Civil Engineering April 2022 | Vol 30 No 3

32 April 2022 Civil Engineering launching shaft. It is fed with directional information from laser beams deployed inside the tunnel. This enables us to effect 3D adjustments with articulated hydraulic rams attached to the AVNs’ cutting heads and gives us a directional tolerance of 15 mm, well within the specified SANS standard of 50 mm,” says Coetzee. Once drilling in both directions is completed a new launching shaft is sunk and the existing launching shaft and the two subsidiary shafts are used to con- struct permanent manholes. Additional manholes are being installed at pipe sec- tions’ mid-points by sinking intermediary shafts and cutting into the crown of installed pipes. In this way manholes are located at approximately 90 m intervals. CASTING THE PIPES Using eight moulds, Concrete Units cast the pipes to a 100D strength rating to withstand the extreme jacking forces. The pipes have a jointing system comprising a cast-in steel band and a sliding rubber ring which is not only watertight but al- lows for directional adjustments. Concrete Units manager, Charl Coetzee, said approximately 20 m of pipe a day has been installed on the project. “To maintain a steady supply, we used eight moulds for the casting. Each pipe mould was measured before casting and the pipes were cast with a 3 mm HDPE lining made by AKS Lining Systems in Cape Town,” said Coetzee. AN INNOVATIVE ALTERNATIVE SOLUTION AVNmicrotunnelling machines have three cutter head options depending on the ex- pected geology, be it soft soils as in the case of the Mitchells Plain project, a combination of soft soils and hard rock, or solely hard rock. During tunnelling, a cone-shaped crusher in the excavation chamber crumbles rock and other material to a conveyable grain size. It is then pumped out through the slurry line to a separation plant where the solids are removed and from where the water is pumped back to the boring machine. Coetzee says microtunnelling is an environmentally friendly system that protects developed and landscaped areas and minimises traffic disruption. “With the rapid urbanisation of our cities, coupled with ageing infrastructure, there is a growing need to install and up- grade bulk water and sanitation services. Trenchless technologies such as micro- tunnelling offer a safe, cost-effective, and low-risk solution. “It significantly reduces the risks often associated with conventional open excavations. These include public and worker safety, risk of damage to existing utilities, as well as the risks associated to the client’s programme and budget. It also provides solutions in cases where conventional, open trench pipe laying would simply not be possible,” he says. “We believe the innovative solutions offered by trenchless technology should be considered and incorporated at the bulk water, stormwater and sanitation master planning stage, and we would like to encourage municipalities to reach out and allow us to demonstrate how this in- novative technology will assist them with upcoming projects.”  Text and photographs by David Beer on behalf of the Concrete Manufacturer’s Association (CMA) david.bigsky@gmail.com Professional Team Client Western Cape Government – Department of Human Settlements Project manager GIBB Consulting engineers WSP / MPAMOT Health and safety Frontline Consultants Environmental consultants GIBB Environmental The cutting head breaking through to the receiving shaft HDPE-lined pipes prior to installation