A clear priority of the Gautrain rapid-rail link is to meet the first 45-month operational completion deadline for the route between Sandton and OR Tambo International Airport, says Bombela Turnkey Contractor CEO Ian Thoms. Since the second operational deadline, for the route between Park station and Sandton, and Midrand and Pretoria stations, follows only nine months after the first, says Thoms, it is clear that both sections of the 80-km route have to be worked on simultaneously.

Construction operations have to date been mobilised at Johannes-burg Park station, Rosebank station, Sandton station, Malboro station, and at the depot site as well as various tunnel sites and viaduct locations. Thoms says that the design of the major earthworks has essentially reached a balance between the cut-and-fill, which is quite important in the civil engineering sector of the project. “We have done a lot of work to balance the cut and fill, so that we don’t end up having to spoil too much material, or excavate borrow pits to get sufficient soft fill material.”

Thoms says operations at OR Tambo Airport have also started. “We have access to the airport itself and are establishing there to start building the piers that support the airport station platforms and approach viaduct.” He adds that the concourse section of the station will be built by Airports Company South Africa as part of the new terminal building, but will be paid for by the Gautrain rapid-rail link.

Thoms says the airport station has been designed in such a way that the escalators carrying passengers from the rapid-rail station down to the departure level of the airport will be equidistant between the international airport and the domestic airport check-ins.

The route through Rhodesfield has mainly been cleared, and construction of the station will take place directly across from the South African Rail Commuter Corporation (SARCC) tracks which connect Metrorail’s Kempton Park and Isando stations. “The SARCC is also looking to build a new station in this area, which will interface conveniently with the Gautrain rapid-rail link,” says Thoms.

At Johannesburg Park station, diaphragm walls are currently being constructed, and the excavation for a starter shaft, which will facilitate driving of drill-and-blast tunnels, is taking place. Some 20 000 m3 out of 27 000 m3 of material has so far been removed from the shaft, and closely spaced steel bars, called soil nails, have been installed to stabilise the excavation.

Only some 8 m of the total 25-m depth remains to be excavated, before the forming of the tunnel portal can start, to permit driving of the drill-and-blast tunnels from Park station towards Rosebank.

At the south end of the planned Rosebank station, an access shaft is being created to lower the tunnel boring machine, which will tunnel its way some 3,6 km towards Johannesburg Park station to meet the drill-and-blast tunnel being driven northwards. At the other end of Rosebank station, a smaller shaft is being sunk to gain access to start the tunnelling towards Sandton station. The shaft is excavated in 3-m stages, and the spaces between the previously placed soldier piles infilled with sprayed shot-crete to stabilise the ground. Once this is done, soft-ground anchors are installed around the shaft, and grouted and stressed to hold the wall back.

Excavation then proceeds for a further 3 m and the ground anchor process is repeated. Eventually, the two shafts of Rosebank station will be connected by the main open cut of the station box, but Thoms says that priority for now is that the tunnelling should start as soon as possible in both directions. The excavation of the station, the building of the slab levels, the closing up of the station roof, and the backfilling of the station are all currently planned to take place over the next two years.

Several emergency shafts will be constructed along the railway, which will provide the emergency services with access to the tunnel level, in case of emergencies during the 15,5 years’ operational period. The E5 emergency shaft at Craddock avenue/Bompas road is currently at 50 m with another 11 m to go before excavation of the shaft will be complete. The shaft is excavated by using drilling and blasting. “Each time we drill the rock at the shaft bottom, and set the charges, we cover the area with sand to avoid fly rock, which could cause damage.”

Sandton station will be the ‘flagship’ station for the Gautrain rapid-rail link, says Thoms. “The design of the station has been changed from its original opencut, to two large shafts at either end of the station, with interconnecting station platform tunnels, owing to the fact that the geology at Sandton is competent rock.”

Owing to environmental considerations, the line has not been allowed to surface in the Katherine street area. “Ideally we would have liked to have raised the alignment, making it shallower with a low gradient. But environmentally it was insisted that we did not daylight at Sandspruit,” says Thoms.

Ensuring that the railway remains under Sandspruit has resulted in the line being depressed to about 45 m below the surface at Sandton station. Thoms says this means the whole-life cost of running the station will be higher than normal, standard, planned metro-type stations, which tend to be located about 20 m below the surface.

The two shafts at Sandton will permit tunnelling southwards back towards Rosebank station, and reception of the tunnel driven from the Mushroom Farm Park access shaft. The shafts will also provide access for driving the station platform tunnels.

The diaphragm wall at the southern tunnelling shaft has been completed in preparation for the capping beams. Eleven out of 26 D-wall panels have been completed in the North shaft, which equals about 42% of the lateral wall of this particular shaft.

Excavation of the Mushroom Farm Park shaft has been completed and the eyes of the tunnel are being formed. “The shaft is 31 m deep” says Thoms. The tunnel eyes are being formed on two sides of the shaft such that the main tunnel drives towards Sandton station, and Marlboro portal can be started soon.

Thoms says the Marlboro tunnel length is about 150 m. Computer-guided drilling jumbos are used to drill the holes, and the rock face is then charged with explosives. “The civil contractors do a full face blast and pull around 4,5 m of advance every blast,” says Thoms. He adds that blasting is now advancing at about 30 m a week. “As we drive the tunnel, we are incrementally feeding in electrical power and other services, including tunnel venti- lation facilities.

“Crossing the N3 is particularly challenging, because it has to be done piecemeal,” says Thoms. Stage by stage, the box has to be built and backfilled. The slip roads have to be moved out of the way, says Thoms, which Bombela is currently busy with. Following the traffic diversion, a section of cut and cover box can be constructed and backfilled, and the slip road reconstructed so the process can start again. Thoms adds that the same procedure will have to be implemented for the main carriageways of the N3, since the tunnel boxes traverse the N3 under the main carriageways.

“This whole crossing will be a series of sequential operations that will last for the next couple of years. Once it is done, we will have a continuous tunnel box under the N3 for over 500 m.”

“Five hundred and eighty thousand cubic meters of soil have been cut and hauled to complete the embankment for the depot.” says Thoms. The site will initially be used as a precasting yard, since a lot of precasting for the bridge and viaduct structures will have to be done. About 10 km of viaduct will have to be built along the alignment, involving some 3 677 precast concrete segments, installed on mobile girders and post-tensioned on a span-by-span basis. Also some 1 100 metre-beams for numerous small bridges, spanning about 20 m, will be constructed using the long line pretensioning system.

The depot also features the train workshop, with inspection pits and facilities for railway jacking equipment. “So when the trains are brought in for maintenance, they can actually jack the train up and drop the bogies off the cars. This means that major maintenance can be done on the bogies and the cars separately.”

The bus depot and the main administration building for the feeder bus services, will also be built next to the rapid-rail depot.

Several utility providers have services crossing or running along the Gautrain rapid-rail route, including Telkom, Eskom, Sasol, and Petronet. “Because the railway demands significant clearance from other high-tension lines, there are a few places along the alignment where Eskom has to build new, higher pylons, to [increase] the height of the transmission lines,” says Thoms.

However, moving these transmission lines is no easy task, and shutting down the transmission lines could have serious implications for the national power grid. “As you know, demand and supply is pretty well balanced at the moment, so what Eskom has had to do is look for times when the demand is lower. Only then could it shut down the power lines to permit connection work to proceed.” Thoms says that this becomes programme sensitive, but added that he was pleased to confirm that Eskom was now moving towards completion of these strategic diversions. Between 800 and 900 utilities would have to be relocated to make way for Gautrain with some 50% already completed or started.

The most cutting edge technology being used on Gautrain is an earth pressure balanced tunnel boring machine, says Thoms, which means that the tunnel boring machine is capable of not only drilling in hard rock, but also excavating soft material. “It is a closed machine, which basically means that it has a thick shield skin, within which the tunnel lining is erected using precast concrete tunnel segments.

” Tunnel boring machines, which are essentially hard rock machines, have previously been used in Southern Africa, particularly on the Lesotho Highlands water scheme, but the one being used for the Gautrain rapid-rail tunnel, being a closed machine, can use compressed air at the front end, which stabilises the tunnel face in soft ground situations, and is equipped with a host of tools and capabilities designed to deal with different ground conditions.

“The machine has been specifically designed and built to suit the conditions between Rosebank station and Sandton station. It will not be used all the way, but only where the ground conditions are highly mixed,” says Thoms.

PUBLICATION: Engineering News
AUTHOR: Esmarie Swanepoel
DATED: 29th June 2007