Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Spanish package deal Flood mitigation work is a key part of one of Spain's largest ever motorway projects.

A vast road, rail and river corridor is undergoing a major transformation on the southern outskirts of Spain's Catalan capital, Barcelona. Running for 20km along the Llobregat river the Ptas30bn (£120M) project combines a new motorway, realignment of railway lines and a flood mitigation scheme.

At the heart of the project is the new motorway, needed to ease the congested N2 trunk road which passes through the 10 towns lining the route.

The N2 currently has to cope with 30,000 vehicles/day. Although it is already bypassed by the toll financed A2 motorway, this too is becoming overloaded, handling 160,000 vehicles/day. N2 traffic is also swollen by local and medium distance travellers not wishing to pay tolls for relatively short journeys.

The new toll-free A7 motorway is the answer. It provides short distance traffic relief, and is expected to handle around 90,000 vehicles/ day, taking about 50% of the traffic off the toll road as well as relieving the N2.

The route for the new road is constrained by the steep sided Llobregat valley. As a result it is very close to the river, crossing flat flood plains and hugging mountain sides.

While the national government was developing plans for the motorway, the Catalan regional government was developing its own proposal for a flood defence scheme for the Llobregat.

Normally the river flows quietly through the valley at between 30m3/sec and 40m3/sec. But heavy rains swell the river exponentially, causing floods. Storms in 1972 increased flows to 3,600m3/s, boosting the case for a major flood defence scheme.

The road scheme was first to get started when in 1995 the £93M main contract was awarded to a joint venture between construction giants Dragados and FCC.

Soon after, the national and regional governments agreed that combining the flood defence and road projects could produce the most efficient solution for the flooding and congestion problems.

Government officials point to the fact that the eastern end of the project involved the most excavation work while most of the fill was needed at the western end. Splitting the project into two halves would thus have split the cut element of the job from the fill, complicating the earthworks.

The contract was therefore extended to include another £27M of flood mitigation works. The resulting project is one of Spain's largest contracts ever to be awarded as a single package.

At the western end, work involves slicing into two mountains to create a path for the new road next to the river. Fill from these areas will be used for embankments which will carry the motorway across flood plains to the east.

Space in the valley is tight, and the new road is wedged between the river, local railways, the A2 and N2. Near Pallej the contractors have had to drill and blast a 200m railway tunnel and a 120m road tunnel to realign the N2 and a local railway line out of the path of the new motorway.

Interchanges between the new and existing roads are also constrained and in some cases have to cross the river. Of the project's seven river crossings, the biggest are two 500m steel box girder structures at the eastern end. Their 50m spans stand on steel encased concrete piles driven between 15m and 30m into the river bed.

Steel figures prominently in the bridge designs, with most structures taking the form of box girder structures. One unusual 318m crossing of the Llobregat appears to be four 58m span arches with two conventional side span girders of 48m and 38m. In fact the structure behaves like girders but incorporates steel curves beneath the deck for aesthetic reasons.

At peak the contractors had 300 workers on site, although numbers are expected to fall this year as work is due to finish in October. 'Initially we were expecting to finish at the end of last year, but the timetable was expanded because the river works were not included in the project at the beginning,' says Public Works Ministry official Pedro Luis Brotons.

Although the project is run as a single main contract, subcontracts for materials such as steel have been divided up. The viaducts will use a total of 12,378t of structural steel and Dragados/FCC was wary about awarding such a large amount of work to a single steel subcontractor. Instead steel contracts have been split between five Spanish suppliers.

In some ways the motorway and flood mitigation projects are inextricably linked. So far around 4M.m3 of river bed gravel has been dredged up and re-used in the motorway embankments, reducing the need for the contractors to find gravel from quarries. 'The material we are using is very good, it's a luxury,' says a government official working on the project.

Heavy rock from the river bed is also being used to armour the embankments, protecting them against fast flowing flood waters.

Re-engineering the river involves changing the profile of the river bed, flattening the slope with fill imported from quarries 10km away. Contractors have also demolished small, 10m high weirs which had silted up over the years.

New proposals for further flood protection measures are under consideration. Consultant GPO Ingenieria which is the government's engineer for the project is currently considering extending Dragados/FCC's contract to include the construction of a box tunnel under part of the motorway route.

The box will act as a sluice to funnel excess river flows around a tight bend in the river. It will have to be built under the line of the road, which is currently planned to run on embankment.

This embankment has still to be built and Dragados/FCC is awaiting a decision. If the contract is extended, it will build the box insitu, cover it and run the road over the top.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.