What Lessons Did Tunnel Builders Learn From The Big Dig?

 

The grandest of ambitions often turn into harsh lessons that projects that begin in their wake must heed if they want to succeed.

 

Tunnels tend to be amongst the largest, most ambitious and most expensive infrastructure projects in the world, and in order for them to be completed efficiently and on schedule, not only do huge tunnel boring machines need to be used to burrow into the earth, but that earth needs to be moved cleanly and efficiently.

 

This is where vulcanised conveyor belts become critical for earthmoving, and ensuring that these belts are serviced, maintained and repaired so they can continue to work at a high rate of efficiency is as vital as ensuring the boring, blasting and drilling equipment functions effectively.

 

This is particularly true for complex projects, and perhaps no tunnel project in history provided as many lessons as the complex ambitions of the Central Artery Project in Boston, Massachusetts, better known today as The Big Dig.

 

Why was it needed? What were the biggest problems? And what lessons were learned from them?

 

Why Was The Central Artery Project Needed?

Unlike other major cities in the United States, Boston more closely resembles an English city in its use of narrow streets and tangled road networks, in no small part due to the city being established and built long before the motor car was invented.

 

As cars not only became a vital form of transport in the United States but progressively got larger and wider, Boston constructed the original Central Artery expressway, but despite displacing 20,000 people and splitting neighbourhoods in a way that is still controversial today, it was quickly overwhelmed by traffic.

 

Known as the “Distressway” and “The Other Green Monster” (the Green Monster being a giant green wall at the hometown Red Sox’s Fenway Park), the John F. Fitzgerald Expressway was despised by residents, its users and local government, who advocated to move the highway underground.

 

Why Did The Central Artery Become A Tunnel?

Ultimately, a tunnel was decided for five main reasons:

 

• • It could provide far more capacity for the 200,000 vehicles that travelled on it per day than the six lanes of the current expressway.

• • The expressway itself was badly designed, with multiple entrances and exits, no merged lanes and tight turns that led to countless traffic accidents.

• • It could be constructed without closing the Central Artery, something that would have brought the city to a standstill.

• • At the same time, it allowed for its eventual demolition, as the expressway had caused damage to the communities displaced by its construction or that lived close to its towering green visage.

• • It could be extended to nearby Logan International Airport.

 

Why Was The Big Dig So Delayed?

The Central Artery Tunnel took 16 years to gain approval, successfully passing Congress by just one vote in 1987, took four more years before it broke ground and yet another 16 years before construction was completed.

 

During that time, it became the most expensive road project in the United States, costing seven times its initial estimated cost of $2.8bn (or roughly three times its cost adjusted for inflation), was delayed by nearly a decade and led to several civil and criminal investigations.

 

Why were there so many problems? There were a lot of answers, but the central issue was the insistence that The Big Dig be constructed under an existing city without disrupting existing traffic, something described by locals as akin to performing open-heart surgery on someone running the Boston Marathon.

 

This required entirely new approaches to creating the tunnel and moving the vast quantities of earth using conveyor belts. The main process used was slurry wall construction, which created deep concrete walls that built the initial foundations of the highway, whilst the rest was excavated.

It also used ground freezing techniques to manage the complexities of tunnelling into reclaimed land, which led to potentially unstable ground that needed to be kept in place to ensure the railways could keep moving.

 

An entire section of the tunnel was deliberately flooded to stop a leak from destroying huge parts of the tunnel, and innovative solutions continued to be found to move vast amounts of earth without disturbing nearby transportation.

 

What Lessons Have Been Learned From The Big Dig?

The constant delays hurt the legacy of The Big Dig project, but they did provide vital lessons for tunnel builders in the future when it came to projects of this scale:

 

 

• • Building under a living city is possible but extremely expensive, requiring innovative solutions to innovative problems as well as safe earthmoving techniques.

• • Projects on a giant scale require a clean sheet approach, rather than the expansion of proven techniques used on a smaller scale.

• • Moving 16m cubic yards of dirt requires wholly innovative approaches in order to minimise disruption and harm to nearby residents.