Landmark operation29 September 2022
Mammoet lowers bridge arch sections, weighting up to 1,150t each, in compact job site accessible only via tunnels using a swivel arch lowering methodology utilising strand jacks for minimum ecological impact.
The continued growth of the port of Bilbao, Spain, resulted in greater pressure on the city’s aging road network. Combined with urban sprawl and increased vehicle usage there was frequent congestion in the city.
To solve this problem the regional government, Interbiak, commissioned the new Supersur relief road to allow traffic bound for the port and the coast to avoid travelling through the city.
One of the final sections of the route to be built required the construction of twin viaducts spanning up to 162m across the scenic Bolintxu valley to link road tunnels on each side.
Apart from the large span of the bridge, a key challenge was that the Bolintxu valley is a protected area of natural beauty and of ecological importance, so the project needed to ensure that any structure crossing it had a minimal footprint and avoided disrupting the area during installation. This meant that a conventional approach of using temporary scaffolding or support towers down in the valley could not be used.
Interbiak and structural engineering consultant Arenas&Asociados had already determined that the design of each bridge would comprise an arch spanning the valley with supports only in the form of abutments on each side. For the best way to install the arch, however, they turned to Dutch heavy lift and transport specialist Mammoet.
“From the beginning we thought the lowering method could be an effective alternative,” explained Alejandro Godoy, project manager at Arenas&Asociados. “Being aware that Mammoet was able to deliver such operations, having seen their work at the Alconetar Arches Viaduct over the Tajo River and the Bolueta Arches over the Nervion River, for the City Metro of Bilbao, they were our first call to establish the best common approach.”
In order to avoid disruption to the valley, and river, below, Mammoet settled on using a swivel arch methodology, which would allow construction of the bridges by lowering an arch segment from each side of the valley.
“Although this technique has been used a handful of times in recent decades, it is not common,” said Veronica Arrayago, Mammoet project manage. “Further, whilst similar bridge spans have been installed this way before, the weight of each arch section – up to 1,150t – had not. So we were facing a landmark operation in every sense of the word to ensure the bridges were put into place safely and efficiently.”
The approach would see each arch erected in two halves – one at each side of the ravine – before lowering and tilting them slowly to meet in the centre. Each semiarch would be built upright, supported by a temporary scaffolding tower and mounted on top of a large swivel. Once both semiarches were completed, they would be simultaneously tilted down and lowered, rotating on their swivels in a smooth and controlled strand jack operation.
“We recognised that moving such large components would present a challenge and demand some of Mammoet’s highest capacity strand jack equipment, especially as the final position of the bridges meant they would face very high tension loads,” continued Arrayago. “However we needed to be creative in the design of the equipment and where it was installed because the space between the tunnel openings and bridge abutments was extremely limited; the tunnels had to be used for tasks such as laying out strands as they were threaded into the jacking equipment. Furthermore, the tunnels were the only access route in and out of the site and would also be in use for other project workflows at the same time – so an extremely thorough plan across all phases was required in order for us to execute successfully.”
Unique equipment proved key
The result was a methodology that allowed each semiarch to be moved directly from where it had been constructed onsite. Two bundles of steel cables were connected to an elevated section of each semiarch, which was then be moved via strand jack to lower the section. The strand jacks were mounted on tailored swiveled steel frames that had been fully engineered, designed, and manufactured by Mammoet for the project, and connected to a temporary foundation behind the abutment.
Due to the massive forces at play during the operation Mammoet used its 1250t strand jacks. “They are the largest in the world and unique to us having been custom designed to our specifications,” said Arrayago. “They proved invaluable for the Bolintxu project as it allowed us to use fewer strand jacks for each arch, reducing time and cost for the client and making best use of the highly limited space on site.”
The commencement of an operation of this kind is triggered by breaking the static balanced position of the semiarches when they are still connected to the temporary scaffolding towers used for their construction. For this purpose, Mammoet deployed six push-pull cylinders of 64t capacity behind each arch to generate the sufficient force for the semiarch to start its way down into the valley.
Each bridge took between two and three days to successfully install. The first day was spent completing the slow and careful equilibrium-breaking process, while the remaining time was used to lower the component smoothly until it was in place ready to connect to the opposite side.
Bilbao now has a faster and less congested route to take traffic away from the centre of the city; crucially this has been done with minimal impact on the local environment and no disruption to the delicate ecology in the Bolintxu valley.