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Tunnelling Terms

A substance added to concrete/shotcrete to accelerate setting.

A tunnel driven from ground surface to provide access to or drainage from underground workings; a length of tunnel driven for an exploration – exploration adit.

The forward progress in the construction of a tunnel, usually measured by the length created, or the rate of segment positioning in terms of a number per hour/day or some other timescale.

A material used to replace excavated soil.

It is in situ ground at the lower face of a tunnel undergoing staged excavation.

Bentonite is clay composed, like fuller’s earth, mainly of the same clay mineral ‘montmorillonite’. It is used for synthetic reasons, expanding properties when water content increases.

Bolt pocket
A pre-formed recess in tunnel segment to accommodate bolts which hold segments together.

The internal diameter of a pipe or other cylinder, single tunnel e.g. Twin Bore.

A hole driven into the ground to get information about the strata, or to release water pressure by vertical sand drains, or to obtain water, oil, gas, salt etc.

Making a hole in the ground by means of a rotating auger.

Box Jack/Jacked box
A fully built structure that is constructed and then thrust into final position from an adjacent jacking point.

Burden (blasting)
The burden of the toe of a blasting hole is its distance from the nearest free face measured at right angle to the hole.

A foundation constructed at surface and sunk to its final position. Caissons can have solid or open bases depending on the ground they are being sunk into (generally open caissons are sunk into softer ground e.g. clay). Caissons can also be filled with compressed air, which provides dry working conditions when using an open-base caisson.

Canopy tube
A metal tube drilled into the tunnel face above the ground to be excavated, the tubes are pumped full of grout once in place. A series of tubes are drilled forming a ‘pipe umbrella’. This umbrella helps poor ground to arch over the tunnel, reducing the risk of crown failure.

Cantilever diaphragm walls
These are walls supporting the sides of an excavation which have no support at the top. The wall is prevented from tipping over by burying the base of the wall below the bottom of the excavation. Sometimes the cantilever diaphragm walls and cut and cover technique are combined with the use of a roof hat which is conciliated over the diaphragm walls and the spoil removed after.

This is sealing a seam to make it watertight. This is often used in tunnels to seal around the segments.

Caulking groove
A detail formed in a construction joint to allow caulking to be placed if required.

A measure of length along the alignment of a project.

Circumferential joint
A joint (typically between segments) which runs circumferentially around the tunnel bore. Except in rock tunnels where the TBM uses grapples.

This is lining placed on the internal surface of the tunnel. Cladding provides a smart finish, can be fireproof and is easily retro-fitted. It is also expensive to replace and can be damaged easily be errant vehicles.

Perfect analytical solutions to (often theoretical) problems. Closed-form solutions allow quick calculation of problems by making assumptions and likening reality to an idealised situation.

A temporary wall built to keep water out of a construction area. These will often be constructed of diaphragm walls or piles sunk into the ground around the construction area. Triad is removed by grate in dry or wet condilim.
Example presentation on building a cofferdam tunnel accross a river

Compressed air
It is used during excavation to apply pressure to the face and prevent the face coming in. Also to prevent water influx and keep the excavation dry. This method carries health and safety implications as it involves miners working in pressurised air conditions.
Used to gain access to face when using enclosed face TBM in certain eradiation.

Compensation grouting
A method of reversing ground settlements by injecting grout into the ground. The volume of grout must be carefully controlled in order to prevent heave whilst eliminating settlement.

A measurement of the movement of the walls of a tunnel inwards. The walls will tend to deflect in with the load of the ground on them, this is often monitored to provide information on the performance of the lining during construction.

Contiguous pile wall
A piled wall where the piles are placed adjacent to each other with small gaps between the piles. This type of wall is used only where ground conditions are suitable.

Used in tunnelling to remove excavated material from a tunnel face or shaft. The conveyor takes the material from the face to the tunnel spoil handling point, where it is dealt with.

Used in rail tunnels to allow trains to access different lines / platforms. Often designed as a large cavern with multiple tunnels joining it.

Cross passage
A small tunnel used to connect between adjacent bores in a multiple-bore tunnel. Cross passages provide a means of escape from an incident bore and allow equipment to be placed out of the main bores. It also provides access for O/M.

The highest point of the internal curved surface of a tunnel cross section.

It is a small channel or drain used to carry water beneath an obstacle (e.g. road).

This is the front side of a tunnel boring machine.

The head at the front of a tunnel boring machine used for cutting into the ground. Cutterheads have different designs depending on the type of ground they are built to operate in. Hard rock conditions are tackled by installing cutting discs which shear the rock off the face. Softer conditions require picks to be fitted to the face, these scoop away the ground.

Cut and cover tunnel
A method of tunnel construction involving excavating a trench, installing the structure and covering it over. This method is typically used for shallow tunnels. Great care is required to ensure the walls of the excavation are well supported while the structure is installed.

The removal of water from granular soils, it is normally carried out within an impermeable cut-off wall and by using well points.

Diaphragm walls
A concrete retaining wall (usually reinforced) which is constructed in panels from the ground surface. Excavation for panels is undertaken by a long-arm excavator, with the ground supported by bentonite mud or similar. Once the reinforcement cage has been lowered into place, concrete is poured into the slot, displacing the mud. Once all panels have been cast, excavation of the ground within may proceed.

Double shield
A tunnel boring machine that is formed of two sections, each being capable of independent forward movement, this allows concurrent excavation and building of a tunnel lining.

Dowel bar/Shear dowel
Reinforcement which crosses a zone of shear, limiting movement.

Drill and blast
The excavation of a tunnel, shaft or cavern in rock using explosive charges placed in holes drilled in the face.

EPB (Earth Pressure Balance machine)
A type of tunnel boring machine which retains a prescribed amount of excavated soil in the cutter head. Hydraulic jacks are used to force this soil against the face of the tunnel, ensuring the ground remains stable. Normally used in granular soils.

Elephant’s foot
The enlargement of a tunnel lining at the joint between the side walls and the invert.

A mechanical arm present in a tunnel boring machine which is used to put the tunnellining segments into place.

Escape sets
Emergency breathing apparatus which may be used in an underground space if the oxygen level reduces.

The outside face of a structural element ie the tunnel extrados.

Extensometer (inclinometer)
A device for measuring the change in distance between two points. Often used for measuring the ground movement induced by tunnelling.

Expanded lining
Primary lining that consists of tunnel segments that are expanded circumferentially against the surrounding ground.

Eye (tunnel eye)
The start of a tunnel, normally at a junction between a shaft and a tunnel.

Face dowel
A rod of steel or fibreglass inserted into the tunnel face to provide temporary support and assist in limiting face movement.

Face loss
The loss of material from the face of a tunnel.

A break in the bedding of rocks, it displaces any deposit vertically by the ‘throw’ and horizontally by the ‘heave’ or lateral shift.

Fibre (reinforced)
Steel fibres of 0.1 – 1.0mm thickness, up to 60mm long, which are used to reinforce concrete, particularly sprayed concrete. FRC can provide superior fire protection and crack control.

Fire curve
A relationship between time and temperature, which is used to assess whether a structure can safely withstand a fire. Standardised fire curves for different types of vehicle fires are used to analyse the structural safety of tunnels.

The buoyancy of a void (e.g. tunnel) beneath the water table.

Fore poling
A system of placing “fore poles” into the ground ahead of the tunnel face to provide a canopy under which excavation can take place.

Support around an opening, i.e. forming a portal for cross passage excavation.

Freezing (ground treatment)/Ground freezing
The process of freezing the ground to enable safe excavation of water bearing deposits.

Gasket (EDPM) (hydrophilic)
An acrylic based strip placed around the outside of a tunnel segment that on contact with water expands to many times its own size to create a water-tight seal. There are both elastometric and hydrophilic gaskets.

Geotechnical engineering
A branch of civil engineering concerned with the engineering behaviour of the ground.

Gripper TBM
A tunnel boring machine with a hydraulic jack system that extends feet radically against the tunnel wall holding the TBM in place whilst another section of tunnel is excavated.

Washers made of flexible material used in bolted segmental tunnel linings to prevent leakage through bolt holes.

Ground anchor
A steel cable or bolt used to restrain movement of a structure by securing it into the ground.

Ground freezing
A technique used to control groundwater and support excavations, where coolant is circulated through tubes inserted into the ground in order to turn any water in the ground into ice.

A construction material, usually composed of water, sand and cement, but also a large number of other materials, used to improve ground conditions, fill voids in the ground or embed reinforcing bars. Fill the annulus around tunnel segments to hold the shape of the ring.

Grout hole
A small diameter hole in a tunnel lining to allow grouting up of any voids behind the tunnel lining.

Grout plug
A plug used to seal grout hole.

The top section of a staged mined tunnel excavation, normally excavated first, followed behind by the bench and invert.

Headrace tunnel
A tunnel carrying water under pressure from a reservoir down to the turbine hall of a hydroelectric power plant.

Headshunt tunnel
A blind tunnel used for the reversing of trains on underground railways.

The movement of the base of excavation, or tunnel invert upwards, caused by the removal of confining pressure of the ground.

A material which expands on contact with water. Hydrophilic gaskets are used on tunnel segment joints.

Immersed tube tunnel
A tunnel assembled under water from preformed structural units, usually floated into position and sunk onto prepared foundations.

Inclinometer (Extensometer)
An instrument used for measuring angles of slope (or tilt), elevation or inclination of an object with respect to gravity.

The inside surface of a tunnel.

The bottom surface of a tunnel.

Jacking station
The thrust pit together with the plant and ancillaries to provide the thrust for pipe jacking.

Jet fan
Apparatus with rotating hades giving current of air for ventilation.

Grouting of the Earth, done under pressure, to stabilise the ground.

A piece of equipment that consists of a frame that rolls on trades or rather wheels and carries drills for excavation of rock tunnels.

The intersection between two or more tunnels where they meet/cross.

The key segment is the last segment to be inserted in a linning ring. It provides the overall stability to the segment in much the same way as a keystone does to an arch.

Heavy planting made to construct walls in excavations and braced cuts.

Lattice Girder
A lightweight curved steel structure installed at the exposed face used to ensure the correct tunnel profile is achieved and also to carry any canopy support (canopy tubes or spiles).

Timber planes or steel plates inserted above a tunnel – supporting ribs to hold back rods or soil.

A Luminaries is a lighting device that consists of one or more lamps or light sources, a fixture that portions and shields them, components that destitute the light, and elements that connect the lamp to the power supply. In general, Luminaries do not radiate light of equal intensity in all directions because of the diazotisation of the lamps or the geometry of the fixtures.

Man rider
A cage-like lift for access into shafts.

Steel bars in a lattice structure which enables shotcrete/fire resistance concrete to bond to the tunnel lining.


A public rail transportation system. It is also a subway system and an electrical underground railway.

Mined tunnel
This is excavated rather than immersed or cut-and-covered by means of drill &blast, TBM, road-header or hand-mining.

Quantified assessment of a tunnel’s infrastructure movement e.g. tunnel structure itself, any rails, any electrical equipment or the tunnel’s environment e.g. temperature and pressure.

New Austrian Tunnelling Method is a philosophy of excavating tunnels in rock. It is based on 7 principles – mobilisation of the strength of rock mass, shotcrete protection, monitoring, flexible support, closing of invert, contractual arrangements to allow for changes in support and construction method during construction, and rock mass classification.

An area/adit off the main tunnel used for emergency supplies – fire extinguishers, fire hoses, telephones etc.

OHLE (Overhead Line Equipment)
Used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point. The devices may be pantographs, bow collectors, or trolley poles. The collectors are electrically conductive, and allow current to flow through to the traction motors of the train or tram, and back to the feeder station via the steel wheels and one or both running rails of the track.

One-pass lining
This is a tunnel with primary (single pass) lining only, normally segmental lining.

This is a larger tunnel diameter than required due to poor rock or soil breaking away into the excavation.

Parallel cut (blasting)
Method of excavating a tunnel from the working face. A central hole is drilled while parallel holes are drilled and charged. The central hole provides the space used when the other filled holes explode, controlling the direction of the energy dissipation.

Pattern bolting
Systematic spacing, e.g. in a grid, of rock bolts in a tunnel crown to provide required level of support, often as defined by the Q system.

Penstocks ( Karahnjukar project)
Part of a hydro plant that sends the water down to the turbines – normally a vertical shaft.

Pilot tunnel
A smaller diameter tunnel bored for investigative purposes before the main tunnel drive. It can be expanded into the final tunnel cross section, for example for the Uetliberg tunnel in Switzerland a 5.00 m pilot tunnel was excavated with a tunnel boring machine and then enlarged to the final cross-section of 14.20m wide by 14.40m high by a tunnel bore extender (TBE) employing undercutting.

Pipe jacking
A method for directly installing pipes behind a shield machine by hydraulic or other jacking, from a drive shaft such that the pipes form a continuous string in the ground.

Internal erosion that leads to sudden collapse.

Plug (shaft)
A thick concrete base of a shaft, that is heavy enough to prevent uplift and flotation for the whole structure.

Polypropylene fibre reinforced concrete
Concrete made with fine polypropylene fibres included in the mix, either used as a sprayed lining or in precast segments. The fibres’ main purpose is to improve the performance of concrete at high temperatures, as may be experienced in a tunnel fire. The fibres melt leaving cavities in the concrete which can be used by released water vapour reducing explosive spalling of the lining.

Entrance, or structure that forms the entrance, to a tunnel.

Pre-cast (concrete)
Uniform units of concrete cast away from the site where they are to be used, more complex shapes can be created to higher tolerances than cast in situ equivalents. Installation on site is greatly simplified and avoids the need for storing composite ingredients of concrete and handling cement materials, for example Beany Drainage in line kerb units and tunnel linings/segments.

Pressure cells
Pressure cells are an example of monitoring equipment that can be embedded into a structure (for example an SCL lining) and used to measure changes in pressure as well as absolute pressure values for radial pressure cells. A cell comprises two thin metal plates sealed around a cavity, attached to a crimping tube which forces a fixed amount of fluid into the cavity. Attached electrical cables and transducers allow the data to be recorded as required.

Primary Lining
Structural tunnel lining that is placed against the ground.

Also called a road header or just header machine, it is a piece of excavating equipment consisting of a boom-mounted cutting head, a loading device usually involving a conveyor, and a crawler travelling track to move the entire machine forward into the rock face. The cutting head can be a general purpose rotating drum mounted in line or perpendicular to the boom, or can be special function heads such as jack-hammer like spikes, compression fracture micro-wheel heads like those on larger Tunnel Boring Machines, a slicer head like a gigantic chain saw for dicing up rock, or simple jaw-like buckets of traditional excavators.

Radial joint
Joints in a pre-cast concrete segmental tunnel perpendicular to the circumference.

A hydraulic operated thrusting member in a
Tunnel Boring machine (TBM) that assists in moving the TBM forward by shoving off pre-cast concrete segmental tunnel lining.

Circular or arch support (usually steel I beams) used to support/strengthen excavations, often used in conjunction with timber boards (ribs and logging).

Ring (number, closure, closure distance)
Pre-cast concrete segmental lining of finite length.

Ring beam
This is a ring-shaped structural member usually carrying bending/vertical gravitational loads.

RMR – (Rock Mass Rating)
The sum of six rock quality parameters (unaxial compressive strength of rock material, rock quality designation (RQD), spacing of discontinuities, condition of discontinuities, groundwater conditions and orientation of discontinuities).
Scale 0-100.

Road header
Excavating equipment consisting of a boom-mounted cutting head, a loading device usually involving a conveyor, and a crawler travelling track to move the entire machine forward into the rock face. Similar to a profiler.

Robot (for spraying concrete)
A robot, attached to the rear of a tunnel boring machine, for spraying a concrete lining onto the perimeter of the tunnel.

Robotic spraying
Spraying of concrete to create a tunnel lining by means of remote or automated control.

Materials consisting of the aggregate of minerals, like those making up the Earth’s crust that has not been broken down into loose material.

Rock arch
This is the phenomenon of rock around an underground excavation behaving as an arch, transferring compressive loads to either side of the excavation. A self supporting excavation shape where the rock is broken to form a natural and stable arch.

Rock bolt
This a long bolt for stabilising rock excavations by transferring loads into the confined strong rock interior.

Running tunnel
This is the basic section of a tunnel between stations,shafts, turnouts etc.

Q system (Tunnel Quality Index)
This is a widely adopted system proposed by Barton et al in 1974 for the determination of rock mass characteristics and tunnel support requirements. Properties, such as blockings, interblock shear strength and the active stress condition of the rock mass are given numerical values based on tables from case studies. The final numerical value for Q varies on a logarithmic scale from 0.001 to 1000.

Secant pile wall
A method of constructing a concrete wall in poor ground by means of a continuous row of concrete-filled boreholes, alternate holes are bored in two successive series such that adjacent piles overlap.

Secondary lining
Lining in addition to primary lining for decoration, improved fluid flow, protection, structural enhancement or other purposes.

Arc shaped preformed component that forms part of the tunnel or shaft lining.

Downward movement of the ground surface.

Spheroid Graphite Iron, a form of segmental cast-iron tunnel lining.

A shaft is a vertical or steeply inclined excavation used as a passage from the surface to the workings, used for ventilation, travelling, hoisting, or all three. Shafts are usually of limited cross section in relation to their depth.

A protective tube used in soft ground, inside which a TBM works, the shield eliminates timbering.

Shield driven
Method of excavation in the front of a tunnel or pipe jack using a shield. (see shield)

A commonly used term for mortar or concrete sprayed through a hose and pneumatically projected at high velocity onto a surface.

Side drift
Continuous ribbed support, usually steel for construction of tunnels.

Single pass
A tunnel which only has one layer of lining.

Usually is a concrete section used for supporting loads, providing cover, or in the case of a base slab, acting as a plug against water filling a shaft.

A mixture of bentonite and water.

Slurry shield
Method using a mechanical tunnelling shield with closed face which conditions the ground and employs

The highest part of the underside or an arch shape.

Soft ground
Normally consisting of sands/gravels, extra consideration is required in tunnelling through this material, as soft soils are unstable over a certain period and must be considered as less predictable than hard rock.

Soil nail
Slender elements (usually steel reinforcing bars) inserted into the ground to act as ground improvement.

Bars inserted into a tunnel face to act as a form of ground improvement.

Earth material from an excavation.

Sprayed concrete (lining)
SCL is an established method of tunnelling using sprayed concrete to support the excavation both temporarily and permanently. (see shotcrete for picture)

Spring line
This is the point where the curved portion of a tunnel roof meets the top of the wall. In a circular tunnel the spring lines are at the opposite ends of the horizontal centreline.

Squeezing rock
Difficult tunnelling ground conditions characterised with (usually) the rock being strongly jointed and fractured and having low strength.

A method of damming up a water course or channel/pipe to control water flow.

Station concourse
This is a large open area where people can gather.

Steel fibre reinforced concrete (fibre)
A concrete mix that contains short discrete steel fibres that are uniformly distributed and randomly oriented throughout the mix opposed to conventional steel rebars used in reinforced concrete.

Steel sets/arches
Steel support structure for tunnel construction.

Step (plate junction)
Where two tunnels lined with plates of different diameters meet, special vertical plates are required to close the vertical faces – so forming a step. Often in situ, concrete is used instead of plates to avoid the heavy cost and time delay in making special plates.

Small tunnel structure, constructed for ease of connection to future tunnels.


A pit in which water collects before being baled or pumped out.

Surge tunnel
A tunnel constructed to take into account the surge pressures associated with the flow of water.

Tail skin
A rear end shield forming a tail seal and used for building the segmental rings.

TAM (tube a manchette)

A Manchette tube is a PVC or metal pipe in which rubber sleeves cover holes that are drilled in the pipe at specific intervals. The tubes are inserted into holes that have been bored into the “work area” (soil, rock, concrete, etc.) known as the “grout zone”. Grout is pumped to a packer that has been slid into the tube, seals on the packer force the grout through the holes in the tube, past the flexible rubber sleeve, and into the grout zone to help stabilize and/or seal it.

Tape extensions
A Tape Extensometer is a portable device, designed to measure the relative distance between reference anchors fixed to the excavation or structure.

This refers to the gradient across the circumferential face of a segmental lining. Rotation of successful rings allows curves to be negotiated.

TBM – (tunnel boring machine)
A machine for excavating circular tunnels, a rotating cutting wheel breaks the ground, which drops through slots in the cutting wheel for removal.

Thrust load

The total amount of force a tunnel boring machine can apply onto the face of an excavation.

Thrust pit
Pit or shaft located at the beginning of a pipe jacking operation.

Timber packer (mining engineering)
One who builds packs or pack walls. In anthracite and bituminous coal mining, one who fills worked-out rooms, from which coal has been mined, with rock, slate, or other waste to prevent caving of walls and roofs, or who builds rough walls and columns of loose stone, heavy boards, timber, or coal along haulage ways and passageways and in rooms where coal is being mined to prevent caving of roof or walls during mining operations. Also known as packer; pillar man; timber packer; waller. Also known as the thin shins placed between segment joints to covert geometry and aid the negotiation of curves.

Specified parameters for construction or manufacture of a particular item or component.

Top heading
A small tunnel dug ahead of the main excavation, they are dug at the crown of the tunnel. Top headings are used in the top-heading-and-bench method, the main advantage being that engineer can use the heading tunnel to gauge the stability of the rock before moving forward with the project.

An underground passage, open to daylight at both ends. If open only at one end, it is called a drift or an adit. A tunnel is a horizontal or sloping underground enclosed way of some length.

Tunnel lining

Permanent or temporary cover to the rock or soil surface at the periphery of a tunnel excavation.

 Tunnel ventilation
Tunnel ventilation and smoke extraction are essential to the design of safe tunnels; its purpose is to provide a safe and comfortable environment for users during the operational stage of the tunnel and to the workers during the construction phase.

Umbrella tube
Another name for canopy tube, a supportive structure made of multiple tubes bored around the drilling face of a tunnel, which are then filled with cement.

Adjective which refers to a location beneath natural (or man made if landscaping has taken place) ground level, as opposed to being at ground level or above ground.

Vacuum erector
A mechanical suction device, used to lift segments of a tunnel into position.

Vitreous enamel panels
Aesthetic panels used to line tunnels after fireproofing etc. The main purpose is to provide an easy to clean external finish to the tunnel, they are also used to maximise reflected light within the tunnel.

Volume loss
This is the volume of the settlement trough and is usually expressed as a percentage of the tunnel face area. Volume loss is the result of convergence and face loss (movement of the walls and face of the tunnel respectively) in the tunnel.

Waterproof membrane
A skin provided external to the immersed tunnel to improve the water tightness of concrete. The membrane may be of steel or other more flexible materials.

Wedgeblock (lining)
The lining used is of an expanded type. The expanded lining or wedge block technique has been developed for impermeable cohesive soils with a stand-up time of several hours (such as the over-consolidated London clay).

Reference: tunnelforum.com