Big Dig is the unofficial name of the Central Artery/Tunnel Project (CA/T), a megaproject that rerouted the Central Artery (Interstate 93), the chief controlled-access highway through the heart of Boston, Massachusetts, into a 3.5 mile (5.6km) tunnel under the city. The project also included the construction of the Ted Williams Tunnel (extending Interstate 90 to Logan International Airport), the Zakim Bunker Hill Bridge over the Charles River, and the Rose Kennedy Greenway in the space vacated by the previous I-93 elevated roadway. Initially, the plan was also to include a rail connection between Boston's two major train terminals.
The final ramp opened 13 January 2006. The project's overall completion is at 99%.
The Big Dig has been the most expensive highway project in the U.S.

Historical background
The project was conceived in the 1970s by the Boston Transportation Planning Review to replace the rusting elevated six-lane Central Artery. The expressway separated downtown from the waterfront, and was increasingly choked with bumper-to-bumper traffic. Business leaders were more concerned about access to Logan Airport, and pushed instead for a third harbor tunnel. In their second terms as governor and secretary of transportation, respectively, Michael Dukakis and Fred Salvucci, came up with the strategy of tying the two projects together—thereby combining the project that the business community supported with the project that they and the City of Boston supported..

Early planning
In addition to these political and financial difficulties, the project faced several environmental and engineering obstacles.
The downtown area through which the tunnels were to be dug was largely landfill, and included existing subway lines as well as innumerable pipes and utility lines that would have to be replaced or moved. Tunnel workers encountered many unexpected geological and archaeological barriers, ranging from glacial debris to foundations of buried houses and a number of sunken ships lying within the reclaimed land.
The project received approval from state environmental agencies in 1991, after satisfying concerns including release of toxins by the excavation and the possibility of disrupting the homes of millions of rats, causing them to roam the streets of Boston in search of new housing. By the time the federal environmental clearances were delivered in 1994, the process had taken some seven years, during which time inflation greatly increased the project's original cost estimates.
Reworking such a busy corridor without seriously restricting traffic flow required a number of state-of-the-art construction techniques. Because the old elevated highway (which remained in operation throughout the construction process) rested on pylons located throughout the designated dig area, engineers first utilized slurry wall techniques to create 120 ft.-deep concrete walls upon which the highway could rest. These concrete walls also stabilized the sides of the site, preventing cave-ins during the excavation process.
The multilane interstates also had to pass under South Station's 7 tracks which carried over 40,000 commuters and 400 trains per day. In order to avoid multiple relocations of the train lines while the tunnelling advanced, as had been initially planned, a specially designed jack was constructed in order to support the ground and tracks to allow the excavation to take place below. Ground freezing was also implemented in order to help stabilise the surrounding ground as the tunnel was excavated. This was the largest tunnelling project undertaken beneath railway lines anywhere in the world. The ground freezing enabled safer, more efficient excavation, and also assisted in environmental issues, as less contaminated fill needed to be exported than if a traditional cut and cover method had been applied.
Other challenges included an existing subway tunnel crossing the path of the underground highway. In order to build slurry walls past this tunnel, it was necessary to dig beneath the tunnel and build an underground concrete bridge to support the tunnel's weight.

Obstacles
The Central Artery/Tunnel Project was managed by the Massachusetts Turnpike Authority with design and construction supervised by a joint venture of Bechtel Corporation and Parsons Brinckerhoff. Due to the enormous size of the project—too large for any company to undertake alone—the design and construction of the Big Dig were broken up into dozens of smaller subprojects with well-defined interfaces between contractors. Major heavy-construction contractors on the project included Jay Cashman, Modern Continental, Obayashi Corporation, Perini Corporation, Peter Kiewit Sons' Incorporated, J.F. White, and the Slattery division of Skanska USA. (Of those, Modern Continental was awarded the greatest gross value of contracts, joint ventures included.)
The nature of the Charles River crossing had been a source of major controversy throughout the design phase of the project. Many environmental advocates preferred a river crossing entirely in tunnels, but this, along with 27 other plans, was rejected as too costly. Finally, with a deadline looming to begin construction on a separate project that would connect the Tobin Bridge to the Charles River crossing, Salvucci overrode the objections and chose a variant of the plan known as "Scheme Z". This plan was considered to be reasonably cost-effective, but had the drawback of requiring highway ramps stacked up as high as 100 feet (30 m) immediately adjacent to the Charles River. The city of Cambridge objected to the visual impact of the chosen Charles River crossing design. It sued to revoke the project's environmental certificate and forced the project to redesign the river crossing again. Meanwhile, construction continued on the Tobin Bridge approach. By the time all parties agreed on the I-93 design, construction of the Tobin connector (today known as the "City Square Tunnel" for a Charlestown area it bypasses) was far along, significantly adding to the cost of constructing the U.S. Route 1 interchange and retrofitting the tunnel.
Boston blue clay and other soils extracted from the path of the tunnel were used to cap many local landfills, fill in the Granite Rail Quarry in Quincy, and restore the surface of Spectacle Island in the Boston Harbor Islands National Recreation Area.
The Zakim Bunker Hill Bridge, designed by Swiss designer Christian Menn, is the terminus of the project, connecting the underground highway with I-93 and US 1. The distinctive cable-stayed bridge is supported by two forked towers connected to the span by cables and girders.
The Leverett Circle Connector, a companion bridge to the Zakim, began carrying traffic from I-93 to Storrow Drive in 1999. The project had been under consideration for years, but was opposed by the wealthy residents of the Beacon Hill neighborhood. However, it finally was accepted because it would funnel traffic bound for Storrow Drive and downtown Boston away from the mainline roadway. The Connector ultimately used a pair of ramps that had been constructed for Interstate 695, enabling the mainline I-93 to carry more traffic that would have used I-695 under the original Master Plan.
When construction began, the project cost, including the Charles River crossing, was estimated at $5.8 billion. Eventual cost overruns were so high that the chairman of the Massachusetts Turnpike Authority, James Kerasiotes, was fired in 2000. His replacement had to commit to an $8.55 billion cap on federal contributions. Total expenses eventually passed $15 billion.

Construction phase
On January 17, 2003, the opening ceremony was held for the I-90 Connector Tunnel, extending the Massachusetts Turnpike (Interstate 90) east into the Ted Williams Tunnel, and onwards to Logan Airport. (The Williams tunnel had been completed and in limited use for commercial traffic and high-occupancy vehicles since late 1995.) The westbound lanes opened on the afternoon of January 18 and the eastbound lanes on January 19.
The next phase, moving the elevated Interstate 93 underground, was completed in two stages: northbound lanes opened in March 2003 and southbound lanes (in a temporary configuration) on December 20, 2003. A tunnel underneath Leverett Circle connecting eastbound Storrow Drive to I-93 North and the Tobin Bridge opened December 19, 2004, easing congestion at the circle. All southbound lanes of I-93 opened to traffic on March 5, 2005, including the left lane of the Zakim Bridge, and all of the refurbished Dewey Square Tunnel.
By the end of December 2004, 95% of the Big Dig was completed. Major construction remained on the surface, including construction of final ramp configurations in the North End and in the South Bay interchange, and reconstruction of the surface streets. Many impact-mitigation projects (transit, pedestrian, bicycle, and parks) also remain, but some are in danger of cancellation due to cost overruns on the rest of the project.
The final ramp downtown—exit 20B from I-93 south to Albany Street—opened January 13, 2006.
In 2006, the two Interstate 93 tunnels were dedicated as the Thomas P. O'Neill Jr. Tunnel, after the former Democratic speaker of the House of Representatives from Massachusetts who pushed to have the Big Dig funded by the federal government.

Final phases

"Thousands of leaks"
Massachusetts State Police searched the offices of Aggregate Industries, the largest concrete supplier for the underground portions of the project, in June 2005. They seized evidence of faked records that hid the poor quality of concrete delivered for the highway project. In May 2006, six executives of the company, including its general manager, were arrested and charged with crimes related to fraud. Immediately after the arrests, Massachusetts Governor Mitt Romney announced he would return $3,900 in political contributions from employees of Aggregate Industries.

Fatal ceiling collapse