17 March 2009
As construction on South Africa’s rapid rail link, known as the Gautrain, has moved northwards from Johannesburg to Pretoria, engineers from around the world have worked smartly to overcome the challenges thrown up by different geological conditions along the route.
The tunnelling that has taken place under the suburbs of Johannesburg has been relatively straightforward. Two methods have been used – dynamiting the tunnels, and the cut and cover method, which involves digging a hole, then covering a roof over it. These are fairly standard engineering jobs when excavating tunnels for an underground train.
But with construction moving further north, dolomitic conditions in the Pretoria area have posed challenges which have produced a range of engineering solutions.
In dolomitic conditions, the ground is unstable and may collapse, forming sinkholes, at any time. The Sudwala Caves in Mpumalanga province and the Wonder Cave in the Cradle of Humankind outside Johannesburg are examples of dolomitic caves. Changes in the water table, often brought about by human intervention through the sinking of boreholes or leaking utility services, cause the sinkholes to develop, often with tragic consequences.
“Designing and installing heavy foundations in these conditions has been a unique challenge for the project team and Bombela mobilised a team of international and South Africa experts in the fields of geology, ground investigation, foundation design and construction, in order to accomplish this,” says a Gautrain statement.
The Gautrain team looked at studies recording 800 sinkholes. “Ninety-five percent of sinkholes are less than 15 metres in diameter,” says Errol Braithwaite, an engineer and technical executive at Bombela, the concession company building the Gautrain. Despite most sinkholes being less than 15 metres across, the engineers are designing the route as if all possible sinkholes will be 15 metres in diameter.
“We have done a lot of ground preparation,” says Braithwaite. “We have been very careful about the dolomite.”
The Gautrain will appear above ground at Marlboro station, in Alexandra, after travelling underground for 15 kilometres, from Park Station in downtown Johannesburg. It will then travel to Pretoria and OR Tambo International Airport above ground, on a viaduct or bridge.
The viaduct through Centurion, immediately south of Pretoria, will be more than four kilometres long. The Centurion Rugby Club had to be relocated to make way for it.
Solutions to dolomite
Where bedrock is relatively shallow, a specially constructed mass concrete mattress is used, making it possible to build in unstable conditions.
In areas of deeper bedrock, voids and cavities in the dolomite are filled with grouting. Piles are then drilled into the bedrock, and a viaduct pier is constructed.
The soil can also be treated in other ways – it can be compacted either by repeated lifting and dropping of purpose-made steel pounders using a crane, or it can be “pre-loaded” by placing a thousand 10-ton concrete blocks on the viaduct foundation and then building the viaduct pier or column above this.
In other areas ground beams or ground bridges have been built, stretching 15 metres in length, covering possible sinkholes. Sensors in the bridge will pick up ground activity, indicating a potential sinkhole opening up. Even if one does open up, the bridge will hold, and repair work can be undertaken beneath it.
Construction of viaducts
Commuters between Johannesburg and Pretoria would have noticed the huge viaducts going up alongside the freeways, and in some cases, crossing them. To support these going over the freeways, the engineers have sunk shafts of seven metres in diameter some 30 metres down to the bedrock.
The viaducts next to the freeways are like huge concrete T-shapes, with two arms growing each way. Whereas viaducts away from the freeways are built using pre-cast concrete segments, put in place using large steel girders and cranes, and strengthened and bound together with steel cabling and high-strength epoxy glue, this is not possible alongside a busy road.
In this case the T-shaped viaduct is built on site, with each side caste in the air simultaneously, balancing one another out carefully until they join with the growing viaducts on either side.
The construction team, composed of professionals from 26 countries, has had to deal with a range of issues. From relocating indigenous frogs and moving 100-year-old graves, to constructing 50 small bridges over the undulating terrain, and moving over 1 000 underground utilities.
Some six-million cubic metres of earth have been moved around, dug out in places and used to build bridges or overpasses in other places. Bridge walls have been built at bridges in high-density areas, to cut the noise level.
Some 1 100 properties have been expropriated to make way for the construction of the Gautrain. While many houses had to be demolished in the Centurion area, Teazers nightclub refused to move and the track had to be re-directed around it.
So far, 13 kilometres of the 80km track have been constructed. In all, 11 289 people are employed in an area of 41 square kilometres of construction.
The rail link between Sandton and the airport will be complete on 27 June 2010, two weeks after the start of the 2010 Fifa World Cup.
But, says Barbara Jensen, the communications and marketing executive for the Gautrain, a R150-million incentive has been built into the contract to get Bombela to finish the Gautrain two weeks early, the day before the World Cup begins.
“We will only be in a position to determine whether we need to accelerate to finish two weeks before 8 June 2010 by June this year,” she says.
The full 80km route will be complete by the end of March 2011.
Source: City of Johannesburg