead rotates, located between the cutterhead and cutterhead support. This is the largest bearing on a TBM – Robbins TBMs have three-axis main bearings protected by forced lubrication to withstand enormous thrust loads.

Main Drive — Conveyor

A drive that supplies the initial force needed to move a conveyor belt and its load.

Main Drive — Tunnel Boring Machines

The assembly that provides the rotational power to the cutterhead and includes the electric motor, gearbox, and either a clutch or torque coupling assembly.

Motorized Small Boring Unit

A mixed ground to hard rock boring attachment smaller than 2.5 meters in diameter that supplies its own torque using an in-shield motor. The Motorized Small Boring Unit is welded to steel casing and an Auger Boring Machine is used to provide thrust.

Mesh handling crane

A mechanized lifting devise used to install ground-supporting steel mesh.

Muck bucket

A scoop shaped part of the front and gage area of the cutterhead that picks up the freshly cut rock/soil or “muck” and drops it onto the muck extraction system, usually a conveyor belt on hard rock TBMs.


Pressure Bulkhead

A retaining wall built into the TBM to hold back water. Sealed cutterhead supports act as a barrier against water inflows.

Probe drill

A pneumatic or hydraulic rock drill mounted on the TBM that is used to drill ahead of the machine to determine ground conditions and water inflows.


Ring beam

Circular steel beams that are erected to support the tunnel where the rock is not self supporting, otherwise known as “bad ground”.

Ring beam erector

A mechanized system used to install ring beams along the tunnel walls.

Rock bolt

Specially designed steel rods with anchor mechanisms used to support unstable rock or to attach items to hard rock, usually “grouted” in place with an epoxy cement.


A Small Boring Unit that functions as either a single shield or double shield boring machine for long utility tunnels. The machine can be either self-propelled (Double Shield Rockhead) or thrust forward using a pipe jacking unit (Single Shield Rockhead). The Rockhead comes in diameters between 1.35 m and 2.0 m. Torque is supplied by a hydraulic or electric drive.

Roof drill

A pneumatic or hydraulic rock drill mounted on the TBM commonly used to install rock bolts for ground support.


Sealed cutterhead support

A cutterhead support that is sealed from the tunnel face to isolate the TBM in unstable or pressurized face conditions. Usually only used on Earth Pressure Balance (EPB) TBMs, or Slurry TBMs.

Segment erector

A mechanized system used to install tunnel lining “segments”. The “segments” are usually made of pre-cast concrete.

Segment hoist system

Mechanism that puts segments into place and is mounted on a monorail between the TBM and the segment train.


Concrete that is sprayed onto a surface. The mixture is often reinforced with rebar, steel mesh, or fibers. Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle.

Shotcrete robot

An automated device for applying shotcrete. Ideal for use in hazardous conditions to keep workers out of harm’s way.

Single Shield TBM

A TBM with a shield – designed for tunneling in variable geology from soil to rock. Instead of the gripper shoes found on an open TBM, a shielded machine advances by “pushing” off the concrete lining segment.

Small Boring Unit

Small diameter rock cutting attachment for use in conjunction with (and normally powered by) an Auger Boring Machine.

Splice stand

See “Belt Splice Stand”

Stabilizer shoes

On an SBU, stabilizer shoes are used to steer OR more importantly keep the unit “on-line” and “on-grade”. The stabilizers also reduce vibrations caused by the rock cutting action.

Steel mesh

Steel wire formed in a grid pattern (like a screen) used to support unstable ground – usually held in place by rock bolting

Straight rollers

Rollers that guide the conveyor belt in a linear direction.

Sub-station transformers

An electrical component used to convert power from its generated state to the required voltage and amperage at the point of utilization.


Tool used with horizontal directional drilling to pull cable or conduit through the bore without twisting it.


Tail shield

A large cylindrical structure mounted to the rear section of the TBM that supports unstable ground. Normally the segments or rock support are mounted inside the tail shield when the rock is too broken or soft.

TBM transformers

An electrical component used to convert power to the required voltage and amperage.

Telescoping shield

A shield that provides protection throughout the entire propel stroke of the TBM. This telescoping section is located between the cutterhead support and the gripper assembly.

Transport dollies

Wheeled carts used to transport the TBM on rail.

Troughing idlers

Rollers used on a conveyor system to shape the belt into a trough so that it can carry more material.


Ventilation fans

High powered fans (usually electric) used to maintain a safe, dust free work environment in the tunnel.

Variable Frequency Drive

The electronics that convert AC current into DC current and then back to AC current to produce variable rotational speeds with an AC motor.


Water-cooled electric motor

An electric motor with a “cooling jacket” that circulates water as a cooling medium.

Wire brush seals

Seals installed in the tail shield where segments are erected to protect the work area from water inflows and to prevent grout or other back filling materials from migrating forward. These slide along the outer diameter of the erected segment rings and are normally grease-impregnated to provide the seal.