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The Casting Basin

The difficult problem of crossing Boston's Fort Point Channel with a highway tunnel was solved in a very big way - in a place called "the casting basin."  (Map of the Project Area.)

As part of the Central Artery/Tunnel Project, the Massachusetts Turnpike (I-90) extends from its current eastern end, through a tunnel beneath South Boston to the Ted Williams Tunnel, Logan Airport, and East Boston. Fort Point Channel, a narrow extension of Boston Harbor into South Boston, lies just east of the I-90/I-93 (South Bay) interchange. To cross the channel, Central Artery engineers decided to use tunnel sections lowered into a trench, much like the technique used to build the Ted Williams Tunnel under the harbor. 

But the Ted Williams Tunnel was built using steel tunnel sections each longer than a football field that were barged to Boston from a shipyard in Baltimore. That method didn't work at the Fort Point Channel because there wasn't enough room to float the sections under channel bridges at Summer Street, Congress Street, and Northern Avenue. The project couldn't build a steel mill in South Boston, so the designers decided to go with concrete tunnel sections, the first use of this technique in the United States.

How to do it? The concrete sections had to be fabricated in a way that allowed them to be conveniently moved into position in the channel. The solution was the casting basin, in effect a huge dry dock 1,000 feet long, 300 feet wide, and 60 feet deep - big enough to hold an aircraft carrier or three Titanics side by side.  

The basin was dug on the South Boston side of the channel, next to the Gillette Company's manufacturing headquarters. (More than 450,000 cubic yards of dirt was excavated to form the basin.) The channel end of the basin was sealed off from the water by a series of round cofferdams, each filled with crushed stone. The six tunnel sections were built on the gravel-covered bottom of the basin, four in the first phase of construction. The completed sections were sealed watertight at either end. Then the basin was flooded. The crushed stone was removed from the cofferdams holding back the channel on the western end of the basin. Also, the steel sheet piles forming the circular structures were removed so the sections could be floated out of the basin and positioned to be lowered into a trench dredged on the bottom of the channel.   

The longest of the six tunnel sections was 414 feet long, the widest 174 feet wide. All were about 27 feet high. The heaviest weighed more than 50,000 tons (more than the Titanic, or about the weight of a battleship). Including ramps, the channel crossing carries 9 lanes of traffic, four eastbound (EB) and five westbound (WB).

Lowering the sections into the channel was a precise business, because the sections couldn't be moved once they were placed, and each had to line up perfectly with adjacent sections. Thus the trench on the bottom of the channel was dredged and graded within extraordinarily narrow tolerances. But there was another challenge under the channel that made sinking the concrete sections even more delicate and precise: The Massachusetts Bay Transportation Authority's Red Line subway tunnel runs under the channel, and the I-90 extension passes just a few feet above it. The weight of the concrete sections would damage the tunnel if nothing were done to protect it.

 

718K PDF showing floated tunnels and tunnel jacking
(source: Bechtel Globe, Aug. 2001)

The solution to that problem was 110 concrete shafts, each six feet in diameter, drilled in the bottom of the channel on either side of the subway tunnel as much as 145 feet into bedrock. The tunnel sections rest on these shafts, with these legs matching up with fittings protruding from the bottom of the sections, like giant Lego blocks.

One further challenge for the designers was adapting the western-most tunnel sections to be the foundation for a ventilation building (VB1), which rises above the shore of the channel atop the highway tunnel near the U.S. Postal Service building.

Tunnel Tube Float-outs

Float-out of the first four tunnel sections took place in early 2000. The first float-out sequence included placing two sections and mooring two more along the shore of the channel next to the U.S. Postal Service building for placement later in the year. When the first four sections were out of the basin, the cofferdams were re-installed, the basin pumped dry, and two more sections were built inside.

After the final two sections were placed (their eastern ends overlap with the channel end of the casting basin) in June 2001, the basin was permanently sealed off next to the channel and the 1,000-foot excavation became a cut-and-cover tunnel path of the highway toward A Street, the South Boston Interchange, and the Ted Williams Tunnel.