South Africa’s Gautrain railway will link Johannesburg with Pretoria across rocky, mountainous terrain. Ed Owen reports on the challenges faced by its engineers.
As attention focuses on South Africa’s 2010 World Cup football stadiums, civil engineers are hard at work on another major project − a 77km railway to link Johannesburg with Pretoria and other townships in the Gauteng Province.
There will also be a spur line to Johannesburg’s Oliver Tambo International airport.
Gauteng’s significance may be lost here in the UK but the province has the largest concentration of wealth in Africa, generating about one-third of South Africa’s wealth, and 10% of Africa’s GDP, according to Ian Thoms, chief executive of Guatrain concession company Bombela.
The aim is that the £1.9bn project will boost Johannesburg’s regeneration. Bombela will design, build, finance and then operate the railway for 15 years following its completion.
Thoms says the client − local government in Gauteng − wanted “value for money, affordability, and risk transfer except the [risk of] land delivery, valuation costs and escalation higher than escalation predicted”. All other risks including geological are Bomela’s, he adds.
Gautrain will pass through some mountainous areas which means the railway needs 10.5km of viaducts as well as 15km of tunnel, mostly between Marlboro and Johannesburg.
“There have been some spectacular sinkhole failures around the world. We classified the ground from very high to medium risk.”
Ian Thoms, Bombela
Viaducts comprise trapezoidal sections that are wider at the top. Typically, spans are 45m and are made up of glued, precast segments.
There are two balanced cantilever viaducts with spans up to 120m.
There are also 60 smaller structures that use M-beams up to 30m long, generally used for road crossings.
To form a total of 3,200 precast units, the team had to build what Thoms believes was the largest casting yard in South Africa. “Or possibly in Africa, but this is difficult to verify,” he says.
One section of viaduct is through dolomitic rock, pocketed with sink-holes large enough to swallow a train. “There have been some very spectacular sinkhole failures around the world,” says Thoms. “We classified the ground from very high to medium risk, and used viaducts whose pier foundations went down to the bedrock on the lowest risk sections.
“Where we had balanced cantilevers, we built a 9m caisson excavation into the rock and took it out,” he said.
The tunnels present further challenges. “The geology is generally very hard − granite and shale − and most of the work is drill and blast. There is a lot of grouting, finished with wet sprayed concrete with fibre reinforcement,” says Thoms.
“We are using an earth-pressure balanced TBM, which is the first time one had been used in South Africa.”
Ian Thoms, Bombela
However, one section is through soft schist. “For 3km of tunnel we are using an earth-pressure balanced tunnel boring machine (TBM), which is the first time one had been used in South Africa,” says Thoms.
The TBM was built to allow the shield to be removed when the tunnel hit hard rock to allow work to take place using drill and blast.
However, some of the tunnels are constrained by the tough geology. “The tunnel after Sandton is single track. This means we will run an interleaved service and the tunnel must use bidirectional signalling,” he said.
The airport link opens on 27 June 2010, with the rest of the line due to open on 27 March 2011.