The Big Dig In Boston

In the heart of Boston, Massachusetts, there is a stretch of highway called the Central Artery. When it was initially constructed, it was intended to hold tens of thousands of cars per day. But as the years progressed, this stretch ended up facilitating up to 200,000 cars a day. Not only did this cause immense amounts of traffic, but it also created collision rates were abnormally high. So, engineers devised a plan to reduce this congestion.

One of the major parts of this plan was rerouting the existing highway. What was initially a 6 lane highway was turned into a 14-lane highway that resided under the road.

Another is a connection between two parts of Boston with a tunnel. With the extension of the I-90 through this tunnel, South Boston and the Logan Airport become connected.

But, constructing in this underground space brought problems. The Red Line subway, an already enormous and widely-used piece of infrastructure, took up much of the tunneling space that this project demanded. So, engineers planned the project under this area.

So, in this underground area, civil engineers had to innovate to maintain stability in this unstable area. Initially, engineers planned for narrow tunnels to be dug in this underground. Then, they used a material called slurry to fill these tunnels. This slurry is a clay-based liquid that was more stable than the soil that initially comprised the underground.

After the slurry was established in this tunnel, they replaced this material with concrete and steel support (while some of the slurry was retained). While these concrete and steel beams were installed, they used monitoring equipment to ensure that the construction was not harming the integrity of the subway.

Along with this, more measures were taken to further ensure stability during the construction process. One of these was ground freezing, which allowed for the stabilization and hardening of unsafe soil during the construction process. Even further, grout was injected in certain areas to harden this unstable soil even more.

When stabilization of this tunnel was complete, the construction of the road was finally able to begin. But since much of the underground work was complete, installing the nearly 8 miles of road was less of a burden to engineers in the designing process.

In urban areas such as Boston, using underground space is vital to providing new infrastructure developments. And in the case of the Big Dig, much of the problems were in the geotechnical department. From grouting to slurry to beam installation, engineers had to pull of many stabilization techniques at once to ensure the success of this project. As a result, this piece of infrastructure is a unique case of combining many soil stabilization techniques to ensure the structural integrity of a large-scale piece of infrastructure.