Editing Big Dig (section)

== Obstacles ==
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. In 1993, the North End Waterfront Central Artery Committee (NEWCAC) created, co-founded by Nancy Caruso, representing residents, businesses, and institutions in the North End and Waterfront neighborhoods of Boston. The NEWCAC Committee's goal included lessening the impact of the Central Artery/Tunnel Project on the community, representing the neighborhoods to government agencies, keeping the community informed, developing a list of priorities of immediate neighborhood concerns, and promoting responsible and appropriate development of the post-construction artery corridor in the North End and Waterfront neighborhoods.

The political, financial and residential obstacles were magnified when several environmental and engineering obstacles occurred. The downtown area through which the tunnels were to be dug was largely [[Land reclamation|land fill]], and included existing [[Red Line (MBTA)|Red Line]] and [[Blue Line (MBTA)|Blue Line]] [[rapid transit|subway]] tunnels 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 [[rat]]s, causing them to roam the streets of Boston in search of new housing. By the time the federal environmental clearances were delivered in 1994,<ref name="Palmer">{{Cite news|first=Thomas |last=Palmer |url=http://libraries.mit.edu/rotch/artery/boston_globe.htm#wins |title=State Wins Federal OK to Start River-Crossing Part of Artery Project |work=[[The Boston Globe]] |date=June 21, 1994 |access-date=December 10, 2007 |archive-url=https://web.archive.org/web/20071105064639/http://libraries.mit.edu/rotch/artery/boston_globe.htm#wins |archive-date=November 5, 2007 |url-status=dead }}</ref> the process had taken some seven years, during which time inflation greatly increased the project's original cost estimates.<ref>{{Cite journal|last=Haynes|first=Wendy|date=June 2008|title=Boston's Big Dig Project: A Cautionary Tale|url=http://paperity.org/p/81488266/bridgewater-review-vol-27-no-1-june-2008|journal=Bridgewater Review |volume=27|issue=1|pages=3–7|url-status=live|archive-url=https://web.archive.org/web/20180317035739/http://paperity.org/p/81488266/bridgewater-review-vol-27-no-1-june-2008|archive-date=March 17, 2018}}</ref>

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 {{convert|120|ft|m|adj=mid|-deep}} concrete walls upon which the highway could rest. These concrete walls also stabilized the sides of the site, preventing cave-ins during the continued excavation process.

The multi-lane Interstate highway also had to pass under [[South Station (MBTA)|South Station]]'s seven railroad tracks, which carried over 40,000 commuters and 400 trains per day. To avoid multiple relocations of train lines while the tunneling advanced, as had been initially planned, a specially designed [[jack (device)|jack]] was constructed to support the ground and tracks to allow the excavation to take place below. Construction crews also used [[ground freezing]] (an artificial induction of [[permafrost]]) to help stabilize surrounding ground as they excavated the tunnel. This was the largest tunneling project undertaken beneath railroad lines anywhere in the world. The ground freezing enabled safer, more efficient [[Earthworks (engineering)|excavation]], and also assisted in environmental issues, as less contaminated fill needed to be exported than if a traditional [[Cut-and-cover tunnel|cut-and-cover method]] had been applied.<ref>{{cite web|url=http://tunnels.mottmac.com/projects/?mode=region&amp;id=3418 |title=Boston Central Artery Jacked Tunnels – Mott MacDonald Project Page |publisher=Tunnels.mottmac.com |access-date=April 8, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140408221143/https://tunnels.mottmac.com/projects/?mode=region&amp;id=3418 |archive-date=April 8, 2014 }}</ref>

Other challenges included existing subway tunnels crossing the path of the underground highway. To build slurry walls past these tunnels, it was necessary to dig beneath the tunnels and to build an underground concrete bridge to support the tunnels' weight, without interrupting rail service.