The R210-million tunnel is being constructed for the ethekweni Municipality with precast concrete segments and will be placed 30 m below sea level and 9 m below sea-bed. “It will replace the current immersed tube tunnel which carries services such as electrical supplies and sewage from the Point road pumpstation to treatment works at the Bluff. This tube tunnel is over 50 years old,” says GOBA Consulting Engineers resident engineer for tunnel and shafts Montso Lebitsa.
Plans to widen and deepen the entrance to the Port of Durban to accommodate the depth of modern container ships has necessitated the installation of the new municipal services tunnel.
For the soft-ground tunnelling project, a 19-m-deep shaft will be sunk on the Point mainland. There will be a 20% downward incline on the Point side and a 20% upwards incline on the Bluff side of the tunnel.
The tunnel will have a 4,4-m internal dia-meter and 13,8-m circumference. It will be constructed with 1,2-m-long and 250-mm-thick precast concrete segments pro-duced at a Point road on-site batch plant. The coupling of five main segments and one key segment will make up each tunnel section.
GOBA’s senior resident engineer, Andrew Officer, says sinking shafts and tunnelling in soft ground inevitably pose challenges. “Special tunnel-boring equipment and skills imported from Herrenknecht, in Germany, are being used. Hochtief-Concor, the main contractor, will bring in skilled operators experienced in similar conditions for the project. “The tunnel-boring part of the project has to be completed in only four months so only a very short learning curve is available in this contract,” says Officer.
As the boring machine advances, segments will be erected within the shield. The final part of the contract will be the decommissioning and removal of existing services in the old tunnel tube.
Officer says spoils from the tunnel boring will be pumped into a separation plant and the material disposed of in a waste site. “Planning allowance has been made for a second, pedestrian tunnel but no decision on its construction has yet been taken,” he added.
Kwazulu-Natal branch manager of Chryso SA Vishnu Beeput says three Chryso products have been specified for the tunnel segment construction.
In the first place, Chrysofluid L, a high-per- formance water-reducing admixture was selec- ted for its ability to reduce concrete perme-ability and enhance durability.
Then, Chrysofluid Premia 100, a new-gene- ration super-plasticiser allows the lower pos-sible water-cement ratio in the concrete mix while still retaining the easy-to-work-with feature. This product is mainly used for precast work where very high early strength and first-class finishes are required.
Finally, Chrysocure WBD, a curing com-pound with Fugitive Dye, which helps to retain moisture in concrete for effective cur-ing, is being used for the concrete segment production. “The dye helps identify the areas treated but fades completely after one or two days,” says Beeput.
Concor-Hochtief Joint Venture is the main contractor for the tunnel project, together with GOBA the consulting engineers, in association with Mott MacDonald, of the UK, Drennan Maud & Partners, and Wilson & Pass Inc.
Specialist input on the concrete specifications for the project was provided by the Durban company, Contest Concrete Technology Services.