Tunnel Leopold II: how to maintain a tunnel for 25 years without stopping traffic?

10 September 2020

A small BESIX innovation with massive impact

At the Leopold II Tunnel in Brussels, tens of thousands of vehicles are passing through each day. Motorists from all over Belgium and beyond are using the major traffic axis to make their way to and from the European capital.

Thirty years after its initial opening, an in-depth renovation of the tunnel is needed. Structural repairs, wiring, waterproofing, asbestos removal and a new road deck are the challenges to be tackled. In 2018, Brussels Mobility enlisted the help of CIRCUL 2020, a consortium made up of BESIX, Jan De Nul Group and Engie-Fabricom, to successfully execute the job.

Apart from the renovation works - currently ongoing - to design and build the infrastructure, the contract includes a 25-year maintenance deal to make sure it is future-proof.

But how will you maintain the longest tunnel in Belgium - 2,600 metres - with 60,000 vehicles travelling along four lanes on a daily basis? How will you carry out inspections? How will you track potential issues? Stopping traffic in such a crucial artery every time a maintenance check-up is required is not an option.

That’s where innovation at BESIX comes in.

Our BESIX engineers have performed a photogrammetric survey inside the Leopold II Tunnel. The aim was to capture an orthomosaic of the concrete slab under the asphalt.

An orthomosaic is a large quality image with high details and resolution. Think of Google Maps, but far more precisely. It is made up by combining ortophotographs, aerial photos that have been “orthorectified”, i.e. corrected in terms of perspective to create a perfectly straight aerial view.

This survey will particularly be valuable in the long term. With each image being geo-referenced, our engineers will be able to easily access the information, locating potential defects within centimetre accuracy.

So how did we carry out this study?

Normally, we would bring out the drones to map out sites in such a high-quality, data-packed way. However, seeing that we’re inside a tunnel, our engineers had to be more creative. So they’ve captured the orthophotographs using a system of three cameras carefully installed on a car. Each camera shot at exactly the same angle to register the data. Accuracy and speed were key during this process, as the concrete slab under the asphalt was only visible for a short period of time before a new layer of asphalt was poured over it. It took the BESIX team about two weeks to prepare the car and map out the tunnel.

The result? A 3D model was generated based on all the orthophotographs. Here’s how that looked 👇

Whenever issues would arise in the future, where defects would be visible, our engineering team can look into that specific underlayer and identify the exact problem, from a distance. In the meantime, inside the tunnel, tens of thousands of motorists would be able to smoothly continue their journey.

It’s innovation at a small scale with a massive impact, developed by BESIX.