Durban’s new service tunnel demands heavy-duty effort

KEVIN MAYHEW
IT WAS a case of handling one of its heaviest loads using its biggest cranes (270t and 220t) in tandem when Elcon undertook the lifting and placing of the heavy sections of phase one of the tunnel boring machine (TBM) for the new service tunnel below the Durban Harbour mouth. It was necessary to instal a new 530m long tunnel approximately 30 metres below the high water level of the channel because of the location of the existing services (electricity, water and sewage) tunnel installed 50 years ago and the need to widen and deepen the harbour mouth to accommodate future container ships using the port. The existing tunnel is located at only 12 metres below the high water level, and had started to restrict entry to the harbour for the new generation container vessels, thus calling for a deeper draft. The R210m tunnel is being constructed for the Ethekwini Municipality with a precast concrete segmental lining and will be placed approximately 30m below sea level to enable deepening of the harbour entrance. It will be completed by early 2007. Says Elcon managing director, David Wilkinson: “The 19m deep shaft for the Point mainland end of the tunnel was dug and we had to then lift the TBM sections off transporters and lower them in sequence into the shaft. One of the exacting elements of this was the predetermined angle at which each component had to be lowered to position it for assembly at the base of the shaft in preparation for drilling,” he said. There will be a 10% downward incline on the Point side before changing to a similar upward incline on the Bluff side of the tunnel, as the TBM needs to exit on the Bluff side of the shaft. The new tunnel will have a 4.4m internal diameter and a 13.8m circumference. It will be constructed with precise concrete segments 1.2m long and 250mm thick that are being produced at a Point Road on-site batch plant. It will accommodate various sewage pipes, and electricity and telecommunications cables routed between the Point area and the Bluff. Resident engineer for the project’s consulting engineers GOBA, Andrew Officer, said the spoil from the tunnel boring would be pumped into a separation plant and the material disposed of in a waste site. He added that planning allowance had been made for a possible second pedestrian tunnel but the decision had been taken not to go ahead with its construction.