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Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Glossary

AASHTOAmerican Association of State Highway and Transportation Officials
ANFOAmmonium nitrate mixed with fuel oil used as an explosive in rock excavation.
ASCEAmerican Society of Civil Engineer
AcceleratorAdmixture to accelerate the process of hardening.
AdmixturesMaterials in liquid or powder form, added to the shotcrete mix influencing the chemical process and consistency of sprayed concrete.
AggregatesGraded mixture of mineral components being added to a concrete mix.
AlluviumA general term for recent deposits resulting from streams.
Aquiclude1. Rock formation that, although porous and capable of absorbing water slowly, does not transmit water fast enough to furnish an appreciable supply for a well or spring.
2. An impermeable rock formation that may contain water but is incapable of transmitting significant water quantities. Usually functions as an upper or lower boundary of an aquifer.
Aquifer1. A water-bearing layer of permeable rock or soil.
2. A formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.
AquitardA formation that retards but does not prevent water moving to or from an adjacent aquifer. It does not yield water readily to wells or springs, but may store groundwater.
Artesian conditionGroundwater confined under hydrostatic pressure. The water level in an artesian well stands above the top of the artesian water body it taps. If the water level in an artesian well stands above the land surface, the well is a flowing artesian well.
 
BenchA berm or block of rock within the final outline of a tunnel that is left after a top heading has been excavated.
BitA star or chisel-pointed tip forged or screwed (detachable) to the end of a drill steel.
BlockingWood or metal blocks placed between the excavated surface of a tunnel and the bracing system, e.g., steel sets. Continuous blocking can also be provided by shotcrete.
Bootleg or SocketThat portion or remainder of a shot hole found in a face after a blast has been fired.
Breast boardingPartial or complete braced supports across the tunnel face that hold soft ground during tunnel driving.
BulkheadA partition built in an underground structure or structural lining to prevent the passage of air, water, or mud.
Burn cutCut holes for tunnel blasting that are heavily charged, close together, and parallel. About four cut holes are used that produce a central, cylindrical hole of completely shattered rock. The central or bum cut provides a free face for breaking rock with succeeding blasts.
 
CCTVClosed-circuit television
CFDComputational fluid dynamics
Chemical groutA combination of chemicals that gel into a semisolid after they are injected into the ground to solidify water-bearing soil and rocks.
Cherry pickerA gantry crane used in large tunnels to pick up muck cars and shift a filled car from a position next to the working face over other cars to the rear of the train.
CohesionA measure of the shear strength of a material along a surface with no perpendicular stress applied to that surface.
ConglomerateA sedimentary rock mass made up of rounded to subangular coarse fragments in a matrix of finer grained material.
Controlled blastingUse of patterned drilling and optimum amounts of explosives and detonating devices to control blasting damage.
CoverPerpendicular distance to nearest ground surface from the tunnel.
CrownThe highest part of a tunnel.
Cut-and-coverA sequence of construction in which a trench is excavated, the tunnel or conduit section is constructed, and then covered with backfill.
CutterheadThe front end of a mechanical excavator, usually a wheel on a tunnel boring machine, that cuts through rock or soft ground.
 
DelaysDetonators that explode at a suitable fraction of a second after passage of the fling current from the exploder. Delays are used to ensure that each charge will fire into a cavity created by earlier shots in the round.
Disk cutterA disc-shaped cutter mounted on a cutterhead.
Drag bitA spade-shaped cutter mounted on a cutterhead.
DriftAn approximately horizontal passageway or portion of a tunnel. In the latter sense, depending on its location in the final tunnel cross section, it may be classified as a "crown drift," "side drift" "bottom drift", etc. A small tunnel driven ahead of the main tunnel.
Drill-and-blastA method of mining in which small-diameter holes are drilled into the rock and then loaded with explosives. The blast from the explosives fragments and breaks the rock from the face so that the reek can be removed. The underground opening is advanced by repeated drilling and blasting.
Dry MixMixture being supplied to the nozzle where the required amount of water and, if required, liquid accelerator is added.
 
FHWAFederal Highway Adminstration
FaceThe advance end or wall of a tunnel, drift, or other excavation at which work is progressing.
Face stabilization wedgeUnexcavated portion of the heading temporarily left in place to enhance face stability.
FibersSteel fibers or synthetic fibers added to mixes to improve flexural strength and post failure characteristics of the shotcrete or concrete.
Final LiningCast-in-place concrete, shotcrete, precast concrete segment, or steel lining placed after installation of the initial support and waterproofing (if applicable).
Fiber Reinforced Shotcrete (FRS)Shotcrete reinforced with either steel (SFRS) or synthetic fibers.
Finishing ShotcreteUnreinforced sprayed concrete to smoothen rough or undulating surfaces or to cover steel fiber reinforced shotcrete. Typically applied on initial shotcrete lining in preparation for the waterproofing installation or as finishing layer for the final surface of permanent support linings.
Finite difference method
Finite element methodThe representation of a structure as a finite number of two-dimensional and/or three-dimensional components called finite elements.
Firm groundStiff sediments or soft sedimentary rock in which the tunnel heading can be advanced without any, or with only minimal, roof support, the permanent lining can be constructed before the ground begins to move or ravel.
Flashcrete (Sealing Shotcrete)Typically unreinforced or steel fiber reinforced sprayed concrete layer to seal off exposed ground surface, typically 30 to 50 mm (1.2 to 2 in) thick.
ForepoleA pointed board or steel rod driven ahead of timber or steel sets for temporary excavation support.
ForepolingDriving forepoles ahead of the excavation, usually supported on the last steel set or lattice girder erected, and in an array that furnishes temporary overhead protection while installing the next set.
Full-face HeadingExcavation of the whole tunnel face in one operation.
 
Gouge zoneA layer of fine, wet, clayey material occurring near, in, or at either side of a fault or fault zone.
GradeVertical alignment of the underground opening or slope of the vertical alignment.
Ground controlAny technique used to stabilize a disturbed or unstable rock mass.
Ground stabilizationCombined application of ground reinforcement and ground support to prevent failure of the rock mass.
Ground supportInstallation of any type of engineering structure around or inside the excavation, such as steel sets, wooden cribs, timbers, concrete blocks, or lining, which will increase its stability. This type of support is external to the rock/soil mass.
Ground Support ClassPrescribed excavation sequence, support and local support based on the type of host material expected in excavation cross section as well as by the anticipated response and behavior of the host material during excavation.
Ground Support SystemSystem of interacting support elements such as shotcrete lining, steel support, rock reinforcement (dowels, bolts, spiling, etc.) in combination with an excavation and support sequence. If required, ground support systems can be supplemented by ground improvement measures (e.g. grouting, ground freezing, dewatering).
GroutNeat cement slurry or a mix of equal volumes of cement and sand that is poured into joints in masonry or injected into rocks. Also used to designate the process of injecting joint-filling material into rocks. See grouting.
Grouted Pipe SpilingPerforated steel pipes installed at the tunnel heading ahead of excavation and grouted as a means to pre-support the ground.
 
Heading and benchA method of tunneling in which a top heading is excavated first, followed by excavation of the horizontal bench.
Ho-ramA hydraulically operated hammer, typically attached to an articulating boom, used to break hard rock or concrete.
Hydraulic jackingPhenomenon that develops when hydraulic pressure within a jacking surface, such as a joint or bedding plane, exceeds the total normal stress acting across the jacking surface. This results in an increase of the aperture of the jacking surface and consequent increased leakage rates, and spreading of the hydraulic pressures. Sometimes referred to as hydraulic fracturing.
 
ITAInternational Tunnel Association
Initial Shotcrete LiningShotcrete layer of a minimum thickness as defined in the ground support class typically reinforced with lattice girders, splice bars and either fibers (steel or synthetic) or welded wire fabric.
Initial SupportSupport required to provide stability of the tunnel opening and to maintain the inherent strength of the ground surrounding the tunnel openings while preventing unnecessary loosening and enhancing the stress redistribution process. This function of support may be enhanced by installation of systematic Tunnel Pre-support and local support where required by ground conditions. It typically consists of reinforced shotcrete, rock reinforcement, pre-support, steel rib or lattice girder sets, or combinations thereof.
InvertOn a circular tunnel, the invert is approximately the bottom 90 deg of the arc of the tunnel; on a square-bottom tunnel, it is the bottom of the tunnel.
Invert strutThe member of a set that is located in the invert.
 
JointA fracture in a rock along which no discernible movement has occurred.
JumboA movable machine containing working platforms and drills, used for drilling and loading blast holes, scaling the face, or performing other work related to excavation.
Jump setSteel set or timber support installed between overstressed sets.
 
LaggingWood planking, steel channels, or other structural materials spanning the area between sets.
Length of RoundLength of the unsupported span of exposed ground opened up during one round of excavation, followed by the installation of the initial support to advance the tunnel.
Local SupportNon-systematic application of initial support measures in addition to the standard support and systematic pre-support as specified by the ground support class for local stabilization and safety during tunneling. Also referred to as additional initial support.
Liner PlatesPressed steel plates installed between the webs of the ribs to make a tight lagging, or bolted together outside the ribs to make a continuous skin.
LithologyThe character of a rock described in terms of its structure, color, mineral composition, grain size, and arrangement of its component parts.
 
Mine strapsSteel bands on the order of 12 in. wide and several feet long designed to span between rock bolts and provide additional rock mass support.
MiningThe process of digging below the surface of the ground to extract ore or to produce a passageway such as a tunnel.
MixMixture of cement, aggregates and, if required, chemical admixtures being processed in a batching plant.
Mixed faceThe situation when the tunnel passes through two (or more) materials of markedly different characteristics and both are exposed simultaneously at the face (e.g., rock and soil, or clay and sand).
Mohr's hardness scaleA scale of mineral hardness, ranging from 1 (softest) to 10 (hardest).
MuckBroken rock or earth excavated from a tunnel or shaft.
 
NCHRP National Cooperative Highway Research Program
NFPANational Fire Protection Association
NHINational Highway Institute
NozzleSpecially manufactured hose through which sprayed concrete is applied. Designed to add water (plus accelerator) through jets to the dry mix or add other admixtures to the wet mix.
NozzlemanPerson who applies the shotcrete by operating the nozzle.
 
Open cutAny excavation made from the ground surface downward.
OverbreakThe quantity of rock that is actually excavated beyond the perimeter established as the desired tunnel outline.
OverburdenThe mantle of earth overlying a designated unit; in this report, refers to soil load overlying the tunnel.
 
PIARCWorld Road Association (previously the Permanent International Association of Roadways Congress)
Partial DriftsTo achieve an early, temporary ring closure and to reduce excavation face size, partial drifts such as sidewall drifts, middle drifts and top heading, bench, and invert drifts can be used. These partial drifts are supported by temporary shotcrete support, such as temporary middle walls, invert supports, etc.
Pocket ExcavationPartial excavation of the tunnel face in unstable ground conditions by which small areas (pockets) of ground are excavated immediately followed by shotcrete installation. A series of pockets are excavated following the drift shape allowing the installation of the shotcrete lining. Typically, a central face stabilization wedge remains in the face that is excavated either during the next excavation round in sequence or after completion of the full shotcrete lining installation.
Passive reinforcementReinforcing element that is not prestressed or tensioned artificially in the rock, when installed, i.e. rock dowel.
Pattern Reinforcement or Pattern BoltingThe installation of reinforcement elements in a regular pattern over the excavation surface.
Phreatic surfaceThat surface of a body of unconfined ground water at which the pressure is equal to that of the atmosphere.
PillarA column or area of coal or ore left to support the overlaying strata or hanging wall in mines.
Pilot drift or pilot tunnelA drift or tunnel driven to a small part of the dimensions of a large drift or tunnel. It is used to investigate the rock conditions in advance of the main tunnel excavation, or to permit installation of ground support before the principal mass of rock is removed.
Pneumatically applied mortar or concreteSee shotcrete.
PortalThe entrance from the ground surface to a tunnel.
Pre-reinforcementInstallation of reinforcement in a rock mass before excavation commences.
Principal stressA stress that is perpendicular to one of three mutually perpendicular planes that intersect at a point on which the shear stress is zero; a stress that is normal to a principal plane of stress. The three principal stresses are identified as least or minimum, intermediate, and greatest or maximum.
 
RaiseA shaft excavated upwards (vertical or sloping). It is usually cheaper to raise a shaft than to sink it since the cost of mucking is negligible when the slope of the raise exceeds 40" from the horizontal.
Ravening GroundPoorly consolidated or cemented materials that can stand up for several minutes to several hours at a fresh cut, but then start to slough, slake, or scale off
Rebar SpilingReinforcement rebars installed at the tunnel heading ahead of excavation and grouted as a means to pre-support the ground. They can be installed in pre-drilled and grout filled holes or rammed into the soft ground
Recessed rock anchorA rock anchor placed to reinforce the rock behind the final excavation line after a portion of the tunnel cross section is excavated but prior to excavating to the final line.
ReinforcementStructural steel reinforcement improving the moment capacity of a concrete section.
Relievers or relief holesThe holes fired after the cut holes and before the lifter holes or rib (crown, perimeter) holes.
RetarderAdmixture for hydration control to delay setting of wet shotcrete.
Rib1. An arched individual frame, usually of steel, used in tunnels to support the excavation. Also used to designate the side of a tunnel.
2. An H- or I-beam steel support for a tunnel excavation (see Set).
Rib holesHoles drilled at the side of the tunnel of shaft and fired last or next to last, i.e., before or after lifter holes.
Road headerA mechanical excavator consisting of a rotating cutterhead mounted on a boom; boom may be mounted on wheels or tracks or in a tunnel boring machine.
Rock AnchorRock anchors are tensioned tendons anchored to the ground over a defined length.
Rock boltA tensioned reinforcement element consisting of a rod, a mechanical or grouted anchorage, and a plate and nut for tensioning by torquing the nut or for retaining tension applied by direct pull.
Rock dowelAn untensioned reinforcement element consisting of a rod embedded in a grout-filled hole and bonded to the surrounding ground along their entire length (fully bonded) either by friction or grout.
Rock massGround mass built up by in situ pieces of rock material of which are limited by discontinuities. Properties controlled by grade of weathering, discontinuities, fillings, and orientation of discontinuities.
Rock reinforcementElements reinforcing a jointed rock mass to enhance the rock mass strength and reinforce the rock's natural tendency to support itself.. Passive (dowels, spiles) or active (bolts, anchors) elements are used. Rock mass reinforcement can be installed either in spot applications or systematically. The reinforcement elements used in SEM tunneling are typically steel or fiberglass bars or pipes in conjunction with shotcrete on the rock surface.
Rock supportThe placement of supports such as wood sets, steel sets, or reinforced concrete linings to provide resistance to inward movement of rock toward the excavation.
RoundA group of holes fired at nearly the same time. The term is also used to denote a cycle of excavation consisting of drilling blast holes, loading, firing, and then mucking.
 
SINTEFFoundation for Scientific and Industrial Research at the Norwegian Institute of Technology
ScalingThe removal of loose rock adhering to the solid face after a shot has been fired. A long scaling bar is used for this purpose.
SegmentsSections that make up a ring of support or lining; commonly steel or precast concrete.
SetThe temporary support, usually of Steel or timber, inserted at intervals in a tunnel to support. The ground as a heading is excavated (see Rib).
ShaftAn elongated linear excavation, usually vertical, But may be excavated at angles greater than 30 deg from the horizontal.
ShearA deformation that forms from stresses that displace one part of the rock past the adjacent part along a fracture surface.
ShieldA steel tube shaped to fit the excavation line of a tunnel (usually cylindrical) and used to provide support for the tunnel; provides space within its tail for erecting supports; protects the men excavating and erecting supports; and if breast boards are required, provides supports for them. The outer surface of the shield is called the shield skin.
Shield tail (or skirt)An extension to the rear of the shield skin that supports soft ground and enables the tunnel primary lining to be erected within its protection.
ShotcreteConcrete applied through a nozzle by compressed air and, if necessary, containing admixtures to provide quick set, high early strength and satisfactory adhesion.
ShoveThe act of advancing a TBM or shield with hydraulic jacks.
SkipA metal box for carrying reek, moved vertically or along an incline.
Soft GroundDeteriorated rock or residual soil with limited compressive strength and stand-up time.
SpallA chip or splinter of rock. Also, to break rock into smaller pieces.
SpilesPointed boards or steel rods driven ahead of the excavation, (similar to forepoles).
SpoilSee muck.
Spot reinforcement or spot dowelling or boltingLocalized reinforcement to secure individual rock blocks and wedges in place. Spot reinforcement may be in addition to pattern reinforcement or internal support systems.
Spray ShadowIn shotcrete applications a shadow generated by objects (e.g. reinforcement, fixing devices). The shotcrete within this shadow area is less compacted and of low quality.
Spring lineThe point where the curved portion of the roof meets the top of the wall. In a circular tunnel, the spring lines are at opposite ends of the horizontal center line.
Squeezing groundMaterial that exerts heavy pressure on the circumference of the tunnel after excavation has passed through that area.
Stand-up-timeThe time that elapses between the exposure of reek or soil in a tunnel excavation and the beginning of noticeable movements of the ground.
Starter tunnelA relatively short tunnel excavated at a portal in which a tunnel boring machine is assembled and mobilized.
StrutsCompression supports placed between tunnel sets.
Systematic Rock Dowelling/BoltingRock reinforcement applied in a systematic pattern designed to suit the ground conditions expected.
 
TBMTunnel boring machine.
Tail voidThe annular space between the outside diameter of the shield and the outside of the segmental lining.
Tie rodsTension members between sets to maintain spacing. These pull the sets against the struts.
TightRock remaining within the minimum excavation lines after completion of a round-that is, material that would make a template fit tight. "Shooting tights" requires closely placed and lightly loaded holes.
Timber setsThe complete frames of temporary timbering inserted at intervals to support the ground as heading is excavated.
Top heading1. The upper section of the tunnel.
2. A tunnel excavation method where the complete top half of the tunnel is excavated before the bottom section is started.
Tunnel Boring Machine (TBM)A machine that excavates a tunnel by drilling out the heading to full size in one operation; sometimes called a mole. The tunnel boring machine is typically propelled forward by jacking off the excavation supports emplaced behind it or by gripping the side of the excavation.
Tunnel Pre-supportSystematic measures including pre-spiling with bars or pipes, grouted pipe arch canopy or steel sheets installed from within the tunnel or prior to tunnel construction.
 
Water tableThe upper limit of the ground saturated with water.
Waterproofing SystemA layered system consisting of a drainage material (i.e. Geotextile) and a flexible, continuous synthetic membrane (typically PVC).
WeatheringDestructive processes, such as the discoloration, softening, crumbling, or pitting of rock surfaces brought about by exposure to the atmosphere and its agents.
Wet MixMixture being supplied to the nozzle readily batched with water and admixtures.
 
Yield ElementStructural element of high deformation capability applied within the Initial Shotcrete Lining to facilitate controlled deformation.
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Updated: 06/19/2013
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000