Gowanus Expressway Project - NYSDOT Home...Description Sequential Excavation Method (SEM) also is known as New Austrian Tunneling Method (NATM) because it was developed in Austria,

Gowanus Expressway ProjectTier 2 Tunnel Studies

Construction Methods

April 2002

New York State Department of Transportation

DRAFT

Tier 2 Studies Construction Methods DDRRAAFFTT

1.1 INTRODUCTION

The New York State Department of Transportation (NYSDOT) currently is exploring options to rehabilitate or reconstruct the Gowanus Expressway and is preparing a Draft Environmental Impact Statement (DEIS). Among the options that will be studied are a no build/maintenance alternative, two viaduct alternatives (Rehabilitation with Operational Improvements and Relief Viaduct with Operational Improvements), and a tunnel alternative. To evaluate the best tunnel alternative for study within the DEIS, the State currently is conducting a three-tiered study and screening process. During Tier 1, forty-six alternative routes were screened, resulting in thirteen alternatives under Tier 2. Tier 2 will select three or four routes for more detailed evaluation, and Tier 3 will select one alternative, which will be studied within the DEIS.

1.2 PURPOSE AND METHODOLOGY

The purpose of this report is to discuss the feasible construction methods for the individual segments of each of the thirteen Tier 2 tunnel alternatives.

The methodology for the tunnel alternative study was largely influenced and guided by the Gowanus Project’s Tunnel Technical Advisory Panel (TTAP), a high-level technical advisory group consisting of international experts in the design, construction, and operation of tunnels throughout the world. The panel includes representatives of tunnel owners and operators, top tunneling engineers and specialists with expertise in state-of-the-art tunnel technology, and government and other public officials. The TTAP’s role is to review assumptions and studies, advise on the project direction, provide guidance and make recommendations to ensure the development of the most optimum tunnel alternative.

In addition to the TTAP, NYSDOT has sought the hands-on advice of other tunneling experts throughout the tunnel alternatives study. These experts have provided specific guidance in critical areas such as the design criteria, construction methodologies, and risk assessment and mitigation measures. By turning to these experts, NYSDOT seeks to ensure that the tunnel alternative will benefit from the latest underground design and construction techniques available now and also factor in any technical advances that would be available in the future at the project’s implementation date.

1.3 CONSTRUCTION METHODS

The basic tunneling methods considered for the thirteen tunnel alternatives are:

• cut-and-cover tunneling techniques, including use of slurry walls and supporting components such as timber lagging, wales, struts, and tiebacks. Open-cut excavations use no walls, supports, or a cover. The sides of the excavation are sloped back until they are naturally stable.

• boring, in which the tunnel would be excavated mechanically with a tunnel-boring machine

• sequential excavation method, a form of mining that involves excavation with roadheaders and other such equipment and includes ground-modification techniques such as ground


freezing, grouting, etc. Other mining techniques also would be employed in the tunnel construction.

• immersed tunnels or staged cofferdams for underwater portions of the alignments.

Portals (entrances to the tunnel) would be constructed using open-cut construction. In addition to these tunneling methods, conventional construction techniques would be used to construct open, surface highways.

1.3.1 Cut and Cover

Description The cut-and-cover method of tunneling entails excavating a trench, constructing a tunnel within the trench, and then covering it with soil. To avoid long-term disruptions, and to restore pedestrian and vehicular traffic as soon as possible, the trench is covered with a temporary deck as soon as it is excavated. In open-cut construction, a trench is excavated (with neither walls nor support), then left uncovered while the tunnel is being constructed. Open cuts are suitable for shallow

excavations as well as in areas without properties nearby.

Pros & Cons Cut-and-cover construction accommodates changes in the width of the tunnel and non-uniform shapes, which are necessary to build interchanges, ramps, and portals. Slurry (concrete) walls, which are generally used to support the sides of the excavation, may serve as either temporary or permanent support for the tunnel. In the case of the Gowanus Tunnel, they will probably be used as part of the permanent tunnel structure. The walls may be held in place with tiebacks—long, angled steel rods, which were used to support the “bathtub” at the World Trade Center site. Other supporting components, such as struts and wales (see definitions below), may be used. The rigidity of the slurry walls helps minimize soils movement (“settlement”), keeping it to approximately 3/8 of an inch, and the need to underpin properties in the area. This is important because loss of ground can cause distress in the buildings next to and above the excavation. In Boston’s Central Artery (“Big Dig”) project, the use of rigid slurry walls successfully reduced the amount of settlement. However, the use of slurry walls may impact a relatively high number of utilities as well as groundwater flow, and there is a short-term impact on the surface prior to installation of the decking.

Other components used to support a cut-and-cover excavation include:

lagging—horizontal planks, usually of timber

soldier and lagging piles—vertical or nearly vertical members, embedded partly or entirely in the ground, used to support a structure. The existing Gowanus Expressway is supported by bundles of concrete-filled, steel batter piles, which are sloped, as well as vertical piles.

rake—diagonal brace, usually made of steel or timber, used for support

sheet piles—made of corrugated steel and driven into the ground vertically


strut—a horizontal compression brace used to support lagging and wales. Struts carry loads from one side of the excavation to the other. In the Big Dig, the use of strutted slurry walls was especially successful.

wales—horizontal beams that transfer loads from the lagging or sheeting to the piles

1.3.2 Tunnel Boring Machines

Description Tunnel-boring machines (TBMs) are steel cylinders with a motor-driven rotating cutter on the front that can excavate rock or soil mechanically. There are several types of TBMs currently in use throughout the world, including earth-pressure balance machines (EPBMs) and the newer, better but more complex slurry machines. Work is now underway on a hybrid TBM, which is expected to be available by the start of construction of the Gowanus Tunnel.

To minimize settlement and risks, TBMs are generally used with at least one diameter of cover (e.g., a 46-foot machine would be at least 46 feet under the surface) and about one to two diameters between machines if more than one tunnel is being bored. TBM technology has advanced rapidly in recent years, with a steady increase in the outer diameter (OD) of the machines: a 46.4-foot OD machine was used under Tokyo Bay, a 46.6-foot OD machine was used under the Elbe River in Germany, and a 48.9-foot OD machine is now being used to bore the Green Heart Tunnel in the Netherlands. The outer diameter of a TBM for the Gowanus would be about 52.5 feet and could handle a maximum of four travel lanes. This type of machine should be available by the time construction of the tunnel begins (currently assumed to be about 2010).

Pros & Cons Tunneling with a TBM is faster and less disruptive than cut-and-cover methods. The machine does require access and egress shafts, extensive room for mobilization and start-up, and provision for muck removal and drying.

TBMs are designed for fixed (generally circular) bores and generally straight runs (to be economically viable, the run should be at least 3,000 feet long). They cannot readily accommodate the non-uniform shapes typically required to build interchanges and ramps. About 70 percent of the Gowanus Expressway has ramps that require acceleration/deceleration lanes or is sufficiently curved to require an appropriate distance for motorists to see around curves (“sight shelf”). These safety features would require widening the tunnel and would have to be excavated from aboveground, meaning there would be few straight TBM runs without interruptions. As with all tunneling methods, ground settlement occurs while using a TBM but can be kept within acceptable limits (1” or less), especially when using a slurry shield. However, a TBM of the size needed for the Gowanus preferably should be located under roadways in order to minimize risks and potential effects on neighborhood buildings. The progress of TBMs can be slowed when they encounter obstructions such as large boulders and buried features such as old seawalls and stone-filled piers. They also cannot bore through the concrete filled steel piles that support the existing Gowanus Expressway without taking extra


measures such as stopping the TBM to allow a crew to manually remove the pile while working in an airlock. 1.3.3 Sequential Excavation and Other Mining Methods

Description Sequential Excavation Method (SEM) also is known as New Austrian Tunneling Method (NATM) because it was developed in Austria, but it is now commonly used throughout the world. It entails dividing the space to be excavated into segments, then mining the segments sequentially, one portion at a time, using supports.

Mining equipment, such as backhoes and roadheaders, is used to excavate the tunnels. Several small openings, or

headings, are made that serve as exploratory tunnels, allowing the condition of the soils to be assessed and determining the appropriate liners for the tunnel and excavating equipment. After the tunnel walls are lined with waterproofing and a wire-mesh cage, they are sprayed with shotcrete, a kind of concrete that is applied pneumatically.

The stack drift, or multiple drift, method of mining involves sequentially excavating several small tunnels, known as headings, one on top of each other. Each tunnel is supported individually before the next tunnel is excavated. This method is slow, but it can be especially effective in certain areas, for example, around a subway or a sewer that cannot be relocated and requires special care to protect it.

In addition, conventional mining methods could be used to build portions of the Tier 2 alignments.

Pros & Cons Whereas TBMs can excavate only a fixed, generally circular, shape, SEM allows a tunnel of any shape to be excavated. The method is especially applicable for areas, such as highway ramps and interchanges, in which the tunnel shape or size needs to change.

To use SEM, the ground must be completely dry, and it will be necessary to dewater (remove ground water) before beginning the excavation. Ground modifications, to strengthen and prepare the soil for tunneling, are also common with this method, including various types of grouting (injection of chemical or cementing agent into the soil), ground freezing, and other such treatments. The method is labor-intensive—it requires not only highly skilled labor but also experienced engineers to supervise the work—and therefore slow. It’s also difficult and riskier to mine exceedingly large shapes since they would be more likely to collapse. Ground-modification techniques include: ground freezing—the use of refrigeration to form areas of frozen soil. It may take 3 to 4 months to effectively freeze the ground prior to mining. The freezing will not work if underground moving water or water / sewerage piping is present within the freeze zone. Limitations include ground heave (upward movement of the soil) and subsidence, both of which could affect the


foundations of nearby buildings or other structures. In addition, extended use of the refrigeration system results in high-energy costs. grouting—injection of chemical or cementing agent to strengthen soil and improve its imperviousness. Conventional grouting does not work well in tight soils such as silt, clay, or organic material. The soil must have voids, or openings, for the grout to permeate it—it cannot be tightly packed, or the grout will fracture the soil and cause extensive heave. If the soil is difficult or unstable, such as silts, clays, and some weak rocks, jet grouting is sometimes an alternative to the conventional chemical/cementing methods. Jet grouting is applied by injecting a cement grout into the soil, creating columns of soilcrete, which is almost as strong as concrete. 1.3.4 Immersed Tunnel Method

Description This method of tunneling involves the underwater immersion of large steel or concrete tunnels. The tunnels, which are constructed on land and designed to be buoyant, are barged to the site, filled with ballast, then lowered into a prepared underwater trench. The ends are closed with watertight bulkheads, which are later removed from the inside when the tunnel segments are sealed together underwater. The completed tunnel is covered over with fill material and topped with a layer of heavy stones or precast concrete castings.

Pros & Cons This type of tunneling would produce few disruptions to the communities surrounding the Gowanus Expressway, but it would be an appropriate method of construction for only one of the thirteen alternatives currently under consideration. To accommodate shipping, the top of the tunnel would need to be approximately 70 feet underwater. This depth and the tunnel location would make connections to inland ramps (e.g., 38th/39th Street) and interchanges difficult.

1.3.5 Staged Cofferdam

Description One way to construct a tunnel under the Gowanus Canal and along parts of the bulkhead area is to use staged cofferdams. This type of cofferdam is built by driving rows of sheet piles into an enclosure, which is then dewatered and excavated. The process is repeated until the body of water is fully crossed.

Pros & Cons This relatively straightforward method of tunnel construction would allow water flow in the

Gowanus Canal to be maintained at all times but would require temporary arrangements to accommodate the oil terminal and other commercial canal users.

Constructing a staged cofferdam involves some of the same components used to build supporting walls in cut-and-cover tunneling. These components, defined under the Cut-and-Cover Tunneling section, include lagging, sheet piling, struts, and wales.


2.0 MAJOR CHARACTERISTICS OF THE TUNNEL ROUTES

2.1 Tier 2 Alternatives

The thirteen alternatives consist of alternatives along the pierhead and bulkhead (TPA, TPX, and TB) and along the avenues (T1, T12, T2 and T3). T1X, T12X, T2X and T3X relocate the connection to the Shore Parkway, from its current location at 65th Street to a point closer to the Verrazano-Narrows Bridge. East of 65th Street, these alternatives are essentially identical to T1, T12, T2, and T3. T3P and T3B differ from T3 only in the Hamilton Avenue area. Along Hamilton Avenue and the nearby interchanges, alternatives TB, TPA, T1, T12, and T2 are or can be identical.

For the alignments in Third Avenue, the inbound tunnel lies mostly west of the outbound tunnel so that all entrances and exits from the tunnel lie in the center of Third Avenue with access to and from the highway’s right lane. The main tunnels are therefore located as far from the center of Third Avenue as they can be without taking additional property, thereby also minimizing conflicts with the existing viaduct foundations.

Characteristics of particular areas that may be factors during tunnel construction include: Pierhead / Bulkhead

• Industrial waterfront • Active marine terminals

• Buried seawalls and old piers may be encountered during tunneling

• Port development and freight tunnel studies are underway, and Gowanus tunnel alternatives must be compatible with these studies

• Location can extend inland connections First and Second Avenues

• Each avenue is only 80 feet wide • Light industrial / residential • First Avenue has railroad, Brooklyn Army Terminal, and sewer chambers • Second Avenue has ConEd substation • Major hospital (Lutheran Medical Center) Third Avenue

• Avenue is about 180 feet wide • Commercial / residential area • Bundles of concrete-filled, steel angled piles supporting the Gowanus Expressway viaduct • Bush Terminal (eligible for historic landmark status)


Sixty-fifth Street Area (common to all alignments)

• Subway (under Fourth Avenue) and major utilities • 65th Street Rail Yard—site of an active Long Island Rail Road yard • Existing Gowanus Expressway, which must be underpinned Hamilton Avenue (common to all alignments)

• Avenue is about 120 feet wide • Commercial / residential area • Existing Gowanus which must be underpinned • Access must be maintained • Hamilton Avenue bascule bridge. The bridge’s foundation is supported by myriad piles that

would need to be either avoided or cut through.

2.2 General Design Considerations

• Soil may include hazardous materials. The Gowanus Expressway area has a long history of manufacturing and industrial uses, including gas tanks and oil refineries. Hazardous materials found within the area include PCBs and hydrocarbons.

• Historic Fort Hamilton (“X” alternatives only). This national historic landmark was built in 1825, and some of the original fort, including the gun batteries, is buried near the current fort. Permission to tunnel or construct a surface road either through or adjacent to the fort must be obtained from the U.S. Army Corps of Engineers and must be approved by the State Historic Preservation Office.

• High water table close to sea level

• Utilities, both sewers and oil-o-static electrical lines. Generally, utilities are either relocated or protected during construction. The area’s sewers include a 180-inch-diameter combined brick sewer along 64th Street that will require special attention during any tunneling work. This sewer, built in 1943, serves 4200 acres. Major sewers are found under other avenues and streets as well, especially Third Avenue, and sewer regulator structures are found under First Avenue (49th and 64th Streets).

The area’s water and natural gas distribution networks and high-voltage oil-o-static lines also will need careful attention during tunneling.

In addition, the Brooklyn shoreline once was located as inland as the western side of Third Avenue; much of the area along the current waterfront was created by fill. Buried seawalls, old piers, and hazardous materials may be found during tunneling. In addition, the makeup of the fill is of unknown and probably inconsistent qualities.


2.3 Geological Conditions The area surrounding the elevated, northern sections of the Gowanus Expressway (including Red Hook, Carroll Gardens, and part of Sunset Park) was covered by a sheet of ice known as the Wisconsin Glacier during the last ice age, which ended some ten to twelve thousand years ago. As a result, glacial till—an unstratified mix of clay, silt, and sand with rocks ranging from pebbles to giant boulders—is prevalent throughout the area. In the northern section of the Expressway, glacial outwash and lake deposits are found in the topmost soil layers, with silty clay and sand about 100 feet below the surface; a 100-foot layer of sand and gravel; cobbles and boulders below that; and finally bedrock about 200 feet down. The remaining, southern length of the Gowanus Expressway (including the southern part of Sunset Park, Bay Ridge, and Dyker Heights) is located along a 150-foot-thick terminal moraine known as the Harbor Hill, the southernmost tip of the Wisconsin ice sheet. The moraine overlies layers of silty clay and sand; gravel, cobbles, and boulders left behind as the glacier retreated; bedrock is found about 200 feet below sea level. 3.0 DESIGN ASSUMPTIONS

� Based on the traffic analysis, the primary routes are four lanes eastbound (with an HOV Lane) and three lanes westbound.

� Geometric design including acceleration /deceleration lanes, shoulders, and sight shelf parameters are based on American Association of State Highway Transportation Officials (AASHTO) requirements.

� Existing surface roads shall be restored in accordance with NYSDOT standards.

� The major routes on First and Second Avenues are double stacked with four lower lanes and three upper lanes except for the split routes running under both avenue (see attached cross-section).

� The major routes on Third Avenue are located east and west of the existing viaduct except that in some cases the tunnel is under the viaduct foundations and underpinning the viaduct during construction is required (see attached cross-section).

� Existing utilities would be replaced, protected, supported or relocated as required on a case-by-case basis and in accordance with the requirements of the New York City Department of Environmental Protection (NYCDEP), American Water Works Association (AWWA), and other utility company requirements.

� Existing active docks would need to be temporarily relocated or kept functional in the case of TB.

� Alternative material / fluid handling would be provided for the upstream users of the Gowanus Canal. Water flow would be maintained during cofferdam construction.

Tier 2 Tunnel Alternatives Report: Tunnel SectionsConstruction Methods DRAFT

80'

52.5' Dia.

52.5

'

52.5' Dia.

52.5

'

Bored Tunnel(shown under Ist/2nd Ave.)(T12, T12X)

OMPACTEDRANULARACKFILL

TILITY RELOC.

EMBRANE WPNNE

XH

AU

ST

XH

AU

ST

UP

PLY

3 LANES @ 3.6m

3 LANES @ 3.6m

PTC (TYP)SPTC (TYP)PTC (TYP)SPTC (TYP)PTC (TYP)SPTC (TYP)SPTC (TYP)PTC (TYP)

.9m

m.3.6m

.3m

.3m BUFFER (TYP.)

.3m

.3m.3m BUFFER (TYP.)

.6m

5.2m

0.8m

.94m

0.5m

1.1m

5.2m

0.3m

1.2m

1.0m UPPLYLY UPPLYY UPPLYLY

Stacked Cut and CoverTunnel under Ist or 2nd Ave.(T1, T1X, T2, T2X)

Cut and Cover Tunnel(shown under 3rd Ave.)(T3, T3B, T3X, T3P, TB. TPA)

Alternative T12, primarily located along First andSecond Avenues, provides connections to the Prospect,Shore, and Brooklyn-Queens Expressways, as well as theBrooklyn-Battery Tunnel. The alternative calls for separatetunnels along the two avenues, with most of the inbound tun-nel under Second Avenue and the outbound tunnel underFirst Avenue.

This alternative uses a combination of tunneling meth-ods, primarily boring with TBMs, cut-and-cover with deck-ing, and sequential excavation. The portals (entrances to thetunnels) and ramps would be open cut—essentially the sameas cut and cover but without a top cover. Open-cut areaswould use cast-in-place concrete retaining walls, with piles

and lagging as part of the shoring (support) system. The sur-face roadways throughout the route would be constructedwith conventional, non-tunneling techniques.

Along 65th Street, between Sixth and Seventh Avenues,the tunnel primarily would be constructed with cut-and-cover methods, using slurry walls supported by tiebacks,wales, and struts. The Gowanus Expressway viaduct wouldbe underpinned, with construction focusing first on theinbound structure and then on the outbound. Near FourthAvenue, the subway would be underpinned, and the under-pinning beams supported by slurry walls.

The inbound tunnel passes directly under and very closeto the 180-inch combined brick sewer in this area. To avoid

damage to the sewer, the stack drift method (a form of min-ing using multiple small tunnels, one on top of the other)would be used. Although this method would be slow andlabor-intensive, it minimizes risks to the sewer.

Ground freezing would be necessary to stabilize the groundand enable the tunnel to cross under the subway and the rampstructures in the 65th Street area; near the Shore Parkway, theground also would be frozen to allow the tunnel to proceedunder the Long Island Rail Road tracks. The Shore Parkwayitself would be underpinned using slurry walls and tiebacks(angled supports used to hold the slurry walls in place).

Along First Avenue (from an area near the BrooklynArmy Terminal to about 40th Street) and along Second

Avenue (to about 38th Street), the tunnel would be machine-bored. Two tunnel-boring machines, both with a slurry shield,would be used: a 45-foot-diameter machine under FirstAvenue, which would be wide enough to accommodate thethree-lane, outbound tunnel; and a 52.5-foot-diametermachine under Second Avenue, which would bore the four-lane (three general-purpose lanes plus the Bus/HOV lane),inbound tunnel. Alternatively, the 52.5-foot-diameter TBMcould be turned around once it was done on Second Avenueand used on First Avenue. In any case, all TBMs would bemobilized underground, within a cut area covered by tempo-rary decking, near the Brooklyn Army Terminal.

The remainder of the alignment, from the 38th/39thStreet ramps to the Brooklyn-BatteryTunnel and Brooklyn-QueensExpressway, would be mostly constructedwith cut-and-cover methods using slurrywalls with tiebacks and/or wales andstruts and temporary decking. TheGowanus Expressway viaduct would beunderpinned. Conventional constructionmethods would be used for surface con-struction, and open-cut construction, sup-ported by piles and lagging, would beused in the BBT/BQE area.

To cross under the Gowanus Canal,staged cofferdams would be constructed.Marine traffic through the Canal would beshut down during construction, but waterflow would be maintained. Pipelines andconveyors also would be constructed andused to transport materials to the Canalusers.

Tier 2 Tunnel Alternatives Report: T12: Tunnel under First and Second AvenuesConstruction Methods DRAFT

Inbound TunnelOutbound TunnelSurface RoadsPortal

Alternative T12X, primarily located along First and SecondAvenues, provides connections to the Prospect and Brooklyn-Queens Expressways, as well as the Brooklyn-Battery Tunnel, inthe same areas as the existing Gowanus Expressway. However,under all “X” alternatives, the Gowanus Tunnel would connect tothe Shore Parkway at a new location, near the Verrazano-Narrows Bridge.

Except for the Shore connection, T12X is identical toAlternative T12. Like T12, T12X calls for separate tunnels alongthe two avenues, with most of the inbound tunnel under SecondAvenue and the outbound tunnel under First Avenue. The alterna-tive uses a combination of tunneling methods, primarily boringwith TBMs, cut-and-cover with decking, and sequential excava-

tion. The portals (entrances to the tunnels) and ramps would beopen cut—essentially the same as cut and cover but without a topcover. Open-cut areas generally would use cast-in-place concreteretaining walls, piles and lagging, and a shoring (support) sys-tem. The surface roadways throughout the route would be con-structed with conventional, non-tunneling techniques.

The “X” alternatives all connect to Shore Parkway near theVerrazano-Narrows Bridge. The approach ramps to the bridgewould need to be underpinned or would use piles and laggingwith struts, rakes, or tiebacks. Most of the underground construc-tion in this area, on the west side of Fort Hamilton, would be cutand cover. Surface and bridge work would be accomplished withnon-tunneling methods.

Near 66th Street, from Seventh Avenue to Fourth Avenue,the inbound and outbound portals would be erected by open-cutconstruction.

Starting on 65th Street, the Gowanus Expressway becomeselevated. The expressway viaduct would be underpinned to allowstaged cut-and-cover construction with decking and slurry wallsto proceed. The inbound tunnel would be constructed first, thenthe outbound. Underground ramps generally would be construct-ed within an open-cut trench, with piles and lagging held in placeby struts, wales, or tiebacks. Retaining walls supporting theramps would be cast-in-place concrete. Surface ramps would beconstructed by conventional, non-tunneling methods.

From Fifth Avenue to Third Avenue, as the tunnel continues

westward and begins to curve to the north, it crosses the FourthAvenue subway, which would need to be underpinned, with thebeams supported on the same slurry walls used to support theexcavation. Part of the ground (about 50 meters) under the FourthAvenue subway tunnel also would need to be frozen. The tunnel’sinbound ramp passes directly under and very close to the 180-inchsewer as well as the subway. To avoid damage to the sewer, thestack drift method (a form of mining using multiple small tunnels,one on top of the other) would be used. Although this methodwould be slow and labor-intensive—each small tunnel would betreated with an envelope of chemical grout before the next one isbuilt—it minimizes risks to the sewer.

Along First Avenue (from an area near the Brooklyn ArmyTerminal to about 40th Street) and alongSecond Avenue (to about 38th Street), thetunnel would be machine-bored. Two tunnel-boring machines, both with a slurry shield,would be used: a 45-foot-diameter machineunder First Avenue, which would be wideenough to accommodate the three-lane, out-bound tunnel; and a 52.5-foot-diametermachine under Second Avenue, whichwould bore the four-lane (three general-pur-pose lanes plus the Bus/HOV lane), inboundtunnel. Alternatively, the 52.5-foot-diame-ter TBM could be turned around once itwas done on Second Avenue and used onFirst Avenue. In any case, all TBMswould be mobilized underground, within acut area covered by temporary decking,near the Brooklyn Army Terminal.

The remainder of the alignment, fromthe 38th/39th Street ramps to the Brooklyn-Battery Tunnel and BQE, would be mostlyconstructed with cut-and-cover methods.The Gowanus Expressway viaduct wouldbe underpinned. Conventional constructionmethods would be used for surface construc-tion, and open-cut construction would beused in the BBT/BQE area.

To cross under the Gowanus Canal,staged cofferdams would be constructed.Marine traffic through the Canal would beshut down during construction, but waterflow would be maintained. Pipelines andconveyors also would be constructed andused to transport materials to the Canalusers.

Tier 2 Tunnel Alternatives Report: T12X: Tunnel under First and Second AvenuesConstruction Methods DRAFT


Alternative TB, primarily located between the waterfront’sbulkhead line and First Avenue, provides connections to theShore Parkway, the Prospect and Brooklyn-QueensExpressways, and the Brooklyn-Battery Tunnel.

Although most of the alternative would be built with cut-and-cover techniques, the portals (entrances to the tunnels)and ramps would be open cut—essentially the same as cutand cover but without a top cover. Open-cut areas would usecast-in-place concrete retaining walls and piles and a shoring(support) system.

The surface roadways throughout the route would beconstructed with conventional, non-tunneling techniques.Along 65th Street, the Gowanus Expressway viaduct would

be underpinned, with slurry walls supporting the underpin-ning beams, to allow staged cut-and-cover construction withdecking and slurry walls to proceed. Excavation equipment inthe 65th Street area would include large excavators, clambuckets, and a front-end loader to construct surface roads anda small track backhoe in the cut section. All excavated mate-rial would be hauled off in trucks to a disposal site.

As the tunnel heads westward, it crosses the FourthAvenue subway. Ground freezing would be necessary to sta-bilize the ground and enable the tunnel to cross under thesubway and the ramp structures in the 65th Street area. TheGowanus tunnel would be mined, with careful monitoring ofthe subway structures to ensure minimal settlement or heave.

The 180-inch sewer in this area also would be mined with thestack drift method (a form of mining using multiple smalltunnels, one on top of the other), probably with chemicalgrouting. Although this method would be slow and labor-intensive—each small tunnel would be treated with an enve-lope of chemical grout before the next one is built—it wouldminimize risks to the sewer.

Tunneling near the Long Island Rail Road tracks wouldneed to be closely coordinated with the LIRR. Near theShore Parkway, the ground also would be frozen to allow thetunnel to proceed under the Long Island Rail Road tracks.The Shore Parkway itself would be underpinned using slurrywalls and tiebacks.

Just behind the Brooklyn Army Terminal, two pierswould have to be removed to construct a cofferdam. The areawould be covered with a weight-bearing fill material inwhich a permanent, watertight bulkhead would be construct-ed. This would enable cut-and-cover construction to proceed.Once the tunnel structures are completed, the piers would bereconstructed and the cofferdam removed.

Along the bulkhead line, the tunnel would be constructedwith slurry walls with tiebacks and/or wales and struts. Thealignment then veers inward and reaches Third Avenue, wherethe Gowanus Expressway viaduct would have to be underpinned.

The remainder of the alignment, including the ProspectInterchange, would be built with cut-and-cover methods, with

temporary street decking, slurry walls,tiebacks, wales, and struts. Conventionalconstruction methods would be used forsurface construction, and open-cut con-struction, supported by piles and lagging,would be used in the BBT/BQE area.

The crossing of the Gowanus Canalwould be accomplished through the con-struction of staged cofferdams. Marinetraffic through the Canal would be shutdown during construction, but water flowwould be maintained. Pipelines and con-veyors would be used to transport materi-als to accommodate the Canal’s usersthroughout the construction.

Tier 2 Tunnel Alternatives Report: TB: Tunnel between Bulkhead and First AvenueConstruction Methods DRAFT


Alternative TPX, primarily located along the Brooklynwaterfront’s pierhead, provides connections to the Prospectand Brooklyn-Queens Expressways, as well as the Brooklyn-Battery Tunnel. However, under all “X” alternatives, theGowanus Tunnel would connect to the Shore Parkway at anew location, near the Verrazano-Narrows Bridge. TPX is theonly one of the thirteen alternatives that would not provide anentrance and exit at 38th/39th Street because it is too far outin the bay to make this inland connection.

The TPX alternative also is the only alternative to bebuilt in part by immersed tunneling technology. Much of therest of the alternative would be be built with cut-and-covertechniques or mined. The portals (entrances to the tunnels)

and ramps would be open cut—essentially the same as cutand cover but without a top cover. Open-cut areas would usecast-in-place concrete retaining walls and piles and a shoring(support) system.

The surface roadways throughout the route would beconstructed with conventional, non-tunneling techniques.

The “X” alternatives all connect to the Shore Parkwaynear the Verrazano-Narrows Bridge. The approach ramps tothe bridge would need to be underpinned or would use pilesand lagging with struts, rakes, or tiebacks. Most of the under-ground construction in this area, just west of Fort Hamilton,would be cut and cover. Surface and bridge work would beaccomplished with non-tunneling methods.

Near 66th Street, from Seventh Avenue to Fourth Avenue,the inbound and outbound portals would be erected by open-cut construction employing piles and lagging with struts,rakes, or tiebacks.

Starting on 65th Street and heading westward, the tunnelgenerally would be built using cut-and-cover constructionwith decking and would involve underpinning the GowanusExpressway viaduct, protecting the Long Island Rail Roadtracks along 65th Street, crossing under the Fourth Avenuesubway, and crossing the 180-inch combined brick sewer.The support system for the excavation would include slurrywalls with tiebacks, wales, and struts, and the underpinningbeams under the viaduct would be supported on the slurry

walls. The tunnel under the subway, as well as under the 180-inch sewer, would be mined with the stack drift method (aform of mining using multiple small tunnels, one on top ofthe other), probably with chemical grouting around the sewerand with freezing of the ground around the subway tunnel.The inbound tunnel and outbound ramp also would be mined,with careful monitoring of the subway structures to ensureminimal settlement or heave.

Beginning just behind the Brooklyn Army Terminal andall along the waterfront (for a distance of about 9,250 feet),the tunnel would be constructed using immersed tunnel tech-nology. Briefly, the immersed tunnel sections, made of steel,would be prefabricated in a shipyard along the northern East

Coast, then barged to the New York Bay.They would be filled with ballast and sub-merged within a prepared trench about 95feet deep, then connected. The processwould be repeated until the desired lengthof the tunnel was achieved. Another possi-ble scenario is for the immersed tunnel tobe built as a concrete box consisting ofthree separate segments: one carryingthree inbound lanes, another with threeoutbound lanes, and a third, two-lane tun-nel for buses and high-occupancy vehi-cles. This type of tunnel would be prefab-ricated in a nearby basin or dry dock (thistype of tunnel cannot be barged), then fin-ished on site.

The alignment would veer inland,reaching Third Avenue, where theGowanus Expressway would be under-pinned. The remainder of the tunnel,including the Prospect Expressway inter-change, would be built with cut-and-covermethods. Conventional construction meth-ods would be used to complete surfaceroads, and open-cut construction would beused in the BBT/BQE area.

The crossing of the Gowanus Canalwould be accomplished through the con-struction of staged cofferdams. Marinetraffic through the Canal would be shutdown during construction, but water flowwould be maintained. Pipelines and con-veyors would be used to transport materi-als and accommodate Canal usersthroughout the construction.

Tier 2 Tunnel Alternatives Report: TPX: Tunnel along Waterfront PierheadConstruction Methods DRAFT


Alternative TPA, primarily located along thewaterfront/port area, provides connections to the ShoreParkway, the Prospect and Brooklyn-Queens Expressways,and the Brooklyn-Battery Tunnel.



Along 65th Street, the Gowanus Expressway viaduct

would be underpinned, with slurry walls supporting theunderpinning beams, to allow staged cut-and-cover construc-tion with decking and slurry walls to proceed. Excavationequipment in the 65th Street area would include large exca-vators, clam buckets, and a front-end loader to construct sur-face roads and a small track backhoe in the cut section. Allexcavated material would be hauled off in trucks to a dispos-al site.

As the tunnel heads westward, it crosses the FourthAvenue subway. Ground freezing would be necessary to sta-bilize the ground and enable the tunnel to cross under thesubway and the ramp structures in the 65th Street area. TheGowanus tunnel would be mined, with careful monitoring of

the subway structures to ensure minimal settlement or heave.The 180-inch sewer in this area also would be mined with thestack drift method (a form of mining using multiple smalltunnels, one on top of the other), probably with chemicalgrouting. Although this method would be slow and labor-intensive—each small tunnel would be treated with an enve-lope of chemical grout before the next one is built—it wouldminimize risks to the sewer.

Tunneling near the Long Island Rail Road tracks wouldneed to be closely coordinated with the LIRR. Near theShore Parkway, the ground also would be frozen to allow thetunnel to proceed under the Long Island Rail Road tracks.The Shore Parkway itself would be underpinned using slurry

walls and tiebacks.Just behind the Brooklyn Army Terminal, two piers

would have to be removed to construct a cofferdam. The areawould be covered with a weight-bearing fill material inwhich a permanent, watertight bulkhead would be construct-ed. This would enable cut-and-cover construction to proceed.Once the tunnel structures are completed, the piers would bereconstructed and the cofferdam removed.

Along the bulkhead line, the tunnel would be constructedwith slurry walls with tiebacks and/or wales and struts. Thealignment then veers inward and reaches Third Avenue,where the Gowanus Expressway viaduct would have to beunderpinned.

The remainder of the alignment,including the Prospect Interchange, wouldbe built with cut-and-cover methods, withtemporary street decking, slurry walls,tiebacks, wales, and struts. Conventionalconstruction methods would be used forsurface construction, and open-cut con-struction, supported by piles and lagging,would be used in the BBT/BQE area.


Tier 2 Tunnel Alternatives Report: TPA: Tunnel along Waterfront AreaConstruction Methods DRAFT


Alternative T3, primarily located under Third Avenue,provides connections to the Shore Parkway, the Prospect andBrooklyn-Queens Expressways, and the Brooklyn-BatteryTunnel in the same areas as the existing GowanusExpressway.



Starting on 65th Street, the Gowanus Expresswaybecomes elevated. The expressway viaduct would be under-pinned to allow staged cut-and-cover construction with deck-ing and slurry walls to proceed. Excavation equipment in the65th Street area would include large excavators, clam buck-ets, and a front-end loader to construct surface roads and asmall track backhoe in the cut section. All excavated materialwould be hauled off in trucks to a disposal site.

From Fifth Avenue to Third Avenue, as the tunnel contin-ues westward and begins to curve to the north, it crosses theFourth Avenue subway. Ground freezing would be necessaryto stabilize the ground and enable the tunnel to cross underthe subway and the ramp structures in the 65th Street area.

The Gowanus tunnel, below the subway, would be mined,with careful monitoring of the subway structures to ensureminimal settlement or heave.

The 180-inch sewer in this area also would be minedwith the stack drift method (a form of mining using multiplesmall tunnels, one on top of the other), probably with chemi-cal grouting. Although this method would be slow and labor-intensive—each small tunnel would be treated with an enve-lope of chemical grout before the next one is built—it wouldminimize risks to the sewer.

Tunneling near the Long Island Rail Road tracks wouldneed to be closely coordinated with the LIRR. Near theShore Parkway, the ground also would be frozen to allow the

tunnel to proceed under the Long Island Rail Road tracks.The Shore Parkway itself would be underpinned using slurrywalls and tiebacks.

Along Third Avenue, the Gowanus Expressway viaductwould be underpinned and the tunnel built with cut-and-cover construction, decking, and slurry walls supported bytiebacks, wales, and struts. Third Avenue has many utilities,and these would either be relocated and/or supported.The108-inch sewer in Sunset Park would need to be supportedfor an approximate length of 1,400 feet. Another sewer, 3.5-feet by 4 feet, would need support where it crosses the cut-and-cover slurry walls. Still another sewer, 84 inches indiameter, would be supported for approximately 2,100 feet.

The remainder of the alignment,including the 38th and 39th Street rampsand the Prospect Interchange, would bebuilt with cut-and-cover methods, withtemporary street decking, slurry walls,tiebacks, wales, and struts. Conventionalconstruction methods would be used forsurface construction, and open-cut con-struction, supported by piles and lagging,would be used in the BBT/BQE area.



Tier 2 Tunnel Alternatives Report: T3: Tunnel under Third AvenueConstruction Methods DRAFT


Alternative T3X, primarily located under Third Avenue,provides connections to the Prospect and Brooklyn-QueensExpressways, as well as the Brooklyn-Battery Tunnel, in thesame areas as the existing Gowanus Expressway. However,under all “X” alternatives, the Gowanus Tunnel would con-nect to the Shore Parkway at a new location, near theVerrazano-Narrows Bridge.

Like T3, almost all of the T3X alternative would be con-structed using cut-and-cover tunneling methods with deckingand slurry walls. The portals (entrances to the tunnel), as wellas the ramps, would be open cut—essentially the same as cutand cover but without a top cover. Open-cut areas would usecast-in-place concrete retaining walls and piles and a shoring

(support) system. The surface roadways throughout the route would be

constructed with conventional, non-tunneling techniques.The “X” alternatives all connect to the Shore Parkway

near the Verrazano-Narrows Bridge. The approach ramps tothe bridge would need to be underpinned or would use pilesand lagging with struts, rakes, or tiebacks. Most of the under-ground construction in this area, on the west side of FortHamilton, would be cut and cover. Surface and bridge workwould be accomplished with non-tunneling methods.

Near 66th Street, from Seventh Avenue to Fourth Avenue,the inbound and outbound portals would be erected by open-cut construction employing piles and lagging with struts,

rakes, or tiebacks. Starting on 65th Street, the Gowanus Expressway

becomes elevated. The expressway viaduct would be under-pinned to allow staged cut-and-cover construction with deck-ing and slurry walls to proceed. The inbound structure wouldbe built first, followed by the outbound structure.

From Fifth Avenue to Third Avenue, the tunnel continueswestward and begins to curve to the north. The tunnel heregenerally would be built using cut-and-cover constructionwith decking and would involve underpinning the GowanusExpressway viaduct, protecting the Long Island Rail Roadtracks along 65th Street, crossing under the Fourth Avenuesubway, and crossing the 180-inch combined brick sewer.

The support system for the excavation would include slurrywalls with tiebacks, wales, and struts, and the underpinningbeams under the viaduct would be supported on the slurrywalls. The tunnel under the subway, as well as under the 180-inch sewer, would be mined with the stack drift method (aform of mining using multiple small tunnels, one on top ofthe other), probably with chemical grouting around the sewerand with freezing of the ground around the subway tunnel.The inbound tunnel and outbound ramp also would be mined,with careful monitoring of the subway structures to ensureminimal settlement or heave.

Along Third Avenue, the Gowanus Expressway viaductwould be underpinned and the tunnel built with cut-and-

cover construction, decking, and slurrywalls supported by tiebacks, wales, andstruts. Third Avenue has many utilities,and these would either be relocated and/orsupported. The 108-inch sewer in SunsetPark would need to be supported for anapproximate length of 1,400 feet. Anothersewer, 3.5-feet by 4 feet, would need sup-port where it crosses the cut-and-coverslurry walls. Still another sewer, 84 inchesin diameter, would be supported forapproximately 2,100 feet.

The remainder of the alignment,including the 38th and 39th Street rampsand the Prospect Interchange, would bebuilt with cut-and-cover methods.Conventional construction methods wouldbe used for surface construction, andopen-cut construction, supported by pilesand lagging, would be used in theBBT/BQE area.


Tier 2 Tunnel Alternatives Report: T3X: Tunnel under Third AvenueConstruction Methods DRAFT


Alternative T3B, primarily located under Third Avenue,provides connections to Shore Parkway, the Prospect andBrooklyn-Queens Expressways, and the Brooklyn-BatteryTunnel in the same areas as the existing GowanusExpressway. It differs from Alternative T3 only alongHamilton Avenue, which would be widened from eight tofourteen lanes to handle Prospect Expressway traffic going tothe BQE or the Battery Tunnel.

Although most of the alternative would be built with cut-and-cover techniques, the portals (entrances to the tunnels) and rampswould be open cut—essentially the same as cut and cover butwithout a top cover. Open-cut areas would use cast-in-place con-crete retaining walls and piles and a shoring (support) system.


Starting on 65th Street, the Gowanus Expresswaybecomes elevated. The expressway viaduct would be under-pinned to allow staged cut-and-cover construction with deck-ing and slurry walls to proceed. Excavation equipment in the65th Street area would include large excavators, clam buck-ets, and a front-end loader to construct surface roads and asmall track backhoe in the cut section. All excavated materialwould be hauled off in trucks to a disposal site.

From Fifth Avenue to Third Avenue, as the tunnel contin-ues westward and begins to curve to the north, it crosses theFourth Avenue subway. Ground freezing would be necessary

to stabilize the ground and enable the tunnel to cross underthe subway and the ramp structures in the 65th Street area.The Gowanus tunnel would be mined, with careful monitor-ing of the subway structures to ensure minimal settlement orheave.


Near the Shore Parkway, the ground also would be

frozen to allow the tunnel to proceed under the Long IslandRail Road tracks. The Shore Parkway itself would be under-pinned using slurry walls and tiebacks.

Along Third Avenue, the Gowanus Expressway viaductwould be underpinned and the tunnel built with cut-and-cover construction, decking, and slurry walls supported bytiebacks, wales, and struts. Third Avenue has many utilities,and these would either be relocated and/or supported.The108-inch sewer in Sunset Park would need to be supportedfor an approximate length of 1,400 feet. Another sewer, 3.5-feet by 4 feet, would need support where it crosses the cut-and-cover slurry walls. Still another sewer, 84 inches indiameter, would be supported for approximately 2,100 feet.



Tier 2 Tunnel Alternatives Report: T3B: Tunnel under Third AvenueConstruction Methods DRAFT


Alternative T3P, primarily located under Third Avenue,provides connections to the Shore Parkway, the Prospect andBrooklyn-Queens Expressways, and the Brooklyn-BatteryTunnel. It differs from Alternative T3 in that it would build adirect tunnel between the Prospect Expressway and theBrooklyn-Battery Tunnel. In addition, Hamilton Avenuewould be widened from eight to ten lanes to handle theProspect Expressway traffic going to the BQE.

Although most of the alternative would be built with cut-and-cover techniques, the portals (entrances to the tunnels) andramps would be open cut—essentially the same as cut and coverbut without a top cover. Open-cut areas would use cast-in-placeconcrete retaining walls and piles and a shoring (support) system.


Starting on 65th Street, the Gowanus Expressway becomeselevated. The expressway viaduct would be underpinned toallow staged cut-and-cover construction with decking and slur-ry walls to proceed. Excavation equipment in the 65th Streetarea would include large excavators, clam buckets, and a front-end loader to construct surface roads and a small track back-hoe in the cut section. All excavated material would be hauledoff in trucks to a disposal site. From Fifth Avenue to ThirdAvenue, as the tunnel continues westward and begins to curveto the north, it crosses the Fourth Avenue subway. Groundfreezing would be necessary to stabilize the ground and enable

the tunnel to cross under the subway and the ramp structures inthe 65th Street area. The Gowanus tunnel would be mined,with careful monitoring of the subway structures to ensureminimal settlement or heave.


Near the Shore Parkway, the ground also would befrozen to allow the tunnel to proceed under the Long Island

Rail Road tracks. The Shore Parkway itself would be under-pinned using slurry walls and tiebacks.

Along Third Avenue, the Gowanus Expressway viaductwould be underpinned and the tunnel built with cut-and-cover construction, decking, and slurry walls supported bytiebacks, wales, and struts. Third Avenue has many utilities,and these would either be relocated and/or supported. The108-inch sewer in Sunset Park would need to be supportedfor an approximate length of 1,400 feet. Another sewer, 3.5-feet by 4 feet, would need support where it crosses the cut-and-cover slurry walls. Still another sewer, 84 inches indiameter, would be supported for approximately 2,100 feet.

The remainder of the alignment, including the 38th and39th Street ramps and the ProspectInterchange, would be built with cut-and-cover methods, with temporary streetdecking, slurry walls, tiebacks, wales, andstruts. Conventional construction methodswould be used for surface construction,and open-cut construction, supported bypiles and lagging, would be used in theBBT/BQE area.



Tier 2 Tunnel Alternatives Report: T3P: Tunnel under Third AvenueConstruction Methods DRAFT


Alternative T2, primarily located under SecondAvenue, provides connections to the Shore Parkway, theProspect and Brooklyn-Queens Expressways, and theBrooklyn-Battery Tunnel.

The relatively narrow width of Second Avenue (80 feet)makes it problematical to use a tunnel-boring machine; itdoes not provide enough space to accommodate both theinbound and outbound tunnels without passing under thebuildings lining the avenue. This would pose potential risksto the foundations of these buildings. To avoid these types ofrisks, which could necessitate property takings, a stackedtunnel could be built using cut-and-cover and slurry wallconstruction. The stacked tunnel would have three lanes on

the top section and four lanes on the bottom. Although most of the alternative would be built with cut-

and-cover techniques, the portals (entrances to the tunnels)and ramps would be open cut—essentially the same as cutand cover but without a top cover. Open-cut areas would usecast-in-place concrete retaining walls and piles and a shoring(support) system.


Starting on 65th Street, the Gowanus Expresswaybecomes elevated. The expressway viaduct would be under-pinned to allow staged cut-and-cover construction with deck-ing and slurry walls to proceed. Excavation equipment in the

65th Street area would include large excavators, clam buck-ets, and a front-end loader to construct surface roads and asmall track backhoe in the cut section. All excavated materialwould be hauled off in trucks to a disposal site.

From Fifth Avenue to Second Avenue, as the tunnel con-tinues westward and begins to curve to the north, it crossesthe Fourth Avenue subway. The tunnel under the subway, aswell as under the 180-inch sewer, would be mined with thestack drift method (a form of mining using multiple smalltunnels, one on top of the other), probably with chemicalgrouting. Although this method would be slow and labor-intensive—each small tunnel would be treated with an enve-lope of chemical grout before the next one is built—it would

minimize risks to the sewer. The double-stacked tunnel under Second Avenue would

be deep, and the slurry walls would be close to the buildings;wales and struts would be used to minimize adverse effectson the buildings. The 72-inch, 120-inch, and 54-inch sewerlines in this section would be supported with slurry walls.Steel beams, steel straps, wood framing, or concrete encase-ment would support these and the sewer regulator (which, intimes of heavy rains, directs sewage overflow into the Bay).

The tunnel would begin to turn eastward, toward ThirdAvenue, after the 38th/39th Street ramps. Along ThirdAvenue, the Gowanus Expressway viaduct would have to beunderpinned. Here again, the tunnel would be built with cut

and cover and slurry walls. To restorepedestrian and vehicular traffic as soon aspossible, a temporary deck would be usedto cover the excavation, with the workproceeding underground.

To pass under the Gowanus Canal,staged cofferdams would be constructed.Marine traffic through the Canal would beshut down during construction, but waterflow would be maintained. Pipelines andconveyors also would be constructed andused to transport materials to the Canal users.

The remainder of the alignment,including the 38th and 39th Street rampsand the Prospect Interchange, would bebuilt with cut-and-cover methods, withtemporary street decking, slurry walls,tiebacks, wales, and struts. Conventionalconstruction methods would be used forsurface construction, and open-cut con-struction, supported by piles and lagging,would be used in the BBT/BQE area. .

Tier 2 Tunnel Alternatives Report: T2: Tunnel under Second AvenueConstruction Methods DRAFT


Alternative T2X, primarily located along Second Avenue,provides connections to the Prospect and Brooklyn-QueensExpressways, as well as the Brooklyn-Battery Tunnel, in thesame areas as the existing Gowanus Expressway. However,under all “X” alternatives, the Gowanus Tunnel would con-nect to the Shore Parkway at a new location, near theVerrazano-Narrows Bridge.

Like T2, almost all of the T2X alternative would be con-structed using cut-and-cover tunneling methods with deckingand slurry walls. The portals (entrances to the tunnel—seemap for locations), as well as the ramps, would be opencut—essentially the same as cut and cover but without a topcover. Open-cut areas would use cast-in-place concrete

retaining walls and piles and a shoring (support) system. The surface roadways throughout the route would be



Near 66th Street, from Seventh Avenue to Fourth Avenue,the inbound and outbound portals would be built by open-cutconstruction employing piles and lagging with struts, rakes,

or tiebacks. Starting on 65th Street, the Gowanus Expressway

becomes elevated. The expressway viaduct would be under-pinned to allow staged cut-and-cover construction with deck-ing and slurry walls to proceed. Excavation equipment in the65th Street area would include large excavators, clam buck-ets, and a front-end loader to construct surface roads and asmall track backhoe in the cut section. All excavated materialwould be hauled off in trucks to a disposal site.

From Fifth Avenue to Second Avenue, as the tunnel con-tinues westward and begins to curve to the north, it crossesthe Fourth Avenue subway. The tunnel under the subway, aswell as under the 180-inch sewer, would be mined with the

stack drift method (a form of mining using multiple smalltunnels, one on top of the other), probably with chemicalgrouting. Although this method would be slow and labor-intensive—each small tunnel would be treated with an enve-lope of chemical grout before the next one is built—it mini-mizes risks to the sewer.

The double-stacked tunnel under Second Avenue wouldbe deep, and the slurry walls would be close to the buildings;wales and struts would be used to minimize adverse effectson the buildings.

The remainder of the alignment, including the 38th and39th Street ramps and the Prospect Interchange, would bebuilt with cut-and-cover methods, with temporary street

decking, slurry walls, tiebacks, wales, andstruts. Conventional construction methodswould be used for surface construction,and open-cut construction, supported bypiles and lagging, would be used in theBBT/BQE area.


Tier 2 Tunnel Alternatives Report: T2X: Tunnel under Second AvenueConstruction Methods DRAFT


Alternative T1, primarily located along First Avenue, pro-vides connections to the Shore Parkway and Prospect andBrooklyn-Queens Expressways, as well as the Brooklyn-Battery Tunnel.

The relatively narrow width of First Avenue (80 feet)makes it problematical to use a tunnel-boring machine; itdoes not provide enough space to accommodate both theinbound and outbound tunnels without passing under thebuildings lining the avenue. This would pose potential risksto the foundations of these buildings. To avoid these types ofrisks, which could necessitate property takings, a stackedtunnel could be built using cut-and-cover and slurry wallconstruction. The stacked tunnel would have three lanes on

the top section and four lanes on the bottom. Although most of the alternative would be built with cut-

and-cover techniques, the portals (entrances to the tunnels)and ramps would be open cut—essentially the same as cutand cover but without a top cover. Open-cut areas would usecast-in-place concrete retaining walls and piles and a shoring(support) system.


In the 65th Street area, between Sixth and SeventhAvenues, the Gowanus Expressway viaduct would be under-pinned, and cut-and-cover construction with decking andslurry walls would be used, first on the inbound structure and

then on the outbound. Near Fourth Avenue, the subwaywould be underpinned.

Finally, the inbound tunnel passes directly under andvery close to the 180” brick sewer in this area. To avoiddamage to the sewer, the stack drift method (a form of min-ing using multiple small tunnels, one on top of the other)would be used. Although this method would be slow andlabor-intensive, it minimizes risks to the sewer.

Ground freezing would be necessary to stabilize theground and enable the tunnel to cross under the subway andthe ramp structures in the 65th Street area; near the ShoreParkway, the ground also would be frozen to allow the tun-nel to proceed under the Long Island Rail Road tracks. The

Shore Parkway itself would be underpinned using slurrywalls and tiebacks (angled supports used to hold the slurrywalls in place).

Along First Avenue, the tunnel generally would be con-structed in stages, using cut-and-cover methods with deck-ing.

The double-stacked tunnel under First Avenue would bedeep, and the slurry walls would be close to the buildings;wales and struts would be used to minimize adverse effectson the buildings. The 72-inch, 120-inch, and 54-inch sewerlines in this section would be supported with slurry walls.Steel beams, steel straps, wood framing, or concrete encase-ment would support these and the sewer regulator (which, in

times of heavy rains, directs sewage over-flow into the Bay).

The tunnel would cross from First toThird Avenue at 22nd Street. To build thetunnel in this area, the GowanusExpressway viaduct would have to beunderpinned. Here again, the tunnel wouldbe built with cut and cover and slurry walls.To restore pedestrian and vehicular traffic assoon as possible, a temporary deck wouldbe used to cover the excavation, with thework proceeding underground.

To pass under the Gowanus Canal,staged cofferdams would be constructed.Although marine traffic through the Canalwould be shut down during construction,water flow would be maintained. Pipelinesand conveyors also would be constructedand used to transport materials to accom-modate the Canal’s users.


Tier 2 Tunnel Alternatives Report: T1: Tunnel under First AvenueConstruction Methods DRAFT


Alternative T1X, primarily located under FirstAvenue, provides connections to the Prospect and Brooklyn-Queens Expressways, as well as the Brooklyn-BatteryTunnel, in the same areas as the existing GowanusExpressway. However, under all “X” alternatives, theGowanus Tunnel would connect to the Shore Parkway at anew location, near the Verrazano-Narrows Bridge.

Like T1, almost all of the T1X alternative would be con-structed using cut-and-cover tunneling methods with deckingand slurry walls. The portals (entrances to the tunnel—seemap for locations), as well as the ramps, would be opencut—essentially the same as cut and cover but without a topcover. Open-cut areas would use cast-in-place concrete

retaining walls and piles and a shoring (support) system. The surface roadways throughout the route would be



Near 66th Street, from Seventh Avenue to FourthAvenue, the inbound and outbound portals would beerected by open-cut construction employing piles and lag-

ging with struts, rakes, or tiebacks. Starting on 65th Street, the Gowanus Expressway

becomes elevated. The expressway viaduct would be under-pinned to allow staged cut-and-cover construction with deck-ing and slurry walls to proceed. The inbound tunnel would beconstructed first, then the outbound. Underground ramps gen-erally would be constructed within an open-cut trench, withpiles and lagging held in place by struts, wales, or tiebacks.Retaining walls supporting the ramps would be cast-in-placeconcrete. Surface ramps would be constructed by convention-al, non-tunneling methods.

From Fifth Avenue to Third Avenue, as the tunnel contin-ues westward and begins to curve to the north, it crosses the

Fourth Avenue subway, which would need to be underpinned,with the beams supported on the same slurry walls used tosupport the excavation. Part of the ground (about 50 meters)under the Fourth Avenue subway tunnel also would need tobe frozen in order to build the tunnel’s outbound ramp andthe HOV lane. The tunnel’s inbound ramp passes directlyunder and very close to the 180-inch sewer as well as thesubway. To avoid damage to the sewer, the stack drift method(a form of mining using multiple small tunnels, one on top ofthe other) would be used. Although this method would beslow and labor-intensive—each small tunnel would be treatedwith an envelope of chemical grout before the next one isbuilt—it minimizes risks to the sewer.

The double-stacked tunnel underFirst Avenue would be deep, and the slur-ry walls would be close to the buildings;wales and struts would be used to mini-mize adverse effects on the buildings. The72-inch, 120-inch, and 54-inch sewer linesin this section would be supported on slur-ry walls. Steel beams, steel straps, woodframing, or concrete encasement wouldsupport these and the sewer regulator(which, in times of heavy rains, directssewage overflow into the Bay).



Tier 2 Tunnel Alternatives Report: T1X: Tunnel under First AvenueConstruction Methods DRAFT


Documents

Gowanus Expressway Project - NYSDOT Home...Description Sequential Excavation Method (SEM) also is known as New Austrian Tunneling Method (NATM) because it was developed in Austria,