Posted on Jun 5, 2007 in Construction Technology & Machines
SPECIALISED TECHNOLOGY IMPORTED FOR UNDERGROUND TUNNEL
The 15km underground line of the Gautrain stretches from Johannesburg Station, under the Johannesburg Hospital, Parktown Ridge, Rosebank, Sandton and Marlboro where it surfaces at the Marlboro Portal.
A portion of three kilometers of the tunnel between Gautrain’s Johannesburg Station and Park Station will be excavated using a high-technology tunnel boring machine (TBM). Described as a moving factory, this 160m long machine will excavate a tunnel of 6.8m diameter with a ground cover of 30m at some places. It weights 885 tons. The rest will be excavated using conventional drilling and blasting.
Suitable for local rock conditions
Currently the custom-made TBM is assembled in Germany as it needs to cope with Johannesburg’s complex and diverse rock conditions underground. Specialist testing is done on rock samples sent to Norway and local geologists. Results will be used in determining the most suitable tunnel alignment for the TBM.
The TBM is ideal for the poor rock conditions, especially in Rosebank. This stretch of tunnelling comprises soft rock and waterlogged soil that is not suitable for drilling and blasting. The TBM is therefore able to operate underwater.
It will be transported via sea and road as it arrives in South Africa in October.
Designing and building the TBM takes about 12 months. Responsible for the construction and use of the machine is Bombela Concession Company partner Bouygues Travaux Publics.
Bouygues is a French world leader in the construction of tunnels. The company has completed more than 200km of tunnel projects internationally. Bouygues completed one of the world’s largest tunnels using a TBM for the 15.2m Groene Hart Tunnel in the Netherlands.
Earlier this year, the company used the world’s largest TBM to excavate a eight kilometer long twin-tube motorway tunnel in China beneath the Yangtze River Estuary. This TBM measures 15.43m in diameter.
Automated tunnelling technology – how does it work?
The type of TBM used for 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 material is kept before being extracted by a pressure-relief discharge system, called “screw conveyor”.
Excavated soil is transported via a 0.8m wide conveyer 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 bores the ground at 1.5m advances at a time, the back end starts lining the walls of the tunnel.
The back end of the machine is able to place pre-cast segment rings forming a watertight concrete cylinder just behind the cutting wheel. In this way 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. They need to work in shifts in order to decompress after entering the pressure chamber. For safety reasons the cutting wheel is 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 disk cutters and four twin disk cutters used for hard rock. The total electric power of the cutting wheel is 2450kW .
Safety and environmental management
In line with Gautrain’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 proven to be a physically safe and an environmentally sound method of tunneling, especially in built-up areas. This method does not disturb surrounding soil and it produces a smooth tunnel wall that is cost-effective. In the geology encountered on this part of the project, as well as working below water table a TBM is by far less time consuming than relying on conventional methods.
Previous use of TBM in Africa
During the construction of the Katse Dam for the Lesotho Highlands Water Project a TBM was used to excavate a 22km long tunnel. The 4.5m 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.