ILLEGAL DREDGING and river flow changes caused by dam construction have emerged as major contributing factors to the collapse of the Ponte de Ferro road bridge in northern Portugal on Sunday which claimed the lives of 70 people.
The structure links the towns of Entre Os Rios and Castelo de Paiva, at the confluence of the rivers Duoro and Tamega near Oporto.
Tragedy struck at around 9.30pm as two cars and a bus carrying 67 passengers were travelling across the 115 year old bridge. All died after a central masonry pier gave way, sending the vehicles plunging into the water.
Scour undermining the foundations is emerging as the main reason for the collapse.
Alterations to the river bed from sand extraction and construction of reservoirs up and downstream of the river have changed the flow regime and river bed over several years.
Excavation had been banned within 10km upstream of the river for the past two years. But evidence of recent excavation work in the river upstream of the bridge could be seen near the structure after the accident.
The river's flow rate had also increased dramatically in recent weeks. Recent heavy rain swelled water depths to 25m, which according to engineers at the accident scene was 5m higher than usual. A combination of the depleted river sands and high water flow rate is likely to have increased the risk of scour.
'In floods, flows can reach up to 5m/sec. With the river over 200m wide, that is a huge capacity of water, with up to 10,000m 3passing the bridge per second.
'Then any excavation for extraction of sand or even minute modifications can affect dramatically the flow regime of the river, ' said Oporto National University civil engineering design Professor Antonio Adao da Fonseca.
The bridge had six masonry piers supporting cast iron trusses spanning 50m between the piers and supporting a road deck above.
Originally designed for horse drawn vehicles, it had become a key traffic artery bridging the Duoro, which is around 200m wide. Loss of the bridge has meant a three-and-ahalf hour detour.
Institution of Civil Engineers archives reveal that at the time of construction, the river bed consisted of layers of sand up to around 25m deep, overlying granite bedrock.
A paper by the bridge's original engineer Antonio Ferreira de Araujo e Silva indicates three types of foundation.
Drawings of two types show an iron caisson filled with concrete, supporting a filled masonry pier. One is anchored into bedrock, with the other resting on the granite and embedded in the sand. Compressed air working was used to drive the caissons down to bedrock.
However it seems that the pier that collapsed was constructed in the dry on deep sand during low water conditions. Piles were used to support the pier, almost certainly timber, and these may not have been driven all the way to bedrock. Adao da Fonseca said timber piles in that location were bound to have deteriorated badly over the decades, due to cycles of wetting and drying as river levels varied.
Recent inspections were carried out on the bridge, including one last month to examine the feasibility of carrying a water main on the bridge.
Interior Ministry engineers said these inspections by the local water board concentrated on the superstructure, and gave it a clean bill of health.
Recent visual inspections of the foundations are understood to have been made impossible by increased flow rates and turbidity. No evidence of detailed underwater inspections or recent repairs to the foundations has emerged.
Public anger forced the resignation of the Portuguese infrastructure minister Jorge Coelho on Monday after it emerged that there had been persistent concerns about the safety of the bridge and that requests for a new bridge by local officials were ignored.
These calls were led by mayor of Castelo de Paiva, Paulo Teixeira. He told NCE: 'When this bridge was built in 1886, it was made to carry animals. Until the disaster it was carrying 1,300 vehicles a day. This is just not good enough - our infrastructure in Portugal is more like the third world than Europe'.
A senior Ministry of Interior engineer at the disaster site told the magazine: 'The river alignment has changed over years because of the construction of reservoirs both upstream and downstream which has changed the water level, and because of sand and gravel extraction.
'Two years ago this was banned for 10km upstream of the bridge.' He added that the possibility that the piers were not embedded in rock combined with the change in the river environment heightened the risk of failure.
Detailed design work for a new bridge had already been well advanced, but the government late last year shelved plans to build it nearby downstream from the existing structure.
Responding to accusations about the previous failure to build a new bridge, the Interior Ministry engineer indicated that plans to build a new bridge would be revived immediately.
'Within three months you'll see tenders being put out to rebuild the new bridge. The Portuguese government needs to prioritise its work but the need to reunify the community makes this a priority now.'