LBA's Tunnelling Glossary

An A-Z compendium of tunnelling terminology


LBA’s Tunnelling Glossary

A: Accelerant - Auger
A Accellerant – Auger
Accelerant / Accelerator A substance added to concrete or shotcrete to accelerate setting.
Additive A chemical substance added in a small amount, usually to a fluid, for a special purpose such as to retard the setting of concrete or shotcrete.
Adit A tunnel, often inclined, driven from ground surface to provide access to or drainage from underground workings, or a length of tunnel driven for exploration.
Advance Forward progress in the construction of a tunnel.
Advance rate Speed of advance of a tunnel heading through the ground.
Angle of repose The angle which the sloping face of a bank of loose earth, gravel, or other material, makes with the horizontal.
Aquaclude A geologic formation containing water which is not easily extractable for use, because of the low permeability of the formation’s constituent material.
Arch Underground ground support, typically steel.
Assurance The extent that defined requirements have been complied with and that controlled processes have been followed in achieving the deliverables.
Auger A device for moving material using a rotating helical flighting. The material is moved along the axis of rotation. They are normally powered and can be used both vertically and horizontally.
Back reamer
Back reamer A cutting head attached to the leading end of a drill string to enlarge the pilot bore during a pull-back operation to enable the carrier, sleeve or casing to be installed.    
Back rip An excavation process involving the removal of a previously installed lining. This method is common during the excavation of SCL tunnels where it may be necessary to remove a shotcrete lining to create a new or larger tunnel.    
Back up Trailing gantries behind a tunnelling machine, containing transformer, hydraulics, materials transfer and grouting equipment, etc… Also a term often used to refer to the crew working and maintaining the back-up services for a tunnelling operation.    
Backfill A material used to replace excavated material, or a filling material for a temporary or disused tunnel either to seal the tunnel or to ease the removal process.    
Backgrouting See ‘Secondary grouting’.    
Backshunt A blind tunnel used for the reversing of trains on underground construction railways to enable the front cars to be loaded / unloaded in the shaft bottom.    
Bagging A term used to describe hose piping or ducting.    
Banksman A skilled, certificated person who directs the operation of cranes, locomotives, surface traffic and other plant.    
Bench The unexcavated ground normally at the lower face of a tunnel undergoing staged excavation, or the second level of a three level staged excavation (top/heading – bench – invert).    
(The) Bends See ‘Decompression sickness’.    
Bentonite Clay composed mainly of the clay mineral montmorillonite. It is typically used in drilling as it can form a slick slurry or gel when water is added. It is a common component of drilling mud. Bentonite is also a principle constituent of TBM slurry.    
Bits Replaceable cutting tools on the cutting head of a TBM or the cutting end of a drill string.    
Bolt hole / pocket A pre-formed recess or penetration within the tunnel segmental lining used for the installation of bolts to hold the segments together.    
Bond A winch rope    
Bore The internal diameter of a pipe or other cylinder.    
Bored pile A pile that is constructed within a pre-drilled hole.    
Borehole A hole driven into the ground to obtain information about the strata, or to obtain water, oil, gas, etc.    
Boring machine A mechanical means of excavating a circular opening in soil or rock, see ‘Tunnel Boring Machine (TBM)’.    
Building bar 1. Extendable bar that when extended supports the shoulder segment of a tunnel ring so as to prevent it falling prior to the insertion of the key to complete the ring. Can be secured to the rear of a tunnelling machine, or fixed to the last rings build.    
  2. Steel sections secured within the lower sections of a tunnelling machine tailskin to aid the construction of the ring centrally to the skin.    
Bullflex seal Trade name for an inflatable grout seal.    
Bulkhead A fixed structure used to divide two compartments. These structures can be found in ventilation tunnels, within the head of a TBM to separate the working chamber or cutting head from the rest of the TBM or used in LPCA installations to separate the pressurised section of tunnel from the free air.    
Banker bar / Benk bar A section of specially forged corrugated steel sheeting used as a headboard.  
Cable tunnel - Concrete Segmentaal Lining
Cable tunnel A tunnel constructed for the installation of power or communications cables.
Caisson A foundation constructed at the surface and sunk to its final position. Caissons can have solid or open bases depending on the ground they are being sunk into, and can be sunk dry or underwater.
California crossing A section of twin track crossing towed along behind the TBM for the crossing, or passing of trains on the construction railway.
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 an umbrella which helps support poor ground as a tunnel advances, reducing the risk of crown failure. The completed arch may be referred to as a ‘Pipe arch’ or ‘Tube hood’.
Cantilever diaphragm walls 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 sufficiently far below the bottom of the excavation.
Casing A pipe used to line boreholes.
Cast iron As in cast iron tunnel lining segment, now more normally made of spheroidal graphite iron.
Caulking Sealing a seam to make it watertight. Often used in tunnels or shafts to seal the joints between segments. Caulking can comprise of lead, hammered into place; or chemical/cementitious compounds. Note that caulking in early tunnels may contain asbestos material.
Chainage Measure of length along the centreline alignment of a project.
Chainman Traditionally, an assistant to a surveyor or engineer. The term deriving from surveying using the measurement of offsets along a chain.
Chemical grouting Method of ground treatment using non-cementitious compounds.
Chimney failure In tunnels excavated in soft ground the arching effect is small. Hence, large deformations can occur that can even cause settlements at the ground surface. This can lead to a chimney-like failure mode which can reach from the tunnel face to the surface. In hard ground, faulting, hydrothermal alteration and weathering can also result in this type of failure.
Cill The base member of a steel frame which is used to form an opening in a tunnel or similar.
Circumferential joint / circle joint The joint in a segmental tunnel lining between the segments of adjacent rings which runs circumferentially around the tunnel.
Cladding Lining placed on the internal surface of the tunnel. Cladding provides a smart finish, can be fireproof, and is easily retro-fitted.
Clay spade See ‘FL22’.
Closed mode The ability of a tunnel boring machine to pressurise the working chamber of the machine, and by preventing or controlling the rate of removal of the excavated material to balance with the rate of forward advance and thus limit face loss and the consequential ground movement
Closed face The term applied to a TBM which is equipped with a cutting head
Cofferdam A temporary wall built to keep water out of a construction area. Commonly used around shafts and chambers, or in hydropower projects.
Collapse As in collapse of ground.
Collar A structure which defines the start of the primary or secondary lining, such as a concrete ring beam cast around shaft segments prior to shaft sinking.
Collector sewer A sewer located in the public highway which collects the wastewaters discharged through building sewers and conducts such flows into larger interceptor sewers to the treatment works.
Colliery arches Curved RSJs, aligned to the shape of the tunnel profile, used to support the ground with timber or steel lagging
Colloidal mixer A type of grout mixer that mixes by recirculating the grout through a pump, injecting tangentially into a chamber with a very fast spinning paddle, creating a vortex. The main attributes of colloidal grout mixers are greater speed of mixing and output capacity than traditional paddle mixers.
Colloidal silicate grout A specially formulated low viscosity chemical grout used to stop water ingress in fine, commonly silty, soils
Compaction The densification of a soil by means of mechanical manipulation.
Compaction grouting Uses controlled displacement to increase the density of soft or loose soils. It is typically used for settlement control.
Compensation grouting The injection of carefully calculated volumes of grout into specific locations within the ground between the tunnel and a structure such that it compensates for the impact of driving the tunnel and prevents settlement occurring at the surface.
Composite lining A tunnel support system consisting of layers of different types of lining, such as a shotcrete primary lining and in-situ concrete secondary lining.
Compressed air The use of low pressure compressed air during excavation to apply pressure to the face and prevent the ground coming in. Also to prevent water influx and keep the excavation dry. This method can carry significant health and safety implications for the workforce.
Compression joint A compression joint is one which uses a fitting to join two or more pipes by compressing a small ring, known as an olive.
Concrete segmental lining Tunnel lining constructed of pre-cast concrete lining segments.
Data Logger - Dumpy level
D Data Logger – Dumpy level
Data logger Device used to electronically record data, such as geotechnical, noise, survey or vibration data.
Disc cutter A wheel-like cutting tool on the front of a tunnelling machine which rotates and pushes onto the rock face as the cutterhead turns, fracturing the rock.
Door Frame Slab (DFS) method A tunnelling method designed by Dr Sauer Corporation whereby a concrete ‘roof’ is cast in a shallow surface trench, raking piles driven along either side of these, and then the tunnel driven beneath this pre-support.
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.
Dowels Steel or glass fibre rods used for ground reinforcement, or plastic or steel inserts used to connect tunnel or shaft lining segments.
Drift An inclined tunnel.
Drill and blast The excavation of a tunnel, shaft or cavern in rock using explosive charges placed in holes drilled in the face.
Drill string System of rods used with cutting bit or compaction bit attached used in drilling a borehole.
Drilling fluid/mud A mixture of water and usually bentonite and/or polymer continuously pumped to the drilling bit of a borehole to facilitate cutting, reduce required torque, facilitate the removal of cuttings, stabilize the borehole, cool the head and lubricate.
Drilling jumbo See ‘Jumbo’.
Drilling rig The machine used for drilling boreholes. Typically, these would be for ground investigation, alignment checks, verification or ground treatment purposes.
Ductile Iron Another name for Spheroidal Graphite Iron (SGI)
Dumper/dump truck A vehicle used to transport materials.
Dumpy level A survey instrument consisting of a telescope and spirit level which, used with a measuring staff, allows differences in height to be recorded.
EPB - Extrados
E EPB – Extrados
Earth Pressure Balance (EPB) machine Type of microtunnelling or tunnelling machine in which by controlling the rate of spoil removal (through a screw conveyor), pressure is created within the cut material in the cutting chamber and this pressure is controlled to provide the correct counter-balance to earth and water pressures in order to prevent heave or subsidence. Typically used in granular soils.
EDM; Electronic distance measurer. Instruments that use light and radio waves, to measure distance The distance is calculated either from the time difference between a transmitted pulse and a return pulse or the phase difference between a transmitted and a reflected beam of radiation. When an EDM is combined together with an electronic theodolite, it becomes a total station.
Elephant’s foot The enlargement of a tunnel lining at the joint between the sidewalls and the invert, or at the base of the top excavation prior to bench and / or invert excavation.
Embankment A bank of earth, rock or other material constructed above the natural ground surface. For transportation projects the spoil from tunnelling is often used to construct embankments.
EPB Earth Pressure Balance.
EPDM Ethylene propylene diene monomer – a material commonly used to manufacture segment gaskets which seal by compression.
Erector A mechanical arm present in a tunnel boring machine that is used to put the tunnel lining segments into place. Lifting of the segment could be by mechanical connection to an insert in the central ‘grout’ hole (if designed as suitable for lifting), or by vacuum means.
Escalator tunnel, shaft or barrel An inclined tunnel constructed for the installation of an escalator. This is also known as an escalator shaft.
Expanded lining Primary lining that consists of tunnel lining segments that by insertion of the key are expanded circumferentially against the surrounding ground, thus removing the need for an annulus filler. Commonly referred to as a ‘Wedgeblock lining’.
Expansion joint An assembly designed to safely absorb heat induced expansion and contraction.
Extensometer A device for measuring the change in distance between two points. Often used for measuring the ground movement induced by tunnelling.
Extrados The outer surface of the tunnel lining.
Face - Freezing
F Face – Freezing
Face Wall into which the excavation is made.
Face dowel A rod of steel or fibreglass inserted into the tunnel face to provide temporary support and assist in limiting face movement.
Face dumpling A wedge of ground used during the excavation process to temporarily support the face. This can also be known as a face wedge
Face loss The movement of ground experienced at the tunnel face during the excavation process. This can either be as a result of ground creep or ground loss.
Face plate A steel plate attached to a ram at the front of a tunnelling machine that can be pushed against the excavated face in order to control ground loss or limit face movement.
Face stability Stability of the excavated face of a tunnel.
Face wedge A wedge of ground used during the excavation process to temporarily support the face
Faceboard In timberwork, the board placed against the excavated face
Falsework Structure of steel, timber or more usually a proprietary system, used to support formwork prior to and during concreting.
Fault A fracture in rocks. A fault can show displacement both vertically and horizontally, it may be locations of poor ground conditions and higher water ingress.
Fibre Steel or polypropylene fibres can be are used to reinforce concrete, particularly sprayed concrete. Fibre reinforced concrete (FRC) can provide superior fire protection and crack control.
Fibreglass dowels Used for ground reinforcement.
Figure of 8 Device used in the slinging of cast iron tunnel lining segments whereby the winch rope is tied back on itself and is held by friction. Easy to rig incorrectly (and dangerously), and modern versions must have safety devices.
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, with different standards applicable within different countries.
Fire life safety Fire life safety engineering involves numerous safety issues including fire prevention, fire suppression, and emergency evacuation/response.
Fire load The amount of combustible matter present that can act as a fuel to feed a hostile fire.
FL22 A medium size of pneumatic jigger, traditionally used for excavation and trimming of London Clay. Generates a high degree of vibration for the user so permissible trigger times are very low under current legislation. Can be fitted with a wide spade-like attachment, and thus is commonly referred to as a ‘Clay spade’.
Floatation The potential buoyancy of a void e.g. tunnel beneath the water table.
Foamed concrete A concrete produced by entraining a mixture of sand, cement and water with air.
Footblock In timberwork, a block of wood placed on the base of an excavation on which a faceboard, or sidetree rests.
Folding wedges A pair of complementary wedges placed opposite each other such that tightening them produces a rectangle of increasing thickness.
Fore poling A system of placing poles or boards into the ground ahead of a tunnel face to provide support to the crown so that excavation can safely take place.
Formwork The term given to either temporary or permanent moulds into which concrete or similar materials are poured.
Fractures A geological term which refers to the cracks visible in the rock mass through which the excavation is being driven. They can be natural or induced by the excavation.
Framing/Frame Term for temporary  support steelwork such as; support around an opening typically for cross passage excavation, consisting of a cill, lintel and jamb sections; cofferdam support steelwork
Freezing / AGF The process of freezing the ground (ground freezing) to enable safe excavation in water bearing ground. Typically coolant (brine or liquid nitrogen) is circulated through tubes inserted into the ground in order to turn any water in the ground into ice. This is a type of ground treatment.
Gasket - Grouting
G Gasket – Grouting
Gasket A strip placed around the outside of a shaft or tunnel segment that creates a water-tight seal when compressed against the adjacent segment. Gaskets are generally either EPDM (rubber), or hydrophilic.
Geomatics Geomatics used as the modern term for Land Surveying. It integrates the acquisition, modelling, analysis, and management of spatially referenced data, i.e. data identified according to their locations.
Geotechnical engineering A branch of civil engineering concerned with the engineering behaviour of the ground.
Gripper A hydraulic jack system on a tunnel boring machine that extends pads out against the tunnel wall, holding the TBM in place whilst another section of tunnel is excavated.
Grizzly A screen comprising parallel bars which prohibits material of a certain size from passing through. Commonly found on material processing plants, TBM cutter heads and conveyor inputs.
Grommets or grummets Washers made of a flexible compressible material used in bolted segmental tunnel linings to prevent leakage through bolt holes.
Ground anchor Steel cable or bolt used to restrain movement of a structure by securing it into the ground.
Ground freezing See ‘Freezing’.
Ground loss Subsidence of the surface or ground adjacent or close to an excavation, caused by soil moving into the excavation. A software package called ‘ground loss’ can be used to predict damage to buildings by determining the impact of excavation on the settlement of the affected region of soil.
Ground movement monitoring The monitoring of ground that may be disturbed by the process of excavation. Monitoring ground movement provides data to allow a construction methodology to be applied through which the movement can be controlled.
Ground treatment  Any treatment applied to the ground which seeks to improve the physical properties for purposes of improving stability or improving the excavatability of the ground.
Groundwater table See ‘Water table’.
Grout A construction material, usually composed of water, sand and cement, but also a large number of other materials, (including resins) used to improve ground conditions, fill voids in the ground, embed reinforcing bars, or to treat water ingress into completed tunnels.
Grout car The wagon in a tunnel train that conveys annulus grout to the TBM
Grout hole A small diameter hole through which grouting can take place.
Grout pan A mechanical mixer with a container for mixing grout in small quantities.
Grout pipe A pipe that transports grout under pressure for injection into a grout hole or a pipe installed in a pre-determined design pattern (vertically or angled) in the ground for the injection of grout.
Grout plug A plug used to seal a grout hole.
Grout pump A pump for pumping grout.
Grout take The quantity of grout injected into a particular hole or location.
Grouting The placement of grout via grout holes normally using a grout pump. Grouting may be undertaken both prior to excavation to improve the ground conditions (ground treatment) and to fill voids behind the tunnel lining after excavation.
Headtree / headboard Structural member of a timber heading which supports the roof of the excavation, by bearing onto the sidetrees
Hand spray Sprayed concrete lining applied by hand, without using a robotic spraying machine.
Handwork Any element of work involving hand (or manual) excavation
HAC Acronym for High Angle Conveyor. A vertical conveyor installed in a shaft which conveys excavated spoil to the surface. The HAC works on a double belt system which compresses the spoil between 2 conveyor belts for vertical lifting.
Inbye Directional term applied in tunnelling which refers to the direction towards the working face
Iron Colloquial name for a segmental tunnel lining. Origins are from when cast iron segmental tunnel linings were used, with the term now applied to linings of all materials.
Mind the Gap LBA
J: Jacking - Junction
J Jacking – Junction
Jacking (1) The actual pushing of a pipe or casing. This is usually done with hydraulic jacks.
Jacking (2) The act of resigning or ending employment
Jacking force Force applied to pipes in a pipe jacking operation.
Jacking frame A structural component that houses the hydraulic cylinders used to propel the tunnelling machine and pipeline. The jacking frame serves to distribute the thrust load to the pipeline and the reaction load to the shaft wall or thrust wall.
Jacking pipes Pipes designed to be installed using pipe jacking techniques.
Jacking pit The excavation that the machinery is set into to install a casing or tunnel. Also known as a thrust pit or thrust shaft.
Jacking shield A fabricated steel cylinder from within which the excavation is carried out either by hand or machine. Incorporated within the shield are facilities to allow it to be adjusted to control line and grade.
Jacking station The thrust pit together with the plant and ancillaries to provide the thrust for pipe jacking.
Jamb The side vertical members of a steel frame for an opening set.
Jet fan Apparatus with rotating blades giving current of air for ventilation. Unlike normal fans, works without ventilation ducting
Jet grouting Specialist grouting technique which removes a column of ground and replaces this with grout, to stabilise the ground or reduce permeability.
Jigger (pick) A hand held vibrating pick driven by compressed air and used to excavate ground in small excavations that cannot be mechanised. Significant health and safety issues with usage relating to white finger and hand arm vibration syndrome as they generate a high degree of vibration for the user so permissible trigger times are very low under current legislation.
Joints A geological term referring to tectonic fractures in the rock mass.
Jumbo A piece of equipment that consists of a frame on which drilling rigs are mounted for drilling multiple boreholes concurrently. Typically used in drill and blast tunnels, or for the drilling of ground support.
Junction The intersection between two or more tunnels or a tunnel with a shaft.
K,L: Key Segment - Look Up
K Key segment – Look-up
Key segment The last segment to be inserted in a lining ring. It is often wedge-shaped to permit installation.
Lagging Timber or steel inserted behind steel arches used to support the ground.
Land surveying The technique and science of accurately determining the three-dimensional location and position of points and the distances and angles between them. These points are used to establish maps, boundaries for ownership and for establishing and marking the position and detailed layout of new structures such as roads and tunnels.
Laser A device emitting coherent radiation which produces a highly focused light source. Used in tunnelling (usually in association with a target) to visibly define the design excavation direction.
Laser scanning Scanning of tunnel profiles and surfaces for survey and as built purposes using high speed lasers. Modern laser scanning can be conducted from fixed or mobile stations. It can also be used for more general tunnel inspection to locate fine cracks and seepages in the tunnel lining.
Lattice girder A lightweight curved steel structure installed at the exposed face used to ensure the correct tunnel profile is achieved, or to aid support of pre-treatment such as a canopy tube. Generally made up of one primary and two secondary steel bars at the intrados and extrados respectively forming a triangular cross-section, with a latticework of smaller steel bars connecting them.
Launch chamber A tunnel structure used for the erection and launching of tunnelling equipment such as a TBM.
Launch seal A mechanical seal, usually comprised of a rubber flange that is mounted on the wall of the drive shaft. The flange seal is distended by the TBM as it passes through, creating a seal to prevent water or lubrication inflow into the shaft during tunnelling. An alternative is the inflatable (Bullflex type) seal.
Layflat Term given to non-rigid ventilation ducting for use in forcing ventilation systems.
Lead A tunnel ring or TBM that has lead, differs in horizontal orientation when compared against the design alignment. The lead is measured by the difference over the tunnel diameter between the actual plane and theoretical plane when viewed from above. The terms ‘left lead’ or ‘right lead’ are used depending on which side of the machine or lining is leading.
Leading edge Surface of a tunnel or shaft lining that is closest to the excavation face. The opposite surface is the trailing edge.
Lintel The top horizontal member of a steel frame.
Liquid nitrogen A substance used for the rapid freezing of ground.
Look-up A tunnel ring or TBM that has look-up is rising more steeply (or falling less sharply) when compared against the design alignment. The look-up is measured by the difference over the tunnel diameter between the actual plane and theoretical plane when viewed from the side. Negative look-up is referred to as over-hang.
M: Man accessible - Muck Wagon
M Man accessible – nominal bore
Man accessible Description of a pipe or excavation which can be physically entered by an operative.
Man rider A suitably tested and certificated cage-like lifting basket for access into shafts or a carriage used to transport TBM workers in an out of the tunnel.
Manchester gate A section of steel across or within a construction railway that is intended to derail (and thus stop) a run-away train. It is opened either by the loco driver or an attendant for the duration a train passes, and returns to closed automatically.
Mesh Steel bars in a grid which when encased in concrete or shotcrete act to strengthen lining.
Metro An underground public rail transportation system. It is also called a subway system, or in London ‘the Underground’.
Microtunnel boring machine (MTBM) A microtunnel boring machine (MTBM) is a microtunnel shield jacked through the earth which excavates the ground in advance of the pipe being installed. It is smaller than a TBM.
Microtunnelling A trenchless construction method typically used for installing pipelines.
Milling head Rotating cutting head usually mounted on an excavator for rock excavation.
Mined tunnel A tunnel excavated from underground rather constructed from the surface (immersed or cut-and-cover tunnels).
Mixed face A soil condition that presents two or more different types of material in the path of the bore.
Monitoring Quantified assessment of movement of the ground, structures or a tunnel.
Muck Spoil. To ‘muck out’ or ‘dig out’.
Muck boxes A container used for the transport of spoil. Also used for muck wagons.
Muck wagon One muck removal system used to remove spoil in tunnelling operations involves a train of muck wagons running on a railway within the tunnel.
N: NATM - Nominal Bore
NATM New Austrian Tunnelling Method is a philosophy of excavating tunnels. It uses the strength of the rockmass together with a shotcrete lining to provide a flexible support system. The performance of underground construction works are monitored during construction. The term is often incorrectly used interchangeably with SCL, as the methods employ similar techniques, though employ different philosophies – NATM permits ground movement to gain advantages of the natural rock arching effect, whereas SCL erects a stiff lining immediately to minimise ground movements.
Niches An area or adit off the main tunnel used for emergency supplies and communications, or a small refuge in which a person can stand to avoid a train.
Nitrous oxides (NOx) Gases (Nitrogen monoxide and Nitrogen dioxide) produced from motor vehicles which have a detrimental effect on the health of workers and as such are monitored. Ventilation is used to minimise their build-up.
Nominal size / bore Size of pipe or shaft used to define the internal working diameter.
OHLE - Over-hang
O OHLE – Over-hang
OHLE Overhead Line Equipment (OHLE) is used to transmit electrical energy to trains or trams 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 (or single pass) lining A pre-cast concrete segmental shaft or tunnel lining with a relatively smooth intrados designed for both temporary and permanent loads, and thus can to be used without a secondary lining.
Open cut The method by which access is gained to the required level underground for the installation, maintenance or inspection of a structure, tunnel, pipe, conduit or cable. The excavation is then backfilled and the surface restored.
Open face shield A tunnelling shield in which excavation is carried out from within a steel tube with the face open to view. Excavation can either be manual or by a boom mounted excavator.
Opening set Specific cast iron lining to form an opening / junction.
Outbye Towards the tunnel exit or away from the tunnel face.
Outfall An outlet from a sewer system or other water based network.
Overbreak This is an unintentionally excavated larger tunnel diameter than required due to poor rock or soil breaking away into the excavation.
Over-hang A tunnel ring or TBM that has over-hang is falling more sharply (or rising less steeply) when compared against the design alignment. The over-hang is measured by the difference over the tunnel diameter between the actual plane and theoretical plane when viewed from the side. Negative over-hang is referred to as look-up.
Packer-proof grouting
P Packer – Proof grouting
Packer An inflatable device used in a borehole to isolate a section of that borehole for say permeability testing or grout injection.
Packing Temporary support between the ground and lining or material placed between tunnel segment joints. Packing is conventionally placed to correct for poor ring build quality or to assist in the negotiation of curved alignments.
Panzer belt Steel chain conveyor commonly used in coal mines and in tunnelling as a feeder from a bunker in or near pit bottom where tunnel skips are tipped.
Parallel cut Drill and blast 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 of rock bolts in a tunnel to provide ground support, often as defined by a rock mass classification system, such as the Q system.
Penstocks Part of a hydropower plant that sends the water down to the turbines, often a vertical shaft.
Perched water table Water table that is positioned above the main water table for an area because of the presence of an impermeable rock such as clay.
Perforex pre-vault method A pre-lining tunnelling method patented by Perforex consisting of saw-cutting in sections of an arch in advance of the face, followed by the filling of these with concrete.
Permeability The rate of flow of a liquid or gas through a porous material.
Pig A hard foam rubber ball which is pushed or blown through a concrete or grout pipe to clean it.
Piles Rigid supports, driven into the ground vertically or predrilled to provide ground support.
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.
Pipe arch See ‘Canopy tube’.
Pipe jacking A method of creating a tunnel by installing pipes directly behind a shield machine so that the pipes form a continuous string in the ground.
Piping Internal erosion within an excavation that leads to sudden collapse.
Piston relief duct Small tunnel or airway used to relieve air pressure underground typically in railway tunnels.
Pit An alternative term for a mine or a quarry (open pit).
Pit boss The person in charge of the miners working underground.
Pit bottom A term used to describe the area at the bottom of the main access shaft.
Pit top The surface site around the access point for the excavation works.
Plane The orientation of a tunnelling machine or a tunnel or shaft lining against the theoretical, viewed as a plane perpendicular to the design alignment.
Plane check A check performed on a tunnel or shaft lining to determine the degree by which the leading edge differs from a perfect plane and by which the best fit plane differs from a plane perpendicular to the design alignment.
Plug A thick concrete base to a shaft that is heavy enough to prevent uplift and flotation for the whole structure.
Plug and feathers A wedge (‘plug’) driven between two steel bars (‘feathers’) in a pre-drilled hole. The feathers expand as the plus is driven in causing the surrounding material to fracture – a method of controlled demolition.
Pointing The filling of caulking grove and /or bolt pockets in tunnel or shaft linings to form a smooth surface
Polyethylene Or Polythene, a tough, light, flexible synthetic resin made by polymerizing ethylene, chiefly used for in construction for plastic sheeting and pipes.
Polypropylene Fibre Reinforced Concrete Concrete made with fine polypropylene fibres included in the mix, either used as a sprayed lining, in precast segments, or in-situ concrete. They 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.
Pony / Ponyman A member of the mining gang who for small works pushes skips around the construction railway
Portal 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. Commonly used for tunnel lining segments.
Pre-lining / Pre-vault / Pre-support A method of excavation that involves the installation of a ground support system prior to the excavation stage. This method is often used to mitigate against surface settlement
Pressure cells Monitoring equipment that can be embedded into a structure and used to measure changes in pressure.
Primary Control Main site control network in which a number of widely separated primary control points are located, established with a high degree of precision. Used to establish a secondary control network.
Primary lining Structural tunnel lining that is placed against the ground.
Probe drilling / probing The act of drilling an exploratory drill hole for the investigation of ground conditions ahead of the tunnel face. Normally undertaken on a cyclic basis in open face tunnel excavations.
Profiler 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. Also called a road header. A number of cutting heads are available for different ground conditions.
Proof grouting The process of proving the grout around a tunnel or shaft by drilling and attempting to grout following either a random or pre-agreed pattern, which may vary depending on initial results.
Q, R: System - Running Tunnel
Q Q system – Running Tunnel
Q system (Tunnel Quality Index) This is a widely adopted system for the assessment 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.
Radial joint Joints in a pre-cast concrete segmental tunnel lining perpendicular to the circumference. Also known as seat joints.
Radon gas A radioactive gas that can build up in tunnels and mines in certain types of ground. It requires monitoring to ensure that safe levels are maintained.
Ram A hydraulic operated thrusting cylinder on a piece of machinery, such as on a TBM that assists in moving it forward by shoving off the tunnel lining.
Resection How a point is determined using the angles subtended from three known points, measured from the unknown point rather than the original fixed points.
Retardant A chemical that slows the setting time of concrete, shotcrete or grout.
Rib Circular or arch support (usually steel I beams) used to support/strengthen excavations, often used in conjunction with timber boards.
Ring Pre-cast concrete segmental lining of finite length.
Ring beam A ring-shaped structural member usually carrying bending/vertical gravitational loads, typically used within a shaft or cofferdam.
RMR (Rock Mass Rating) A methodology for quantifying rock quality. The sum of six rock quality parameters (uniaxial 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. Also termed a profiler.
Roadbyte (or railbyte) A length of timber or steel spanning across the knees of a tunnel which can be decked over to form a flat surface for a temporary road or railway.
Robot A robot, is a remotely controlled, sometimes tracked machine, for spraying a concrete lining onto the perimeter of the tunnel.
Robotic spraying Spraying of concrete using a robot.
Rock 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 anchor These generally consist of steel elements (bars or strands) grouted in a drilled hole. The bars or strands are subsequently tensioned. This provides lateral or vertical force to resist movement of a retaining structure. Anchors are often used for excavation support, or as a part of permanent retaining walls, or to resist up-lift forces on foundations.
Rock arch 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 excavated to form a natural stable arch.
Rock bolt This a long bolt for stabilising rock excavations by transferring loads into the confined stronger rockmass interior. Often used in associated with mesh and shotcrete.
Rock mechanics Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock and rock masses.
Rockmass A term used to describe the rock surrounding and above a tunnel or other underground excavation
Roll Measurement of the degree of rotational movement of a tunnelling machine or tunnel lining when looking along the design centreline. TBM roll is generally a reaction to cutterhead rotation and tunnel rings are rotated more slowly by the thrust rams. Maintaining minimum ring roll can be critical if pre-cast inserts are located in the segments for tunnel services.
Roller bolt A device used during the erection of segmental linings consisting of a roller attached to a segment bolt. A winch rope is passed over the roller and the segment pulled towards it.
Roller cone bit or reamer A bit or reamer in which the teeth rotate on separate, internal shafts that are usually aligned perpendicular to line. Used for boring rock.
RQD Rock quality designation (RQD) is a method for recording the in situ fracture density of the rockmass, normally measured in core recovery. See also RMR.
Running tunnel The basic section of a railway tunnel between stations, shafts, caverns and other ancillary excavations.
S: Secant pile wall - Survey equipment
S Secant pile wall – Survey equipment
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 drilled in two successive series such that adjacent piles overlap.
Secondary Control Site control stations located within the main primary network, established with slightly less precision, used primarily for the site setting out and survey work.
Secondary grouting The process of systematic re-grouting behind tunnel or shaft segments carried out after the initial grout has set. Carried out sufficiently behind the working face so as not to be disturbed by induced movements. Often referred to as ‘backgrouting’.
Secondary lining A permanent lining in addition to the primary lining. This lining can have many uses including: architectural, improved fluid flow, protection, or structural enhancement.
Seepage Water escaping through or emerging from the ground along rather a extensive line or surface, as contrasted with a spring, the water of which emerges from a single spot.
Segment Arc shaped preformed component that forms part of the tunnel or shaft lining.
Segment bogie Part of the tunnel train that conveys segments to the working face
Sequential excavation method Sometimes used as an alternative description for the NATM or SCL tunnelling method.
Settlement Downward movement of the ground surface, an above or below ground structure.
Sewer An underground pipe or conduit for transporting storm water and/or wastewater.
SGI Spheroidal Graphite Iron, the replacement to segmental cast-iron tunnel linings. SGI is more ductile than cast iron and is hence sometimes referred to as ‘ductile iron’.
Shaft A shaft is a vertical or steeply inclined excavation used as a passage from the surface to the workings, or from one level of workings to those below (a ‘sub-shaft’). Can be used for access and egress (plant, materials and manpower), or ventilation, or both. Shafts are usually of limited cross section in relation to their depth.
Shear dowel Reinforcement which crosses a zone of shear, limiting movement.
Sheet piling Thin interlocking sheets of steel which can form a continuous barrier in the ground. Steel sheet pile walls are constructed by driving steel sheets into a slope or excavation. Their most common use is within temporary deep excavations. They are considered to be most economical where retention of higher earth pressures of soft soils is required.
Shield A protective steel tube used in soft ground tunnelling, inside which a tunnel is excavated by machine or hand. This can limit temporary support measures to the face only.
Shield driven A tunnel or pipe jack excavated using a shield.
Shoring To prop up or support the excavated ground using timber or steel typically as part of the temporary works.
Shotcrete A commonly used term for concrete sprayed through a hose and pneumatically projected at high velocity onto a surface. See also SCL.
Shoulder The part of a tunnel profile between the crown and the sidewall.
Shutter/shuttering A shutter or shuttering is used to confine a concrete pour. Consists of formwork and, where necessary, falsework.
Sidetree / Sideboard In timberwork, the board placed against the sides of the excavation, generally sidetrees are the vertical pieces of a timber frame, whereas a sideboard is horizontal, bearing on two adjacent steel frames.
Sidewall The sides of a tunnel.
Single pass A tunnel which only has one layer of lining.
Site Any location where work has been or will be done.
Site access Roadway or entrance used to permit the access and egress of plant into the site.
Slab Usually 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.
Sleepers A steel or timber member used to fix and to maintain the spacing between two railway tracks.
Sling A strap or bond used during lifting operations.
Slinger / signaller Modern term for a banksman.
Slinger belt A high speed conveyor.
Slurry Typically a mixture of bentonite and water. A term that also describes the consistency of spoil if very wet.
Slurry chamber Located within the working chamber of a slurry TBM behind the cutting head. Excavated material is mixed with slurry in the working chamber for subsequent transport to the surface.
Slurry line A series of hoses or pipes that transport the slurry spoil from the face of a slurry machine to the surface for separation.
Slurry separation A process where excavated material is separated from the circulation slurry. Undertaken by using a slurry treatment plant, or STP
Slurry shield / TBM A mechanical tunnelling shield equipped with a closed face. This type of TBM allows for the conditioning of the ground and balances the ground / water pressure by the pressure of the mixed spoil and slurry in the cutting chamber. The excavated spoil is removed from the TBM using hydraulic means and is then pumped to surface for treatment.
Soft ground Normally consisting of sands/gravels or clays. 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 mixing Soil Mixing involves the introduction of cementitious or specially formulated solutions directly into the ground through the use of purpose built blending injection augers. The system is designed to achieve reduced permeability and/or increased strength within the soil mass. Soil Mixing can improve the engineering performance of soft cohesive deposits, expansive clays and loose granular soils through the direct blending of cementitious compounds or lime based slurries.
Soil nail Slender elements (usually steel reinforcing bars) inserted into the ground to provide ground improvement.
Soldier A substantial vertical steel or timber section used to strut walings from, or smaller vertical sections used behind formwork.
Spalling A process that describes the flaking of the surface layer of the rockmass, but can also be used to describe concrete where spontaneous chipping, fragmentation, or separation of a surface or surface coating occurs.
Specifications The portions of the contract documents consisting of written technical descriptions of materials, equipment, construction systems, standards and workmanship as applied to the work.
Spiles Bars inserted from a tunnel face at a outward raking angle, so as to act as a form of collapse prevention when excavating the next section beneath them. Bars may be solid or hollow and then grouted, and can be self-drilling or pushed into the face.
Spiling The methodology of using spiles.
Spoil Earth, rock and other often waste materials displaced by a tunnel or casing, and removed as the tunnel or casing is installed.
Sprayed concrete lining (SCL) SCL is an established method of tunnelling using sprayed concrete to support the excavation both temporarily and permanently. The term is often incorrectly used interchangeably with NATM, as the methods employ similar techniques, though employ different philosophies – NATM permits ground movement to gain advantages of the natural rock arching effect, whereas SCL erects a stiff lining immediately to minimise ground movements
Square marks Marks around a tunnel or shaft lining describing a circle perpendicular to the design direction of excavation. Measurement of the tunnelling machine or lining against these demonstrates differences in direction (lead / look-up / over-hang), permitting their correction.
Square work The support of conventionally excavated tunnels using either steel or timber frames.
Squat The degree by which a tunnel ring is smaller in vertical diameter than horizontal diameter, measured as the difference between the design and actual vertical diameters.
Squeezing rock Difficult tunnelling ground conditions characterised by the rock being strongly jointed and fractured, having low strength and moving towards the excavation.
Stage A working platform within the tunnel or shaft
Steel fibre reinforced concrete A concrete mix that contains short discrete steel fibres that are uniformly distributed and randomly oriented throughout the mix, as opposed to conventional steel reinforcing bars used in reinforced concrete. Also used in shotcrete.
Steel sets/arches Steel support structure for tunnel construction.
Stone A term for a concrete tunnel lining segment (no longer in regular use)
Storm water sewer A sewer that conveys storm water runoff from two or more properties and storm water runoff from roofs, paved areas and roads within the catchment area of the sewer.
Stress The load applied per unit area of material.
Stress relief The process of tunnelling causes stress relief in the existing ground. Stress relief can be modelled and monitored.
Structural Physical adequacy to support imposed loads.
Stub Small tunnel structure off the main tunnel.
Subsidence The settlement of the ground, pipelines or other structures. The effects may not be evenly distributed and differential settlement may occur.
Sump A small depression, hole or reservoir into which liquid, normally water, collects before being pumped away. Can be natural or manmade, often found at the bottom of shafts.
Surge shaft A shaft constructed to take into account the surge pressures associated with the flow of water.
Surge tunnel A tunnel constructed to take into account the surge pressures associated with the flow of water in a hydropower project.
Surveillance Activities including audit, monitoring/inspection, investigation, data capture/trend analysis, and document review.
Survey equipment Includes a range of precision equipment used during the setting out or accurate positioning of the works eg total station, theodolite, dumpy level, laser, etc.
T: Tailrace tunnel - tunnelling
T Tailrace tunnel – tunnelling
Tailrace tunnel The part of a hydropower tunnel below the powerhouse through which the spent water flows.
Tailseal In water bearing ground, seals placed between the rear of the tailskin and the segmental lining to prevent the inrush of water and ground. The efficacy is maintained by the continuous application of a special grease during forward progress.
Tailskin The rear section of a shield within which segmental rings are built, protecting the workforce from the ground.
TAM (tube a manchette) A Manchette tube is a 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 treatment zone or grout zone. Grout is pumped to a packer and past the flexible rubber sleeve, into the grout zone to help stabilize and/or seal it. The packers are then moved to the next position and the process repeated. In this manner specific locations can be targeted with each injection.
Tape extensometer A portable device, designed to measure the relative distance between reference points fixed to the excavation or structure. Often used for measuring the ground movement induced by tunnelling.
Taper This refers to the gradient across the circumferential face of segmental lining (ie one side of a ring is slightly longer than the other). Rotation of successful rings allows curves to be negotiated.
TBM (Tunnel Boring Machine) A machine for excavating circular tunnels through a variety of soil and rock strata. A rotating cutting head breaks the ground, which drops through slots in the cutting wheel for removal. Tunnel diameters can range from a metre to 19 metres. Tunnels of less than a metre or so in diameter are typically done by horizontal directional drilling rather than TBMs. TBMs were first used in the 19th century.
Teeth The cutting tools fixed to the head of a TBM or road header and used to excavate or break the rockmass.
Theodolite A surveying instrument to measure both horizontal and vertical angles, as used in triangulation networks. Combined with an EDM, it becomes a total station.
Thrust Force applied to a tunnelling machine, pipeline or drill string to propel it through the ground.
Thrust block A block of steel of concrete at the rear of a shaft or pit from which a pipe jack is pushed forward, or a manual or remote operated locking mechanism that engages stations in the track to provide a thrusting base for a machine to advance and retract.
Thrust load The total amount of force applied by a tunnel boring machine onto the face of an excavation or the last ring built.
Thrust pit Shaft from which a pipe jack is driven.
Thrust ring A fabricated ring that is mounted on the face of the jacking frame. It is intended to transfer the jacking load from the jacking frame to the thrust bearing area of the pipe section being jacked.
Timber heading A heading constructed with timber as a temporary support.
Tolerances Specified parameters for construction or manufacture of a particular item or component.
Top heading A small tunnel dug ahead of the main excavation, at the crown of the tunnel. Top headings are used in the top-heading-and-bench method, the advantages being smaller excavation and early support to minimise settlement, or that engineer can use the heading tunnel to gauge the stability of the rock before moving forward with the project.
Top plate The tunnel or shaft lining segment either side of a key, and is shaped appropriately to receive the key.
Total station A Total Station integrates the functions of a theodolite for measuring angles and an EDM for measuring distances with digital data and information recording.
Track A set of longitudinal rails mounted on cross members that support and guide a tunnel boring machine. Also underground railway track.
Trailing edge Surface of a tunnel or shaft lining that is furthest from the excavation face. The opposite surface is the leading edge.
Triangulation The process of determining the location of a point by measuring angles to it from known points, rather than measuring distances to the point directly.
Triangulation network A network of observed triangles at the largest appropriate scale are established first, then points within the main network can be accurately located using the main stations, minimising survey error.
Tube hood See ‘Canopy tube’.
Tunnel An underground passage that can be horizontal or sloping, lined or unlined, but is fully enclosed except for the two ends. Tunnels are constructed for many purposes including transportation, water distribution, sewage collection, hydropower, mineral extraction and for the installation of services.
Tunnel Boring Machine (TBM) See ‘TBM’.
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. Ventilation provides a safe and comfortable environment for users during the operational of a tunnel and to the workers during the construction phase.
Tunnelling Any of a variety of construction methods used for excavating an opening beneath the ground without continuous disturbance of the ground surface.
Uncased borehole - ZED
U Uncased borehole – ZED
Uncased borehole Any borehole without a lining or pipe inserted, i.e. self-supporting, whether temporary or permanent.
Underground A location beneath natural (or man made) ground level, as opposed to being at ground level or above ground.
Underground utility Active or inactive services or utilities below ground level.
Vacuum erector A mechanical suction device, used to lift segments of a tunnel into position.
Vent duct Ducting through which air passes to the tunnel face or heading. Flexible vent duct may be used for forced ventilation (air is blown), whereas rigid or semi-rigid duct must be used for extract ventilation (air is sucked).
Ventilation Good ventilation is vital to modern tunnel design, both to ensure the health of the workforce during construction and passenger comfort in rail and road tunnels during their operation. Forced ventilation is normal.
Verification Monitoring activity to ensure that outcomes comply with the requirements.
Vitreous enamel panels Aesthetic panels used to line tunnels after the structural lining has been installed. 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.
Voids Holes on the outside of the tunnel lining that normally require grouting.
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.
Waling A horizontal steel or timber section providing support to a series of piles or faceboards. The waling is restrained at two or more locations by struts back to the other side of the excavation or to vertical soldiers.
Water level The level of the surface of a body of water. Also a water filled tube used to measure the level difference between two points.
Water table The elevation of groundwater beneath which the ground is wholly saturated with water. A ‘perched water table’ is one that is locally higher in a confined section of ground due to the presence of an aquaclude.
Waterproof membrane A skin provided between the tunnel primary and secondary linings to improve the water tightness of concrete. The membrane may be a sheet UPV type, or a sprayed cementitious material.
Wedgeblock (lining) Another term for an ‘Expanded lining’ – Primary lining that consists of tunnel lining segments that by insertion of the wedge-shaped key are expanded circumferentially against the surrounding ground, thus removing the need for an annulus filler.
Yankee bob / Brob Short piece of steel with two opposing right angle bends such that it presents a zig-zag profile. Used to hold face timbers in place beneath headboards when excavating a timber heading.
ZED A company who manufacture guidance systems for tunnelling machines. There are now several competitors, but as ZED were first, tunnel machine guidance systems are sometimes generically referred to as ‘ZEDs