The TBM-whisperer

Interview with Mariachiara Di Nauta, mechanical digging expert with the Webuild group

A team made up of a handful of people. All highly specialized: experts in geotechnics, engineering and, naturally, mechanical digging. Their job is putting together the technical specifications of, search around the world for, and then order the TBMs, the mechanical moles that Webuild uses to dig great tunnels for public works. Works that include high speed railways, subway systems, highways and hydraulic tunnels, that is, everything that runs below meters and meters of rock or dirt, at the foot of a mountain or beneath the surface of a lake.

Mariachiara Di Nauta, 38, with a degree in civil and environmental engineering and a specialization in geotechnics, is one of them: a member of the team of experts that for the Webuild group makes up the technical profile of the TBM necessary for each project, based on the characteristics of the project and the nature of the structure to be built.

“We start from the definitive project,” Di Nauta explains. “We study every detail of the tunnel that the mechanical mole is expected to dig. First of all we conduct a careful study of the hydrogeological conditions of the planned route, and then we verify that the type of TBM the clients expects to use is right for the types of terrain it will cross on its path and the best price-wise. Once the type of TBM is chosen, we define the technical specs of the TBM. On the basis of the study of the hydrogeological conditions crossed by the route and the technical capacity of the machine, we then evaluate the progress in terms of meters/day on average, that will then be the reference point for the work program.”

How much can a TBM cost?

“The price of a TBM depends on the terrain it must dig and the diameter.

A TBM for a subway or railway tunnel can cost as much as 15-20 million euros. Those with very large diameters, of 13 or 14 meters, can cost up to 40 million euros.”

How does the TBM market work?

“Generally TBM’s are purchased with a buy-back scheme allowing the supplier to buy it back at the end of the project, unless we want to reuse it on some other site. For example, since there are many tenders in Sicily for TBM projects, we are looking for synergies between all these jobs. So for the same TBM that we expect will work on one segment, we study a program so it can be used for another project as well.”

How many TBMs are operating for the Webuild group?

“The projects with TBMs are truly many. During 2023 and 2024 the number of TBMs in use by Webuild will gradually reach 45 units, a number that will only increase as we add more projects.”

How does it feel to be in a tunnel, tens of meters below ground?

“It’s complicated to explain what we actually do in the tunnel. When I talk about it I see the amazement in people’s faces, especially when I tell them I have worked 200 meters underground beneath a lake. You must not be claustrophobic, because I would enter in the digging work space with the miners, and for me that was ok. I felt I could handle it.”

How did you develop a passion for this job?

“Until the end of my university studies I had no idea what I would do. Then in the last quarter my course was about tunnels and great underground public works and I fell in love like crazy for this subject.”

Since joining Webuild, engineer Di Nauta worked on some of the most challenging mechanized digging projects in the world. She was part of the boring of a hydraulic tunnel beneath Lake Mead, in Nevada, dug to bring drinking water to the city of Las Vegas.

“A unique experience,” she recalls. “Because we were working 200 meters beneath the bed of the lake.”

She also worked on the Anacostia River Tunnel, another hydraulic tunnel that Webuild realized in Washington D.C. After this experience, she was selected as part of the team of experts that supports the bids office in the section dedicated to TBMs.

“I follow all the tenders and I support the work sites that require mechanized digging,” she explains. “And I ensure that the project designers follow our guidelines, our standards and the know how that we accrued over the years thanks to projects completed all over the world.”

Why do you like this job so much?

“I’m still very fascinated when I watch this stream that walks and leaves behind itself something completed. It’s as if it were a magic box. In there there’s caos and a lot of noise, yet she advances and magically leaves in her path something ready to use, stable, that one day will be used by thousands of people, and you know how it was done.”