Riverside jobsite accident opened hole that swallowed people and equipment
While the recovery of the bodies from victims killed in a the worst incident in the history of the São Paulo subway system reached its conclusion, officials began taking steps to understand what went wrong.
On Friday, Jan. 12, the wall of a shotcrete-lined shaft 40 meters in diameter and 30 meters deep gave way without warning. The collapse of the shaft, located on a subway construction site near the Pinheiros River, swallowed several pedestrians and a minibus as well as several dump trucks on the site itself.
The incident occurred at the Pinheiros Station construction site – part of the São Paulo’ subway system’s $1.4-billion Linea 4 expansion project.
As of Jan. 19, the bodies of six people had been recovered. Officials are continuing to search for the remains of at least one other victim. In addition, several families have been relocated due to the damage the collapse caused to their homes which must now be torn down.
The Linea 4 project is being built by Consórcio Via Amarela or Yellow Line Consortium. It consists of Brazilian companies Odebrecht, OAS, Queiroz Galvão, Camargo Correa and Andrade Gutierrez.
The turnkey project involves a 20-year contract to operate and maintain the subway line. A different consortium will operate the line after it is delivered “in rough.”
São Paulo Metrô is owned by the State of São Paulo and is operated by Companhia Do Metropolitano De São Paulo. Engineers with Metrô have been overseeing the work on the Linea 4 project since construction began.
The Metro is the largest subway in South America and is considered to be one of the most advanced subways in the world. The Yellow Line is the fifth leg on a system which transports more than 2.5 million people daily.
The project includes the construction of a 12.8-kilometer concrete-lined tunnel, nine meters in diameter. The new line is designed as an integrating link between the subway and the suburban commuter rail system as well as the city’s bus networks.
The first stage of the project is the completion of the entire double underground track, five stations, one yard, one workshop and 13 train sets. It is a 48-month job expected to be completed by June of 2008 to the cost of $700 million.
The second stage of the project calls for the construction of six more stations and additional train sets and is slated for completion in 2012.
Celso Rodrigues, Production Coordinator for Consórcio Via Amarela, said that the collapse of the shaft did not affect the tunnel excavated for the subway and that work is continuing at 22 locations on the line. It is not known when work on the Pinheiros Station will resume.
On Tuesday, Manoel Xavier Lemos, a director of the São Paulo Subway Workers' Union, said construction on the entire project should be stopped until a comprehensive inquiry into the project’s safety is complete. He told the Associated Press that at least 10 other accidents have occurred on the job since it started in late 2004.
According to Consórcio Via Amarela officials, a house and a shop located near the Oscar Freire station sank into the ground in late 2005. There were no injuries. One worker was killed last year when a wall fell in on a worker.
Butana station, across the river from Pinheiros station where accident occurred, was connected to it Dec. 20.
The job has already struggled with significant delays due to soil issues. Excavation of the line near the Morumbi Station was delayed in September of last year when residual fuel was discovered in the material. Engineers theorized it was runoff from improperly sealed gas station tanks.
Consórcio Via Amarela said their primary efforts since the accident have been to recover the bodies of the victims and no detailed examination of the collapse has begun. Instituto de Pesquisas Tecnológicas do Estado has already been commissioned to perform an independent inquiry into the collapse for the São Paulo state government.
On Monday a statement released by the Consortium suggested that recent heavy rains may have contributed to the cave-in, a theory São Paulo state officials dismissed.
São Paulo state's deputy governor, Alberto Goldman, said he believed a "major engineering flaw" was to blame. São Paulo state Attorney General Rodrigo Pinho also questioned the consortium’s explanation.
"A serious mistake was made, and what we have to do now is determine if it was in the project or in its execution," he told the Associated Press.
The incident occurred when workers were constructing a platform tunnel 18 meters in diameter and 45 meters long as part of the station adjacent to the subway line. A geotechnical engineer familiar with the project explained that the heading had previously been excavated and workers had begun work removing the first of two four meters stages of bench excavation.
The geology near the station consists of alluvial deposits, Tertiary sediments, and finally Precambrian gneiss.
Vertical displacements of material had occurred above the excavation begun at approximately 3 mm and accelerated to as much as 20 mm before the incident. The heading was supported by shotcrete and steel girders but, according to the engineer, at the time of the collapse, workers were installing additional rock bolts to the shaft to improve support.
Officials with Consórcio Via Amarela would not confirm details on the incident other than to say that additional measures were been taken in the station when the incident occurred.
“Details of those measures are now part of the technical investigations, so we cannot discuss details,” Rodriguez said.
Following the failure of the tunnel roughly half of the 30 meter diameter shaft occurred. The failed slope reached an offset of about 40 meters swallowing a walkway and part of the street. The victims of the incident were pedestrians and a passing minibus. No workers were injured, officials said.
The cave-in also affected the base of 55-ton crane on the site that had to be shored up to prevent collapse before rescue efforts could begin.
The collapse did not affect the subway tunnel itself, officials said. Approximately 5.3 kilometers of the subway line will be excavated using the New Austrian Tunneling Method (NATM) that consists of digging short sections of tunnel with regular excavation methods.
The controversial method can be cost effective but also requires a high degree of accuracy from the builders, John Anderson, a British expert on NATM-excavated tunnel collapses.
The collapse resembles an incident in 1993, when a road tunnel under construction at Avenida Santo Amaro collapsed due to unstable ground conditions. There were no fatalities but power was cut off for half a million city residents.
While the collapse at the Pinheiros Station clearly calls for a detailed examination of all the other stations on the line, Anderson said it did not necessarily mean any of the others were in danger of collapse.
“Each one will have to be looked at entirely upon its own merits,” he said.
Approximately 7.5 kilometers from the Faria Lima station to the line’s terminus at Luz station will be dug by a Herrenknecht-built EPB-Shield TBM. Consortium officials expect the 9.5 meters in diameter TBM to bore through the through the sand, clay and gneiss material at the rate of 14 meters per day. It is slated to begin work at the end of the month.