Anthony L. Ricci, P.E.Chief Bridge Engineer

Central Artery/Tunnel ProjectMassachusetts Turnpike Authority

Boston, Massachusetts

This article is the sixth and final paper in the series of articles on Boston, Massachusetts' mammoth Central Artery Tunnel Project. Thismulti-billion dollar project is the largest highwayconstruction job ever undertaken in the UnitedStates. The first five articles discussed the variousinnovative ways in which precast/prestressed concrete is playing in the successful constructionof this project. This concluding article summa-rizes the major precasting techniques being utilized and outlines the project milestonesplanned and accomplished so far, which are already having a beneficial impact on the transportation needs of the people of Boston.

Amajor transportation infrastructure undertaking, billedas “The Big Dig” (Central Artery/Tunnel Project), istransforming traffic operations in and around Boston,

Massachusetts. This $14 billion project, the biggest andmost complex transportation system ever undertaken in theUnited States, has worldwide significance.

Precast/prestressed concrete is playing a major role in theconstruction of the Central Artery/Tunnel Project at many ofthe interchanges, viaducts and ramps, temporary structures,and even in tunnels, which are all being built while main-

Central Artery/Tunnel Project:Boston’s Engineering Marvel –Where We Are Now

14 PCI JOURNAL

Vijay Chandra, P.E.Senior Vice PresidentParsons Brinckerhoff Quade &Douglas, Inc.New York, New York

PART 6

March-April 2001 15

taining traffic. The project has broughtout the best that precast concrete tech-nology has to offer — in many casesutilizing cutting edge technologies.

Numerous innovative constructiontechniques are being used in this pro-ject, including:• Precast concrete tunnels jacked

under railroad embankments.• Precast concrete immersed tube

tunnels.• Standardized designs for precast

New England Bulb-Tee (NEBT)girders and slab-on-pile structures.

• Standardized temporary structuresutilizing precast elements.

• Precast concrete segmental boxgirders integrated into cast-in-placecolumns to provide seismically re-sistant connections.

• Precast concrete segmental boxescut at a skew to connect on eitherside of straddle bents.

The purpose of the Central Artery/Tunnel Project is to provide an efficient,uncongested vehicular route for thepeople of Boston. The city’s existingCentral Artery, which includes an ele-vated section of I-93 between CongressStreet and Route 1 in Charlestown, isone of the most congested segments ofinterstate highway in the nation.

Jammed with more than 190,000 ve-hicles daily — more than twice the ca-pacity it was originally designed tohandle when it opened in 1959 — theCentral Artery experiences eight hoursof congestion each weekday. Theseconditions were expected to worsen,to the point that by the year 2010, itwas estimated that there would be a15-hour rush period.

Massachusetts transportation offi-cials initiated the current CentralArtery/Tunnel Project (see Fig. 1)in August 1986. As planned, majorelements of this extensive projectinclude:• Replacement of a 4.3 mile (6.9 km)

segment of the congested CentralArtery (I-93) between the SoutheastExpressway to the south andCharlestown to the north.

• An improved 1.7 mile (2.7 km) di-rect connection between LeverettCircle and Charlestown.

• A 3.9 mile (6.3 km) extension of theMassachusetts Turnpike (I-90) toLogan International Airport.

Fig. 1. Overall project map.

Fig. 2. South Bay Interchange model.

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• A 1.4 mile (2.3 km) South BostonBypass Road.

PROJECT MILESTONESDesign of the project began in 1986

with major utility relocations initiatedin 1991. Major project milestones, to-gether with actual, planned completiondates, are given in Table 1.

I-90 Extension to Logan Airport

Presently, I-90 (known locally asthe Massachusetts Turnpike), extends

westwards to Seattle, Washington. InBoston, it terminates at the intersec-tion with I-93 near the Fort PointChannel.

One of the crucial parts of the over-all project has been the extension of I-90 to Logan Airport. This will im-prove access to the airport as well asprovide a relief valve to the over-loaded Sumner and Callahan Tunnels.

This extension of I-90 includes amajor four-level interchange with I-93; crossing under a busy railroadcorridor with jacked tunnels; going

under Fort Point Channel and overthe Massachusetts Bay Transporta-tion Authority (MBTA) Red Linetunnel with concrete immersed tubetunnels; running through SouthBoston in a depressed roadway sec-tion; crossing under the busy ship-ping lanes accessing Boston Harbor;providing access to the various air-port terminals via an interchange;and connecting to the Sumner andCallahan Tunnels and Route 1A viaanother interchange.

The complicated four-level all-di-rectional interchange with I-93 (seeFig. 2) is being built in stages whilemaintaining the traffic flow of an ex-isting three-level interchange and thendemolishing the existing structures instages. This is being accomplished byusing temporary structures and previ-ously built structures as detours. Mostof this interchange uses precast con-crete technology with many innova-tions (see Figs. 3 and 4). For furtherdetails, see the July-August 2000, PCIJOURNAL.

I-90 needed to be depressed throughSouth Boston due to impacts toGillette’s main manufacturing facility,and needed to cross under Amtrak andMBTA commuter railroad tracks nearSouth Station. The tracks make a 90-degree sweep in the project area.

Crossing under the tracks is accom-plished by a technology called “tunneljacking.” Precast concrete tunnels arefirst constructed in a pit and pushedwith large capacity jacks into previ-ously frozen ground (see Fig. 5).There are three 78 ft (24 m) wide, 38ft (12 m) high tunnels of varyinglengths from 167 to 350 ft (51 to 107m), which have been precast andpushed under the tracks. Tunnel jack-ing for I-90 is complete.

Crossing the shallow Fort PointChannel is accomplished by immersingprecast concrete tunnels (see Fig. 6) ofunusual trapezoidal shapes to matchthe main line section and incomingramps. Lengths vary from 280 to 417 ft(85 to 127 m), widths vary from a min-imum of 66 to 164 ft (20 to 50 m),heights vary from a minimum of 26 to31 ft (7.9 to 9.4 m), and they weigh asmuch as 50,000 tons (45,400 t).

The tubes are precast in a dry dockalong the I-90 alignment adjoining theFig. 3. Precast segmental erection.

Actual/PlannedProject Completion Dates

South Boston Bypass Road, Ted Williams Tunnel and part of Logan Interchange Late 1995

Leverett Circle Connection to Charlestown Late 1999

I-90 Eastbound, Westbound, and Ramp L Mid 2002

Logan Airport Interchange Mid 2002

I-93 Northbound Late 2002

East Boston Route 1A Interchange Mid 2003

I-93 Southbound (two lanes) Late 2003

I-93 Southbound (all lanes) Late 2004

I-90/I-93 Interchange (final completion) Late 2004

Existing Artery Removal and Surface Restoration 2005

Table 1. Design of the project began in 1986 with major utility relocationsinitiated in 1991. Major project milestones are listed in table.

March-April 2001 17

Fort Point Channel, and then floatedout into the channel. Four of the sixtubes have been completed and twohave already been installed. The twoinstalled tube sections comprise thelower portion of a ventilation struc-ture. Both the tunnel sections and ven-tilation building were cast together indry dock and then floated. This wasdone to avoid underwater work for theventilation building after placement ofthe tubes.

The ventilation building is currentlyunder construction. Four of the tubesare being placed over underwater

drilled shafts so as not to stress the100-year-old Red Line transit tubeswhich are located close to the bottomof the new tunnels — within 4.9 ft(1.5 m). Two of the constructed tubesare in the channel awaiting placement.The last two tubes are under construc-tion in the dry dock.

I-90 traverses through South Bostonas a depressed highway and connectsto the previously completed and oper-ating Ted Williams Tunnel, which isan immersed steel tube tunnel underthe ocean at the entrance to BostonHarbor (see Fig. 7).

Exiting the tunnel in East Boston to-wards Logan Airport, a semi-circular in-terchange, which is under construction(see Fig. 8), provides access to the air-port terminals and connects to anotherinterchange (East Boston Route 1A).Construction of the East Boston Route1A Interchange recently began and willeventually connect I-90 back to the Sum-ner and Callahan Tunnels and Route 1A.

I-93 Reconstruction

Most of I-93 up to the North End willbe relocated into tunnels. After rehabil-

Fig. 4. Precast segments connection to straddle pier. Fig. 5. Precast tunnel jacking.

Fig. 6. Precast immersed tube tunnel under construction.

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itation, the existing Dewey SquareTunnel will be serving southbound traf-fic only. At the I-90 interchange, thenorthbound roadway splits off and runsthrough its own alignment under At-lantic Avenue and below the Red Lineat South Station before rejoining thesouthbound main line. North of SouthStation, the northbound and south-bound roadways come together and run

parallel with four or more lanes of traf-fic under the present elevated CentralArtery viaducts in downtown Boston.

The new tunnels are being builtwhile maintaining traffic on the ele-vated viaducts and surface roadways.The northbound roadway is presently50 percent complete. Finishes for thenorthbound and southbound tunnelswill include precast concrete walls and

Fig. 7. Ted Williams Tunnel under Boston Harbor.

Fig. 8. Logan Airport Interchange under construction.

ceiling panels (see Fig. 9), which arenow being installed.

The southbound roadway is partiallycomplete. After both northbound andsouthbound tunnels are in service, theelevated Central Artery will be razedand a park-lined boulevard will be de-veloped along the alignment.

At Boston’s North End, the tunnelsemerge into daylight at CausewayStreet before proceeding to a cable-stayed bridge over the Charles River.The bridge, named the Leonard B.Zakim Bunker Hill Bridge, is the firstof its kind in the United States (seeFig. 10). It is also the widest cable-stayed bridge in the world.

The bridge is asymmetric, bothtransversely and longitudinally, withinverted Y-shaped towers. The mainspan is 745 ft (230 m) with back spansof 250 and 420 ft (75 and 130 m). It isa hybrid structure with concrete backspans and composite precast slabs onsteel superstructure in the main span.The back spans of the bridge are com-plete and the main span is 80 percentfinished.

North of the Charles River, thecable-stayed bridge merges into a

March-April 2001 19

Fig. 9. Precast concrete walls and ceiling being installed.

complicated three-level interchangethat accommodates I-93 as well as aconnection to the Tobin Bridge (Route1) and ramps from Leverett Circle.The interchange utilizes mostly pre-cast segmental box girders, built usingspan-by-span and balanced cantileverconstruction techniques.

Span-by-span erection is used infairly straight sections, with balancedcantilever erection used for the curvedramps over the railroad tracks servingNorth Station (see Fig. 11).

The northbound and southboundLeverett Circle ramps, after spanningover the North Station platforms andthe Charles River, merge into theabove interchange.

Temporary BridgesBecause of the need to maintain

traffic during construction, as well asthe congested urban environmentthrough which the project is con-structed, many temporary bridge struc-tures are needed. Early in the process,for a few structures, we proceededwith proprietary products that tendedto increase costs. However, sincemany temporary structures totaling ap-

proximately $120 million were neces-sary, we changed strategies to reducethis cost.

Rather than limiting ourselves toone type of structure, we developedfour types — all simple spans — andproduced standard drawings to facili-tate adaptability in most locations. Theconcrete alternates included boxbeams, full depth precast slabs, and

Inverset units. This alternative concepthas also fostered increased competi-tion and reduced costs.

After the completion of this project,these temporary structures will becomethe property of the Massachusetts High-way Department for use in replacingsecondary and rural bridges. For moreinformation, see November-December2000, PCI JOURNAL.

Fig. 10. Leonard B. Zakim Bunker Hill Bridge under construction.

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Transition Bridges

When ramps or viaducts were lo-cated close to the ground, rather thanusing concrete or steel box girders, weutilized comparatively shorter spansand a correspondingly shallower su-perstructure. This improved access forinspection purposes, enhanced aesthet-ics, and alleviated the traditionallyhumid conditions below these struc-tures. Rather than allowing individualsection designers to develop their owntypes of structures, the project engi-neers decided to develop cohesivestandards.

Two types of structures were stan-dardized: For spans from 75 to 110 ft(23 to 34 m), NEBTs with compositedecks (see Fig. 12), and for very shortspans up to 25 ft (7.6 m) (see Fig. 13),precast slabs on piles. The bulb-tee so-lution was developed for simple spansup to four-span continuous structures.

Section designers used the stan-dard drawings by first calling outbeam types from tables, supple-mented by general plan, elevation,and other details that could not beanticipated.

The precast slab-on-pile standardsincluded full depth precast slabs onprecast pier caps supported on pre-cast square piles. Most of these struc-tures have been constructed and arenow in service. For further details,see January-February 2001, PCIJOURNAL.

Miscellaneous StructuresNot every aspect of this project

uses new or innovative technologies.Wherever possible, and appropriate,

Fig. 11. Segmental concrete construction over North Station tracks. Fig. 12. New England bulb-tee girder construction.

Fig. 13. Slab-on-pile erection.

conventional concepts have beenutilized. For example, at the Broad-way (see Fig. 14) and DorchesterAvenue Bridges, precast side-by-side boxes were used with compositedeck slabs.

For the 800 ft (245 m) long Broad-way Bridge, built over railroad tracks,the structure is made continuous forsuperimposed dead and live loads.

Additionally, at Spectacle Island,where an old landfill was used formuch of the project’s excavated tun-nel material, a berthing facility wasneeded. The resulting pier, necessaryto dock boats and ferries once the is-land is landscaped and transformedto recreational use, utilized a totallyprecast solution featuring doubletees on precast piles and caps. Referto September-October 2000, PCIJOURNAL.

PRECAST CONCRETESHOPPING LIST

The Central Artery/Tunnel Projectincorporates hundreds of millions ofdollars worth of precast concrete com-ponents. The sheer scope of the differ-ent segments and volume of casting isstaggering. Table 2 summarizes thetypes of precast components used insome of the interchanges.

CONCLUDING REMARKS

The Central Artery/Tunnel Projecthas been at the forefront of adaptingthe latest technologies, innovative newconcepts, and new techniques as situa-tions allowed — or demanded —while also respecting conventionalmethods. The recently opened sections

March-April 2001 21

of highway are already improving thetransportation efficiency of the peopleof Boston.

This series of articles, of which thisis the concluding paper, owes much tothe ingenuity of designers, contractors,and precasters working together tomake this massive undertaking a suc-cess. The authors wish to once againthank them all.

The authors would also like to thankthe PCI JOURNAL editors for pub-lishing this series of articles, as well asthe many PCI members for their con-tinued interest and support.

At this time of writing, the BostonCentral Artery/Tunnel Project contin-ues to make rapid progress. Approxi-mately 67 percent of construction isnow complete.

Location Quantity

I-90/I-93 South Bay Interchange 12,000 linear ft (3660 m) of 12-in. (305 mm) piles51,700 linear ft (15,760 m) of 14-in. (355 mm) piles47,000 linear ft (14,330 m) of 16-in. (405 mm) piles594,000 sq ft (55,185 m2) of segmental box pieces71,400 sq ft (6630 m2) of NEBT130,000 sq ft (12,080 m2) of box beams98,900 sq ft (9190 m2) of curtain walls43,900 sq ft (4080 m2) of slab on piles

Massachusetts Avenue Interchange 125,900 linear ft (38,375 m) of AASHTO-PCI girders5600 sq ft (520 m2) of box beams67,700 sq ft (6290 m2) of curtain walls

I-90 Logan Airport Interchange 13,100 linear ft (3990 m) of 12-in. (305 mm) piles64,400 linear ft (19,630 m) of 16-in. (405 mm) piles25,000 sq ft (2230 m2) of precast slabs140 precast caps28,400 sq ft (2640 m2) of curtain walls42,200 sq ft (3920 m2) of segmental concrete box pieces

Leonard B. Zakim Bunker Hill Bridge 104,400 sq ft (9700 m2) of precast slabs

I-93 Interchange 7000 linear ft (2130 m) of 16-in. (405 mm) piles11,700 sq ft (1090 m2) of box beams503,600 sq ft (46,785 m2) of segmental boxes12,800 sq ft (1190 m2) of slab on piles17,500 sq ft (1625 square m) of curtain walls

Table 2. Summary of precast components in some of the interchanges.

Fig. 14. Aerial view of Broadway Bridge.