First hole through has been achieved on the 16km of tunnel being excavated for the Gautrain rail link in South Africa, which is being developed as a public-private partnership (PPP) project.
The breakthrough on the single bore project was on the drill and blast section between Marlboro Portal and the shaft at Mushroom Farm Park, which is near Sandton station. Excavation at the ends of the 4,215m long tube started in February and June last year, respectively. The longest drive on the double-track (74m2) bore was 2,728m from Marlboro.
Tunnelling activity on the project peaked this year with a total of eight faces, including a TBM drive. Just over 13km, or approximately 80%, of the tunnel excavation will be done by drill and blast. About two-thirds of the tunnels are single track with bores of 45m2. A total of 15 jumbos are being used on the project: 2 x 3-boom, 9 x 2-boom, and 4 x single boom rigs.
Geology along the alignment consists manly of weathered granite and there is also silt, clay and boulders. There are several faults. Groundwater is at least 25m down and cover varies 15m-80m.
The contractor is the Bouygues-led joint venture Bombela Civils, and the tunnel was designed by Atkins. The entire, 77km long rail project is being developed by Bombela Concession Co and will have three underground stations (Sandton, Rosebank and Park) and seven surface stations, all of which will be 180m long.
In the other underground sections of the project, to the south – Sandton to Rosebank to Park stations – there has been good progress made. Between Sandton and Rosebank there are three drill and blast faces, and total distance excavated was 1,661m by the end of September – the latest data available, or 38% of the section. The section is single track.
On the 5,554m long section between Rosebank and Park station, in Johannesburg, there are both TBM and drill and blast construction methods employed. Just over half of the section, which is also single track, has been bored by the end of September.
While the drill and blast excavation has still some way to go, the 6.68m diameter Herrenknecht TBM (S-386) had completed 2,233m, or almost 80%, of its run from Rosebank by the end of the September. Coming in the opposite direction, the drill and blast face had progressed 693m.
Progress with the shield was interrupted in July when a large hole opened in the road above the shield, which at that point was not operating in EPB mode. At the location below Oxford Road, investigations blamed a leaking sewer that altered the soil characteristics. Concerns over the stability of other utilities led to road closures as the TBM proceeded with its drive (T&TI, August, p4).
The team celebrate the breakthrough
