GOWANUS EXPRESSWAY

TUNNEL PRIMER

BUILDING THE HOLLANDTUNNEL, 1920S: Sandhogs take a break to pose within the tunnel shield.

New York State Department of TransportationJoseph H. Boardman, CommissionerDouglas A. Currey, P.E., Regional Director

he New York State Department of Transportation is currently studying the replacement of the Gowanus Expressway

with a tunnel. This investigation involves looking at a number ofpotential alignments, or routes, for the tunnel and identifying whichones best meet transportation needs and address project goals andcommunity concerns.

As this investigation proceeds, it will require increasing levels ofengineering and technical analysis. In discussing these analyses, certain terms and concepts specific to tunnel design, construction,and operations will be employed.

This booklet is meant to help you understand some of these termsand to briefly explain several of the construction methods that maybe used to build a tunnel.

T

TUNNELING UNDER CAIRO, EGYPT, 2000: Cairo’s new subway line, scheduled to open in 2000, is being built beneathits downtown and under the Nile River. Above, sections of a tunnel-boringmachine waiting to be lowered underground, where they will be assembled.

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Original New Jersey portal to theLincoln Tunnel, 1957

Access shaft to a water tunnel,Chicago, Illinois

Courtesy of the Port Authority of New York & New

Jersey

Tunnel workers preparing a section of the shield used to constructthe Third Tube of the Lincoln Tunnel, early 1950s

ENGINEERING DRAWINGSBefore construction of a tunnel can occur, a variety of tunnelroutes, or ALIGNMENTS, must be investigated. To see how thealignment fits in with existing conditions, engineering drawingscalled plans, profiles and cross-sections are created, showingdifferent views of the tunnel. A PLAN shows the top of thetunnel, a PROFILE shows the sides, and a CROSS-SECTIONshows a cut through the tunnel.

THE ANATOMY OF A TUNNELThese are some of the terms used to describe the individualparts of a tunnel:

CROWN: the top of a tunnel.

INVERT: the bottom of a tunnel.

PORTAL: the entrance to a tunnel.

SHIELD: a structure used in soft ground to provide support at the face of the tunnel for the soil above the tunnel, to provide space for erecting supports, and to protect the workersexcavating and erecting supports.

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HOW A TUNNEL IS BUILT Tunnels are constructed using many methods, depending upon thekind of soil and/or rock through which they will pass, how deepthey need to be, and the obstructions that may be encounteredalong the route. These methods include tunnel boring withmachines (TBMs), cut-and-cover construction, immersion ofprefabricated tunnels, and sequential excavation method (SEM).

BORED TUNNEL This type of tunnel is circular and may be constructed with atunnel-boring machine (TBM), which mechanically excavates soilor rock. A large-diameter, reinforced steel cylinder, a TBM containsdrive motors for a rotating cutter, which is mounted on one end,and hydraulic machinery for the thrusting jacks mounted along theperimeter of the cylinder on the other end. A TBM moves forwardby pushing with the jacks against the tunnel lining or against thesurrounding rock. The tunnel is lined with rings, usually made ofconcrete or steel, bolted together to provide integrity and water-tightness to the lining. Each time there is space for another ringof lining, the TBM stops, the jacks are retracted, and the ring isinstalled. In a rock tunnel, the TBM body moves forward until thejacks reach the end of their stroke. The jacks are then retractedinto the body of the machine before gripping the rock again andrepeating the boring cycle.

These are some of the technical terms associated with tunnelboring:

ADVANCE RATE: the amount of progress the tunnel-boringmachine makes—the distance it covers in a day, usuallymeasured in feet or meters.

FACE: the soil being excavated directly in front of the tunnel-boring machine. A MIXED FACE is a condition with more thanone type of material, such as clay, sand, gravel, cobbles, or rock.

LINING: a temporary or permanent structure, made of concreteor other materials, to secure and finish the tunnel interior or tosupport an excavation.

Muck

TBM BODY

TunnelLiningRings

The front of the TBM is made up of a CUTTER HEAD, which usesrotating cutting tools to break up rock and/or other materials.Immediately behind the head is the machine BODY, whichremains stationary while the cutters rotate. The MUCK (excavatedmaterial) from the excavation is either dropped onto a conveyorbelt (above right) or removed by a screw conveyor, which carries itto the back of the machine. As the material is cut by the cutterhead, thruster JACKS maintain forward pressure, and the machineis advanced until the jacks reach the end of their stroke.

All TBMs contain these essential components, but designs differaccording to the type of ground to be encountered (for example,soft soil, a mixture of soils, rock).

Face

ConveyorSystem

RotatingCutterHead

A 46-foot-diameter tunnel-boring machine used underthe Elbe River, Germany, is the largest in the world.

Courtesy of Herrenknecht AG Tunneling Systems

Muck removal by conveyor belt, Bangkok, Thailand

CUT-AND-COVER TUNNELIn this method of tunnel construction, a deep trench is excavated,the tunnel is constructed within the trench, and then covered bysoil. Sometimes a temporary deck is placed over the excavation atan early stage so that vehicular and pedestrian movements can bereinstated while construction continues below.

The following are some of the technical terms that apply tocut-and-cover tunneling:

BENTONITE: a clay mineral that can absorb large amounts ofwater. When mixed with water, it forms a SLURRY used tosupport deep trenches or bore holes against collapse until theycan be filled with concrete.

DECKING: a plank cover over a work area that serves as atemporary surface for pedestrian and vehicular traffic (includingconstruction equipment); usually made of wood, concrete, or steel.

OPEN CUT: a method of construction in which the excavatedtrench is left uncovered while the tunnel is constructed.

BOTTOM UP: a method of construction in which a tunnel isbuilt within an excavated trench in a conventional way orsequence: base slab first, then the walls and, finally, the roof.The completed structure is then backfilled and the ground surfacereinstated. In TOP DOWN construction, the tunnel walls arebuilt first, using special machinery, within a narrow trench. Next,the roof is built in a shallow excavation, and the ground surfaceis reinstated. The rest of the tunnel is then excavated andconstructed underneath the roof.

Cut-and-cover construction of the extensionof the 63rd Street Tunnel, Queens, New York:Tiebacks (steel bars or cables) are angledinto the completed slurry walls and anchoredinto rock to support the walls.

A clamshell excavator, left, digs a slurry wallpanel in downtown Boston. The plasticsheeting protects the building from splashesof soil and bentonite slurry.

One method of supporting the sides of a cut-and-cover tunnel is SLURRY WALL CONSTRUCTION, in which a wall is constructed within a narrowtrench. During excavation (A), the trench is keptfilled with a heavy clay and water mixture (bentonite slurry), which supports the sides of the trench. When excavation is complete,prefabricated cages (B) of reinforcing steel, andsometimes also steel beams, are lowered into theslurry. Concrete, which is heavier than slurry, ispiped to the bottom of the hole, displacing theslurry upward (C). Excess slurry is pumped awayand cleaned, stored, and reused.

Slurry

UnexcavatedMaterial

RebarCage

Slurry

Slurry

ConcreteDisplacing Slurry

SlurryPumpedOut andRecycled

A B C

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Tunnel elements are lowered underwater,where they are placed in a trench

dredged in the sea bottom.

A steel-shell segment, used to construct an immersed tunnel as partof the Boston Central Artery project

IMMERSED TUNNELA tunnel below a body of water may be built using this method.First, the individual elements of the tunnel, made of steel orconcrete and designed to be buoyant, are constructed on land,usually in a shipyard or dry dock. The ends of the elements areclosed with temporary bulkheads, making them watertight. Eachelement, which may be as large as a football field, is thenfloated to the tunnel site, sometimes on a barge. The tunnelelement is readied for immersion, then lowered into a trenchthat has been dredged under water. Each successive element isaligned with the previously placed element and joinedunderwater using a watertight seal. After the elements arejoined, the bulkheads are removed from the inside. Thecompleted tunnel is covered with fill material and topped with alayer of heavy stones or precast concrete castings.

These are some of the technical terms commonly used to discuss this method of tunnel construction:

BALLAST: a stabilizing weight (stones or concrete), eithertemporary or permanent, often added to the tunnel elementswhile floating, being immersed, or in their final position.

COFFERDAM: a temporary structure, constructed in waterand pumped dry; used to keep water out so that work can beperformed in dry conditions.

QUAY (pronounced “key”): A wharf or reinforced bank whereships are loaded or unloaded, built parallel to the bank of awaterway. A tunnel west of Third Avenue in Brooklyn mayencounter buried quays built along earlier shorelines.

This cofferdam, used to build immersed tunnelelements under dry conditions, was then floodedso the completed sections could be floated out.

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A waterproofing membrane is installedbetween the ground and the tunnel’sconcrete lining.

A roadheader may be used to excavatetunnels through soft rocks and soils.

SEQUENTIAL EXCAVATION METHOD(SEM) TUNNELSEM, also known as New Austrian Tunneling Method (NATM), wasdeveloped in Austria but is now used worldwide. This undergroundmethod of excavation divides the space (cross-section) to beexcavated into segments, then mines the segments sequentially,one portion at a time. Whereas TBMs can only excavate a fixed(generally circular) shape, SEM permits a tunnel of any shape orsize to be excavated. This makes it useful in areas where thetunnel shape or size needs to change, such as highway ramps andsubway stations.

The excavation can be carried out with common mining methodsand equipment (often a backhoe), chosen according to the soilconditions; tunnel-boring machines are not used. Groundconditions are assessed at the face of the tunnel or from the sideof a small tunnel, which helps to decide how to proceed in thebest way and determines the choice of equipment and lining. SEM involves careful sequencing of the excavation as well asinstallation of supports. SHOTCRETE (a kind of concrete sprayedfrom high-powered hoses) may be used to line the tunnel orsupport the face, and GROUTING (the injection of a cementingor chemical agent into the soil) may be used to increase the soil’sstrength and reduce its permeability.

SEM requires extremely dry conditions; dewatering is oftennecessary before the excavation can proceed. Because of therequirements of this method, the rate of excavation is slow.

These are some of the technical terms associated with thistype of tunnel construction:

DEWATERING: the removal of ground water from the area to be excavated.

DRAWDOWN: a lowering of the normal ground-water level(water table) as the result of dewatering.

HEADING: A smaller tunnel used when a larger tunnel isexcavated in several stages. Smaller headings are used when full tunnel excavation at once would not be prudent. Sometimesmultiple headings are used simultaneously to increase thenumber of excavation faces and compensate for SEM’s slowerexcavation rate.

Shotcrete, a pneumatically sprayed concrete,will be applied on these tunnel walls, whichhave been lined with waterproofing and acage of wire-mesh fabric.

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Brooklyn-Battery Tunnel ventilationbuilding, Governor’s Island, New York

Underpinning the IND tunnel, NorthernBoulevard, with transfer beams and

new supports

Muck in the Lincoln Tunnel awaiting removal, 1935

OTHER TUNNELING TERMSCROSS-PASSAGE: a short passage or tunnel connecting twoadjacent tunnels, often used to allow people to escape from onetunnel to another in an emergency.

CUT-OFF WALL: an underground wall, either temporary orpermanent, used in tunnel excavations to prevent the passage ofground water.

GRADE: a measure of the inclination of a road or slope.

MOLE: slang term for a tunnel engineer.

PILE: a long, slender, below-ground column, usually of steel, timber or reinforced concrete, serving as a foundation for a structure bytransferring the structure’s weight to bedrock or other subsurfacematerials more capable of withstanding the loads.

PILOT TUNNEL: an exploratory tunnel, usually smaller and drivenahead of the main tunnel.

SANDHOG: slang term for a tunnel worker.

SHAFT: a vertical excavation, often used to provide access to atunnel from the surface.

UNDERPINNING: the installation of new supports beneath thefoundation of a building or other structure to protect it fromsettlement caused by adjacent tunneling or other construction;sometimes used in place of an old foundation that was removed.

VENTILATION BUILDING: a building that houses ventilation fansand other mechanical and electrical equipment necessary to ventilate,power, and light a tunnel.

GEOTECHNICAL TERMSBACKFILL: material such as sand, gravel or crushed stone used to fill the remainder of an excavation after a tunnel or other underground structure has been constructed within the excavation.

GLACIAL DEPOSITS: a mixture of geological debris (such as rocks, stones, gravels, silts and clays) carried, plowed and mixed byglacial ice and meltwater.

ORGANIC SILT: a silt, highly compressible and with a low bearingcapacity (strength), containing plant and occasionally animal remains.

OVERBURDEN: the soil between the ground surface and the roof of a tunnel.

SETTLEMENT: the movement or settling of soils.

SILT: soil whose grains are finer than sand but coarser than clay.

SOFT GROUND: soils and weak rock that can be easily removed.

TERMINAL MORAINE: an area where glaciers melted during theirsouthward advance, leaving behind an accumulation of boulders,stones and other debris.

Courtesy of the Port Authority of New York & New

Jersey

HOW TO CONTACT US

We welcome your comments. To contact us or for moreinformation, please write or call:

New York State Department of TransportationGowanus Expressway Project Community Office920 Third Avenue (34th Street)Brooklyn, New York 11232

Phone: (718) 832-5060Fax: (718) 832-5097

Visit our project website at:http://www.gowanusproject.com

New York State Department of TransportationJoseph H. Boardman, CommissionerDouglas A. Currey, P.E., Regional Director

Fort McHenry Tunnel portal, Maryland

Trial assembly of the cast-iron ring segments used to

line the Lincoln Tunnel

Courtesy of the Port Authority of New York & New

Jersey

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