Before its completion in 2009, the connection between the towns of Olot and Vic had long been one of the main priorities for the counties of Garrotxa and Osona in Catalonia. Due to the unique importance of the project and singular surroundings: the Olot Volcanic Area Natural Park as well as several other protected landscape areas and villages, the environmental impact had an extreme importance and was closely followed by local residents and environmental organizations.
One of the main structures on this new road was necessary to gain the sufficient altitude to reach the tunnels that would connect the two valleys. The Vall d’en Bas Viaduct would be long: 565 m and very visible. To minimize the visual impact, it was required to be as transparent as possible: a sober structure with moderately long spans. With these premises in mind, the decision towards a composite steel-concrete solution was compulsory.
The viaduct is composed of nine spans up to 80 m long. The total width of the deck is 14.5 m, with 3 traffic lanes. The deck is a steel tub girder with a variable depth within the first six spans and a constant depth within the last three shorter spans. This depth varies from 1.75 m at the center of the spans to 3.85 m at the intermediate supports. The variable depth and the constant web slope results in an also variable bottom flange width from 6 m over the supports to 6.8 m at mid span. The steel section includes two 4.1 m long cantilever outriggers that complete the overall width, spaced at 4.3 m. The top concrete slab has a constant thickness of 80 cm and was designed to act compositely with the main box girder, including the transverse diaphragms and cantilever outriggers. The slab was cast in place on top of 25 cm thick precast reinforced concrete panels which also served as formwork.
The construction method was required to be environmentally friendly. Consequently, once the drilled piles and the abutments had been built, the steel girder, including the cantilever outriggers was assembled on site. The girder segments were put in place using cranes, without the use of shoring. Splices were located in such a way as to produce the length of 3/4 of one span and 1/4 of the following span and were all welded on site. An exhaustive quality control process was undertaken, whereby all the welds, both off and on site, were checked using techniques such as ultrasonic testing, X-rays and magnetic particles.