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Triumph, Tragedy Mark Boston’s Big Dig Project

Mon June 04, 2007 - National Edition
Pete Sigmund

An aerial shot of the peak of work at the I-90/I-93 interchange. Tubes are being immersed in the Four Point Channel and a roadway is built under the railroad.
An aerial shot of the peak of work at the I-90/I-93 interchange. Tubes are being immersed in the Four Point Channel and a roadway is built under the railroad.
An aerial shot of the peak of work at the I-90/I-93 interchange. Tubes are being immersed in the Four Point Channel and a roadway is built under the railroad. An army of iron fills the skies and the ground in the South Bay Interchange area. A Testa crew uses a Cat excavator equipped with a massive shear to demolish a temporary ramp structure near the Central Artery. Testa workers clear the way for the underground I-93 north- bound lanes. Crews construct a canal for North Point Park.

(Editor’s Note: This article is the last in a 10-part series on iconic United States construction projects.)

One highway project stands out in the history of U.S. construction for its complexity, problems and ultimate achievement: Boston’s “Big Dig.”

Now 99 percent complete, this excavation, tunneling, and road and bridge project has burrowed underneath the length of Boston. As wide as an interstate highway, it has been painstakingly built through unstable landfill, nine railroad tracks, scores of glass and steel office towers, and many fragile, older brick buildings without disturbing them and without seriously disrupting the city’s business or destroying historic artifacts, including remains of buildings from early settlements.

Though it has been marred by mishaps, lawsuits and spending issues, the Big Dig has accomplished great things in the 16 years since construction began.

Officially called the Central Artery/Tunnel Project (CA/T), it represents major construction advances. These include ingenious “jacking boxes” to excavate beneath operating railroad tracks; the largest use of slurry construction techniques in North America; and fabricating, in a huge casting basin, six concrete tunnel sections, the heaviest of which weighed more than the Titanic.

Other notable achievements include:

• a massive underpinning of the old elevated highway so it wouldn’t collapse during the underground construction.

• building the Ted Williams Tunnel under Boston’s inner harbor using steel sections longer than a football field.

• development of an extremely advanced electronic traffic monitoring system.

• spanning the Charles River with the first bridge in the country employing an asymmetrical and hybrid (steel and concrete) design. It’s the widest cable-stayed bridge in the world and a spectacular gateway to the city.

The Central Artery/Tunnel Project, managed and operated by the Massachusetts Turnpike Authority and part of the Metropolitan Highway System, has been compared to such iconic construction marvels as the Panama Canal and the Chunnel under the English Channel.

Yet it has been plagued by problems: leaks, accusations of financial mismanagement and, most recently, a tragedy, when tiles fell from the roof of a tunnel ramp onto a car, killing a female passenger.

Dire Need

The Big Dig could be called the project which unclogged the throat of a city choking with traffic.

The main obstruction was the Central Artery (I-93), an elevated six-lane highway through the center of downtown, which was like a funnel full of slowly-moving, or stopped, cars (and swearing motorists). Begun in 1951 and opened in 1959, this 1.5-mi.-long (2.4 km) “green monster” at first carried approximately 75,000 vehicles a day. This had grown, tortuously, to 190,000 vehicles by the 1990s, when its traffic crawled for 10 hours a day. Traffic jams of 16 hours a day were predicted for 2010.

The artery also was an unsightly barrier cutting off neighborhoods, impeding traffic flow, and separating Boston’s North End and waterfront neighborhoods from downtown, thus limiting participation in the city’s economic life. Its construction displaced more than 20,000 people and demolished more than 1,000 structures. Replacing it by building on the surface would have compounded the problems, crowding downtown Boston with a horde of construction equipment, cars and trucks, operating noisily in clouds of dirt and fumes.

Going Underground in a Big Way

The solution was to go underground.

Environmental impact studies on new underground highways under the city, that would replace the monster with smooth, efficient traffic arteries, began in 1982 and a plan was given final approval in 1991. The work even included archeological surveys of historic sites like the Spectacle Island Indian area, whose historic artifacts had to be preserved.

Design studies continued through the decade, with Congress approving the scope of the project in 1987 and allocating $755 million for the work in 1990.

The first contracts were advertised and awarded in 1991. Construction of the four-lane Ted Williams Tunnel under Boston’s inner harbor began in September 1991.

Design and construction management on the Big Dig was under a Bechtel/Parsons Brinckerhoff joint venture.

The project has involved some 60 design packages and more than 100 construction contracts — often as high as $400 million, and covering everything from finishing to ventilation buildings.

“The unique challenge was to construct this project in the middle of Boston without crippling the city,” said the Massachusetts Turnpike Authority Web site ( “Work of this magnitude and duration has never been attempted in the heart of an urban area. Unlike any other major highway project, CA/T was designed to maintain traffic capacity and access to residents and businesses — to keep the city open for business — throughout construction.”

The Web site points out that “highway projects of the 1950s and 1960s, when the interstates were first built, gave very little consideration to the communities in the path of the new roads, with disruption and dislocation the rule of the day.”

The Big Dig’s large-scale, novel, subsurface approach, on the other hand, would reconnect the city, reinvigorate communities and free the original artery area for parks and open space.

Truly a “Big” Dig

Originally planned for completion in 2004, the project has taken 16 years — truly a big dig involving thousands of workers and pieces of equipment.

In its complexity and many facets, the Big Dig has resembled a World War II operation

Costing at least $14.7 billion to date, the dig is 7.8 mi. (12.5 km) long.

The army of construction workers, which eventually grew to 5,000 people, used thousands of pieces of construction equipment (including approximately 150 cranes) to build the highway under a city. The workers completed 161 lane-mi. (259 km) of highway — approximately half of which was in tunnels — and placed 3.8 million cu. yds. (2.9 million cu m) of concrete. This was the equivalent of 2,350 acres (951 ha) 1 ft. (0.3 m) thick, and enough to build a sidewalk 3 ft. (0.9 m) wide and 4 in. (10 cm) thick, from Boston to San Francisco and back three times.

A staggering amount of soil was excavated, too — more than 16 million cu. yds. (12 million cu m).

The Big Dig also relocated 29 mi. (47 km) of gas, electric, telephone, sewer, water, and other utility lines, and installed approximately 5,000 mi. (2,050 km) of fiber optic cable and 200,000 mi. (321,870 km) of copper telephone cable.

Gigantic Concrete Lobster Claw

The new Central Artery/Tunnel is shaped like a gigantic lobster claw. The gargantuan project includes highways, tunnels, bridges, four major new interchanges and many access ramps and connectors.

The right (east) portion of the claw, opened in January, 2003, extends I-90 (the Massachusetts Turnpike) approximately 3.5 mi. (5.6 km) from its previous terminus near South Station all the way to Logan Airport, saving motorists as much as 45 minutes. This extension (four lanes plus a high occupancy vehicle parallel tunnel) runs under railroad tracks, the Fort Point Channel and South Boston before connecting to the new Ted Williams Tunnel and the airport. The massive, complex, tunneling and excavation was a significant engineering achievement in itself, involving jacking and other advanced techniques.

The other claw, eight or 10 lanes wide, now replaces the city’s monstrously congested Central Artery. This reconstruction of I-93, enormously complex, passes beneath downtown Boston and goes northward over the graceful new 10-lane Leonard B. Zakim Bridge to Sullivan Square. It’s approximately 3 mi. (4.8 km) long, including a 1.5 mi. (2.4 km) underground portion (the Tip O’Neill Tunnel).

At the foot of the claws, in South Boston, a new I-90/I-93 interchange, completed in 2005, allows access to both routes.

Huge Tubes Connected

for Ted Williams Tunnel

The 8,500-ft.-long (2,591 m) Ted Williams Tunnel under Boston Harbor was opened to traffic in December 1995, as was the first link in extending I-90 to the airport. It cost $1.9 billion — more than 13 times a 1968 estimate — and won the 1996 Outstanding Civil Engineering Achievement Award from the American Society of Civil Engineers.

First, in 1992, the world’s largest dredging vessel, the “Super Scoop,” owned by the Dutra Group, dug a 50-ft.-deep (15 m), 100-ft.-wide (30 m) trench beneath the harbor. A total of 890,000 cu. yds. (680,000 cu m) of material was moved in the dredging, which continued around the clock for almost two years.

Next, 12 steel immersed tube tunnel sections, prefabricated at Bethlehem Steel Company’s shipyard in Baltimore, Md., were barged 400 mi. (644 km) to the site from Maryland. Each section was 325 ft. (99 m) long, weighed 7,500 tons (6,800 t), and contained twin tubes, each 40 ft. (12 m) in diameter, enough to accommodate two traffic lanes.

Assembly teams at the site placed 25,000 tons (22,700 t) of concrete roadway deck and 33,000 tons (30,000 t) of steel reinforcement bars inside each section. After the sections were barged out to the trench, a laybarge lowered them into place by cable, using a laser beam and a global positioning system to determine exact location. Divers then aligned their final placement.

After pumps sucked out water, sand and rock for an airtight seal, welders connected the sections and ironworkers removed the steel bulkheads at the section ends.

Weak Soil on Tunnel Approaches

Land tunnels also had to be built for approaches to the tunnel in South Boston and East Boston. Completed in 1995 after three years of work, these approaches used 450,000 cu. yds. (344,000 cu m) of concrete.

Because the approaches went through weak landfill, engineers stabilized the soil with a three-step Japanese technique in which the soil was liquefied, mixed with concrete and frozen. Engineers also designed the South Boston approach tunnel with a 17-in.-thick (43 cm) roadway surface, called a gravity slab, which was tied by cables to the bedrock below to prevent the tunnel from sinking.

A 250-ft. (76 m) diameter, 85-ft.-deep (26 m) cofferdam, the largest ever built in North America, also was constructed for a short tunnel connecting the south approach with the main tunnel.

The cofferdam had 13-ft.-thick (4 m) walls to restrain tidal forces.

A South Boston bypass road also was completed early in the project to accommodate trucks which would otherwise go through South Boston neighborhoods.

Casting Basin Larger

Than Three Titanics

The I-90 extension also goes under the Fort Point Channel, a narrow extension of Boston Harbor just east of the I-90/I-93 interchange. Because there wasn’t enough room to float tunnel sections under bridges, engineers decided to cast the six concrete tunnel sections near the site. They did this in a huge casting basin — in effect a mammoth dry dock 1,000 ft. (305 m) long, 300 ft. (91 m) wide and 60 ft. (18 m) deep. This huge hole could hold an aircraft carrier, or three Titanics side by side. More than 450,000 cu. yds. (344,000 cu m) of dirt was excavated to form the basin.

The longest section was 414 ft. (126 m) long, the widest was 174 ft. (53 m) wide and the heaviest weighed more than 50,000 tons (45,400 t). All six were 27 ft. (8.2 m) high.

Completed sections were sealed watertight at either end. The basin was then flooded and the sections were floated out and positioned to be lowered into a trench which had been dredged and graded at the bottom of the channel, with extremely narrow tolerances, only a few feet above a subway tunnel.

Tunnel engineers had to make sure the weight of the sections wouldn’t damage the subway. They did this by setting the sections on 110 concrete shafts, each 6 ft. (1.8 m) in diameter, which were drilled as much as 145 ft. (44 m) into the bedrock at the bottom of the channel.

Slurry Walls Did the Trick

The Big Dig placed more than 26,000 linear ft. (7,900 m) of slurry walls, one 10-ft. (3 m) trench at a time. The largest use of slurry anywhere in North America, the walls were the single most-important construction technique on the project.

According to the Web site, “slurry walls were essential to the success of the Central Artery project because the special excavating equipment can work in confined spaces in a dense old city such as Boston — the machinery was first developed in Europe — particularly under the elevated highway where there was no headroom for tall conventional excavators.”

The slurry walls were built into concrete walls which go from the surface to bedrock, defining the area to be excavated and eventually forming the actual walls of the artery.

This is how they did it. First, an excavator dug a trench (approximately 3 by 10 ft. [.9 by 3 m]) to bedrock. As equipment removed the earth, liquid slurry was pumped into the hole. This kept the walls intact before workers lowered huge reinforcing beams into the trenches and pumped in concrete to fill the holes and displace the slurry. It took approximately two days to dig, reinforce, and fill each of these slurry panels, which form the wall.

“The walls produced a rigid work area for excavating the tunnel without the need for a much wider conventional trench with sloping sides, which would be impossible in the narrow corridor where the elevated highway stood,” the Web site said.

Holding Up the Old Highway

To keep traffic moving, the old Green Monster stayed put (at a estimated cost of $600 million) while the new Central Artery/Tunnel (an interstate) tunneled right under it. The monster’s original support columns were cut away and replaced by 67 rows of three steel supports, which rested on the new concrete walls which had been built with the slurry technique.

The entire weight of the elevated structure was shifted to the new supports in a load-shifting process called “underpinning.”

After the new I-93 Central Artery opened to traffic in 2003, the old elevated structure was finally demolished in 2004, creating 27 acres (11 ha) for streets, parks and other refurbished open space.

Massive Use of

Jacking Techniques

I-90 had to pass 20 ft. (6 m) beneath nine active railroad tracks carrying commuter and Amtrak trains into South Boston’s busiest rail terminal. How do you dig a tunnel big enough for an interstate while trains rumble by just a few feet overhead? The Big Dig used huge jacking boxes which were the largest application of jacking techniques in the world.

Simply put, jacking pits were dug, tunnel boxes 80 ft. (24 m) wide and 40 ft. (12 m) high were built inside the pits, and hydraulic jacks then pushed the boxes under the tracks.

The “jacked” tunnels thus pushed through were 380 ft. (116 m) long for I-90 eastbound, 260 ft. (79 m) long for I-90 westbound and 150 ft. (45 m) long for an I-90 ramp.

More than 50 jacks, each with a maximum pushing capacity of 10,000 psi were used in the first tunneling operation.

The ground ahead of the tunnel boxes, compared to “day-old pudding,” needed to be frozen so that it would be stable and not allow the tracks above to settle. Engineers built a freezing plant near the tracks which pumped a brine mixture, which stayed liquid below 32F, into plastic pipes within hundreds of steel pipes which had been driven into the ground. Over weeks, the circulating brine drew the heat from the soil, and froze the ground outward from the pipes.

Once frozen, the soil was excavated without settling. A rotating grinder on a machine called a road header chewed out the frozen soil. The jacking pits later became the path of the highway.

Spectacular Bridge

The 10-lane, $100 million Leonard B. Zakim Bunker Hill Bridge over the Charles River was opened to I-93 traffic at the northern end of the project in 2001. This cable-stayed bridge is the widest ever built — 76 ft. (23 m). It’s the first hybrid (steel and concrete) cable-stayed span in the United States. The main span consists of a steel box girder and steel floor beams, while back spans contain post-tensioned concrete.

It’s also the first bridge to use an asymmetrical design. Employing inverted-Y-shaped towers, the Leonard B. Zakim has been compared to a gigantic two-masted schooner sailing into the city’s skyline.

A parallel four-lane Leverett Circle Connector Bridge also was constructed in 1999 as part of the project. Nine box girder sections, with the largest cross sections in North America, were barged into place and raised by cranes or jacks.

Advanced Traffic Control System

The turnpike authority said the Central Artery/Tunnel project’s Operations Control Center (OCC) “contains the most advanced electronic traffic monitoring and incident response system in the world.” The OCC employs a “Smart Highways” computer system receiving data from more than 45,000 collection points to monitor traffic and accidents, detect and respond to fires, and check ventilation, lighting and air quality.

Intelligent transportation system devices monitor all traffic in the I-90/I-93 system of tunnels, ramps and surface highways in downtown Boston, as well as in the Sumner, Callahan, Prudential and City Square tunnels and on I-90 out to Route 128. Equipment includes more than 1,400 loop detectors, embedded in roadway pavement, which measure traffic density, and more than 600 closed-circuit TV cameras that show what’s happening.

Tragedy Strikes

Late in the evening of July 10, 2006, a concrete ceiling panel weighing 3 tons (2.7 t) and measuring 20 by 40 ft. (6.1 by 12.2 m) fell on a car traveling on the two-lane ramp connecting northbound I-93 to eastbound I-90 in South Boston, killing Milena Del Valle, who was a passenger, and injuring her husband, Angel Del Valle, who was driving.

In the subsequent investigation, engineers said the failure was caused by stress in the vertical ceiling bolts and inadequate epoxy holding the bolts to the ceiling.

More than 3,300 bolts have been replaced since then as part of a different bracketing system. Hundreds of additional diagonal bolts have been been installed to prevent possible future collapses during earthquakes.

While the investigation (and lawsuits) continue, an HOV lane on I-90 was expected to open in early June as the final section to be repaired and go back into service.

The family of the victim has filed a lawsuit against nine companies and the Massachusetts Turnpike Authority in a case which legal observers believe could bring record punitive damages.

History of Problems

The falling tile was the latest, and most tragic, of the major problems to plague the Big Dig.

The Boston Globe said in 2003 that at least $1.1 billion in Big Dig cost overruns, or two-thirds of the cost growth to date, have been tied to Bechtel errors.

In September 2004, water gushed into a northbound tunnel still under construction, backing up rush hour traffic for miles. A report by investigating engineers said the Central Artery project includes hundreds of leaks, pouring millions of gallons of water into the underground highway system, and that Bechtel managers were aware the tunnel was deficient when it was built in the 1990s.

In April 2005, a day after the Federal Highway Administration said the tunnels were safe, rocks and other debris rained down on the I-93 southbound tunnel from overhead vents.

In May 2005, Big Dig officials said two leaks had been sending 20 to 30 gal. (76 to 113 L) of water a minute into the Fort Point Channel section of the I-90 tunnel. An inspection in July found that a 1,500-ft. (457 m) stretch of the tunnel had weak areas.

The state of Massachusetts is trying to get some of its money back. It sued Bechtel Corp. and Parsons Brinckerhoff for $146 million in 2004, alleging that the firms made inaccurate cost estimates. Then in 2006, it demanded $106 million in refunds from these and other contractors, and the design firms in the consortium. Also in 2006, six managers from Aggregate Industries NE Inc. were indicted on charges of running a conspiracy that delivered 5,000 truckloads of tainted concrete, approximately 1.2 percent of the concrete used on the Big Dig.

End of Long Road

Now, at last, the Big Dig is in its last stages: finishing, painting, park construction and final inspections by turnpike engineers before the contractors hand it over the turnpike authority.

Despite its problems, the Big Dig takes its place among the world’s greatest construction achievements.

For more information, visit (click on Big Dig). CEG

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