The high speed railway linking the three major cities of the Basque region in Spain: Vitoria, Bilbao and San Sebastian will consist of three branches with a central node serving as an interchange. This will be located near Elorrio, an industrial town in a mountainous region, 39 km away from Bilbao. The two-track alignment is designed for an operational speed of 220 km/h, which results in a minimum radius of 2500 m and a maximum gradient of 2.5 %. Due to these requirements and a complex topography, many structures are needed. One of the segments within the central node will need five viaducts and a tunnel despite being just 3 km long. Pedelta carried out the original design of these viaducts in 2011, the longest of which will run across the Zumelegi valley.
The original design for the Zumelegi Viaduct consisted of a continuous eight-span viaduct, with a total length of 451 m and a longest span of 65 m, simply supported over piers, having the tallest of these a height of 74 m. The deck was to be a constant depth post-tensioned concrete box, to be built span-by-span with a movable scaffold system (MSS).
Pedelta was later asked to modify the design to meet the requirements for the future widening of the road below. The new configuration of the viaduct required increasing the span length to minimize the interferences of the piers with the future northbound carriageway of the road, which led to the decision to eliminate one of the piers, and redistribute the span lengths, obtaining a final layout with seven spans: 51+75+80+85+85+67+38 m, with a total length of 481 m. Due to this change, the post-tensioned concrete box will have a variable depth within the five central spans. The deck, which is 14 m wide and curved in plan, will carry two rail tracks. The piers, composed of a bicellular hollow box will have a maximum height of 70.36 m.
The Zumelegi viaduct, according to the new design, will be cast in place using the balanced cantilever method. As a result, construction will minimize the environmental impact and not interfere with the traffic underneath. Once completed, it will become one of the longest HSR bridges in Spain built with this method by length of their main span. Construction is expected to begin in 2020.