TBM and large infrastructure: the head of the mechanical mole Stefania at Milan’s Triennale

The achievements of the technological giants that dig invisible tunnels beneath our cities

There’s nothing wrong in calling it “mole” or “mechanical mole.” It reminds you of the meticulously hard work of those small mammals, master diggers of gallery systems in the search of food, capable of advancing underground, leaving behind them a new road and opening in front of them new communication galleries. The extraordinary capacity of the little moles is amplified by the imposing technology of the mechanical moles, technically called TBM, Tunnel Boring Machine. A name that seems to almost diminish its value, but there’s nothing boring about these diggers.

Ever since the first experiments at the beginning of the last century, the TBM has opened new horizons and, as the years brought the development of new solutions, these machines became ever more sophisticated electronically, efficient at greater depth, huge in size and yet easier to handle, comfortable and safe for the workers handling them as they guide the digger. TBMs can perforate, or rather mill, with extreme precision, stone and concrete to create rail, road or hydraulic tunnels. Their cost varies on the diameter of the boring head and the length of the TBM, and can reach several tens of millions of euros. A cost that can bring a multiplier effect in benefits for the residents of an area where a tunnel has been dug.

The show “Building the future, infrastructure and benefits for people and regions,” promoted by Webuild and Milan’s Triennale, which opens to the public March 3-26, will include the head of the Stefania TMB, one of the six mechanical moles used to dig the tunnels of the city’s new Blu Line. It is a technical prodigy capable of burrowing underground at a speed of 18,5 meters a day, part of the “family” of 40 TBMs currently in use by Webuild in its many projects.

The TBM Webuild uses for its projects

With 3,400 kilometers of tunnels dug around the world, Webuild has taken on a primary role in these strategic mobility and environmental infrastructure projects that bring wellness to the areas and residents affected. Iconic works such as the Cityringen in Denmark, a ring of 17 subway stations, 30 meters deep, beneath the city of Copenhagen, and the construction of the Brennero base tunnel between Italy and Austria, at 64 km long destined to the longest rail tunnel in the world, are unique experiences approached with innovative projects and technical solutions.

TBMs are custom built for each project and, usually, are dismantled on site once the work is completed.  In the last decade, “the largest, fastest and deepest” dig in the world, according to the American Society of Civil Engineers (ASCE), was one of the most complex projects carried out in the heart of the USA, beneath Lake Mead, near Las Vegas. The Lake Mead Intake N. 3 project, conceived to bring water to the parched city of gambling, was completed in 2016 by the Webuild group via the boring of a hydraulic tunnel 4 km long dug at a depth of 200 meters in the center of the lake.  The TBM had to withstand a pressure of 15 bar, compared with an average that ranges from 1 to 3, and at the end joined perfectly with the vertical shaft build to pump water to Las Vegas.

Another American project, next to Washington D.C., called for Webuild and its unit, Lane Construction, to solve the problematic pollution of the Anacostia river by digging a 3.8 kms long water tunnel, at an average depth of 30 meters, that conveys rainwater separately from waste water.  The city’s old sewage system was often clogged when hit by torrential rains. The city had to take measures to avoid that a toxic mix of untreated waste water end up in Rock Creek and in the Potomac and Anacostia rivers. During the crossing of the Anacostia, the TBM bore through clay and sandy ground, under a pressure as high as 3,5 bar. In November 2017 the project received the Sustainability Initiative of the Year award from the International Tunneling and Underground Space Association (ITA-AITES) and in 2018 the Award of Merit for Water/Waste Water from ENR Global Best Project Award.

Decades of experience garnered on all continents

From America to Australia, from Europe to Africa to Asia, Webuild’s experience spans decades and has seen its TBMs in action on every continent.  Given its landscaped, Italy has been at the forefront of tunnel digging techniques. The first tunnel dug by a TBM was bored 54 years ago by SELI, by far one of the most experienced companies in the world, which became part of Webuild group in 2021.

In time, TBMs became crucial to digs for subways, highways, aqueducts and sewage treatment plants. The need to build ever longer tunnels came in hand in hand with ever more complex geological complexities and variables, in sites with rocky, loose or crumbly terrain. TBMs had to evolve and transform to adapt to individual projects, with one or more frontal digging shields, different equipment and technical teams depending on the tunnel’s coating, the removal of the detritus and the underground conditions, that sometimes have been extreme.

The TBM “mole” is capable of opening new horizons for an ever more sustainable future. Once a project is completed, the technicians of a water tunnel, the passengers of a subway or the motorists driving through a highway tunnel will be able to admire a new infrastructure jewel, a work of art, built thanks to the incredible talent of men and women and their TBM.