An 885-t tunnel boring machine (TBM), which will be used for tunnelling in the construction of the Gautrain rapid-rail link, is scheduled to arrive in South Africa in October. A 3-km portion of the tunnel between Gautrainâ€™s Johannesburg station and Park station will be excavated using the high-technology TBM.
Described as a moving factory, this 160-m-long machine will excavate a 6,8-m-diameter tunnel with a ground cover of 30 m in some places. The rest will be excavated using conventional drilling and blasting. The 15-km underground line of the Gautrain stretches from Johannesburg station, under Johannesburg hospital, Parktown Ridge, Rose-bank, Sandton and Marlboro, where it surfaces at the Marlboro portal.
Currently the custom-made TBM is being assembled in Germany where it is being readied to cope with Johannesburgâ€™s complex and diverse underground rock conditions. Specialist testing is done on rock samples sent to Norway by local geologists, and the results will be used to determine the most suitable tunnel alignment for the TBM.
The TBM is ideal for poor rock conditions, especially those found in the Rosebank area. This stretch of tunnelling comprises soft rock and waterlogged soil, which is not suitable for drilling and blasting, while the TBM is able to operate underwater.
The design and manufacture of the TBM is estimated to take about 12 months. Bombela Concession Companyâ€™s partner, Boygues Travaux Publics, will be responsible for the construction and use of the machine during operation.
The type of TBM used for the Gautrain is called an earth pressure balance machine. At the front end of the TBM is a rotating cutting wheel that excavates the ground. Behind the cutting wheel is a chamber where excavated materials are kept before being extracted by a pressure-relief discharge system, called a screw conveyor.
The excavated soil is transported through a 0,8-m-wide conveyor belt, fitted inside the machine to the opening of the tunnel where it will be picked up by tipper trucks. After the front end of the machine has bored the ground at 1,5-m advances at a time, the back end starts lining the walls of the tunnel.
The back end of the machine is able to place precast segment rings forming a watertight concrete cylinder just behind the cutting wheel. Thus the tunnel structure is completed section by section as the TBM slowly moves along.
The machine is driven by a computerised and automated cabin that controls and accurately steers the system. Several maintenance teams will service and change the tools on the cutting wheel, and will need to work in shifts in order to undergo decompressing, after entering the pressure chamber. For safety reasons the cutting wheel will be serviced from inside the TBM.
The cutting wheel is driven by seven motors and comprises 150 drag teeth for soft rock. It also comprises 450 single-disc cutters and four twin-disc cutters used for hard rock. The total electric power of the cutting wheel is 2 450 kW .
In line with the Gautrainâ€™s adherence to the environment management plan, the groundwater level and quality will be monitored at strategic, dedicated boreholes during construction.
Tunnel boring has also proved to be a physically safe and environmentally sound method of tunnelling, especially in urban areas. This method does not disturb surrounding soil and it produces a smooth tunnel wall that is cost-effective. In the geology encountered in this part of the project, as well as for working below the water table, a TBM is by far less time consuming than relying on conventional methods.
During the construction of the Katse dam for the Lesotho Highlands Water Project, a TBM was used to excavate a 22-km-long tunnel. The 4,5-m-diameter tunnel was completed 20 months ahead of schedule in 1993.
While skills are available locally, French experts will transfer skills during the installation and use of the TBM.
PUBLICATION: Engineering News
AUTHOR: Esmarie Swanepoel
DATED: 29th June 2007