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Goals For Gautrain

Gautrain is set to open in time to take hordes of sports lovers to World Cup venues. Gemma Goldfingle finds out about the underground section of Africa’s first high-speed train line.

Football fans from across the globe will be able to make the commute to Johannesburg’s hotels in comfort and ease as the first section of Gautrain, travelling from OR Tambo - the city’s main airport - to the city centre, opens on the 9 June, two days before the start of the world’s biggest football tournament.

The section was not originally planned to open in time for the World Cup so the early finish is a major coup for the Bombela consortium in charge of building, financing and operating the line.

In its entirety the Y-shaped 77km line runs north-south from Pretoria, the executive base of government, to the country’s financial centre Johannesburg, with an extra leg running east-west linking the city centre to the airport.

This smaller section, which will transport travellers into the city in 15 minutes, is the first to open and is probably the most challenging section to build.

Atkins began working on the full multi-disciplinary design for the airport to city leg in 2005 on behalf of Bombela.

This consortium includes French concessionaire specialist Bouygues Travaux Publics, Bombardier, one of South Africa’s biggest contractor Murray & Roberts, French rail operating company RATP Développement, and the Loliwe Group.

“We are breaking new ground with the design. As this is South Africa’s first high-speed line the consequences of constructing it were unknown.”

The design includes three underground stations across the city centre, seven permanent emergency access shafts, a temporary shaft for construction purposes and 13.5km of tunnel. Work on the line started in 2007.

“This is the biggest civil engineering project in Africa,” says Atkins project director and designer of the section Rob McCrae.

“We are breaking new ground with the design. As this is South Africa’s first high-speed line the consequences of constructing it were unknown.”

The majority of the tunnel is constructed using drill and blast, with a 2.9km bored section and 300m of cut and cover.

From the airport the route starts in very strong halfway house granite underlying weathered granite all the way to Sandton in the city centre, making it adequate strength - up to 480MPa - to drill and blast.

This was the preferred option as it was less time consuming and expensive than using a tunnel boring machine (TBM).

“We were keen to maximise local skills on the project too. There is a wealth of mining skills in the area that we could tap into,” says McCrae.

Gautrain1

From Rosebank onwards the geology varies between rock, sand and soft, water clogged soil, making boring the most appealing option.

There is also a high water table in the area. A 145m-long Herrenknecht earth pressure balance shield TBM, named “Imbokodo” - Zulu for hard rock - is used to excavate the 6.8m diameter tunnel at depths ranging from 20m to 40m.

This type of TBM has never been used in South Africa.

The cutter head allows the machine to excavate through soft ground, hard rock, or mixed conditions.

The machine is “earth pressure balanced”, which means the excavated ground and conditioning agents are pressurised in the chamber behind the cutter head to balance the pressure of the ground and water in front of the machine.

This ensures that ground surface settlement is minimised.

During tunnelling, excavated material is removed by screw conveyor onto a conveyor belt system that transports it along the tunnel to the launch chamber.

Park station

Park is the south-west terminus of Gautrain located in Johannesburg’s central business district next to the city’s main rail station and this will provide a vital link between suburban and national rail.

The station is already one of the busiest transport terminals in South Africa.

The 195m long, 16.7m wide station is constructed to the west of the current station in a location that minimises affects on foundations and basements of existing structures.

The station will have two underground levels to minimise the transfer times to trains.

Rising groundwater was a big issue at this site and could rise to just 5m below ground level, 7m higher than the rest of the route.

It was too risky to use the TBM at this section so it was replaced with drill and blast tunnelling.

Rising groundwater was a big issue at this site and could rise to just 5m below ground level, 7m higher than the rest of the route.

It was essential that the station box was watertight to counteract this hazard so 800m thick diaphragm walls were constructed to depths of 27m.

“Diaphragm walls are extremely unusual in South Africa,” says McCrae. “Contiguous piled walls are the norm. Specialised international contractors had to be brought in to construct the d-walls as local contractors were unfamiliar with the technique.”

Atkins wanted to use bottom-up construction for the entire station, which McCrae says was by far the quickest method.

But a small section, where vital road diversions needed to be carried out above, had to be carried out top down.

Temporary steel propping was used while casting the base slab of the diaphragm wall, which has been constructed under bentonite.

Rosebank station

Next stop on the line is Rosebank, located in an upmarket residential area of Johannesburg.

The station is sited beneath the busy Oxford Road next to the Rosebank Mall. Much like Park station, two underground levels have been built.

With groundwater less of a risk, piled retaining walls have been used to build the station box.

More than 540, 750mm piles were installed to a depth of 22.5m with 2m centres used to form the temporary perimeter wall of the excavation.

More than 2,900 ground anchors and shotcrete were used for support and temporary propping.

A chamber also needed to be constructed to launch the TBM from.

This was done in January 2007 in parallel with the station box to fit into Gautrain’s tight construction programme. The TBM launched at the end of 2007.

Sandton station

Sandton is the Johannesburg hub of Gautrain.

The station is located next to the city’s commercial quarter and is surrounded by expensive real estate.

Settlement is not an issue as the station is the deepest across the route at 40m.

As Sandton is the heartbeat station of the line, the designers opted for statement architecture with a huge atrium, which had to be engineered around.

Two rectangular shafts of 45m and 50m deep at each end of the excavation form the station.

A cavern, an insitu concrete structure in rock excavation, links the two shafts at rail level where two of the platforms are built with cross passages connecting them.

A smaller cavern has been built to house the third platform.

Gautrain_3

From the southern shaft, which houses plant rooms and services, the tunnel has been excavated towards Rosebank.

It will also be used for baggage handling when Sandton station’s airport check-in facility is operational.

Passengers will be able to check their luggage in at the station with the baggage diverted down the southern shaft into a special compartment of the train to be processed at OR Tambo airport.

The northern shaft will form the impressive entrance atrium with the criss-crossing escalators travelling 40m down from the ticket hall to the platforms benefiting from a constant stream of daylight.

Mass dampers, like those fitted to correct London’s wobbly Millennium Bridge, have to be fitted to the escalators to prevent vibration.

Diaphragm walls have been used to create the two shafts.

A trench cutter dug more than 20m to form the 800mm panels of the 24m by 21m southern shaft.

Gautrain2

The northern shaft is larger at 61m by 22.4m so huge permanent steel props were needed while cutting the 33m deep, 1.2m wide diaphragm wall.

Drill and blast was used to create the cavern in between the two shafts.

“There is a whole lot of rock to support in the cavern so a mass of reinforcement was used,” says McCrae. “Both ground anchors and reinforced shotcrete is used. Luckily there wasn’t much water within the rock, although that we did have to remove what water was present.

There was an area of bad geology with an igneous intrusion causing much weaker rock at the northern end of the station so some special steel arches were designed to support the ground,” adds McCrae.

Sandton is the last underground station on the Gautrain route.

As the line heads further east it rises above ground just before the next station 4km away in Marlboro.

With much fanfare planned for the grand opening of the Johannesburg section on 9 June, the next milestone is the completion of the main Pretoria to Johannesburg section, which is scheduled to open in early 2011.

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