“Systemic failure” of design, construction and risk management was responsible for the São Paulo tunnel collapse that killed seven people in January 2007, a lead member of the investigation team said last week.
Speaking about the collapse for the first time in the UK, São Paulo Instituto de Pesquisas Tecnológicas (IPT) professor André Assis said that the blame lay with the contractors, designer and client.
“These were big international contractors and designers. All parties had knowledge of risk assessment. But in this case it was never applied,” he said. Assis also dismissed attempts by the contractors to blame unforeseen ground conditions.
“All parties had knowledge of risk assessment. But in this case it was never applied.”
André Assis, IPT
Thirteen people from the consortium, designer and client will go on trial in September accused of construction negligence and manslaughter. If found guilty they could face up to six years in prison.
Before the collapse, consortium Consórcio Via Amarela (CVA) had been working on construction of the new Pinheiros station on São Paulo’s new Metro Line 4. Its members were Odebrecht, OAS, Queiroz Galvão, Camargo Corrêa, Andrade Gutierrez and Alstom. Engecorps was design subcontractor. CVA was working to a design and build contract for client Companhia do Metropolitano de São Paulo.
Construction of Pinheiros station included a 40m diameter, 6m deep shaft, plus two 18.6m wide, 14.2m high and 46m long platform tunnels.
The first signs of failure
At 2.30pm on 12 January 2007, the first signs of failure appeared as small concrete blocks began to fall from the tunnel, near to the shaft. By 2.54pm the tunnel had failed catastrophically (NCE 31 July 2008).
Assis said the IPT found a catalogue of shortcomings − in construction and design − which caused the disaster.
“I will be honest with you, the accident was not foreseeable because the system had so many mistakes.
“There was so little control and the designer could not impose himself on the big contractors.”
In its own report into the collapse CVA blamed unforeseen ground conditions exacerbated by a water table which had risen following heavy rain and a leaking water main. But Assis dismissed both suggestions as unfounded.
“Everyone knows that when there is an accident of this kind the primary guilt is often with the geology. But here, there was 10 years of studying [of ground conditions] before bidding began.
“Ground conditions should not be blamed for the disaster.”
André Assis, IPT
“There was no geological change [from what was expected prior to construction] and ground conditions should not be blamed for the disaster,” he said. He added that rainfall was average for the time of year and that the high water table was entirely due to the major river running alongside the site.
Assis said the real blame lay with “non-validated” changes to design and construction procedures.
The tunnel was being excavated using the New Austrian Tunnelling Method (NATM) and the height and sequencing of bench work had been set at design stage.
But during construction, bench heights were increased from 4m to over 5m in places. The direction of excavation was also reversed and the rate of excavation doubled from 900mm per day the previous month to 1.8m per day at the time of the failure. Benches were also excavated in the wrong order leaving walls unsupported.
“The reason for changing this construction sequence was never given,” said Assis. “But they were running for more production.”
The quest for faster construction was also behind the failure to install enough rock bolts, said Assis. The design incorporated a contingency for the use of rock bolts for the tunnel crown where bedrock was shown to be variable.
However the contractors failed to order enough bolts and site workers continued to drill 120 boreholes, knowing there were only enough rock bolts for 15% of the holes.
Failures and shortcomings
Design shortcomings related to an oversimplification of the geomechanical model, which failed to sufficiently account for the tunnels’ full overburden load and weaknesses in the sides of the tunnels caused by discontinuities in the rock.
Assis said a failure to consult monitoring data compounded all the errors. “The design concept was wrong, but it would have worked if data available was used.
“But there is no evidence of any back analysis of data from monitoring − it was there but never looked at,” he said.
“The first action of the workers was to take photos of the cracks with their cell phones. But no-one closed the road.”
André Assis, IPT
He added that although he did not blame the client, Companhia do Metropolitano de São Paulo, site representatives could have stopped work at any time, but failed to do so.
The legal case against the 13 accused is particularly critical of the action − or lack of action − taken once the crack propagation was spotted ahead of the collapse.
All construction workers were safely evacuated from below ground, but no-one acted to close the road above ground. All seven killed were on or near the road at the time of collapse.
“The first action of the workers was to take photos of the cracks with their cell phones. But no-one closed the road,” said Assis.
No-one at CVA or Companhia do Metropolitano de São Paulo was available to comment as NCE went to press.