In a remote corner of western Spain, the world’s biggest concrete-arch railroad bridge awaits the arrival of tracks and signaling for the planned high-speed corridor between Madrid and Portugal’s capital, Lisbon. Budget shortfalls delayed progress on the Almonte bridge, but they failed to stop the dramatic, cable-stayed arch from taking shape across the Alcántara reservoir’s mouth in the Extremadura region.

With a 384-meter main span, the bridge is now the world’s longest in steel or concrete for high-speed rail, according to the state’s railroad infrastructure manager, Administrador de Infraestructuras Ferroviarias (ADIF). It is also the third-longest concrete bridge for any use. In Spain, the arch overtook a similar structure having a 261-m span, built five years ago at Contreras Reservoir in Villagordo Cabriel, about 115 kilometers west of Valencia.

About 35 km north of Cáceres, the new bridge crosses the roughly 400-m-wide mouth of the Almonte River, where the river enters the Alcántara reservoir. It lies some 130 km from Badajoz, near the Portuguese border, around two-thirds along the planned 450-km corridor from Madrid.

Between the tops of roughly 70-m-tall pylons on either side of the river, the deck is propped up by columns at roughly 42-m intervals. From each abutment, the shape of the arch springs up as a pair of hexagonal sectioned legs, which merge at roughly quarter spans into a single box with an octagonal profile. Its depth reduces from 6.9 m at abutments to 4.8 m at the crown.

Barred for environmental reasons from placing even temporary piers in the river, the designers studied options to span its full width. Among their key criteria were structural efficiency and low maintenance needs, says Guillermo Capellán, technical director at Arenas y Asociados, Madrid. Arenas and Bilbao-based IDOM Ingeneria y Consultoria formed ADIF’s design team.

The designers chose a concrete arch from various options, says Capellán. Having worked on some 15 such structures in Spain, “we’ve had very successful experience with arches,” he says. For example, on the 217-m Third Millennium arch bridge built in Zaragoza nine years ago, “construction was easy to do,” he says. There, as at Almonte, “we used high-strength, self-compacting concrete, and we have no cracking,” he adds.

In terms of arch construction techniques, Arenas claims significant experience in the cable-stayed cantilever method used on Almonte, says Capellán. “The construction process is exactly the same as we developed. … We are very proud of that,” he says.

But transferring the method from paper to reality required early contractor participation, says Pedro Cavero de Pablo, railway division managing director at Madrid-based contractor FCC Construcción S.A. FCC led the joint venture with Portugal’s Conduril Engenharia S.A., Ermesinde, which built the 6.3-km Garrovillas-Alcántara reservoir section of the line, including the Almonte bridge. The contract was awarded in April 2010, according to FCC.

The contract was “almost design-build,” says Cavero de Pablo. “This was a very smart decision by [ADIF] as … the different technical means used during the construction have a very significant influence on the final design.”

After ADIF awarded the contract, it required FCC-Conduril to build its own numerical model of the bridge to calculate the structural effects of its proposed construction methods, says Jimenez. FCC’s technical department handled this work, and Arenas checked it independently using its own model.

While the analyses were done independently, “both teams worked side by side,” adds Jimenez. The Alan G. Davenport Wind Engineering Group at the University of Western Ontario did wind model tests for the bridge.

Almonte’s chosen erection technique has been used several times on other Spanish bridges, says Jimenez. By casting the arch in cantilevers from both abutments, the system was highly flexible, allowing the geometry of the growing structure to be fine-tuned, he adds.

To build the arch, concrete teams used traveling formwork, supplied by Castellón-based Rubrica Ingenieria Y Architectura S.L. They first cast the two hexagonal section legs at the arch ends and progressively modified the travelers’ geometry to form the single, octagonal central section.

The formwork’s complicated, variable geometry and the high rebar density made concrete vibration impossible in places. So, the contractor used self-compacting concrete with 80-MPa compressive strength at 28 days, a type never before used in Spain, according to FCC.

As the arch cantilevers grew, the contractor initially tied them tied back with cables to the abutment piers and then to a 54.6-m-tall temporary steelwork tower at abutment piers on either side of the bridge. Tuned mass dampers steadied the longest cables. Because the arch is so long, the contractor had to place two tower cranes on each growing half to serve the work fronts.

Spanish and Portuguese workers began building the arch in April 2012. They closed it in August 2015 and began deck construction over the span a few months later, completing the crossing last November.

When work was allowed to go ahead, the pace was “faster than we expected,” says Cavero de Pablo. But budgetary constraints forced the project into periods of “hibernation.” Coupled with land-acquisition snags, it took almost twice as long to build the bridge than the planned 32-month schedule, he adds.

The delays and other factors contributed to a cost hike of about 10% for the bridge, says Jimenez. But ADIF and the joint venture have yet to agree to a final price on the contract for the whole 6.3 km. The original overall budget was roughly $91 million, according to FCC.

ADIF adopted a slower pace of construction on this work after the sovereign debt crisis forced Portugal to shelve its part of the corridor some five years ago, undermining the project’s key purpose. However, Spain decided to push ahead, focusing on the economically depressed western half, where some 200 km is in construction or complete, explains Jimenez.

With track and signal work now in procurement, 140 km of line, including the Almonte viaduct, will be ready for diesel trains in 2019, says Jimenez. And if electrification work goes to plan, the first high-speed trains could be crossing the Almonte arch in a matter of seconds by this decade’s end.

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