The project was conceived in the 1970s by the Boston Transportation Planning Review to replace the rusting six-lane elevated central arterial. The highway separated the center from the coast and was increasingly congested. Business owners were most concerned about access to Logan Airport and advocated for a third tunnel into the port.

Planning for the project officially began in 1982 and environmental impact studies in 1983. After years of lobbying for federal funding, Congress passed a public works law funding the Big Dig in 1987, but President Ronald Reagan vetoed it for being too expensive. When Congress overrode the veto, the project got the green light and the first stone was laid in 1991.[21]​.

In 1997, the state legislature created the Metropolitan Highway System and transferred responsibility for the central artery and "CA/T" tunnel project from the Massachusetts Department of Highways and the Governor's Office to the Massachusetts Thruway Authority (MTA).[22][23]The MTA, which had little experience managing a company of the scope and magnitude of the CA/T project, hired a joint venture to perform preliminary designs, manage design consultants and construction contractors, track project cost and schedule, advise the MTA on project decisions, and (in some cases) act as a representative of the MTA. Over time, the MTA combined some of its employees with those of the joint venture into an integrated project organization. This was intended to increase management effectiveness, but hampered the MTA's ability to independently monitor project activities since the MTA and the joint venture had become partners.[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 the Central Artery development. In 1993, the North End Waterfront Central Artery Committee (NEWCAC), co-founded by Nancy Caruso, was created to represent the residents, businesses and institutions of Boston's North End and Waterfront neighborhoods. The goal of the NEWCAC Committee was to reduce the impact of the Central Artery/Tunnel project on the community, represent neighborhoods with government agencies, keep the community informed, develop a priority list of immediate neighborhood concerns, and promote responsible and appropriate development of the post-construction arterial corridor in the North End (Boston) and Waterfront neighborhoods. "Waterfront–SEU (Washington Metro)").

Political, financial and residential obstacles were magnified when several environmental and engineering hurdles emerged. The area of ​​the city center through which the tunnels had to be excavated was largely fill terrain, and included existing Red Line and Blue Line subway tunnels, as well as countless pipes and utility lines that would have to be replaced or moved. The tunnel workers encountered numerous unexpected geological and archaeological obstacles, from glacial debris to buried house foundations to several sunken ships lying in the reclaimed land.

The project received approval from state environmental agencies in 1991, after addressing concerns such as the release of toxins from the excavation and the possibility of disturbing the homes of millions of rats, causing them to roam the streets of Boston in search of new homes. When federal environmental authorizations were obtained in 1994,[25]​ the process had lasted about seven years, during which inflation significantly increased the project's original cost estimates.[26]​.

Redeveloping such a busy corridor without severely restricting traffic flow required a number of cutting-edge construction techniques. Since the old elevated highway (which remained in operation throughout the construction process) rested on pylons located throughout the designated excavation area, engineers first used diaphragm wall techniques to create 37 m deep concrete walls on which the highway could rest. These concrete walls also stabilized the sides of the work, preventing collapses during the excavation process.

The multi-lane interstate highway also had to pass under South Station's seven railroad tracks, which served more than 40,000 travelers and 400 trains a day. To avoid multiple relocations of the train lines while tunnel excavation progressed, as initially planned, a specially designed jack was constructed to support the ground and tracks and allow excavation underneath. Construction crews also used ground freezing (an artificial induction of permafrost) to help stabilize the surrounding terrain while excavating the tunnel. It is the largest tunnel construction project under railway tracks in the world. Freezing the ground allowed for safer and more efficient excavation, and also helped with environmental issues, as less contaminated fill had to be exported than if a traditional cut and cover method had been applied.[27]​.

Other problems were the subway tunnels that crossed the route of the underground highway. To build diaphragm walls beyond these tunnels, it was necessary to excavate beneath the tunnels and build an underground concrete bridge to support the weight of the tunnels, without interrupting rail service.

La gestión del proyecto corrió a cargo de la Autoridad de Autopistas de Massachusetts (Massachusetts Turnpike Authority), que unió financiera y jurídicamente la Big Dig y la ampliación de Boston de la Turnpike de los años 60 para crear el Sistema Metropolitano de Autopistas (Metropolitan Highway System)[28]​El diseño y la construcción fueron supervisados por una empresa conjunta de Bechtel Corporation y Parsons Brinckerhoff. Debido a la enorme envergadura del proyecto -demasiado grande para que cualquier empresa lo acometiera sola-, el diseño y la construcción del Big Dig se dividieron en docenas de subproyectos más pequeños con interfaces bien definidas entre los contratistas. Entre los principales contratistas de construcción pesada del proyecto se encontraban Jay Cashman, Modern Continental, Obayashi Corporation, Perini Corporation, Peter Kiewit Sons' Incorporated, J. F. White y la división Slattery de Skanska USA. (De todas ellas, Modern Continental fue la adjudicataria del mayor valor bruto de los contratos, empresas conjuntas incluidas).

La naturaleza del cruce del río Charles había sido fuente de gran controversia durante toda la fase de diseño del proyecto. Muchos defensores del medio ambiente preferían un cruce del río totalmente en túnel, pero éste, junto con otros 27 planes, fue rechazado por ser demasiado costoso. Finalmente, con el plazo inminente para iniciar la construcción de un proyecto separado que conectaría el puente Tobin con el cruce del río Charles, Salvucci hizo caso omiso de las objeciones y optó por una variante del plan conocida como "Esquema Z". Este plan se consideró razonablemente rentable, pero tenía el inconveniente de requerir rampas de autopista apiladas de hasta 100 pies (30 m) de altura inmediatamente adyacentes al río Charles.[29]​.

La ciudad de Cambridge se opuso al impacto visual del diseño elegido para cruzar el río Charles. La ciudad interpuso una demanda para revocar el certificado medioambiental del proyecto y obligó a los planificadores a rediseñar de nuevo el cruce del río.[30]​.

El ingeniero suizo Christian Menn se hizo cargo del diseño del puente. Sugirió un puente atirantado de cuna que soportaría diez carriles de tráfico. El plan fue aceptado y comenzó la construcción del Leonard P. Zakim Bunker Hill Memorial Bridge. El puente tiene un diseño asimétrico y para su construcción se utilizó un híbrido de acero y hormigón. El característico puente, diseñado por el arquitecto Miguel Ángel Rosales,[31]​[32]​[33]​ está sostenido por dos torres bifurcadas unidas al vano por cables y vigas. Fue el primer puente del país en emplear este método y, en su momento, fue el puente atirantado más ancho del mundo,[9]​habiendo sido superado desde entonces por la sustitución del vano este del puente de la bahía de San Francisco-Oakland.

Mientras tanto, proseguía la construcción del acceso al puente Tobin. Para cuando todas las partes se pusieron de acuerdo sobre el diseño de la I-93, la construcción del conector Tobin (hoy conocido como "City Square Tunnel" por la zona de Charlestown que rodea) estaba muy avanzada, lo que aumentó considerablemente el coste de construcción del enlace de la US Route 1 y la adaptación del túnel.

La arcilla azul de Boston y otras tierras extraídas del trazado del túnel se utilizaron para tapar muchos vertederos locales, rellenar la cantera Granite Rail Quarry de Quincy y restaurar la superficie de Spectacle Island, en el Área Recreativa Nacional de las Islas del Puerto de Boston.

El Storrow Drive Connector, puente complementario del Zakim, empezó a transportar el tráfico de la I-93 a Storrow Drive en 1999. El proyecto se había estado estudiando durante años, pero contó con la oposición de los residentes adinerados del barrio de Beacon Hill. Al final, el conector utilizó un par de rampas que se habían construido para la interestatal 695,[34]​ lo que permitió que la I-93 principal soportara más tráfico del que habría utilizado la I-695 según el plan director original.

Cuando se inició la construcción, el coste del proyecto, incluido el cruce del río Charles, se estimó en 5.800 millones de dólares. Los sobrecostes fueron tan elevados que el presidente de la Massachusetts Turnpike Authority, James Kerasiotes, fue despedido en 2000. Su sustituto tuvo que comprometerse a limitar las aportaciones federales a 8.550 millones de dólares. El gasto total acabó superando los 15.000 millones de dólares. Los intereses elevaron este coste a 21.930 millones de dólares.

Engineering Methods and Details

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 find the safest way to build the tunnel without endangering the existing elevated highway. Finally, they created horizontal stays as wide as the tunnel, cut the struts from the elevated highway, and lowered it onto the new stays.[35] Three alternative construction methods and corresponding structural design were studied to address existing conditions, safety measures, and constructability. In addition to the coded loads, construction loads were calculated to support final design and field execution.[36]​.

On January 18, 2003, the groundbreaking ceremony was held for the I-90 Connector Tunnel, which extends the Massachusetts Turnpike (Interstate 90) east to the Ted Williams Tunnel and then to Boston Logan International Airport. The Ted Williams Tunnel had been completed and was 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, the burying of the elevated Interstate 93, was completed in two stages: the northbound lanes opened on March 29, 2003 and the southbound lanes (in an interim configuration) on December 20, 2003. A tunnel under Leverett Circle connecting eastbound Storrow Drive to I-93 North and the Tobin Bridge opened on December 19, 2004, thus easing congestion at the roundabout. All southbound lanes of I-93 opened to traffic on March 5, 2005, including the left lane of the Zakim Bridge and the entire renovated Dewey Square tunnel.

By the end of December 2004, 95% of the Big Dig had been completed. Major surface work remained, including construction of final ramp configurations at the North End and South Bay Interchange, and reconstruction of surface streets.

The final downtown ramp, exit 16A (formerly 20B) from I-93 south to Albany Street, opened on January 13, 2006.[37]​.

In 2006, the two tunnels on Interstate 93 were named the Thomas P. O'Neill Jr. Tunnel, in honor of the former Democratic speaker of the Massachusetts House of Representatives who pushed for the Big Dig to be funded by the federal government.

La Commonwealth de Massachusetts estaba obligada por la Ley Federal de Aire Limpio a mitigar la contaminación atmosférica generada por las mejoras de la autopista. El Secretario de Transportes, Fred Salvucci, firmó en 1990 un acuerdo con la Conservation Law Foundation en el que se enumeraban 14 proyectos concretos que el Estado se comprometía a construir. Esta lista se ratificó en un acuerdo judicial de 1992.[38]​.

Entre los proyectos que se han llevado a cabo figuran:[38]​[39]​.

Sin embargo, se eliminaron algunos proyectos.

Surface treatments

Some surface treatments that were part of the original project plan were eliminated due to the enormous cost overruns of the highway portion.[42]​.

$99.1 million was allocated for remedial improvements[43]​ in the Charles River watershed, including the construction of North Point Park in Cambridge and Paul Revere Park in Charlestown.[43]​ The North Bank Bridge, which provides pedestrian and bicycle connectivity between the parks, was not funded until the Recovery and Reinvestment Act of 2009. Nashua Street Park, on the Boston side, was completed in 2003, by McCourt Construction, with $7.9 million in funding from MassDOT.[44] As of 2017, $30.5 million had been transferred to the Massachusetts Department of Conservation and Recreation to complete five projects. Another incomplete but necessary project is the South Bank Bridge over the MBTA commuter rail tracks at North Station (connecting Nashua Street Park with the proposed South Bank Park, which is currently a parking lot under the Zakim Bridge at the Charles River Locks).[45]

Improvements in the lower Charles River include the new walkway at Lovejoy Wharf (built by the developer of 160 North Washington Street, Converse's new headquarters), the Lynch Family Skate Park (built in 2015 by the Charles River Conservancy), the rehabilitation of historic operations buildings for the Charles River Dam and Lock, a maintenance facility, and a planned pedestrian walkway across the Charles River next to the MBTA commuter rail drawbridge at North Station (connecting Nashua Street Park and North Point Park).[43]​MassDOT funds South Bank Park,[43]​and the replacement of the North Washington Street Bridge (construction between August 2018 and August 23).[46]​EF Education funds public green space improvements as part of its three-phase expansion in North Point.[47]​Remaining funding can be used to build the North Point Inlet Pedestrian Bridge and a pedestrian walkway over Leverett Circle. Before being replaced by surface access during the reconstruction of the Science Park MBTA Green Line station, Leverett Circle had pedestrian bridges with stairs that provided elevated access between the station, Charles River parks, and the sidewalk to the Boston Museum of Science. Replacement ramps would meet the requirements of the Americans with Disabilities Act and facilitate access to the station.[43]​.

public art

Although not a legal requirement, public art was part of the urban design planning process (and later design development work) through the Arteries Art Program. The objective of the program was to integrate public art into road infrastructure (retaining walls, fences and lighting) and into the essential elements of the pedestrian environment (walkways, landscape elements of parks and bridges). As the overall costs of the project increased, the Artery Arts Program was seen as a potential liability, even though it had the support and interest of the public and professional arts organizations in the area.

Early in the highway design process, a temporary art program was initiated and more than 50 proposals were selected. However, only a few projects began development before the program's funding was cut. Permanent public art funded includes supergraphic text and facades of old West End houses cast into the supporting concrete walls of the elevated freeway abutments near North Station, by Bretteville artist Sheila Levrant; Harbor Fog, a sensor-activated sculptural environment of fog, light and sound, by artist Ross Miller on plot 17; a historic sculpture celebrating the 20th 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.

During public meetings, many citizens requested extensive landscape planting as well as a planting maintenance program.

The Big Dig separated mixed traffic from the Massachusetts Turnpike and the Sumner and Callahan tunnels. Although only one net lane was added in each direction to I-93 north-south, several new east-west lanes were enabled. Massachusetts Turnpike/I-90 east-west traffic now passes directly through the Ted Williams Tunnel toward Logan Airport and Route 1A beyond. Traffic between Storrow Drive and the Callahan and Sumner tunnels continues to use a short stretch of I-93, but additional lanes and direct connections have been made available for this traffic.

The result was a 62% reduction in vehicle travel hours on I-93, the airport tunnels, and the connection from Storrow Drive, from an average of 38,200 hours per day before construction (1994-1995) to 14,800 hours per day in 2004-2005, once the project was largely completed.[48] Savings to travelers were estimated at 166 million dollars annually in the same period of 2004-2005.[49]​ Travel times on the northbound central artery during the afternoon rush hour were reduced by 85.6%.[50]​.

A 2008 Boston Globe report stated that wait times for most trips actually increased as a result of demand induced by increased highway capacity. As more drivers chose to use the new roads, traffic jams only moved out of the city, not reduced or eliminated (although some trips are now faster). The report states: "Ultimately, many motorists traveling to and from the suburbs at rush hour spend more time stuck in traffic, not less." The Globe also says its analysis provides a more complete picture of the traffic situation than a state-commissioned study conducted two years earlier, which credited the Big Dig with helping save at least $167 million a year by increasing economic productivity and lowering motor vehicle operating costs. That study did not analyze highways outside the Big Dig construction zone and did not take into account new traffic jams elsewhere.[51]

Towards the end of the Big Dig in 2003, it was estimated that the demolition of the Central Artery freeway would cause a $732 million increase in property values ​​in Boston's financial district, with the replacement parks providing an additional $252 million in value.[52] 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. It is estimated that the Seaport development alone generated $7 billion in private investment and 43,000 jobs.[53]​.

As part of the project, an elaborate Operations Control Center (OCC) control room was built in South Boston. Staffed 24/7/365, this center monitors, reports on traffic congestion and responds to emergencies. Hundreds of cameras continuously monitor video and thousands of sensors monitor traffic speed and density, air quality, water levels, temperature, equipment status and other conditions inside the tunnel. The OCC can activate emergency ventilators, change electronic signs, and dispatch service teams when necessary.[54][55][56]​.

Leaks

As early as 2001, Turnpike Authority officials and contractors were aware of thousands of leaks from roof and wall cracks, extensive water damage to steel supports and fireproofing systems, and overloaded drainage systems.[57] Many of the leaks were caused by Modern Continental and other subcontractors' failure to remove gravel and other debris before pouring concrete. This information was not made public until MIT engineers (students and faculty volunteers) conducted several experiments and discovered serious problems in the tunnel.[58]​.

On September 15, 2004, a major leak in the north tunnel of Interstate 93 forced the tunnel to close while repairs were carried out. This also forced the Turnpike Authority to release information about its failure to disclose previous leaks. A follow-up report reported "extensive" leaks that were more serious than state authorities had previously acknowledged. The report stated that the tunnel system had more than 400 leaks. However, a Boston Globe report disputed this claim, stating that there were almost 700 leaks in a single 300 m section of tunnel under South Station. Turnpike officials also stated that the number of leaks investigated had dropped from 1,000 to 500.[58]

The problem of leaks is further aggravated by the fact that many of them involve corrosive salt water. This is due to the proximity of Boston Harbor and the Atlantic Ocean, which causes a mix of salt and fresh water leaks into the tunnel. The situation is made worse by road salt spread in the tunnel to melt ice during frost, or introduced by vehicles passing through it.[59]​ Salt water and salt spray are well-known problems that must be resolved in any marine environment. It has been reported that "hundreds of thousands of gallons of salt water are pumped monthly" into the Big Dig, and a map has been prepared showing "hot spots" where water leaks are especially severe.[60] Salt-accelerated corrosion has caused failures in ceiling light fixtures (see below), but can also cause rapid deterioration of embedded rebar and other steel structural reinforcements holding the walls and roof of the Big Dig. tunnel.[59]​.

Poor materials

In June 2005, Massachusetts State Police raided the offices of Aggregate Industries, the main supplier of concrete for the underground portions of the project. They seized evidence that Aggregate "Aggregates (West Virginia)") had supplied concrete that did not meet contract specifications. In March 2006, Massachusetts Attorney General Tom Reilly announced plans to sue the project's contractors and other companies for poor work performed on the project. The state of Massachusetts filed more than 200 complaints regarding leaks, cost overruns, quality issues, and safety violations. In total, the State has claimed about 100 million dollars from contractors (1 dollar for every 141 dollars spent).[61]​.

In May 2006, six company employees were arrested and charged with conspiracy to defraud the United States.[62] In July 2007, Aggregate Industries settled the case with a $50 million settlement. $42 million of the settlement went to civil cases and $8 million was paid in criminal fines. The company will provide $75 million in maintenance insurance and pay $500,000 for routine checks on areas suspected of containing substandard concrete.[63] In July 2009, two of the defendants, Gerard McNally and Keith Thomas, both directors, pleaded guilty to charges of conspiracy, mail fraud and filing false reports.[64] The following month, the remaining four, Robert Prosperi, Mark Blais, Gregory Stevenson and John Farrar, were found guilty of conspiracy and fraud charges.[65] All four were sentenced to probation and home confinement, and Blais and Farrar were also sentenced to community service.[66]

Deadly roof collapse

A fatal accident raised safety concerns and shut down part of the project for most of the summer of 2006. On July 10, 2006, concrete roof panels and debris, weighing 26 short tons (24 tons) and measuring 6.1 by 12.2 meters (20 by 40 feet), fell on a car traveling on the two-lane ramp connecting northbound I-93 to I-90 in eastbound in South Boston "South Boston (Virginia)"), killing Milena Del Valle, who was a passenger, and injuring her husband, Angel Del Valle, who was driving.[67]​Immediately after the fatal roof collapse, Governor Mitt Romney ordered a "top to bottom" safety audit by engineering firm Wiss, Janney, Elstner Associates, Inc. to look for other risk areas. Said Romney: "We simply cannot live in an environment in which a project of this magnitude can endanger human lives, as we have already seen."[68]​The collapse and closure of the tunnel greatly collapsed traffic in the city. The traffic jams are believed to have contributed to the death of another person, a heart attack victim who died on the way to Boston Medical Center when his ambulance became stuck in one of the traffic jams two weeks after the collapse. On September 1, 2006, one eastbound lane of the connector tunnel was reopened to traffic.[70][71]​

Following extensive inspections and repairs, the east and west lanes of Interstate 90 reopened in early January 2007.[72] The last piece of the highway 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 the epoxy glue used to hold the roof in place during construction was not suitable for long-term bonding,[73] which was determined to be the cause of the roof collapse. The Power-Fast epoxy adhesive used in the installation was designed for short-term loads, such as wind or seismic loads, not long-term loads, such as the weight of a panel.[74][75][76]​.

Powers Fasteners, the manufacturers of the adhesive, revised their product specifications on May 15, 2007 to increase the safety factor from 4 to 10 on all of their epoxy products intended for aerial applications. The safety factor of Power-Fast Epoxy was increased from 4 to 16.[76]​On December 24, 2007, the Del Valle family announced that it had reached a settlement with Powers Fasteners in which the family would be paid $6 million.[77]​In December 2008, Powers Fasteners agreed to pay $16 million to the state to resolve involuntary manslaughter charges.[78]​.

"Ginsu railings"

Public safety workers have dubbed the safety railings in the Big Dig tunnels "ginsu guardrails" because the square edges of the support posts have caused mutilations and deaths of passengers ejected from crashed vehicles. After an eighth death related to safety railings was reported, MassDOT officials announced plans to cover or remove the supposedly dangerous features, but only near curves or exit ramps.[79] This partial removal of hazards has been criticized by one safety specialist, who suggests that the guardrails are just as dangerous on straight sections of the tunnel.[79]​

Lightning

In March 2011, it emerged that senior MassDOT officials had failed to disclose a problem with the O'Neill Tunnel luminaires. In early February 2011, a maintenance crew found a light fixture lying in the center lane of the northbound tunnel. Assuming it was simply road debris, the maintenance crew picked it up and took it to its original facility. The next day, a supervisor passing by the shipyard realized that the 54kg lamp was not road debris, but one of those used to illuminate the tunnel. Subsequent investigation revealed that the luminaire's mounting apparatus had failed due to galvanic corrosion of incompatible metals, caused by direct contact of aluminum with stainless steel in the presence of salt water.[60]​[81]​The electrochemical potential difference between stainless steel and aluminum ranges between 0.5 and 1.0 V, depending on the alloy involved, and can cause considerable corrosion within months under unfavorable conditions.

Once the reason the fastener had failed was discovered, a thorough inspection of the other fasteners in the tunnel revealed that many other fasteners were also in the same state of disrepair.[82] Some of the worst fasteners were temporarily propped up with zip ties.[59] In moving forward with the temporary repairs, members of the MassDOT management team decided not to share the news of the systemic failure and repair of the fasteners with the public or Governor Deval Patrick's administration. fixings.[83]​.

In April 2012, it appeared that all 25,000 luminaires would need to be replaced, at an estimated cost of $54 million.[59] Replacement work was carried out primarily at night, requiring lane closures or occasional closure of the entire tunnel for safety, and was estimated to take up to two years to complete.[59]

The project was conceived in the 1970s by the Boston Transportation Planning Review to replace the rusting six-lane elevated central arterial. The highway separated the center from the coast and was increasingly congested. Business owners were most concerned about access to Logan Airport and advocated for a third tunnel into the port.

Planning for the project officially began in 1982 and environmental impact studies in 1983. After years of lobbying for federal funding, Congress passed a public works law funding the Big Dig in 1987, but President Ronald Reagan vetoed it for being too expensive. When Congress overrode the veto, the project got the green light and the first stone was laid in 1991.[21]​.

In 1997, the state legislature created the Metropolitan Highway System and transferred responsibility for the central artery and "CA/T" tunnel project from the Massachusetts Department of Highways and the Governor's Office to the Massachusetts Thruway Authority (MTA).[22][23]The MTA, which had little experience managing a company of the scope and magnitude of the CA/T project, hired a joint venture to perform preliminary designs, manage design consultants and construction contractors, track project cost and schedule, advise the MTA on project decisions, and (in some cases) act as a representative of the MTA. Over time, the MTA combined some of its employees with those of the joint venture into an integrated project organization. This was intended to increase management effectiveness, but hampered the MTA's ability to independently monitor project activities since the MTA and the joint venture had become partners.[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 the Central Artery development. In 1993, the North End Waterfront Central Artery Committee (NEWCAC), co-founded by Nancy Caruso, was created to represent the residents, businesses and institutions of Boston's North End and Waterfront neighborhoods. The goal of the NEWCAC Committee was to reduce the impact of the Central Artery/Tunnel project on the community, represent neighborhoods with government agencies, keep the community informed, develop a priority list of immediate neighborhood concerns, and promote responsible and appropriate development of the post-construction arterial corridor in the North End (Boston) and Waterfront neighborhoods. "Waterfront–SEU (Washington Metro)").

Political, financial and residential obstacles were magnified when several environmental and engineering hurdles emerged. The area of ​​the city center through which the tunnels had to be excavated was largely fill terrain, and included existing Red Line and Blue Line subway tunnels, as well as countless pipes and utility lines that would have to be replaced or moved. The tunnel workers encountered numerous unexpected geological and archaeological obstacles, from glacial debris to buried house foundations to several sunken ships lying in the reclaimed land.

The project received approval from state environmental agencies in 1991, after addressing concerns such as the release of toxins from the excavation and the possibility of disturbing the homes of millions of rats, causing them to roam the streets of Boston in search of new homes. When federal environmental authorizations were obtained in 1994,[25]​ the process had lasted about seven years, during which inflation significantly increased the project's original cost estimates.[26]​.

Redeveloping such a busy corridor without severely restricting traffic flow required a number of cutting-edge construction techniques. Since the old elevated highway (which remained in operation throughout the construction process) rested on pylons located throughout the designated excavation area, engineers first used diaphragm wall techniques to create 37 m deep concrete walls on which the highway could rest. These concrete walls also stabilized the sides of the work, preventing collapses during the excavation process.

The multi-lane interstate highway also had to pass under South Station's seven railroad tracks, which served more than 40,000 travelers and 400 trains a day. To avoid multiple relocations of the train lines while tunnel excavation progressed, as initially planned, a specially designed jack was constructed to support the ground and tracks and allow excavation underneath. Construction crews also used ground freezing (an artificial induction of permafrost) to help stabilize the surrounding terrain while excavating the tunnel. It is the largest tunnel construction project under railway tracks in the world. Freezing the ground allowed for safer and more efficient excavation, and also helped with environmental issues, as less contaminated fill had to be exported than if a traditional cut and cover method had been applied.[27]​.

Other problems were the subway tunnels that crossed the route of the underground highway. To build diaphragm walls beyond these tunnels, it was necessary to excavate beneath the tunnels and build an underground concrete bridge to support the weight of the tunnels, without interrupting rail service.

La gestión del proyecto corrió a cargo de la Autoridad de Autopistas de Massachusetts (Massachusetts Turnpike Authority), que unió financiera y jurídicamente la Big Dig y la ampliación de Boston de la Turnpike de los años 60 para crear el Sistema Metropolitano de Autopistas (Metropolitan Highway System)[28]​El diseño y la construcción fueron supervisados por una empresa conjunta de Bechtel Corporation y Parsons Brinckerhoff. Debido a la enorme envergadura del proyecto -demasiado grande para que cualquier empresa lo acometiera sola-, el diseño y la construcción del Big Dig se dividieron en docenas de subproyectos más pequeños con interfaces bien definidas entre los contratistas. Entre los principales contratistas de construcción pesada del proyecto se encontraban Jay Cashman, Modern Continental, Obayashi Corporation, Perini Corporation, Peter Kiewit Sons' Incorporated, J. F. White y la división Slattery de Skanska USA. (De todas ellas, Modern Continental fue la adjudicataria del mayor valor bruto de los contratos, empresas conjuntas incluidas).

La naturaleza del cruce del río Charles había sido fuente de gran controversia durante toda la fase de diseño del proyecto. Muchos defensores del medio ambiente preferían un cruce del río totalmente en túnel, pero éste, junto con otros 27 planes, fue rechazado por ser demasiado costoso. Finalmente, con el plazo inminente para iniciar la construcción de un proyecto separado que conectaría el puente Tobin con el cruce del río Charles, Salvucci hizo caso omiso de las objeciones y optó por una variante del plan conocida como "Esquema Z". Este plan se consideró razonablemente rentable, pero tenía el inconveniente de requerir rampas de autopista apiladas de hasta 100 pies (30 m) de altura inmediatamente adyacentes al río Charles.[29]​.

La ciudad de Cambridge se opuso al impacto visual del diseño elegido para cruzar el río Charles. La ciudad interpuso una demanda para revocar el certificado medioambiental del proyecto y obligó a los planificadores a rediseñar de nuevo el cruce del río.[30]​.

El ingeniero suizo Christian Menn se hizo cargo del diseño del puente. Sugirió un puente atirantado de cuna que soportaría diez carriles de tráfico. El plan fue aceptado y comenzó la construcción del Leonard P. Zakim Bunker Hill Memorial Bridge. El puente tiene un diseño asimétrico y para su construcción se utilizó un híbrido de acero y hormigón. El característico puente, diseñado por el arquitecto Miguel Ángel Rosales,[31]​[32]​[33]​ está sostenido por dos torres bifurcadas unidas al vano por cables y vigas. Fue el primer puente del país en emplear este método y, en su momento, fue el puente atirantado más ancho del mundo,[9]​habiendo sido superado desde entonces por la sustitución del vano este del puente de la bahía de San Francisco-Oakland.

Mientras tanto, proseguía la construcción del acceso al puente Tobin. Para cuando todas las partes se pusieron de acuerdo sobre el diseño de la I-93, la construcción del conector Tobin (hoy conocido como "City Square Tunnel" por la zona de Charlestown que rodea) estaba muy avanzada, lo que aumentó considerablemente el coste de construcción del enlace de la US Route 1 y la adaptación del túnel.

La arcilla azul de Boston y otras tierras extraídas del trazado del túnel se utilizaron para tapar muchos vertederos locales, rellenar la cantera Granite Rail Quarry de Quincy y restaurar la superficie de Spectacle Island, en el Área Recreativa Nacional de las Islas del Puerto de Boston.

El Storrow Drive Connector, puente complementario del Zakim, empezó a transportar el tráfico de la I-93 a Storrow Drive en 1999. El proyecto se había estado estudiando durante años, pero contó con la oposición de los residentes adinerados del barrio de Beacon Hill. Al final, el conector utilizó un par de rampas que se habían construido para la interestatal 695,[34]​ lo que permitió que la I-93 principal soportara más tráfico del que habría utilizado la I-695 según el plan director original.

Cuando se inició la construcción, el coste del proyecto, incluido el cruce del río Charles, se estimó en 5.800 millones de dólares. Los sobrecostes fueron tan elevados que el presidente de la Massachusetts Turnpike Authority, James Kerasiotes, fue despedido en 2000. Su sustituto tuvo que comprometerse a limitar las aportaciones federales a 8.550 millones de dólares. El gasto total acabó superando los 15.000 millones de dólares. Los intereses elevaron este coste a 21.930 millones de dólares.

Engineering Methods and Details

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 find the safest way to build the tunnel without endangering the existing elevated highway. Finally, they created horizontal stays as wide as the tunnel, cut the struts from the elevated highway, and lowered it onto the new stays.[35] Three alternative construction methods and corresponding structural design were studied to address existing conditions, safety measures, and constructability. In addition to the coded loads, construction loads were calculated to support final design and field execution.[36]​.

On January 18, 2003, the groundbreaking ceremony was held for the I-90 Connector Tunnel, which extends the Massachusetts Turnpike (Interstate 90) east to the Ted Williams Tunnel and then to Boston Logan International Airport. The Ted Williams Tunnel had been completed and was 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, the burying of the elevated Interstate 93, was completed in two stages: the northbound lanes opened on March 29, 2003 and the southbound lanes (in an interim configuration) on December 20, 2003. A tunnel under Leverett Circle connecting eastbound Storrow Drive to I-93 North and the Tobin Bridge opened on December 19, 2004, thus easing congestion at the roundabout. All southbound lanes of I-93 opened to traffic on March 5, 2005, including the left lane of the Zakim Bridge and the entire renovated Dewey Square tunnel.

By the end of December 2004, 95% of the Big Dig had been completed. Major surface work remained, including construction of final ramp configurations at the North End and South Bay Interchange, and reconstruction of surface streets.

The final downtown ramp, exit 16A (formerly 20B) from I-93 south to Albany Street, opened on January 13, 2006.[37]​.

In 2006, the two tunnels on Interstate 93 were named the Thomas P. O'Neill Jr. Tunnel, in honor of the former Democratic speaker of the Massachusetts House of Representatives who pushed for the Big Dig to be funded by the federal government.

La Commonwealth de Massachusetts estaba obligada por la Ley Federal de Aire Limpio a mitigar la contaminación atmosférica generada por las mejoras de la autopista. El Secretario de Transportes, Fred Salvucci, firmó en 1990 un acuerdo con la Conservation Law Foundation en el que se enumeraban 14 proyectos concretos que el Estado se comprometía a construir. Esta lista se ratificó en un acuerdo judicial de 1992.[38]​.

Entre los proyectos que se han llevado a cabo figuran:[38]​[39]​.

Sin embargo, se eliminaron algunos proyectos.

Surface treatments

Some surface treatments that were part of the original project plan were eliminated due to the enormous cost overruns of the highway portion.[42]​.

$99.1 million was allocated for remedial improvements[43]​ in the Charles River watershed, including the construction of North Point Park in Cambridge and Paul Revere Park in Charlestown.[43]​ The North Bank Bridge, which provides pedestrian and bicycle connectivity between the parks, was not funded until the Recovery and Reinvestment Act of 2009. Nashua Street Park, on the Boston side, was completed in 2003, by McCourt Construction, with $7.9 million in funding from MassDOT.[44] As of 2017, $30.5 million had been transferred to the Massachusetts Department of Conservation and Recreation to complete five projects. Another incomplete but necessary project is the South Bank Bridge over the MBTA commuter rail tracks at North Station (connecting Nashua Street Park with the proposed South Bank Park, which is currently a parking lot under the Zakim Bridge at the Charles River Locks).[45]

Improvements in the lower Charles River include the new walkway at Lovejoy Wharf (built by the developer of 160 North Washington Street, Converse's new headquarters), the Lynch Family Skate Park (built in 2015 by the Charles River Conservancy), the rehabilitation of historic operations buildings for the Charles River Dam and Lock, a maintenance facility, and a planned pedestrian walkway across the Charles River next to the MBTA commuter rail drawbridge at North Station (connecting Nashua Street Park and North Point Park).[43]​MassDOT funds South Bank Park,[43]​and the replacement of the North Washington Street Bridge (construction between August 2018 and August 23).[46]​EF Education funds public green space improvements as part of its three-phase expansion in North Point.[47]​Remaining funding can be used to build the North Point Inlet Pedestrian Bridge and a pedestrian walkway over Leverett Circle. Before being replaced by surface access during the reconstruction of the Science Park MBTA Green Line station, Leverett Circle had pedestrian bridges with stairs that provided elevated access between the station, Charles River parks, and the sidewalk to the Boston Museum of Science. Replacement ramps would meet the requirements of the Americans with Disabilities Act and facilitate access to the station.[43]​.

public art

Although not a legal requirement, public art was part of the urban design planning process (and later design development work) through the Arteries Art Program. The objective of the program was to integrate public art into road infrastructure (retaining walls, fences and lighting) and into the essential elements of the pedestrian environment (walkways, landscape elements of parks and bridges). As the overall costs of the project increased, the Artery Arts Program was seen as a potential liability, even though it had the support and interest of the public and professional arts organizations in the area.

Early in the highway design process, a temporary art program was initiated and more than 50 proposals were selected. However, only a few projects began development before the program's funding was cut. Permanent public art funded includes supergraphic text and facades of old West End houses cast into the supporting concrete walls of the elevated freeway abutments near North Station, by Bretteville artist Sheila Levrant; Harbor Fog, a sensor-activated sculptural environment of fog, light and sound, by artist Ross Miller on plot 17; a historic sculpture celebrating the 20th 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.

During public meetings, many citizens requested extensive landscape planting as well as a planting maintenance program.

The Big Dig separated mixed traffic from the Massachusetts Turnpike and the Sumner and Callahan tunnels. Although only one net lane was added in each direction to I-93 north-south, several new east-west lanes were enabled. Massachusetts Turnpike/I-90 east-west traffic now passes directly through the Ted Williams Tunnel toward Logan Airport and Route 1A beyond. Traffic between Storrow Drive and the Callahan and Sumner tunnels continues to use a short stretch of I-93, but additional lanes and direct connections have been made available for this traffic.

The result was a 62% reduction in vehicle travel hours on I-93, the airport tunnels, and the connection from Storrow Drive, from an average of 38,200 hours per day before construction (1994-1995) to 14,800 hours per day in 2004-2005, once the project was largely completed.[48] Savings to travelers were estimated at 166 million dollars annually in the same period of 2004-2005.[49]​ Travel times on the northbound central artery during the afternoon rush hour were reduced by 85.6%.[50]​.

A 2008 Boston Globe report stated that wait times for most trips actually increased as a result of demand induced by increased highway capacity. As more drivers chose to use the new roads, traffic jams only moved out of the city, not reduced or eliminated (although some trips are now faster). The report states: "Ultimately, many motorists traveling to and from the suburbs at rush hour spend more time stuck in traffic, not less." The Globe also says its analysis provides a more complete picture of the traffic situation than a state-commissioned study conducted two years earlier, which credited the Big Dig with helping save at least $167 million a year by increasing economic productivity and lowering motor vehicle operating costs. That study did not analyze highways outside the Big Dig construction zone and did not take into account new traffic jams elsewhere.[51]

Towards the end of the Big Dig in 2003, it was estimated that the demolition of the Central Artery freeway would cause a $732 million increase in property values ​​in Boston's financial district, with the replacement parks providing an additional $252 million in value.[52] 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. It is estimated that the Seaport development alone generated $7 billion in private investment and 43,000 jobs.[53]​.

As part of the project, an elaborate Operations Control Center (OCC) control room was built in South Boston. Staffed 24/7/365, this center monitors, reports on traffic congestion and responds to emergencies. Hundreds of cameras continuously monitor video and thousands of sensors monitor traffic speed and density, air quality, water levels, temperature, equipment status and other conditions inside the tunnel. The OCC can activate emergency ventilators, change electronic signs, and dispatch service teams when necessary.[54][55][56]​.

Leaks

As early as 2001, Turnpike Authority officials and contractors were aware of thousands of leaks from roof and wall cracks, extensive water damage to steel supports and fireproofing systems, and overloaded drainage systems.[57] Many of the leaks were caused by Modern Continental and other subcontractors' failure to remove gravel and other debris before pouring concrete. This information was not made public until MIT engineers (students and faculty volunteers) conducted several experiments and discovered serious problems in the tunnel.[58]​.

On September 15, 2004, a major leak in the north tunnel of Interstate 93 forced the tunnel to close while repairs were carried out. This also forced the Turnpike Authority to release information about its failure to disclose previous leaks. A follow-up report reported "extensive" leaks that were more serious than state authorities had previously acknowledged. The report stated that the tunnel system had more than 400 leaks. However, a Boston Globe report disputed this claim, stating that there were almost 700 leaks in a single 300 m section of tunnel under South Station. Turnpike officials also stated that the number of leaks investigated had dropped from 1,000 to 500.[58]

The problem of leaks is further aggravated by the fact that many of them involve corrosive salt water. This is due to the proximity of Boston Harbor and the Atlantic Ocean, which causes a mix of salt and fresh water leaks into the tunnel. The situation is made worse by road salt spread in the tunnel to melt ice during frost, or introduced by vehicles passing through it.[59]​ Salt water and salt spray are well-known problems that must be resolved in any marine environment. It has been reported that "hundreds of thousands of gallons of salt water are pumped monthly" into the Big Dig, and a map has been prepared showing "hot spots" where water leaks are especially severe.[60] Salt-accelerated corrosion has caused failures in ceiling light fixtures (see below), but can also cause rapid deterioration of embedded rebar and other steel structural reinforcements holding the walls and roof of the Big Dig. tunnel.[59]​.

Poor materials

In June 2005, Massachusetts State Police raided the offices of Aggregate Industries, the main supplier of concrete for the underground portions of the project. They seized evidence that Aggregate "Aggregates (West Virginia)") had supplied concrete that did not meet contract specifications. In March 2006, Massachusetts Attorney General Tom Reilly announced plans to sue the project's contractors and other companies for poor work performed on the project. The state of Massachusetts filed more than 200 complaints regarding leaks, cost overruns, quality issues, and safety violations. In total, the State has claimed about 100 million dollars from contractors (1 dollar for every 141 dollars spent).[61]​.

In May 2006, six company employees were arrested and charged with conspiracy to defraud the United States.[62] In July 2007, Aggregate Industries settled the case with a $50 million settlement. $42 million of the settlement went to civil cases and $8 million was paid in criminal fines. The company will provide $75 million in maintenance insurance and pay $500,000 for routine checks on areas suspected of containing substandard concrete.[63] In July 2009, two of the defendants, Gerard McNally and Keith Thomas, both directors, pleaded guilty to charges of conspiracy, mail fraud and filing false reports.[64] The following month, the remaining four, Robert Prosperi, Mark Blais, Gregory Stevenson and John Farrar, were found guilty of conspiracy and fraud charges.[65] All four were sentenced to probation and home confinement, and Blais and Farrar were also sentenced to community service.[66]

Deadly roof collapse

A fatal accident raised safety concerns and shut down part of the project for most of the summer of 2006. On July 10, 2006, concrete roof panels and debris, weighing 26 short tons (24 tons) and measuring 6.1 by 12.2 meters (20 by 40 feet), fell on a car traveling on the two-lane ramp connecting northbound I-93 to I-90 in eastbound in South Boston "South Boston (Virginia)"), killing Milena Del Valle, who was a passenger, and injuring her husband, Angel Del Valle, who was driving.[67]​Immediately after the fatal roof collapse, Governor Mitt Romney ordered a "top to bottom" safety audit by engineering firm Wiss, Janney, Elstner Associates, Inc. to look for other risk areas. Said Romney: "We simply cannot live in an environment in which a project of this magnitude can endanger human lives, as we have already seen."[68]​The collapse and closure of the tunnel greatly collapsed traffic in the city. The traffic jams are believed to have contributed to the death of another person, a heart attack victim who died on the way to Boston Medical Center when his ambulance became stuck in one of the traffic jams two weeks after the collapse. On September 1, 2006, one eastbound lane of the connector tunnel was reopened to traffic.[70][71]​

Following extensive inspections and repairs, the east and west lanes of Interstate 90 reopened in early January 2007.[72] The last piece of the highway 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 the epoxy glue used to hold the roof in place during construction was not suitable for long-term bonding,[73] which was determined to be the cause of the roof collapse. The Power-Fast epoxy adhesive used in the installation was designed for short-term loads, such as wind or seismic loads, not long-term loads, such as the weight of a panel.[74][75][76]​.

Powers Fasteners, the manufacturers of the adhesive, revised their product specifications on May 15, 2007 to increase the safety factor from 4 to 10 on all of their epoxy products intended for aerial applications. The safety factor of Power-Fast Epoxy was increased from 4 to 16.[76]​On December 24, 2007, the Del Valle family announced that it had reached a settlement with Powers Fasteners in which the family would be paid $6 million.[77]​In December 2008, Powers Fasteners agreed to pay $16 million to the state to resolve involuntary manslaughter charges.[78]​.

"Ginsu railings"

Public safety workers have dubbed the safety railings in the Big Dig tunnels "ginsu guardrails" because the square edges of the support posts have caused mutilations and deaths of passengers ejected from crashed vehicles. After an eighth death related to safety railings was reported, MassDOT officials announced plans to cover or remove the supposedly dangerous features, but only near curves or exit ramps.[79] This partial removal of hazards has been criticized by one safety specialist, who suggests that the guardrails are just as dangerous on straight sections of the tunnel.[79]​

Lightning

In March 2011, it emerged that senior MassDOT officials had failed to disclose a problem with the O'Neill Tunnel luminaires. In early February 2011, a maintenance crew found a light fixture lying in the center lane of the northbound tunnel. Assuming it was simply road debris, the maintenance crew picked it up and took it to its original facility. The next day, a supervisor passing by the shipyard realized that the 54kg lamp was not road debris, but one of those used to illuminate the tunnel. Subsequent investigation revealed that the luminaire's mounting apparatus had failed due to galvanic corrosion of incompatible metals, caused by direct contact of aluminum with stainless steel in the presence of salt water.[60]​[81]​The electrochemical potential difference between stainless steel and aluminum ranges between 0.5 and 1.0 V, depending on the alloy involved, and can cause considerable corrosion within months under unfavorable conditions.

Once the reason the fastener had failed was discovered, a thorough inspection of the other fasteners in the tunnel revealed that many other fasteners were also in the same state of disrepair.[82] Some of the worst fasteners were temporarily propped up with zip ties.[59] In moving forward with the temporary repairs, members of the MassDOT management team decided not to share the news of the systemic failure and repair of the fasteners with the public or Governor Deval Patrick's administration. fixings.[83]​.

In April 2012, it appeared that all 25,000 luminaires would need to be replaced, at an estimated cost of $54 million.[59] Replacement work was carried out primarily at night, requiring lane closures or occasional closure of the entire tunnel for safety, and was estimated to take up to two years to complete.[59]