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A zoom with a view

Roads Portugal

A new motorway will ease access to one of Portugal's remotest and most beautiful areas, but the terrain makes for difficult engineering, discovers Andrew Mylius.

Threading four lanes of new motorway through the steep, rocky hills north east of Porto, Portugal, is tricky by anyone's standards. As the viticulturists who produce their famous Port wines here know, flat land must be carved from the region's schist and granite slopes.

Motorway IP3, linking the town of Viseu with the Spanish border to the north, will cling to the valley sides on benches that will involve a total of 18.3Mm 3cut and fill. Where gradients fall away too steeply, and to negotiate the folds and side-valleys crossing its path, the road leaps onto viaducts.

Over much of IP3's 155km length shotcrete and rockbolts are being used to retain rock faces. Slopes are cut back using a combination of excavation on weathered schist, and drill and blast on competent rock.

Geogrid earth reinforcement and gabion walls have been used to steepen earth embankments - in places the motorway will be perched on sheer walls averaging 15m tall and in two locations 40m high.

IP3 concessionaire Norscut, which is 50% owned by French contracting giant Eiffage with French firms Egis Projects/ CDC Ixis and Contacto holding 25% each, has let much of the construction work to subcontractors. On one central length of the route however - section C - the technical challenge presented by the ground conditions has been simply too enticing to hand over and Eiffage is undertaking most of the work itself.

Section C marks a transition from granite to schist, where the slopes of the V-shaped valleys can reach almost 45degrees. It crests a hill just south of the town of Vila Real and then descends towards the River Corgo.

'We need to lose 600m height in 11km - the motorway's on a 6infinity gradient, which is steep, ' says Andrew Else, director of contracts for consultant Symonds, which is overseeing the ú360M project on behalf of the lending banks. Along much of the alignment it is impossible to place the carriageways side by side, and they are staggered one above the other. Section C also has seven viaducts, culminating with a multi-span, reinforced concrete, balanced cantilever structure over the Corgo. 'Section C is constant cut, fill, viaduct; cut, fill, viaduct, ' Else sums up.

Where possible the client, the Instituto das Estradas do Portugal - the Portuguese highways agency - favours viaducts over embankments as they help maintain continuity of the agricultural and natural landscapes. Depending on the viaduct's length, the distance between the hollow section piers is either 33m or 44m. They are footed on concrete pad foundations cast directly onto bedrock.

French subcontractor Spie has been brought in to tackle the smaller reinforced concrete viaducts on section C, the longest of which measures just over 300m. Elsewhere on IP3, viaduct decks are cast insitu using a gantry-mounted shuttering system supplied by Peri.

But with seven viaducts to build, decks are also being push launched from abutment casting yards for three of the structures. 'We've diversified to speed up construction, ' explains Norscut technical director Didier Payerne.

Each dual carriageway is carried on a separate viaduct. In places the height difference between decks can be as great as 10m and where it is using the gantry to cast decks insitu, Spie is forced to cast the higher deck first: The formwork system consists of wings which fold down, enclosing the gantry's belly. Pre-formed rebar cages are then placed and concrete cast. The formwork wings are folded out and up before the gantry is advanced for the next pour. Were the lower deck constructed first, the wings would snag on it when it came to constructing the uphill viaduct, Payerne says.

Culture clash: French take charge

There has been a minor clash of cultures on the Corgo Viaduct, by far the largest and most demanding of the structures under construction. The elegant five span, reinforced concrete, balanced cantilever bridge across the Corgo river was designed by Portuguese grand master of bridge engineering Professor Armando Rito, to normal Portuguese design codes. But Eiffage project director for the bridge Daniel Mary would like him to know, for future reference, that the French do things differently - and in his view better.

Portuguese designers provide little structural redundancy, Mary explains. 'They think only about the dead load and live loading of the finished structure.' However, French engineers would normally design in some extra beef to help them deal with forces in play only during construction - which on the Corgo Viaduct are fairly significant.

The 615m long structure curves on a very tight 500m radius in plan. The deck also curves slightly in side elevation. The three main spans are 142m each, with side spans of 94.5m. Eiffage calculated that, as designed, the hollow box section piers would bend unacceptably under eccentric loading as the cantilevered deck advanced.

'Every time there's a pour it adds weight to the deck which wants to pull the pier over, ' Mary explains.

The tallest pier stands at 80m, and would have flexed 60mm.

Rather than adding more rebar to the piers, Eiffage has installed temporary cables which are being incrementally stressed to counteract the sideways bending moment exerted by the temporary eccentric of the cantilever deck.

Once the deck has been completed and forms a single, continuous element it will become self-supporting and the cables can be removed. But that is still some way off, and far from the end of Mary's worries.

The viaduct deck is also underreinforced, in Mary's view, and is prone to vertical deflection. To combat this, Eiffage pared back the mass of the deck by some 800t and is taking extreme care to achieve high quality concrete.

Temperatures are being kept low during the baking Portuguese summer by lifting concrete from the batching plant in 3t skips rather than pumping it up - concrete could reach a staggering 85infinityC if pumped.

Superplasticisers are being used to reduce water content.

Once the concrete reaches a strength of 15MPa it is prestressed to prevent cracking.

The cables are stressed further when the concrete reaches 35MPa to combat creep. Final concrete strength is 55MPa.

Mary expects that in around eight years time restressing will be needed to restore the viaduct's hog, which will have dropped by approximately 150mm.

Formwork, supplied by French firm Ersem, is a self-advancing system that is cantilevered forward from C-beams stressed to the completed deck sections with macalloy bars. Internal formwork is a retractable 'springform' system held in place during casting by walers.

The first pour at the piers consumes 110m 3concrete, diminishing to 60m 3at mid-span.

At peak capacity Eiffage has been placing 10m 3per hour.

Mary is concerned about torsional and lateral oscillation in the columns induced by wind, and by deflection resulting from thermal loading caused by the hot Portuguese sun. As the cantilevers of the first deck near mid-span, he is worried about joining them.

To ensure proper alignment for the final concrete pour, 'Portuguese bridges require anchorage during closure', he gestures resignedly. Tendons running the width of the deck will be used to clamp steel beams to its outside edges. These will span the gap to be stitched, holding the cantilevers rigidly in place.

The final pour takes place at night to prevent any thermal stresses in the deck being locked in - expansion of the deck's top side over the cooler under side could have reduced the desired hog at centre span.

Construction of the Corgo Viaduct faces one challenge of Eiffage's own making. Work started at the piers closest to either abutment, and has been advancing towards the central spans. Because the deck has a concave curve in elevation, rotational moments are in play as the mass of the concrete responds to gravity. This is ever so slightly pushing the first and last piers over longitudinally, towards the centre of the bridge.

It is now impossible to correct the bend induced in the piers, Mary admits. But pouring the stitch between deck sections at night serves the double benefit of preventing contraction of the deck pulling the piers over still more, he says.

Ecological angle

Norscut is waiting on the Portuguese government's environment department to finalise the alignment of a stretch of motorway between Sections D and E, which could add ú85M to the construction cost. The concessionaire was proposing to route IP3 close to an existing local road through an area of national parkland on the eastern flank of a deep valley near the small town Vila Pouca de Aguiar.

Technically the simplest solution, with only two relatively short viaducts required, the environment department is concerned about its impact - particularly on wolves and butterflies. It has put forward a preferred route which will avoid the sensitive areas. Even though this will require construction of a 700m long tunnel and 2km viaduct which would rival France's Millau Viaduct (NCE 1 May).

Norscut in turn has resubmitted alternative alignments that skirt the edge of the national park before striking out across the valley on a 1.3km viaduct.

'Whatever happens we will be building a long bridge, ' observes Norscut technical director Didier Payerne.

Norscut is trying to figure out if it is best to increase the shadow toll rate per vehicle it will collect from the government, or whether it should bid to extend its 30 year concession beyond 2025.

It is also concerned that delays in starting on the disputed section will eat up most of the 15 month cushion that has been built into the construction programme to guard against unforeseen problems.

Landed with trouble

Moving material from where it is cut to where fill is needed is proving a major logistical challenge along the length of IP3. On stretches of Section 3, where the carriageways run at different levels, it is a question of moving earth and rock down hill. With slopes approaching 45infinity manoeuvring plant is no easy feat.

But the real problem is delays in land acquisition. Eiffage has been unable to drive haul routes between key sections of the motorway because client has not completed compulsory purchase of the route. There are on average 50 to 60 parcels of land to buy on every kilometre of the route, Payerne reports.

Eiffage is being forced to transport fill material over relatively large distances via tortuous local roads. There is a major cost in fuel, and it is impossible to transport the volumes needed, so delays in land purchase are impacting on the construction schedule and the environment.

Road range IP3 is a shadow tolled road.

Concessionaire

Norscut will be paid according to only two categories of vehicle - light and heavy. But the client wants to find out in more detail about the kind of traffic using the road. Sensors in the road surface will be able to put vehicles accurately into any of eight classes, from motorcycle to HGV, by measuring speed, wheel base, axle load and magnetic length overall. 'We expect to see 4M vehicle kilometres per year to start with, growing to 6-8M a year in 20 years time, ' says Payerne.

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