The 145-m long, 885 t machine is expected to take 14 months to complete the structure.
The remaining 12 km of tunnel for the rapid-rail link - from Park station to the Marlboro portal - will be excavated using more conventional drill-and-blast methods.
The R300-million TBM has been named 'Imbokodo', or 'hard rock'.
It was designed and custom-built by Herrenknecht, in Germany, in order to cope with the complex underground geological conditions south of Rosebank, which renders drilling and blasting impossible.
Geology in this area features a high-water table as well as varying degrees of hard rock, sand, and soft, water-clogged soil.
At the front of the Gautrain TBM machine is a rotating cutting wheel, which excavates the ground. Behind this 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 shaft opening.
The front cutter head of the machine advances at roughly 220 m a month.
The concrete tunnel lining is built ring by ring in the rear.
These precast segment rings form a watertight concrete cylinder.
In this way the tunnel structure is completed, section-by-section, as the TBM inches forward like a giant earthworm.
The cutter head is driven by seven motors and features 150 drag teeth for soft rock. It also comprises 45 single-disc cutters and four twin disc-cutters for hard rock conditions.
A computerised guidance monitoring system provides information to the TBM pilot on a continuous basis.
It also allows the pilot to steer the machine, and to continuously monitor the actual position of the TBM in relation to the theoretical centre line of the tunnel at any given location along its route.
It is not the first time a TBM is used in South Africa.
The undersea tunnel in the Durban harbour was built with a refurbished Herrenknecht TBM, used previously in Hong Kong.
Although this type of TBM would have been appropriate for the Rosebank tunnel, the machine's diameter of 4,4 m was too small to accommodate the Gautrain. (The Gautrain TBM has a diametre of 6,81 m).
The Durban harbour tunnel carries water, sewer, power and communication services.
The Lesotho Highlands Water Project also made use of six hard-rock TBMs excavating three tunnels.
These water transfer and delivery tunnels each had an internal diameter of between four metres and five metres.
The tunnels ranged in length from 32 km to 45 km.
However, again, none these hard-rock TBMs were the right size for the Gautrain project, as well as not being suitable for the soft-ground conditions in Rosebank.
It is not yet clear which will make more economic sense for the Gautrain TBM: The cumbersome task of returning the machine to the surface in order to sell it to an interested party, or to simply leave it underground.
Bombela, the consortium which is to build and operate the Gautrain, consists of international partners Bombardier and Bouygues Travaux Publics, and local stakeholder Murray & Roberts, along with empowerment company the Strategic Partners Group.
The R25-billion rail project is a public-private partnership between Bombela and the Gauteng government, and will link Tshwane, Johannesburg, and the OR Tambo International Airport by 2011.
WHAT IS IN A NAME?
Aurora, Delilah, and the Emerald Mole
According to global tunnelling tradition, a tunnel-boring machine (TBM) cannot start work until it is given a name. This tradition also sees most TBMs being named after women.
The Gautrain TBM name, 'Imbokodo', flows from the women's protest march to the Union Buildings on August 9, 1956. This march saw the birth of the phrase 'wathinta abafazi, wathinta imbokodo' or, 'if you strike a woman, you strike a rock'.
Other TBM names around the world include 'Xiaolongnu', which is a 80-metre long machine constructing the Kowloon southern link, in Hong Kong.
This TBM is named after a famous heroine of the popular Chinese martial arts book 'The Return of the Condor Heroes'.
Project officials chose the name because they believed the TBM will do her work with the "agility and effect of a super kung fu master, yet with a feminine touch of tenderness and softness as she tunnels her way through soft ground and hard rock".
'Terra Nautilus' and 'Pipi' are the names given to two TBMs used on the Gold Coast Desalination project, in Australia.
A seven-year old boy gave the name 'Emerald Mole' to a TBM which will be used to build light-rail tunnels in Seattle, US.
A biblical 'Delilah' is boring a tunnel for a sewage improvement project, in the UK.
In turn, 'Aurora' was named after the Roman goddess of daybreak. This custom-made, 120 metre long TBM built the Green Heart tunnel for a high-speed rail project in the Netherlands.
DIGGING DEEP The TBM ready and waiting at the Rosebank station - See Gautrain TBM naming.JPG - PIC BY DUANE
GIANT PUZZLE The TBM being assembled at the Rosebank station - See GT TBM Assembly Nov 07.2.JPG