DECEMBER 2022 Ӏ JOB OF THE MONTH
j “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;
10 CRANES TODAY
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.
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