Editing Big Dig (section)

== Construction phase ==
[[File:Boston-big-dig-area.png|thumb|Construction sites of the "Big Dig"]]

The project was managed by the Massachusetts Turnpike Authority, with the Big Dig and the Turnpike's Boston Extension from the 1960s being financially and legally joined by the legislature as the [[Metropolitan Highway System]].<ref>{{cite web |url=http://www.mass.gov/legis/laws/mgl/81a-3.htm |title=MGL Chapter 81A, Section 3 |publisher=Mass.gov |access-date=April 8, 2014 |url-status=live |archive-url=https://web.archive.org/web/20100726222848/http://www.mass.gov//legis/laws/mgl/81a-3.htm |archive-date=July 26, 2010 }}</ref> Design and construction was supervised by a joint venture of [[Bechtel Corporation]] and [[Parsons Brinckerhoff]]. 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. Major heavy-construction contractors on the project included [[Jay Cashman]], [[Modern Continental]], [[Obayashi Corporation]], [[Perini Building Company|Perini Corporation]], [[Peter Kiewit Sons' Incorporated]], [[J. F. White Contracting Co.|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.

[[File:Boston_Central_Artery_(Big_Dig)_Construction_Contract_Progress_Map_-_Janaury_2004.jpg|right|thumb|275x275px|Construction Status by contract number as of January 1, 2004]]
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. 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 {{convert|100|ft|m}} immediately adjacent to the Charles River.<ref name="bg-scheme z">{{Cite news|title=Caution: High bridge ahead; Artery plan includes an awful scheme for a great wall across the Charles |last=Campbell |first=Robert |date=June 24, 1990 |work= [[The Boston Globe]] |id= Accessed August 4, 2009 |quote= The grotesque monstrosity that will be the new Central Artery requires a certain power of imagination to be grasped. Maybe that's why people are only now beginning to get upset about it.}}</ref>

The city of [[Cambridge, Massachusetts|Cambridge]] objected to the visual impact of the chosen Charles River crossing design. The city sued to revoke the project's environmental certificate and forced the project planners to redesign the river crossing again.<ref name="bg-cambridge">{{Cite news |title= Cambridge sues to halt bridge plan |last=Palmer Jr |first=Thomas C. |date= March 12, 1995 |work= [[The Boston Globe]] |id= Accessed August 4, 2009}}</ref>

[[File:ZakimBridge20040307.jpg|right|thumb|300px|The [[Leonard P. Zakim Bunker Hill Memorial Bridge|Leonard P. Zakim Bridge]]]]
Swiss engineer [[Christian Menn]] took over the design of the bridge. He suggested a cradle [[cable-stayed bridge]] that would carry ten lanes of traffic. The plan was accepted and construction began on the [[Leonard P. Zakim Bunker Hill Memorial Bridge]]. The bridge employed an asymmetrical design and a hybrid of steel and concrete was used to construct it. The distinctive bridge is supported by two forked towers connected to the span by cables and girders. It was the first bridge in the country to employ this method and it was, at the time, the widest cable-stayed bridge in the world,<ref name="CEG" /> having since been surpassed by the [[Eastern span replacement of the San Francisco–Oakland Bay Bridge]].

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, Boston, Massachusetts|Charlestown]] area it bypasses, was far along, significantly adding to the cost of constructing the [[U.S. Route 1 in Massachusetts|US Route 1]] interchange and retrofitting the tunnel. <!--Shift to active voice shortened this sentence a little, but it's still unclear whether the Rt1 work and tunnel retrofit were built, or were abandoned because of the increased cost.-->

Boston blue [[clay]] and other soils extracted from the path of the tunnel were used to cap many local [[landfill]]s, fill in the [[Quincy Quarries Reservation|Granite Rail Quarry]] in [[Quincy, Massachusetts|Quincy]], and restore the surface of [[Spectacle Island (Massachusetts)|Spectacle Island]] in the [[Boston Harbor Islands National Recreation Area]].

The [[Storrow Drive 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, Boston|Beacon Hill]] neighborhood. It was finally accepted because it would funnel traffic bound for Storrow Drive and downtown Boston away from the mainline roadway.<ref>{{cite web| url=http://www.bostonroads.com/crossings/zakim/ | title=Leonard P. Zakim-Bunker Hill Memorial Bridge | access-date=July 18, 2006 | publisher=BostonRoads.com| archive-url= https://web.archive.org/web/20060626115922/http://www.bostonroads.com/crossings/zakim/| archive-date= June 26, 2006 | url-status= live}}</ref> The Connector ultimately used a pair of ramps that had been constructed for [[Interstate 695 (Massachusetts)|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. The total expenses eventually passed $15 billion. Interest brought this cost to $21.93 billion.

=== Engineering methods and details ===
[[File:BigDigSupportsCentralArtery.agr.jpg|thumb|Temporary supports hold up the elevated Central Artery during construction.]]
Several unusual engineering challenges arose during the project, requiring unusual solutions and methods to address them. At the beginning of the project, engineers had to figure out the safest way to build the tunnel without endangering the existing elevated highway above. Eventually, they created horizontal braces as wide as the tunnel, then cut away the elevated highway's struts, and lowered it onto the new braces.<ref name="AP 122507">{{Cite news |url=https://news.yahoo.com/s/ap/20071225/ap_on_re_us/big_dig_the_end |archive-url=https://web.archive.org/web/20071228035119/http://news.yahoo.com/s/ap/20071225/ap_on_re_us/big_dig_the_end |archive-date=December 28, 2007 |title=Boston's $14.8B Big Dig finally complete |first=Steve |last=LeBlanc |agency=[[Associated Press]] |date=December 25, 2007}}</ref>

Three alternative construction methods were studied with their corresponding structural design to address existing conditions, safety measures, and constructability. In addition to codified loads, construction loads were computed to support final design and field execution.<ref>{{citation |author=Chalhoub Michel Soto |title=Construction Method and Supporting Structural Design for the Boston Area Central Artery Project |date=Aug 1999 |publisher=report#005081999 Integrated Services-CEM Eng Lib}}</ref>