The Big Dig was the most expensive highway project in the United States, and was plagued by cost overruns, delays, leaks, design flaws, accusations of poor execution and use of substandard materials, criminal charges and arrests, and the death of one motorist.

[8] Commissioner of Public Works William Callahan promoted plans for the Central Artery, an elevated expressway which eventually was constructed between the downtown area and the waterfront.

Governor John Volpe interceded in the 1950s to change the design of the last section of the Central Artery, putting it underground through the Dewey Square Tunnel.

There was chronic congestion on the Central Artery (I-93), the elevated six-lane highway through the center of downtown Boston, which was, in the words of Pete Sigmund, "like a funnel full of slowly-moving, or stopped, cars (and swearing motorists).

[11] MIT engineers Bill Reynolds and (eventual state Secretary of Transportation) Frederick P. Salvucci envisioned moving the whole expressway underground.

[12][13] Another important motivation for the final form of the Big Dig was the abandonment of the Massachusetts Department of Public Works' intended expressway system through and around Boston.

The Sargent moratorium led to the rerouting of I-95 away from Boston around the Route 128 beltway and the conversion of the cleared land in the southern part of the city into the Southwest Corridor linear park, as well as a new right-of-way for the Orange Line subway and Amtrak.

The most expensive was the building of the Phase II Silver Line tunnel under Fort Point Channel, done in coordination with Big Dig construction.

After years of extensive lobbying for federal dollars, a 1987 public works bill appropriating funding for the Big Dig was passed by the US Congress, but it was vetoed by President Ronald Reagan for being too expensive.

[24] In addition to political and financial difficulties, the project received resistance from residents of Boston's historic North End, who in the 1950s had seen 20% of the neighborhood's businesses displaced by development of the Central Artery.

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.

To avoid multiple relocations of train lines while the tunneling advanced, as had been initially planned, a specially designed jack was constructed to support the ground and tracks to allow the excavation to take place below.

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.

Because of the enormous size of the project—too large for any company to undertake alone—the design and construction of the Big Dig was broken up into dozens of smaller subprojects with well-defined interfaces between contractors.

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.

Secretary of Transportation Fred Salvucci signed an agreement with the Conservation Law Foundation in 1990 enumerating 14 specific projects the state agreed to build.

Permanent public art that was funded includes: super graphic text and facades of former West End houses cast into the concrete elevated highway abutment support walls near North Station by artist Sheila Levrant de Bretteville; Harbor Fog, a sensor-activated mist, light and sound sculptural environment by artist Ross Miller in parcel 17; a historical sculpture celebrating the 18th and 19th century shipbuilding industry and a bust of shipbuilder Donald McKay in East Boston; blue interior lighting of the Zakim Bridge; and the Miller's River Littoral Way walkway and lighting under the loop ramps north of the Charles River.

[49] Additionally, as a result of the Big Dig, a large amount of waterfront space was opened up, which is now a high-rent residential and commercial area called the Seaport District.

Continuous video surveillance is provided by hundreds of cameras, and thousands of sensors monitor traffic speed and density, air quality, water levels, temperatures, equipment status, and other conditions inside the tunnel.

This information was not made public until engineers at MIT (volunteer students and professors) performed several experiments and found serious problems with the tunnel.

[60] In July 2009, two of the accused, Gerard McNally and Keith Thomas, both managers, pled guilty to charges of conspiracy, mail fraud, and filing false reports.

[61] The following month, the remaining four, Robert Prosperi, Mark Blais, Gregory Stevenson, and John Farrar, were found guilty on conspiracy and fraud charges.

[64] Immediately following the fatal ceiling collapse, Governor Mitt Romney ordered a "stem-to-stern" safety audit conducted by the engineering firm of Wiss, Janney, Elstner Associates, Inc. to look for additional areas of risk.

[69] The final piece of the road network, a high occupancy vehicle lane connecting Interstate 93 north to the Ted Williams Tunnel, reopened on June 1, 2007.

On July 10, 2007, after a lengthy investigation, the National Transportation Safety Board found that epoxy glue used to hold the roof in place during construction was not appropriate for long-term bonding.

After an eighth reported death involving the safety handrails, MassDOT officials announced plans to cover or remove the allegedly dangerous fixtures, but only near curves or exit ramps.

Further investigation revealed that the fixture's mounting apparatus had failed, due to galvanic corrosion of incompatible metals, caused by having aluminum in direct contact with stainless steel, in the presence of salt water.

[57][78] The electrochemical potential difference between stainless steel and aluminum is in the range of 0.5 to 1.0V, depending on the exact alloys involved, and can cause considerable corrosion within months under unfavorable conditions.