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Spain's biggest concrete span | Almonte Viaduct

Almonte 23 jpg

The world’s longest high speed rail bridge is approaching completion in southern Spain.

Another title for this bridge is “the world’s third-biggest concrete arch”. In this regard, the Almonte viaduct’s span of 384m only falls behind the Wanxian bridge in China, which has a 421m arch, and the KRK bridge in Croatia, whose arch spans 390m.

World records aside, the 996m long Almonte viaduct stretch is a key section of the high-speed line linking Madrid and Lisbon. When high speed trains actually start operating on this line, well, that is a different question.

The Madrid-Lisbon high speed line has much promise, but construction has yet to begin on the Portuguese portion of the line. The European Union (EU) has provided much of the funding for this project, upwards of €500M (£431M) for the Spanish portion alone.

Source: FCC

Valuable experience

The challenges and scale of the new bridge are similar to those expected on High Speed 2 in England. It is no coincidence that FCC have created a joint venture with Laing O’Rourke and Murphy to bid for work on the UK project.

FCC in particular has worked on many high speed rail projects in Spain, laying 700km of track in the process – 32% of the country’s high speed network.

One reason companies like FCC are looking abroad for contracts is Spain’s economic woes. The Almonte Viaduct is a good display of how of how infrastructure investment throughout Spain was affected by the well-known economic challenges seen across Europe. Originally set to take two years and eight months, the construction programme was slowed down and stretched to five and a half years. Work on the viaduct started in 2011, and the last part of the decking will be cast next week, finalising the bridge’s main construction phase.

Almonte clip image001

Almonte clip image001

The concrete arch extends, using the temporary towers and piers as cantilevers.

The build

FCC Construcción has built the bridge as part of the contract for a 6.3km section of the route in the region of Extramadura. The contract includes more than 1M.m3 of earthworks, four large viaducts, two overpasses and one underpass.

The Almonte Viaduct is the largest of the bridges: of the £69.8M contract, it accounts for £39.1M of the project’s value.

The bridge takes the railway across a 50m deep section of the Almonte river. Early on in planning,  Spanish high speed rail infrastructure manager ADIF, and its design joint venture — Arenas y Asociados, Madrid, and IDOM Ingeneria y Consultoria, Bilbao – ruled out putting piers into the river. As determined by the new line’s environmental statement, piers were not permitted to be placed within the river. In addition, the proximity of the viaduct to the Alcantara reservoir, which the river flows into, meant the water levels can vary, depending on electricity production and related water demands.

Almonte 5 vert jpg

Almonte 5 vert jpg

The formwork travellers, working on the bridge’s deck in late June, meet in the centre of viaduct.

With river piers ruled out, early options for the span were a beam bridge supported by v-shaped abutments on either side of the reservoir, or a cable-stayed bridge. But a concrete arch and deck design was deemed the optimal solution – aesthetically and for whole-life cost.

The giant arch, has been built by two 240t formwork travellers working along the arch from each end. They have been designed specifically for this arch due to the complexity of the arch geometry. The two vehicles produced cast-insitu segments, with the segments’ cross sectional shape changing from octagon to a hexagon as they reach the apex. As the arch was built out from the river banks, its two halves were held in place with temporary cable stays rigged to the abutment piers and temporary steel towers resting on top of them and supported by cables attached to the bridge deck on the arch approach spans.

The bespoke 54.6m tall temporary towers were designed by FCC Technical Services. They sit on pin bearings, allowing independent movement from the rest of the structure, and taking loads of up to 700t. In total there were 208 cables.

The arch segments are 6.9m deep at the base, then gradually become slimmer as they reach the apex.

Almonte 4 vert jpg

Almonte 4 vert jpg

Two hexagonal concrete box sections of the arch extend from their abutment, each measuring 6.9m deep.

During the detailed design stage the number of segments forming the arch was cut from 52 to 32  to reduce the number of movements of the travelling formwork. It was a costly change and, among other factors, it led to a 30% overrun in total costs. Another costly hurdle was the need to include 2,000t of extra reinforcement for temporary loads.

Inside each arch segment is a piece of innovation: a developed-but-not-yet-launched sulphate-resistant, high-strength cement called UltraVal, from Cementos Portland Valderrivas Group, an FCC affiliate. Extensive prototyping and testing occured before it was used on the bridge.

“We have 60,000m3 of all types of concrete, from common 20MPa to 30MPa, to 80MPa self compacting concrete for the arch, to 60MPa concrete for the deck,” says FCC Construcción head of railway infrastructure Pedro Cavero de Pablo.

Two concrete pumping lines were placed along the cantilever to supply the travelling formwork. These were fed by a pair of static pumping machines positioned at the beginning of each leg of the semi arches. A vast fleet of truckmixers carried the concrete from the batching plant located at the site facilities to the arch site.

Finally, the bridge deck was built, by a movable scaffolding system. The deck is connected to the columns with elastomeric bearings, allowing the deck to move longitudinally with respect to the columns. The longitudinal forces from trains braking go through a fixed point which is 42m long, connecting the arch and the deck.

 

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