With the high congestion at Durban harbour, the National Ports Authority (NPA) is planning to widen and deepen the harbour entrance. Currently, the movement of vessels in and out of the harbour is limited to one direction at a time. This leads to cargo ships having to find anchorage close to the city’s shoreline at times, and delays at Durban’s port, which harm the local economy and smooth trading.

At present, a number of essential municipal services cross the harbour entrance in an immersed-tube tunnel built some 50 years ago. These services include a 900-mm-diameter sewer-pumping mains, another 750-mm-diameter sewer-pumping main, a 450-mm-diameter water main as well as various power and communication cables.

Due to the widening and deepening of the harbour entrance, the existing tunnel will be too short and to shallow. eThekwini Water & Sanitation (EWS) is therefore planning to replace it with a new and larger services tunnel.

“We are in the process of designing the new tunnel, which will have a 4,5 m internal diameter and will carry the two 1 000-mm-diameter sewer-pumping mains, a 450 mm water main, a number of 11 kV and other power cables, telecommunication cables, plus cable ducts for the new Durban lighthouse. “In addition, we are investigating the development of a second tunnel, known as the ‘people mover’ tunnel, to allow people to cross the harbour entrance between the point and the bluff, ” says Goba Moahloli Keeve Steyn’s (GMKS’s) executive chairperson Trueman Goba.

The R150-million project’s design will be completed by the end of the year, with construction due to start in early 2005. “The project has to be fast-tracked in order to meet the tight deadlines set by the NPA’s port-widening programme,” says Goba.

Construction of the tunnel will take place in various stages. Goba explains: “The first stage will be the sinking of a 12-m-diameter shaft to provide access to the tunnel, and another shaft at the other end of the tunnel to receive the tunnel-boring machine (TBM) on completion of the tunnel drive.

“The tunnel itself will be excavated in the marine sediments using a slurry-type TBM or an earth-pressure balance (EPB) TBM, and lined with a pre-cast segmental lining.” The shielded TBM is pushed forward while a rotating cutter-head in front of a sealed bulkhead cuts the sand and sediments. A slurry is injected under pressure and mixed with the spoil ahead of the bulkhead in such a manner that the pressure of the spoil equals the pressure of the material ahead of the TBM. Spoil is extracted through a pump-line to the surface at the tunnel shaft. The key is to maintain the balance of pressure ahead of the bulkhead as excavated material is removed and the TBM moves forward.

Behind the machine, but still within the shield, pre-cast concrete segments are erected in a ring to form the lining of the tunnel. The assembled lining is able to carry the full weight of the water and sediment on top of it.

Ingress of water behind the TBM is prevented by use of an innovative system of seals between the TBM shield and the newly-erected segment rings. The segments are also fitted with gaskets, and cement grout is injected around the outside of the segment rings to prevent seepage into the tunnel. The services tunnel is due for completion in June 2006.

“This is a very interesting and tech-nically challenging project from an engineering perspective as the tunnel has to be constructed under difficult conditions due to the soft sediments under the ocean as well as the water pressure. “This type of machine and tunnel-ling technology is new to South Africa, although it has been used on a number of projects overseas,” Goba concludes.

GMKS is also involved in many other projects, one of which is the rehabilitation and upgrading of the N12 between Gillooly’s interchange and the R21 interchange for the South African National Roads Agency. The company is also doing design work and project management of various coal-mine infrastructure projects in Mpumalanga. The firm is also a partner in the consulting joint venture for the design, project management and construction supervision of the Berg Water Project, which includes the recently-awarded Berg river dam, near Franschhoek, in the Western Cape.