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Tunnels are one of the breakthrough innovations in construction that bridge the gaps and connect far-off places. But maintenance of tunnels is still a challenging process, especially if the concerned route doesn't have any alternatives.
One such challenge is being tackled head-on by the team working on the renovation of the 2,600m-long Leopold II Tunnel in Brussels, Belgium. Built in 1987, the tunnel caters to a massive traffic of 60,000 vehicles a day traveling along its four lanes.
Naturally, after 30 years of its opening, the tunnel displayed a certain degree of degradation, from ceiling to road deck, along with the entry and exit ramps. But the biggest challenge for the team was to carry out the work without disturbing the ongoing traffic in the tunnel. The most important questions posed were: How to carry out inspections? How to track potential issues?
Stopping traffic in such a crucial artery for every maintenance check-up was not a feasible option. To solve this problem, the engineers at BESIX performed a photogrammetric survey inside the tunnel, with the aim to capture an orthomosaic of the concrete slab under the asphalt.
An orthomosaic is an enhanced image with very accurate details and high resolution. Just like Google Maps, but far more precise. The orthomosaic is made up of a combination of ortophotographs, which are aerial photos that have been improved to create a perfectly straight aerial view.
As each image will be geo-referenced, it will allow the engineers to easily access the information, locating potential defects within centimeter accuracy. The precision and accuracy of orthophotos means orthomosaics have the same quality as maps and can be used to measure true distances.
How was the Survey Carried Out?
Usually, the orthophotographs are captured by drones by mapping out sites in a high-quality, data-packed way. But the engineers had to devise a more creative way as the survey was to be carried out inside a tunnel.
They captured the orthophotographs using a system of three cameras installed on a car, with each camera shot at the exact same angle to collect the data. The system had to maintain accuracy and speed as the concrete slab under the asphalt was visible only for a short period of time before a new layer of asphalt was poured over it.
Image Courtesy: BESIX
It took around two weeks to prepare the car and map out the entire tunnel. As a result, a 3D model was generated based on all the orthophotographs.
This would help the engineers identify the exact cause of a defect by enabling them to look into that specific underlayer whenever a defect is visible. With this data, BESIX will be able to maintain the tunnel for a period of 25 years, without even having to conduct another survey and without interrupting the traffic in the tunnel.
Orthomosaic images are particularly useful as they can be created and updated comparatively quickly to give up-to-date maps of the on-ground conditions. They provide accurate information regarding which part of land needs to be excavated, and which obstacles need to be removed, or holes need to be filled. This allows construction companies to save time by enabling accurate planning for deliveries, equipment movement, or excavation on a site.
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