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Glossary Many tunneling and geological terms can have multiple, and even somewhat contradictory, meanings. The following definitions have been written or chosen to be consistent with usage in this report. Underlined words are separately defined under their own entries. However, a few words are generally not noted in this manner because they occur so frequently in other def initions that underlining would prove a distraction. There- fore, before proceeding to other entr ies in the glossary, the reader may wish to refer to the definitions for "earth, n "ground, " crock, " "shaft," "soil, " and "tunnel. " ~A" LINE A dimensional line in a tunnel, inside of which rock projections are not permitted. ADIT (a) A short length of tunnel driven from the surface to the main tun- nel for access or mucking out. (b) A short transverse tunnel con- necting two parallel main tunnels; often called a "cross edit. n ADVANCE The distance excavated during a given time (shift or day) in tunnel- ing, drifting, or in raising or sinking a shaft. ARCH The configuration of the upper portion of a tunnel section above the springline; the crown, roof, or back of a tunnel. "B. LINE A dimensional line in a tunnel, outside of which excavation is not paid for ; may also be referred to as the pay line. BACK The overhead portion of a tunnel, i.e., ceiling; less commonly used than the synonymous term roof. BEDDING The arrangement of rocks in layers, strata, or beds of varying thick- ness and character; usually applied to sedimentary rock, in which case it is synonymous with "stratification. n 169

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BED ROCK (a) Any solid rock exposed at the surface of the earth or overlain by unconsolidated material. (b) Rock of relatively great thickness and extent in its native location. BENCH The lower portion of a tunnel which is constructed by first excavating and supporting an upper portion and then excavating the lower portion. A system used in larye-diameter or questionably stable tunnels. BLOCKING Blocks of wood or concrete installed between the lagging or steel sets and the rock surfaces of a tunnel to transfer stress to the sup- ports. BLOCKY ROCK Rock having joints or cleavage spaced and oriented in a manner such that it readily breaks into loose blocks under excavation conditions. BLOW OUT The sudden escape of air from a tunnel driven under compressed air. BOoMHEADER 1 A relatively soft-rock mining machine that can mine selectively and cut any shape or size of tunnel by use of a cutting head on a hydrau- lically controlled boom or arm that is generally centrally positioned on the unit and extending cantilever-fashion in front of the machine in such a way that it can be vertically raised or lowered or swung in an arc from side to side. BORE In reference to construction operations, the making of a relatively large hole in earth or rock with an excavating device, while removing the muck mechanically or with the aid of gravity. Distinguished from drill. BOREABILITY A value expressing the boring properties of rock in terms of the pen- etration rate with certain numbers/types of cutters and amount of . pressure applied. Equivalent to "tunnelability," the ease or diffi- culty with which a rock type can be penetrated by a tunnel boring machine. BOREHOLE, BORING - An exploratory hole made in the earth with a drill, auger, or drive sampler for the purpose of determining soil, rock or groundwater con- ditions. BOULDER A detached and rounded or much-worn mass of rock greater than 10 inches in diameter, typically carried some distance from the parent rock by natural forces and worn by a stream, ocean waves, or glacier, or by weathering in situ. BREAST BOARDS, BREASTING Boards placed and braced temporarily across the face of a tunnel drive to support incompetent materials. BRIDGE ACTION TOME The time that elapses between the exposure of an area at the roof of a tunnel and the beginning of noticeable, unprovoked inward movement of the ground above this area; sometimes described as "stand-up time." 170

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CAVERN OR CHAMBER A relatively short, underground room-type opening of large cross- sectional area, generally built to house a special structure such as a hydroelectric power plant, hardened defense facility or storage for waste. CHANGED CONDITION Physical site condition revealed by excavation to be substantially different from the condition that could reasonably be anticipated from information in the contract documents . Common teas is for litiga- tion by contractor, sometimes resulting in an extra paid by owner. CHEMICAL GROUT A combination of chemicals that gel into a semi-solid after they are injected through drilled holes to strengthen incompetent ground (gen- erally soil), or to prevent groundwater from flowing into the excava- tion. CIRCULAR TUNNEL A tunnel of circular cross section, generally made with a full circu- lar shield or tunnel boring machine. , COMPETENT GROUND Ground that can stand for relatively long periods with no support or only minimal support when a tunnel is excavated through it. COMPRESSED AIR Air supplied to the tunnel at greater than atmospheric pressure, either to operate pneumatic tools or to facilitate tunneling in very soft or wet ground. See also high air and low air. CONSOLIDATED MATERIALS Earth materials, generally of sedimentary origin, which have been firmly densified or converted into rock by compaction, deposition of cement in pore spaces, and/or by physical and chemical changes in the constituents. CONSOLIDATION (a) In classical geology, any or all of the processes whereby loose, soft, or liquid earth materials become firm and coherent. (b) In soil mechanics, the adjustment of a saturated soil in response to in- creased load involving the squeez ing of water from the pores and de- crease in void ratio. CONTRACT MODIFICATION Change in a construction contract that either increases or decreases the scope of work, amount of materials, or length of performance time originally envisaged. CONVENT TONAL MINING Traditional, labor-intensive excavation such as hand mining in soft ground and dr ill-and-blast mining in rock. Distinguished from the more highly mechanized methods of mining. CORE RECOVERY In rock core drilling, the amount of the drilled rock withdrawn as core (i.e., recovered); generally expressed as a percentage of the cored interval or coring "run." Example: a 5-ft coring run that yields 4 ft of rock core constitutes a recovery of 80 percent. 171

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COVER The material, including soil and/or rock, as measured along a perpen- dicular from the tunnel crown to the ground surface. See also over- burden. CRoWN The highest point of an arched tunnel cross section; the roof or back of a tunnel. CROWN BARS Timbers or steel members cantilevered from previously installed sets nearest the heading to temporarily support a rock tunnel roof while the next set is being installed. CUTANDCOVER TUNNEL A tunnel constructed by excavating a trench from the surface and then decking it over, usually with timber, so that traffic can be main- tained while the structure is built within the trench. CUTTERHEAD The rotating front end of a tunnel boring machine, serving as a mount for the ground-abrading cutters. DEEP SHAM A shaft of relatively {but not formally defined) great depth, usually associated with mines and underground waste storage and less often with civil engineering projects which are relatively shallow. In this report, a 1,225-ft deep penstock shaft used as a study project might not be considered a "deep" shaft by some specialists. DIFFERING SITE CONDITION Same as changed condition. DISC CUTTER The most coIrunon type of roller cutter, taking the form of a single circular disc or cutter blade of hardened steel alloy which revolves freely about its axis as it rolls around the rock face. May carry double or triple disc blades on a s ingle cutter mounting. DRIFT A mined passageway or portion of a tunnel. In the latter sense, de- pend~ng upon its location in the final tunnel cross section, it may be classified as a "crown drift," "side drift," "invert drift," etc. DRILL In reference to construction operations, the making of a relatively small circular hole in earth or rock with a cutting tool, while re- moving the cuttings by means of a circulating fluid. Distinguished from bore. DRILLABILITY A specif ic value expressing the drilling properties of a rock in terms of the penetration rate with a certain type of bit and feed pressure. DRILL-AND-BLAST A method of excavating rock by drilling small-diameter holes on a planned layout, packing these with explosives, and then firing to a fixed program to shatter the rock in a desired form. Distinguished from machine mining, mechanical excavation. 172

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DRILL JUMBO In drill-and-blast tunnel construction, a rubber-tired or track mounted movable frame with platforms to support men and drills. DRIVE To excavate horizontally or at an inclination, as in a drift, tunnel, edit, or entry. Distinguished from sink or raise. EARTH (a) Loose material of the earth's surface; the disintegrated parti- cles of solid matter, as distinguished from rock. (b) Material which can be removed and handled economically with pick and shovel or by hand, or which can be loosened and removed with a power shovel. See soil for related definition. EARTH-BALANCE SHIELD A closed-face shield designed for tunneling in fine-grained soils by trapping excavated materials against the face and removing them at a rate slow enough to maintain pressures that counterbalance earth pressures, stabilize the face, and prevent ingress of water. EARTH TUNNEL A tunnel driven in relatively easily excavated earth or soil rather than in rock. Also commonly referred to as a soft-ground tunnel. EXPLORATORY SHAFT A shaft constructed for the purpose of studying ground conditions in the vicinity of a future underground opening. EXTRA Additional payment made to a contractor as a result of work or use of materials beyond the scope of the original contract. EXTRADOS The exterior surface of an arch; in a tunnel it is the arch surface lying against the excavated rock or soil surface. FACE The advance end or wall of a tunnel at which work is progressing. FAULT A fracture or fracture zone in the ground along which there has been displacement of the two sides relative to one another, parallel to the fracture. The displacement may be a few inches or many miles. FEELER HOLE A small-diameter exploratory hole drilled ahead of the tunnel face in order to determine ground conditions. FINAL LINING Long-term shaft or tunnel support installed for permanent stability or other user requirement, often incorporating the initial support elements; also referred to as "permanent lining," "permanent support," "final support." FIRM GROUND Consolidated sediments or soft sedimentary rock in which the tunnel heading can be advanced without any (or with only minimal) roof sup- port, and the permanent lining can be constructed before the ground begins to move or ravel. 173

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FLOWING GROUND Soil below the water table so affected by seepage pressures toward the tunnel working face that what might otherwise be running or ravelling ground is transformed into a flowing mass that advances like a thick liquid into the heading. FOLIATION General term for a planar arrangement of textural or structural fea- tures in any type of rock, especially the planar structure that re- sults from flattening of the constituent grains of a metamorphic rock. See also schistosity. FOREPOLE, FOREPOLING Sharpened planks or steel sections driven from the arch to extend at an upward angle ahead of a soft-ground tunnel face to provide tempo- rary support and overhead protection while another increment is being mined. Generally driven over the last set near the face, with the butt end wedged beneath the next to last set. FREE AIR Air at atmospheric pressure. FRICTION ROCK STABILI ZER A 3- to 8-ft long steel bar with a slot along its entire length, in- serted in drilled holes of slightly smaller diameter around the pe- riphery of a tunnel. The slot causes the stabilizer to be in com- press~on and exert an outward anchoring force to tie rock-blocks or strata together and prevent their loosening or falling out. FULL FACE Tunnel excavation to full cross-sectional size with each blast or shove. Distinguished from heading, bench, and multiple drift. GOUGE GRADE Finely abraded or pulverized rock particles and claylike altered rock found between the walls or within the fractures of a fault or shear zone; the result of grinding movements that crush the affected rock. (a) The overall vertical alignment of an underground opening. (b) Locally, the same as "gradient," i.e., the rate of incline or decline in terms of degrees from the horizontal, percent of rise to the hori- zontal distance, or in feet of vertical projection per mile of hori- zontal projection. GRIPPER SHIELD A shielded rock or hard-earth tunnel bor ing machine equipped to move forward by reacting (i.e., exerting shove forces) against the tunnel walls through a hydraulic gripper reaction system. GRIPPER TBM A rock tunnel boring machine which generally utilizes roller disc cutters as excavation tools and which moves forward by reacting (idea, exerting shove forces) against the tunnel walls through a hy- draulic gripper reaction system. GROUND The medium, whether soil or rock, through which a tunnel is driven. 174

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GROUND ANCHOR Part of a ground support system consisting of a tendon inserted in a drilled hole, secured at the remote end, usually by means of a grout- ed plug, and tightened or tensioned against the ground retaining mem- ber in the system. See also rock bolt. GROUT A pumpable slurry of neat cement or a mixture of neat cement and fine sand, commonly forced into holes drilled from a tunnel to strengthen incompetent soil/rock or to prevent groundwater from flowing into the excavation. See also chemical grout. GUNITE A form of mortar consisting of fine sand, cement, and water which is sprayed on freshly excavated rock by air pressure to prevent deteri- oration of the rock, and In some instances to provide structural sup- port. HAND MINING (a) Tunnel excavation by means of hand-held tools rather than by heavy, mechanized cutting or digging equipment. (b) Term sometimes applied to drill-and-blast operations in order to emphasize a dis- tinction from TBM-mining. HARD ROCK In construction, rock having a strongly bonded nature such as to re- quire excavation by blasting or the use of specially hardened cutters; generally includes igneous and metamorphic rock and the more strongly bonded sedimentary rocks. HEADING (a) The wall of unexcavated ground at the advance end of a tunnel; similar in use to face. (b) A small advance tunnel driven for the purpose of enlarg ing to create the main tunnel; similar to drift, but generally driven above the spr ingline as a top heading. HEADING-AND-BENCH CONSTRUCTION A tunneling method in which a top heading is excavated, followed (within one to a few blasts or shoves) by excavat ion of the lower bench. Distinguished from top heading construction. HEAVY GROUND Very incompetent rock, usually found in faults or in shear zones; highly weathered or decomposed material having a tendency to move into the open tunnel area. HIGH AIR Compressed air used in tunnels to operate pneumatic tools. Generally supplied at 100 psi. HIGH ROCK A rock surface extending above the invert of a tunnel that would otherwise be driven entirely in soft-ground conditions. HOLE THROUGH To "daylight" a tunnel at a portal or to meet another tunnel face which results in a continuous tunnel. HORSESHOE TUNNEL A tunnel of roughly horseshoe-shaped cross section, or tented like an inverted "U." Many variations on the basic configuration are pos- sible. 175

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INCoMPETENT GROUND Essentially the opposite of competent ground or firm ground. See heavy ground for related definition. INITIAL SUPPORT Relatively short-term tunnel or shaf t supper t ins tailed f or s tabili ty and safety during construction operations, with elements generally let t in place and incorporated into the f inal lining. Initial sup- port is often referred to as primary support. INTRADOS The interior curve of an arch, as of a tunnel lining. INVERT The lowest point of a tunnel, i.e., the floor. On a circular config- uration, it is approximately the bottom 90 degrees of the arc of the tunnel. On a square-bottom configuration, it is the bottom of the tunnel. JOINT In rock, a naturally occurring fracture or parting along which there has been no vis ible movement parallel to the fracture plane or sur- face. JUM80 See drill jumbo. JUMP SET One steel rib or unit of timber framing installed between two over- stressed sets or between two pre-existing sets . LAGGING Wood planking or other structural materials spanning the area between ribs. LINER PLATE Iron or steel plates which can be fastened together to support the arch, sides, and in some cases the invert of a tunnel. LINING A casing of brick, concrete, shotcrete, iron, steel, or wood placed in a tunnel or shaft to provide support and/or to finish the interior. LoW AIR Compressed air used to facilitate tunneling in soft or very wet ground by counterbalancing external hydrostatic pressures. Supplied at 5 to 40 psi. MACHINE MINING Continuous tunneling by means of boomheaders, tunneling machines, TBMs, etc. Distinguished from drill-and-blast. MASSIVE (a) In geology, the homogeneous structure of a rock without any planar, directional arrangement of textural or structural features. (b) A durable body of rock that is essentially free of fractures and other discontinuities, and possesses a strength that does not vary appreciably from point to point. 176

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MECHANICAL EXCAVATION The removal of soil or rock by means of heavy cutting or digging equipment (not hand-held). and-blast excavation. MINED TUNNEL A tunnel excavated without removing the overlying rock or soil and, except for shaft connections, open to the surface only at one or both ends during construction. MTXED-FACE TUNNEL A tunnel requiring excavation of both earth and rock materials in the same heading at the same time. Some owners may extend the definition of rock to include boulders larger than 3 ft in diameter because of similar difficulties of removal. MOLE See tunnel boring machine (TBM). MUCK Excavated soil or rock that must be removed from the tunnel or shaft in order to continue advancing. The removal operation is termed "mucking" or "mucking out." MULTIPLE-DRIFT EXCAVATION A tunneling method in which two or more parallel drifts are pre-exca- vated in order to install partial ground support before the full tun- nel cross section is opened up between them. Distinguished from hand mining and drill- OPEN-CUT TUNNEL A tunnel constructed by excavating a trench from the surface, build- ing the structure within the trench, and then backfilling to restore the surface. OVERBREAE; The quantity of rock that is actually excavated beyond the perimeter established as the desired tunnel outline (i.e., the pay line), owing to the irregular pattern of rock breakage. OVERBURDEN In this study, essentially the same as cover, the total depth of soil and/or rock overlying the tunnel crown. Distinguished from another common def inition as the mantle of soil or loose mater ial overlying bed rock. PAY LINE A dimensional line in a tunnel, outside of which excavation is not paid for. PENETRATION RATE The optimum speed with which a drill or excavator can advance through the ground in a short time before it is slowed or stopped by mechani- cal breakdown, ground instability, or the like. PILOT TUNNEL A small tunnel excavated over the entire length or over part of a larger tunnel, to explore ground conditions and/or to assist in final excavation. May also be referred to as a "pilot drift." 177

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POLING BOARD OR PLATE See forepoling. POPPING ROCK An overstressed rock condition involving the spontaneous and violent detachment of rock slabs. See also rock burst. PORTAL The entrance from the ground surface to a tunnel. POSTS The vertical members of a steel rib or timber support system. PRE-SPLITTING A technique of inducing cracks roughly following the periphery of the rock shape to be excavated by the use of closely spaced holes and re- duced explosive charges prior to main blasting; a subdivision of smooth blasting. PRIMARY SUPPORT See initial support. PULL The length of rock broken when a round is fired at the face. QUARTER ARCH POINTS Those areas between the tunnel crown and springline covering approxi- mately the spread from the 1:00 to 2:00 o'clock and the 10:00 to 11:00 o'clock positions. RAISE (a) To excavate a shaft upwards, in distinction from sinking. (b) A vertical or inclined shaft driven upward from an underground opening, most frequently to connect with another underground opening or the surface. RAISE BORE To raise a shaft by means of a rotating mechanical device generally powered and/or guided upward by a drill stem fed through a small down- drilled pilot hole. RAISE BORE AND REAM To raise bore a shaft of moderate size, then enlarge to a greater di- ameter by pushing or pulling a mechanical "reaming" device through the initial opening. RAISE BORE AND SLASH To raise bore a shaft of moderate size, then enlarge to a greater di- ameter by means of blasting, or "slashing." RAVELLING GROUND Poorly consolidated or cemented materials that can stand up for sev- eral minutes to several hours at a fresh cut, but then start to slough, slake, or scale off. RIB A part of the tunnel support, usually of structural steel, curved to suit the shape of the tunnel section. See also set. ROADHEADER See boomheader (currently a synonymous term). 178

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ROCK (a) Ordinarily any consolidated or coherent and relatively hard, nat- urally formed mass of mineral matter; stone. (b) In engineering, a natural aggregate of mineral particles connected by strong and perma- nent cohesive forces (i.e., interlocking crystals, closely packed grains, natural cement). {c) Any material which requires blasting or the use of powerful, hardened equipment for effective removal. ROCK BOLT A round steel bar, sometimes very long but usually less than 25 ft long, equipped with an expandable anchor at the far end, inserted in drilled holes around the periphery of a tunnel to tie rock-blocks or strata together and prevent their loosening or falling out. It may be locked into the hole mechanically or with some type of grout. It may be tensioned or untensioned. ROCK BURST A spontaneous and violent detachment of a slab or slabs from over- stressed rock. See also popping rock. ROCK DOWEL A 5- to 40-ft long steel reinforcing bar inserted in drilled holes around the periphery of a tunnel and anchored or sealed with mortar or polyester resin to tie rock-blocks or strata together and prevent their loosening or falling out. ROCK QUALITY DES TGNATION (ROD) A modified core recovery percentage in which only sound pieces of rock core 4 in. or more in length are counted as recovery. ROD is considered a more accurate gauge of a rock's engineering "quality" or competence than is the gross recovery percentage. It is stated as the cumulative percent of the core run occurring in pieces greater than 4 in. long. ROCK TUNNEL A tunnel driven in consolidated natural material {i.e., "rock") which requires use of rock excavation methods such as blasting, channeling, wedging, or barring, or a tunneling machine making use of specially hardened cutters. ROLLER CUTTER A cutter consisting of a circular metal disc with hardened rim or teeth, mounted on bearings set in the rotating face of a TBM and rolled in an arc across the rock face under force; the resulting con- centration of force or of stress spells the rock. See also disc cutter. ROOF The overhead portion of a tunnel, i.e. ceiling; a more common term than the synonymous back. - ROOF BOLT Physically the same as a rock bolt. ROUND A cycle of rock excavation consisting of drilling blast holes, load- ing, firing, and then mucking. ROUTE FOOT A measurement of alignment that distinguishes between single tube and multiple tube configurations for the purpose of equating the utility of borehole surveys. For example, one borehole may be used to survey 179

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(theoretically) twice as many linear ft of tunnel for a double tube system as for a single tube system. Thus, one route ft in a double tube system comprises two linear ft of tunnel alignment; in a single tube system, route ft and linear ft are the same. RUN-IN Relatively sudden, uncontrolled flow of material into a tunnel from the face or the tunnel circumference. RUNNING GROUND Perfectly cohesionless materials (such as dry sand or clean, loose gravel) above the water table which run from any unsupported lateral face until a stable pile is built up at the angle of repose. SCALING The removal of loose pieces of rock adhering to the solid tunnel sur- face after blasting. SCHISTOSTTY The foliation in schist or other coarse-grained, crystalline rock due - to the parallel, planar arrangement of mineral grains of the platy, prismatic or ellipsoidal type, such as mica. SEGMENTS Sections of iron, steel, or precast concrete which can be bolted or keyed together to make up a ring of support or lining. Iron or steel segments are generally referred to as liner plates; concrete segments may be referred to as "panels." SET SHOTCRETE SHOVE One steel rib or unit of timber framing to support the sides and roof of a tunnel. SHAFT An excavation of limited area compared with its depth, constructed for access, ventilation, or conveyance of water to an underground opening. The term is often specifically applied to an approximately vertical shaft as distinguished from an incline or inclined shaft. SHEAR ZONE A local geologic structure resulting from the relief of earth stress- es by the formation of a multitude of minute, closely spaced frac- tures with slight slipping or faulting along each. SHIELD A movable steel tube, framework, or canopy shaped to fit the excava- tion line of a tunnel and used to provide immediate support for the tunnel and protect the men excavating and providing the longer-term supports. May be fitted with a cutting device for excavating the tunnel and/or a form and mechanical devices for placing the tunnel lining. See also soft-ground _hield and gripper shield. A form of quick-setting concrete with aggregate generally no larger than 3/4 in., sprayed on freshly excavated rock by air pressure to provide early, flexible support and sometimes applied more thickly to provide permanent support. The act of advancing a mole or shield with hydraulic jacks. 180

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SINE To excavate a shaft downwards from the surface, in distinction from raising. SLABBY ROCK Rock cut through by finely parallel joints and/or cleavage planes so that it breaks into tabular plates upon exposure in an excavation. SLAKING The crumbling and disintegration of rock or hard soil upon exposure to air or water. SLICKENSIDES The polished and sometimes striated surfaces on the walls of faults and shear zones, resulting from rubbing during earth movements. Sometimes referred to by construction people as "slicks. n SLURRY SHIELD A closed-face shield designed for tunneling in very soft, wet, or running ground by use of circulating, pressurized clay slurry against the face to counterbalance earth pressures, prevent ingress of water, and also to carry away the cuttings. SMOOTH BLASTING A technique of using carefully controlled shot hole drilling and spe- cially prepared charges in peripheral blast holes to reduce overbreak. See also pre-splitting. SOFT-GROUND SHIELD Any tunnel shield which moves forward by reacting {i.e., exerting shove forces) against the tunnel lining and generally utilizing drag type excavation tools that can be mounted on a backhoe, rotating wheel or oscillating arm. SOFT-GROUND TUNNEL Same as earth tunnel. The ground may be hard or soft in consistency, the word "soft" differentiating it only from "hard" rock. SOFT ROCK In construction, rock having a weakly bonded nature such as to permit excavation by air-operated hammers or other equipment only slightly more powerful than earth excavation equipment. Generally includes the more weakly bonded of the sedimentary rocks, such as clay shales. SOIL (a) In geology, any loose surface material overlying solid rock. (b) Broadly and loosely, the regolith, or blanket of unconsolidated rock material that lies on the bed rock. See earth for related definition. SPALLING The breaking off of thin surface sheets or plates in rock under ex- cessive tension. Spires, SPILING Essentially the same as forepoling, but may also include steel bars drilled ahead of a rock tunnel face. SPRINGLTNE The point where the curved portion of a tunnel roof meets the top of the wall. In a circular tunnel the springlines are at the opposite ends of the horizontal centerline. SQUEEZING GROUND Weak material, generally clayey, that behaves plastically under the weight of overlying ground and tends to close a tunnel opening by slowly advancing into it without perceptible volume increase. 181

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STAND-UP TOME See bridge action time. STOPE {a) A highly inclined or vertical excavation driven from the main tunnel or drift in an upward direction. (b) Excessive overbreak oc- curring for only a short distance and extending to a considerable height above the crown of a tunnel; may also be referred to as a "chimney." SWELLING GROUND Material that expands in volume by absorbing or adsorbing water so that it tends to move into a tunnel opening or to exert great pres- sure upon the supports. TAIL VOID The annular space at the back {tail) end of a shield between the outside diameter of the shield and the outside of the primary lining. TEMPORARY SUPPORT Essentially the same as initial support, except that the elements can be (and sometimes must be) removed because of non-contribution to or incompatibility with the final lining. TOP HEADING The upper portion of a tunnel, often extending from springline to crown, pre-excavated in order to install arch support before opening the tunnel to full size. TOP HEADING CONSTRUCTION A tunneling method in which a complete top heading is excavated end- to-end before excavation of the lower bench is begun. Distinguished from heading-and-bench construction. TUNNEL An elongate, essentially linear excavated underground opening, gen- erally with a length greatly exceeding its width or height. TUNNEL BORING MACHINE (TBM) A machine that excavates a circular tunnel by cutting and/or abrading the heading to full size in one operation. Also referred to as a mole. The term has so commonly been associated with rock tunneling that when a TBM is used in earth it is often prefaced by the quali- fier "soft-ground.' TUNNELING MACHINE A continuously excavating machine utilizing one or more rotating cut- terheads which are revolved under force against the tunnel face. WATER TABLE The upper limit of the portion of the ground at which water levels stand, as measured in piezometers or observation wells . 182