SETTING SAIL Immersed tubes will be installed this summer. (Photo by Willam J. Angelo for ENR)

Phase One of the three-stage project, a $50-million new busway down Washington Street from Dudley Square to the New England Medical Center MBTA station, is already completed. Phase Two, the $601-million tunnel complex linking South Station to Logan International Airport, is scheduled for completion early next year. The final phase, a $952-million tunnel linking the Medical Center Station to South Station, is scheduled for a 2010 completion. All work will be done utilizing unit-price lump-sum bids.

One particularly difficult portion of the project is a $110-million section involving a 325-ft-long NATM tunnel under 100-year-old Russia Wharf. That binocular tunnel lies west of a rectangular 180-ft-long cut-and- cover transition tunnel linked to a rectangular 700-ft-long, three-piece, concrete immersed tube tunnel across Fort Point Channel, that is in turn tied to another 100-ft cut-and-cover piece. Cambridge, Mass.-based Modern Continental Construction Co. Inc. is the general contractor and is doing the NATM portion in joint venture with Innsbruck, Austria-based Beton-und Monierbau.

Russia Wharf is a three-building complex, all seven stories high, supported on 45-ft-long wooden piles. Before mining on the overlapping 27-ft-dia tunnels could start last September, the soil under the wharf had to be frozen. About 180 pipes were placed up to 25 ft deep early last year to carry a brine solution. "We had to go NATM/freezing because it was the least invasive and most cost-effective process," says Edward L. Karpinski Jr., MBTA deputy director. "We looked at cut and cover, a soft ground shield and boring machine, but costs and impacts didn't work for us."

"We think it's the first time ground freezing and NATM have been combined under a structure," says Stephen A. DelGrosso, Modern Continental's senior project manager. "The combination works great but we had to underpin parts of the structure with a steel frame and 120-ft-deep rock-socketed mini piles and two drilled monoshafts each 150 ft deep." Some of the piles are incorporated into the center wall and the rest are outside the primary tunnel lining.

COMBO New Austrian Tunneling Method was used with ground freezing for first time under a structure. (Photo courtesy of Modern Continental)

Shotcrete applied up to 12 in. thick comprises the primary tunnel lining. A PVC lining incorporating water barriers and grout injection hoses ensures tunnel dryness. Another 12-in. shotcrete secondary lining then will be applied, along with a 2-in. shotcrete finish layer. A cast-in-place 3-ft-thick concrete invert is now being poured.

"The biggest challenge was digging between the piles and mitigating potential building movement by maintaining the frozen ground," says DelGrosso. "We used a roadheader attachment on an excavator for the crown and excavation bucket on the bench." The underpinning has some eight-ton structural steel sections that had to be placed through windows using chain falls, come-alongs and a trolley system, DelGrosso adds.

The contractors had to turn down the temperature of the ground freezing operation to about 5°F because the soil contained more organics than expected, which raised concerns about the strength of the frozen carrying beam across the tunnel, says DelGrosso. Excavation on the eastbound tunnel is completed and work on the westbound side will start this summer. Other portions of the Phase Two tunnel, all cut- and-cover sections, are either finished or are being completed by J.F. White Contracting Co., Framingham, Mass., Perini/Kiewit/Cashman joint venture, Framingham; and a Medway, Mass.-based Kiewit/Atkinson/Cashman joint venture. Three engineering firms, DMJM+Harris, Stone & Webster, and STV, are also involved.

The immersed tubes are being set behind a replica of the Tea Party ship. Workers drove two lines of 75-ft-long sheet piles across the tidal channel to create a trench for the concrete immersed tubes, which are being built in a nearby casting basin. While dredging the 55-ft-wide trench down about 40 ft through organics, clay and till, they hit the top of a hard, glacial rock, which required divers to drill cores and use a hydraulic splitter to break up a 200-sq-ft section. "It is roughly 20,000 psi and an 8-ton chisel didn't work," says DelGrosso. "And we couldn't dislodge it, so we had to use divers, which has caused delays and a probable claim."

This summer, the tubes will be floated into the channel under two bridges and over the sheeting. "Each tube will be set in a lay barge, ballasted and lowered with winches," says DelGrosso. The tubes will sit on hydraulic pin-jack foundations. Once set to grade, sand will be pumped under the tubes.

While Phase Two is winding down, preliminary engineering already has started on Phase Three by the locally based joint venture of URS/DMJM+Harris. "We're looking at a number of alternatives to try and lock in federal funding because we need at least 50% participation," says Peter F. McNulty, joint venture project manager. "Technically speaking, the big issues are location of shafts and portals and the complexities of twin driven 24-ft-dia tunnels crossing underneath portions of the Central Artery/Tunnel and several transit stations."

CONNECTED Immersed tubes will link up with dug tunnels at Fort Point Channel. (Photo by Willam J. Angelo for ENR)

The work is all through clay and sand and mostly about 60 ft deep, he says. Construction is scheduled to start by 2006.

"Ridership is already up 50% along the Washington corridor," says David W. Ryan, MBTA assistant general manager. Because of opposition to a catenary system, the surface system uses busses powered by compressed natural gas. But the CNG's can't go in tunnels, so dual diesel/electric powered busses will be used there, says Ryan. But by 2010 only one type of vehicle will be used systemwide—either dual mode, all electric, or an evolved CNG.

Ryan says the project is having a major economic impact, with at least $500 million of new private development under way along the Washington Street corridor. When completed, the Silver Line will tie into three other mass transit lines plus South Station, which handles Amtrak, commuter rail and a nearby bus terminal. "That's its real appeal," says Ryan.