SCOUR UNDERMINING the piled foundations of a masonry pier is thought to have been the main reason for collapse of the Ponte de Ferro road bridge in northern Portugal early last month, which cost 70 lives.
Illegal dredging and river flow changes caused by dam construction are thought to have increased the risk of scour.
The 115-year-old Ponte de Ferro bridge linked the towns of Entre Os Rios and Castelo de Paiva, at the confluence of the rivers Duoro and Tamega near Oporto. The bridge consisted of six masonry piers supporting cast iron trusses spanning 50m between the piers and supporting a road deck above.
The river's flow rate had increased dramatically after recent floods and heavy rain.Reservoir construction up and downstream of the bridge and dredging have also changed the flow regime and riverbed over several years.
Excavation of sand from the river bed had been banned within 10km upstream of the river for the past two years. But evidence of recent excavation just upstream of the bridge could be seen after the accident.
'In floods, flows can reach up to 5m/sec, ' said Oporto National University civil engineering design Professor Antonio Adao da Fonseca.'With the river over 200m wide, that is a huge capacity of water, with up to 10,000m 3ofwater passing the bridge per second. Any excavation for extraction of sand or even minute modifications can affect dramatically the flow regime of the river.'
The collapsed pier is believed to have been founded on timber piles driven into the river bed sand, which at the time of construction was 25m thick, overlying granite bedrock.These piles are thought to have been subjected to cyclical wetting and drying before the reservoirs permanently raised the water level. The other piers are founded on cast iron caissons keyed into or resting on the granite.
Visual inspections of the foundations are understood to have been made impossible by increased flow rates and turbidity in recent months.No evidence of detailed underwater inspections or recent repairs to the foundations has emerged.
A geotechnical engineering expert from the Laboratorio Nacional de Engenharia Civil (National Civil Engineering Laboratory), principal research officer Rui Correia, has been appointed to lead the official investigation into the disaster.