Gautrain goes underground


8 January 2008

Just over a year since groundwork began on the multi-billion rand rapid rail link between South Africa’s commercial and administrative centres, construction work is heading underground, with a 160-metre, 885-ton tunnel-boring machine set to tackle the rock under central Johannesburg.

Christened “Imbokodo”, meaning rock, the Gautrain’s tunnel-boring machine (TBM) is to begin underground tunnelling of a three-kilometre section between the suburb of Rosebank and Park Station in central Johannesburg.

“Today marks the start of a countdown towards the completion of the construction of the Gautrain rail link,” Gauteng Premier Mbhazima Shilowa said at the ceremony to name the machine last December.

The 80-kilometre rail link will run between central Johannesburg and OR Tambo International Airport, with a separate line linking the system to Pretoria.

Work is ongoing at 45 sites across the two cities, and it is expected that the line from the centre of Johannesburg to the airport will be completed in 2010, ahead of the 2010 Fifa World Cup, while the line linking to Pretoria will be completed in 2011.

The project has so far spent some R7-billion of its R25-billion budget, most of which has been on groundwork along the train route and at the construction sites of future stations – all easily visible to motorists and passersby.

Some underground blasting has also taken place further up the route from Sandton onwards, but the presence of between 300 and 400 metres of granite rock in the three-kilometre section between Rosebank and central Johannesburg made it necessary for a TBM to be used.

“For the most part the tunnel will be driven through decomposed granite below the water table,” the Gautrain website states. “The tunnel will also need to traverse in the soft zones that will often occur be below the water table.”

Designing and building Gautrain’s R300-million Earth Pressure Balance Shield TBM, which is 160 metres long and weighs 885 tons, took about 12 months. After its pre-assembly at the Herrenknecht factory in Swanau, Germany, it was transported by ship to Durban Harbour and overland to Rosebank Station, where it was re-assembled by a team of international and local experts.

TBM operation
According to the Gautrain website, the benefits of using TBMs for tunnelling are recognised worldwide, as the machines are designed to bore tunnels in specific areas and to cope with site-specific ground conditions.

“A computerised guidance monitoring system is used to steer the machine accurately underground while the machine and tunnel lining resist the soil pressures during and after the tunnel construction,” the Gautrain website states. “This means that the TBM is an environmentally sound method of tunnelling, especially in built-up areas.”

At the front of the machine is a rotating cutting wheel, 6.8-metres in diameter, which excavates the ground. Excavated material accumulates in a chamber behind the wheel, before it is transported back to the station shaft by a conveyor belt, where it is picked up by tipper trucks.

The wheel bores into the earth at 1.5-metres at a time, while the tunnel is lined with concrete rings at the rear of the machine.

“These pre-cast segment rings form a watertight concrete cylinder. In this way the tunnel structure is completed, section by section, as the TBM slowly moves along like a giant earthworm,” the Gautrain website states.

Some 12 000 of the precast segments that will be needed for the tunnelling operation will be manufactured at a dedicated factory in Boksburg on Gauteng’s East Rand.

According to the Gautrain website, the TBM will be active for some 14 months, tunnelling 24 hours per day and seven days per week, after which it will be dismantled over a three-month period. The TBM will then be available for use in other projects around the world.

Similar Earth Pressure Balance Shield TBMs have been successfully used to construct a 3.6-kilometre underground highway tunnel that crosses the Madrid city centre in Spain, an 8.9-kilometre high-speed railway tunnel from Karlsruhe to Basle in Germany, and in the US to extend the Los Angeles subway network by 10 kilometres.

SAinfo reporter. Additional reporting by Lucille Davie, City of Johannesburg

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