TRENCHLESS TECHNOLOGY
Rehabilitating Cape Flats 1 and 2 Trenchless technologies will be employed extensively for the refurbishment of one of Cape Town’s largest sewer systems, with all works expected to be completed by 2025. Vuyo James, senior professional officer: Planning, Design and Projects at the City of Cape Town’s Water and Waste Directorate, says the project is the largest undertaken to date in South Africa. By Alastair Currie
T
he Cape Flats 1 and 2 (CF 1 and 2) sewer lines, each measuring some 14 km in length and running in parallel, were originally installed in 1962 and 1969, respectively, to meet growing demand. Then and now, they perform a vital service, which has become even more crucial given ongoing residential, commercial, and industrial expansion within the City of Cape Town. This was the motivation for the construction of the new Cape Flats 3 (CF 3) sewer line, completed in 2016, which has significantly reduced the load on the overall system. The additional services provided by CF 3 now make it possible for a systematic and phased refurbishment of CF 1 and 2, without putting service delivery on the line. As a far more modern system, CF 3 was installed with larger-diameter pipe networks compared to CF 1 and 2 – the latter ranging from 1 050 mm up to 1.8 m. CF 3 also benefits from the latest advances in protective polyurethane lining systems preinstalled by the precast pipe manufacturer that supplied the project.
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IMIESA September 2021
Athlone is the starting point for CF 1 and 2, culminating at the Cape Flats Wastewater Treatment Works (WWTW) in Zeekoevlei. The treated effluent is subsequently discharged into the Zeekoevlei zone, an approximately 258 ha water body that borders the Atlantic Ocean.
Designed to last The original networks for CF 1 and 2 are composed of precast reinforced concrete pipe in varying diameters along their length – as is typical of a sewer system. However, it’s interesting to note that the City’s design team went a step further by encasing the entire pipeline installation with a concrete outer layer as an added protective measure during their construction. As was common practice in the 1960s, the manholes are built predominantly of clay brick, incorporating a chamber, or chimney, for inspections. “Considering that these pipes have been in operation for close on 60 years in a highly corrosive environment, they have stood the test of time exceptionally well,” says James.
Vuyo James, senior professional officer: Planning, Design and Projects, City of Cape Town Water and Waste Directorate
“From the CCTV inspections undertaken to date, it has been confirmed that there are no pipe collapses on CF 1 and 2. However, there are collapses at the manholes, which can be expected given some six decades of performance. Manhole chambers tend to be subjected to far higher concentrations of hydrogen sulfide (H2S) gas build-ups than the pipes themselves.”
Trenchless versus opentrench excavation The City of Cape Town’s engineers have gained extensive experience in the deployment of trenchless technology for new pipeline installations, like CF 3, as well as major bulk line refurbishments. This made going the trenchless route a logical choice. “A major benefit of trenchless construction is minimal, if any, disruption, which is especially important in urban areas,” James explains. “Plus, the cost of electing for an open-trench
