On July 20, 2022, the heat from an exploratory drill inside Montréal’s century old Mont-Royal railway tunnel, advance work for the city’s modern Réseau Express Métropolitain (REM) automated Métro line, caused an explosion. It was completely unexpected, and mystifying, as there should only have been solid rock there. Fortunately no one was injured and no fire resulted, but all work was suspended for a time.
Engineers investigated and discovered that there was actually some unexploded dynamite left in tunnel crevices, which had been used extensively to blast through this mountain on the north side of downtown Montréal a century ago. But the dormant explosive material was ignited by the heat from the drill tip. Fortunately, all drilling was performed by remote-controlled equipment.
Enter the Dragados
Jacobo Trigo is the Dragados Construction Technical Support and Surveying Director on the REM tunnels, working on relining the Mont-Royal tunnel. Having been blasted out from 1912 to 1917 by Canadian Northern Railway (CNoR), predecessor of Canadian National Railway (CN), the tunnel required modernisation, waterproofing, and reinforcement in the downtown section.
REM’s contractor NouvLR has been excavating caverns on the Mont-Royal tunnel alignment for two stations downtown on the REM line (in lime green), to interchange at McGill on the (dark) Green Line, and Bonaventure – Gare Central (Central Station) on the Orange Line.
When the engineers evaluated the original 1918 McGill station tunnel segment, they found that de-icing road salt from the streets above had leached into the soil and corroded the tunnel lining’s steel rebar. So corroded was it that the tunnel lining was holding itself up, which could have become a disastrous structural failure. The original tunnel builders also did not install a waterproof membrane, as is modern practice, as the tunnel had been dug in a time before widespread car use, so road salt wasn’t yet being applied. However, with the large snowfalls Montréal is renowned for, de-icing salt has been used liberally for past half century. Consequently, there has been a lot of salt water leakage into the tunnel, which of course is corrosive to steel rebar. In particular, this was a problem at the two new REM underground stations downtown at Métro McGill and at Gare Centrale (Métro Bonaventure). Fortunately, this tunnel segment was rebuilt.
Monsieur, the tunnel lining is wafer thin
On August 12, 2022, this writer was part of a media tour that visited the Mont-Royal tunnel between McGill and Gare Central stations. The platforms and elevator shafts were being roughed in at McGill station, and it was 150m south to Gare Centrale station, the hub of Montreal’s commuter rail network, as well as the VIA Rail intercity terminal.
Work was underway in this section to spray shotcrete, part of the work to create a waterproof lining within the original tunnel. However, the gauge of the trains and the overhead catenary left little space for the tunnel liner – a clearance of a mere 3.75cm on average. The standard method was to manually spray shotcrete in a halting and laborious process: spray – stop – measure – spray – repeat. But the tunnel face was not even – so the shotcrete could not be applied as a blanket coat.
Traditional laser surveying sensors would indicate to workers measurement markings which were applied manually on the tunnel surface. However, as soon as the shotcrete was sprayed, the wall markings were covered over. So work would have to stop to to be rescanned, then the data sent back to the office to compare them against the theoretical drawing point clouds to determine if the new shotcrete layer was sufficient, or if more were necessary, and approximately how much more were required. This survey – spray – measure – repeat process was time consuming, repetitive, and frustrating. Trigo, the aforementioned Construction Technical Support and Surveying Director, realised that a prototype created by Dragados, his company since 2006, could be used on REM. He put this prototype into operational service.
It was a multifaceted endeavor that required the implementation of new features, rigorous testing, collaborative decision-making, continuous improvement, training and support, and a deep understanding of various technical, logistical, and safety considerations. The most demanding one was to embrace new technology in an ongoing project due to established norms, resistance to change, and a lack of familiarity with modern tools. Successful adoption of new technology requires patience, persistence, and a well-thought-out strategy. Thanks to the dedication and hard work of many people over many months, this resulted in success. [Added paragraph]
Dragados can build it
Dragados (Enrique Fernandez, Alfredo Muñoz and Miguel Segarra) built an automated system to measure the tunnel surface, perform its own calculations of the surface level in near real-time, then project it onto the tunnel face. Furthermore, it wanted to use off-the-shelf components as much as possible. Dragados built the first system, called SOGUN, in the lab to prove the concept. Advances in computing power provided the capability to calculate the surface data points much more quickly at the source. From that unit, a smaller prototype unit was developed that could be lowered through a work shaft to the tunnel and easily be towed into position. [Replaced paragraph]
Scanning and projecting the surface level
The system uses a Lidar (Light Detection and Ranging) sensor – similar to radar but using light instead of radio signals – laser beam pulses to digitize the tunnel surface. The on-board computer then computes the 3D surface scan in under a minute. The SOGUN unit processes the data and compares the scanned surface with the required final surface profile. At that point, the SOGUN laser light projection system paints different colours and patterns to onto the tunnel surface to differentiate the surface levels – which shows exactly where more shotcrete is required. The unit also provides data to indicate to the shotcrete sprayers precisely how much concrete should be sprayed.
As the shotcrete is applied, the system automatically scans and remeasures the surface levels, and adjusts the projected colour topography to indicate to the workers what the new surface levels. Typically, there are hundreds of thousands of data points in the scanned point clouds, but it’s only a 35 second process to scan the tunnel face, compute the surface profile, then project the surface topography as a colour field. So far, this technology has enabled a tenfold productivity improvement over the manual method of the survey – mark – spray shotcrete – stop – resurvey process. Furthermore, the projection allows continuous spraying as the system provides near immediate feedback to the workers. This makes the process like a video game, much quicker, and actually fun for the workers, as well as producing a more accurate surface profile.
Expectations of this technology for the prototype were high, but so far the experience has been even better than expected. The construction team thinks of it as a kind of magic and took to it right away. It creates a fun, new work environment as the workers really enjoy the dynamic nature, finding it motivating to get work done so quickly. Given that tunnels are always dusty and/or muddy affairs, working in relentless dark on 12-hour shifts up to 14 days consecutively, this new unit also gives workers colours they don’t see underground, other than reflective hi-vis construction vests and orange construction equipment.
The projection is amazing and beautiful in a sullen, dark tunnel:
And it resemble the colourful Ben-Day dots of 20th Century comic strips:
And here’s a video of the tour in the tunnel starting at the McGill station cavern.
A hitherto unexploited feature of this new technology is its ability to scan, measure, and project unto unusual shapes in tunnels, such as vent shafts, maintenance access tunnels, and arches. It compares the construction drawings to the physical reality of the tunnels, even complex geometries.
The SOGUN operator selects the colours and patterns to indicate the different surface heights. The idea is to support the construction team, not to make them learn a particular data set or technology.
Work at workers’ pace, not the equipment’s
Another benefit is that workers can can schedule their own breaks – they don’t have to wait for surveying equipment to be set up to measure or re-measure the tunnel surface, nor wait for the calculations to be performed. They are a lot happier as they can eat or relax on their own timetable, not the equipment’s.
Development
This real-time scanning and projecting technology is a first for the tunnelling industry. It is the result of three years of development in Spain. The first pilot implementation has been in Montréal. But it can be used for any tunnel construction or repair, as well as for gauge clearance. You may recall that LR had previously featured Dragados Bank Station Part 3: A Dance of Dragados.
This SOGUN unit has also been used to help construct the 350m long new tunnel that NouvLR is constructing for the REM branch to Aéroport Montréal (Pierre Elliot) Trudeau (Prime Minister Justin Trudeau’s père). Dragados also plans to use it on other projects in the US, Spain, and Chile in the coming months. [Updated paragraph]
Baaam!!! It’s Pop Art
The SOGUN projections are, surprisingly, quite similar to the works of Roy Lichtenstein, who was a contemporary of Andy Warhol in the Pop Art movement of the 1960s. Lichtenstein used the colour and style of comic book and children’s colouring book Ben-Day dot printing technology. This was a relatively simple mechanical colour printing method that had been patented in 1879 and named after its inventor, printer Benjamin Henry Day, Jr. The method small dots of colour (typically cyan, magenta, yellow, and black) that are printed repetitively but spaced at various distances from each other to create shading effects. The dot colours could also also combined to create additional colours. Being a simple process, it was inexpensive to print, which led to it being used predominantly in newspapers and comics.
Ben Day may well have been inspired by the French post-impressionist technique of painting known as pointillism pioneered by painter Georges Seurat. The Frenchman was himself inspired by contemporary scientific research into perception of optical effects, which he adapted to his paintings. His overlapping fields of small coloured dot are perceived in the human eye as hues. His most famous painting, above, A Sunday Afternoon on La Grande Jatte, is one of the best known Impressionist works.
Whilst impressionist painters meticulously applied each dot by brush, resulting in merging, intricate colour fields, Ben-Day dots were printed, and were hence of uniform size and distribution.
It is not often that there is a parallel between the hard sciences of infrastructure construction and the realm of fine art. One example has been the the surreal Métro of Charleroi, and with the Dragados SOGUN technology, demonstrate that the sciences and the arts are not as far apart as they initially seem.
Un grand merci à la Caisse de dépôt et placement du Québec (CDPQ) pour le tour du tunnel de Mont-Royal, et pour l’entrevue avec Jacobo Trigo de Dragados.
Great article!
Fantastic, relatively simple and genuinely useful technology.
They will love this in Subterranea Britannica