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

Sand extraction - a major factor in Europe's 'worst ever' road disaster

Exceptional weather along with technical, political and economic factors are all being touted as reasons for the collapse of the Ponte de Ferro bridge in Portugal. Mark Hansford investigates.

Extraction of sand from the River Douro has emerged as a major contributing factor in last week's collapse of the Ponte de Ferro road bridge in Entre-os-Rios, northern Portugal.

Up to 70 people died when a coach and two cars plunged into the swollen waters on the night of Sunday 4 March. Scour undermining the piled foundations of one of the bridge's six masonry piers during flood conditions is seen as the most likely cause of the tragedy, said to be Europe's worst ever road disaster.

Government engineers had already blamed the local sand extraction industry for dramatically altering the flow regime under the bridge (NCE 8 March), and NCE has since learnt that cargo ships transporting the sand between sites are frequently seen striking the bridge piers.

A geotechnical engineering expert from the Laboratorio Nacional de Engenharia Civil (National Civil Engineering Laboratory), principal research officer Rui Correia, has been appointed to head the official investigation into the disaster, but the circumstances that led to the collapse are as much political as they are technical, and lead right to the Portuguese government.

The River Douro, or river of gold, is so named because of the high concentration of sand that is suspended in its waters. Sand extraction is big business along the entire length of the river.

However, Entre-os-Rios's position at the confluence of the rivers Douro and Tamega - both hundreds of metres wide - makes it a particularly good location for extraction of what is a valuable material.

Where the rivers converge the water slows near to the south bank and the sand settles out, making extraction easy. The Ponte de Ferro is less than 100m downstream of this point.

The state-run Institute of Navigation recognised the potential risk this posed to bridges along the river, and acted to solve the problem in 1998 by imposing a '10km' rule for sand extraction.

This banned sand extraction from the river at any point up to10km upstream of a bridge.

However, the sand extraction industry is a major part of the local economy around Entre-osRios and, rather than abandon the site at Ponte de Ferro, the operators merely changed its function.

Officially, the actual extraction has shifted to sites downstream of the bridge, with the site at Ponte de Ferro used as a distribution centre for transferring the sand from ship to truck.

This move may have only served to worsen the situation by encouraging 50m long cargo ships to manoeuvre in the fast flowing waters next to the 115 year old bridge.

The local population is not happy. Antonio Grilo, art and design teacher at the local school, says: 'They have already broken our roads with lorries carrying loads 10% to 20% heavier than legally permitted. Now it's the bridge.

'With the speed of the river, and its currents, the ships are always hitting the piers. The locals are saying that maybe the pier that collapsed got hit one time too many.'

Oporto National University civil engineering design professor Antonio Adao da Fonseca, who is president of the European Council of Civil Engineers, is even concerned about the impact on the river's flow regime: 'With the speed of the water in floods reaching 5m/s, any modification to the upstream banks may well have an impact.'

And as the extraction equipment at the site near the bridge appears to show few signs of two years of inactivity, da Fonseca also questions whether the extraction firm is adhering to the rules: 'Temptation comes to us all the time. If you have such a nice piece of sand, it may be hard not to extract it.

'The institute's resources are good, but they are only so good.

It is hard to enforce a rule if the public do not understand the implications of breaching it.'

While Grilo bemoans the sand extraction industry and the modern developments plaguing his country, another modern development is also likely to have contributed to the collapse - electricity generation.

The construction of three hydroelectric dams by national power company Electricidade de Portugal within 15km of Entre-os-Rios raised the water level at the bridge to 18m to 20m.

The Carrapatelo and Crestuma dams were built in the late 1960s and 1970s spanning the Douro upstream and downstream of the Ponte de Ferro.

The Torrao dam across the Temega was constructed 15 years ago.

Da Fonseca is reluctant to blame the electricity company.

'EDP has huge resources and would have studied everything thoroughly, and would have considered the implications on the river bed. But they would only have been theoretical studies.'

However, he concedes that the rapid flow in the River Douro makes it a special case.

'Dams tend to stabilise the river bed by creating a cushion of sand. But the quantity of water in the Douro is too high to be controlled. The Douro has six dams in all - three in Spain, three in Portugal - and none have the capacity to retain significant quantities of water, ' says da Fonseca.

Recent floods have raised the level of the water by at least 2m, pushing it close to the bridge's design limit.

A paper by the bridge's original engineer, Antonio Ferreira de Araujo e Silva, reveals that the design was based on a maximum depth of 22m. This could easily have been exceeded during the recent wet weather.

The recent floods are also likely to have swept away sand from around the base of the pier.

'Sand carries vertical loads and provides a stabilisation effect, ' explains da Fonseca. 'So if it is taken away, and the piles are not able to withstand the load, collapse could occur.'

Da Fonseca feels strongly that establishing the presence of stabilising sand and the condition of the timber piles are the key factors for the investigation.

Urgency is therefore vital: 'If they wait, what will they see?' he asks. 'When the flood goes the sand will settle back again.'

Concern surrounds the timber piles because the original drawings indicate that the piles under the collapsed pier may have been subjected to cyclical wetting and drying before the reservoirs permanently raised the water level.

However, no evidence of a thorough inspection of the foundations has yet emerged.

The most recent inspection of the bridge, carried out in January to assess the feasibility of running a water main across the bridge, focused solely on the superstructure. No structural problems were uncovered, but the study advised against the water main.

The last thorough inspection was carried out in 1998 by the Junta Autonoma de Estrados (JAE), the authority responsible for the construction and maintenance of highways in Portugal.

Again, no reference was made to corrosion of any kind on the deck, and the piers were found to be in good condition. The only safety concerns surrounded the narrowness of the deck, which allowed only two-way traffic for cars.

Significantly though, the latest inspection did not did not extend to the foundations.

'This is a lesson for everybody throughout Europe, the US and beyond, ' says da Fonseca. 'Old bridges demand more thorough examinations. These sort of collapses are becoming more common.'

However, da Fonseca feels the structure of the JAE puts bridges in Portugal at particular risk: 'The majority of bridges like Ponte de Ferro are under the authority of the railways, which have a good record of inspection.'

He says inspection of rail bridges in Portugal is well structured, with all bridges getting a superficial annual inspection, backed by a thorough one every five years.

This contrasts with the JAE, which has yet to instigate a structured maintenance regime since the authority was overhauled in 1998.

This saw the JAE split into three bodies - one responsible for new construction, one for maintenance and a holding company, the Instituto de Estradas de Portugal. In the process the bridges group was dismantled and merged with roads.

Da Fonseca does not approve:

'Bridges should not be under the same authority as roads.

They are different issues and require a different expertise.'

He is not alone in his view. The move was not popular, and has created a major problem: 'Most experienced bridge engineers either retired or left, ' da Fonseca says. 'Now technical decisions are being made by non-technical people.

'Managers have to have a good knowledge of what they are managing. This is a panEuropean problem, ' he adds.

'A good engineer from JAE should have stood up and called for a proper inspection. It is not a big issue to reinforce piles - we now need to inspect all bridges.'

The lack of investment in maintenance is all the more frustrating given that Portugal qualifies for significant amounts of EU funding as a developing country. But little of this reaches communities like Castelo de Paiva.

'You're talking about power, ' says da Fonseca. 'Where is the power in Portugal? Lisbon and Porto. It is a real problem, the old country against Porto and Lisbon. Castelo de Paiva is near Porto - but only near.'

Meanwhile, the Ministry for the Interior told NCE that replacing the bridge, and reunifying the shattered community of Entre-os-Rios, will be a top priority.

Without the bridge, getting from one bank to the other is a major problem. The speed of the river makes a ferry impractical, and the trip by road is a long and arduous one.

Crossing by Entre-os-Rios' second road bridge upstream on the Tamega involves a two hour 60km trip, with the only alternative a three hour 90km journey via Porto.

With statistics like these it is easy to understand the frustration of the mayor of nearby Castelo de Paiva, who told NCE: 'We are not in Europe here. This is Africa.'

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.