Heart of Darkness

Despite last year's fatal tunnel collapse, work on Sao Paulo's metro continues, deep beneath the Brazilian city's busy streets. CJ Schexnayder reports.

Greater São Paulo is now the seventh largest metropolitan area in the world with almost 20M residents. Yet the urban area has been largely unplanned and the transportation infrastructure reflects this.

More than 6M vehicles clog the streets and the steadily increasing population and boom in business only promise to make things worse.

During peak periods, even relatively short jaunts turn into nightmares of endurance with traffic on major routes and busy side streets reaching a complete standstill.

Last year, the Metrô de São Paulo or simply "Metro" saw a 9% surge in ridership with a record 919M transits – an average of 3.23M people every day. It is not an anomaly. Between 2004 and 2007 the number of riders increased by more than 30%.

Yet, the 61km system remains woefully limited. To address this, the city has implemented a $10.5bn (£5.2bn) effort to upgrade the entire public transport system.

A series of line extensions and upgrades will increase the total amount of track to 240km by the end of 2010. The heart of this effort is the 12.8km Line 4 subway, also known as the Yellow Line which is designed to enhance the system by intersecting with three existing Metro lines.

The work has been undertaken by Consórcio Via Amarela (Yellow Line Consortium) comprising
Odebrecht, OAS, Queiroz Galvão, Camargo Correa and Andrade Gutierrez.

The turnkey project involves a 20-year contract to operate and maintain the subway line.

Since the start of work in March last year there have been considerable delays and cost overruns.
The 5.4km section being excavated using New Austrian Tunnel Method (NATM) has been particularly fraught with complications.

On Friday 12 January 2007 the excavation of Pinheiros Station near the Pinheiros River suddenly collapsed claiming the lives of seven people – pedestrians and people in vehicles on the nearby street who were sucked into the 30m-deep hole.

The incident occurred when workers were constructing an 18m diameter, 45m long platform tunnel next to the subway line. A heading had previously been excavated and the first stages of bench excavation were underway.

In official statements, the consortium suggested that heavy rains may have contributed to the cave-in. São Paulo officials have repeatedly dismissed that explanation and blamed the accident on an engineering flaw.

In March, the consortium unveiled a report by independent consultant Nick Barton. This found a previously undetected 15t portion of fractured rock was located over the excavation although no previous studies indicated it existed.

"What we found was a geological surprise," said the consortium's  legal representative Márcio Pellegrini at the Brazilian Congress of Tunnels in São Paulo last month.

A review of the accident by the Instituto de Pesquisas Tecnológicas do Estado (IPT) handed over to prosecutors in June says that the excavation work on the station failed to account for the geology of the site. The report found that the lack of sufficient pins and brackets in the roof and side walls of the excavation may have contributed to the instability of the tunnel.

The consortium's final report and review of the accident is expected this month. Discussions between the consortium and the state government on the cost implications of the failure will not be completed until that report is finalised and a review by the state prosecutor's office completed, say officials.

Work at the station was halted for weeks to recover the bodies and conduct the inquiry into the cause of the collapse. When it resumed, the priority was to stabilise the section with extensive reinforcements. A system of anchors extending 32m into the soil was put in place and the excavations through the section were performed using pre-grouting methods.

In addition to the accident, the consortium has had to contend with vast areas of contaminated soil, groundwater flooding and a lack of access points to the work. The result has been considerable delays and cost overruns.

"The kinds of soils we have run into has been very complicated," says consortium project manager Jose Luiz Pinho Correa. "In lots of areas it has been sand, mixed with other materials and when there is water as well… it's a problem."

The initial contract was for 42 months concluding in September 2007 but early delays pushed that back to November this year.

After the Pinheros Station accident,  the expected completion date has been re-set to March 2010.
Costs have also increased and £34M claim is being discussed by the consortium and the state.

Earlier this year, the consortium sought £83M from the Metro of São Paulo for changes to construction on the section although the outcome of that request has not been decided.

The good news is that there is only 100m of excavation on the section left to complete, says project director Marcio Pellegrini.

As difficult as the conventional excavation section of the project has been, work on the 7.5km section of the project being constructed with a Herrenknecht-built earth pressure shield tunnel boring machine (TBM), has proceeded with few complications. The 9.5m diameter beast is the largest TBM currently operating in Latin America.

Since beginning its work in March last year, the TBM has progressed at an average of more than 14m per day and is expected to complete the remaining 3.5km by August 2009.

Burrowing as deep as 40m below the streets of São Paulo, the TBM is expected to remove a total of more than 650,000m3 of material.

Click here for Line 4 map

NATM Failures: the facts

Last year's S‹o Paulo metro collapse (right) was the fourth involving NATM on the city's network since 1973. Worldwide 67 NATM failures have been recorded over the same time period.
 
The other failures occured in:
December 1981. Tunnel collapses during excavation in soft ground, leading to local instability. Buildings have to be demolished.
November 1991. Tunnel in soft ground collapses and floods.
November 1993. A sink hole opens up following a collapse in soft ground.

The Health & Safety Executive's 2006 NATM report shows that:
90% of NATM tunnel incidents occur close to the face, before the structure is complete.
Over half of collapses result in a surface crater.
The primary cause of failures is unpredicted ground conditions.

The report emphasises:
"the necessity for a good prior understanding of the ground and for thorough contingency and emergency pre-planning."
"It is also possible for ground conditions to change rapidly over short tunnel advances and this can result in sudden unstable open tunnel faces."