This week, I met Ailie MacAdam, Senior VP and MD of the global rail division of Bechtel. I was introduced to her by the awesome Peter MacLennan from Crossrail, who thought that I might like to meet a female engineer who has played a leading role on huge civil engineering projects.
Ailie is a chemical engineer by training, and joined Bechtel directly on completion of her Masters. In that time, she has worked on everything from the Boston Central Artery project (often called the “Big Dig”), the UK’s High Speed 1 rail project and the central section of the Crossrail project.
The Big Dig was first up on the agenda. Boston had a “world-class traffic problem”, thanks to the presence of an elevated six-lane highway called the Central Artery (CA) that ran straight through with city. Designed in 1959 to comfortably carry about 75,000 vehicles daily, by the early 1990s, more than 200,000 vehicles used the route every day – congestion was a nightmare, road accident levels were dangerously high and .the city was beginning to suffer. So, the Big Dig began. One of the most ambitious road infrastructure projects in the world, it took 15 years of construction work, and involved re-routing the CA, replacing it with new roads and an eight-lane underground expressway, re-routing all utilities that ran through the footprint, creating many new public spaces. Altogether, the project constructed 161 miles of highway lanes, about half of which were in newly-built tunnels, excavating 12,200,000 cubic metres of material in total (enough to fill 10 Wembley Stadiums to the brim).
Aside from the huge cost of the project, the most-often discussed aspect of the Big Dig is the engineering that went into it, especially the tunnels, so I thought I’d tell you a little about them. Most of the digging area did not consist of dense clays or solid bedrock, oh no. Instead, it was a messy combination of left-over landfill, glacial debris and sunken ships! So, tunnelling through this material can’t be done by tunnel-boring machines (for more on TBMs, read this post).
And there was one other small complication. The new tunnel would largely follow the footprint of the elevated artery (the highway), without shutting down the highway. The Washington Post artfully described this process as “…like performing open-heart surgery on a patient running a marathon”! So, how did they do it? Well, Ailie and her team used a process called open-cut-and-cover tunnelling.
Figure: Cut and Cover Tunnel – (L) Top-Down Construction (R) Bottom-Up Construction (Image sourced here)
In “bottom-up” construction, a trench is excavated from the surface. Temporary support walls and struts are used to support the sides of the trench, and a tunnel is constructed within it. A roof is added too, of course. As sections of the tunnel are constructed, the surrounding trench is backfilled. Once the tunnel is complete, the surface material is restored and life above ground can go on as normal.
However, most of the Big Dig’s tunnelling used a top-down approach. In “top down” construction, d-walls (diaphragm walls) are inserted first – these will eventually become the tunnel walls. Then the team will excavate all the material between the d-walls, (forming the tunnel trench) until they reach what will become the tunnel floor. Slabs are added to the top of the trench and braced for extra support, and the surface materials restored. Work continues on inside the tunnel, but with minimal interruption to those at surface level.
In addition to just the complications of digging these massive eight-to-ten lane tunnels, the project also relocated almost 47 km of gas, electric, telephone, sewer, water, and other utility lines, turning what was a chaotic mess, into something that is beautiful to my (control-freak) eyes!
Boston’s underground utilities (a) Before the Big Dig and (b) after. Image from here
This huge project was just one of the many fascinating things Ailie and I discussed at our meeting. As well as talking about some of her adventures in infrastructure, we also pondered the (annoyingly) enduring question of how best to encourage more women into engineering. Like me, Ailie was brought up in a house where an interest in science and maths was encouraged, and no-one ever tried to dissuade her from pursuing a career in engineering. But, the numbers of female engineers speak for themselves. According to a recent RAEng report, the UK professional engineering workforce ranks particularly badly when compared with other European counties – female engineers account for only 8%! (It’s worth noting though that more than 14% of Bechtel’s engineers are women)
These days, Ailie is a committed STEMNET Ambassador, spending lots of time in schools across the country, to bring the joys of engineering to the next generation. With figures suggesting that the UK will need at least 1.82 million more people with engineering skills by 2022 in order to meet demands, Ailie’s assertion that “As a country we are short of engineers, so if we ignore 50% of the potential workforce we’re shooting ourselves in the foot” seems fair!
Another thing we agreed on was that a complex issue like gender diversity won’t ever have a single, neat solution, but Ailie also suggests that it is vitally important to reach out to young girls before they choose their path through education. Speaking in 2013, she said engineers need “…to dispel myths, show girls that engineering is a viable option and how rewarding it can be.” This sentiment was echoed in our own conversation too, when she concluded by saying “I don’t desperately want all school-age children to be engineers. I just want them, both boys and girls, to learn what engineering is all about, so that they can make an informed choice.”