Set to start tunnelling in January 2008, the 145-m long, 885 t Imbokodo, with its 13 gantry trailers, is expected to take 14 months to tunnel the 3-km, and will work 24 hours a day, seven days a week.
‘Imbokodo' (literally, ‘the hard rock that grinds') was the name chosen for South Africa's TBM, in the spirit of the women who marched to Pretoria in 1956 singing "Wathina abafazi, wathina imbokodo" ("If you strike a woman, you strike a rock").
Traditionally, tunnel boring cannot start until local people have chosen a suitable name for the TBM, which, according to custom, and to signal good luck for the project ahead, must be a woman's name.
Gautrain Management Agency CEO Jack van der Merwe said that the name Imbokodo was a symbol of South Africa's heritage of willpower in its drive to overcome obstacles.
The TBM, which uses electricity as its power source, took 12 months to design and build. At the front of the machine is a rotating cutting wheel, which excavates the ground, and behind the wheel is a chamber where the excavated material accumulates, before being extracted by a pressure relief discharge system called a ‘screw conveyor'. Excavated soil is then transported on a conveyor belt back up to the Rosebank south shaft, where it will be picked up by tipper trucks.
Urban tunnelling requires that the ground surface is left undisturbed, particularly since it has to be done below deep infrastructure foundations. A computerised guidance monitoring system is used to steer the machine accurately underground.
The trenchless closed construction method has its advantages, and life above ground will continue with no economic losses, no road diversions and traffic disruptions. Noise pollution and dust can be avoided, and no groundwater lowering is necessary. Construction is largely independent of weather conditions.
Once the tunnel linking Rosebank and Park stations is complete, it is unclear what will become of Imbokodo - it could be left underground once its work is finished, or, if economically feasible, it could be made available for use on another project elsewhere in the world.
As the machine cannot move backwards, getting it out from underground could prove very costly, and it is a possibility that "once she has submerged into the belly of the earth it is better to let the old lady lie there when her work is finished", Gauteng premier Mbazima Shilowa commented at the naming ceremony.