Upgrading the
PLANKENBRUG MAIN OUTFALL SEWER
Renowned for its university and surrounding wine farms, Stellenbosch has experienced considerable growth in recent years, placing immense pressure on its services infrastructure. This includes the bulk sewer network, which required an urgent intervention.
B
esides the capacity challenges, the existing bulk sewers bordering the Plankenbrug River, which flows through the town, were contributing to high levels of pollution caused by overflowing manholes, and leaking pipes and manholes. Studies carried out on the water quality of the Plankenbrug River have recorded E. coli counts far in excess of the accepted maximum limit. According to SABS (1984) and DWAF (1996) guidelines, the maximum acceptable E. coli level in rivers is less than 2 000 microorganism counts per 100 mℓ. However, at least one Plankenbrug study recorded 3 500 000 counts per 100 mℓ. Besides being devastating to the environment, this level of toxicity clearly poses a significant health risk. AECOM was subsequently appointed by Stellenbosch Municipality to render full professional engineering services for the Plankenbrug Main Outfall Sewer and Associated Works (Phases 2 & 3) project. Construction commenced in September 2017 and the project was completed in November 2020, within budget and at a total cost of R82.1 million (excluding VAT). CSV Construction was appointed as the main contractor. Approximately 4 300 m of bulk sewers were replaced and another 1 500 m network
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IMIESA November/December 2021
Due to the challenging terrain, a total of six route options were evaluated during Phase 2
sewers were installed, with pipe sizes up to DN 1 000 mm. The upgraded system caters for a future estimated sewage flow of over 900 ℓ/s, according to Stellenbosch Municipality’s Sewer Master Plan. “AECOM approached the design of the new sewer in a systematic manner, according to best practice sewer design codes. This included Stellenbosch Municipality’s own design guidelines,” explains Christian Jordaan, associate engineer at AECOM. A major challenge was that sections of the sewer had to be installed alongside the river within a highly built-up area, which in some locations offered less than 7.5 m working width. AECOM’s design proposal recommended the replacement of the existing problematic sewers with the following pipe specifications along the route: • 1 245 m of HDPE-lined concrete pipe (DN 900 mm to DN 1 000 mm) • 3 094 m of GRP pipe (DN 350 mm to DN 600 mm) • 1 486 m of uPVC collector connections (DN 110 mm to DN 355 mm).
Gravity flow and pumped options Due to the challenging terrain, a total of six route options for Phase 2 were evaluated during concept design. Three of these were gravity flow and three were pumped options. The pumped options considered, among others, the possibility of converting one of the existing sewers to a pumped main by repairing the pipeline by means of cured-in-place pipe. “AECOM conducted a cost evaluation on the different route options, finding that a gravityflow pipeline along the river embankment was the most feasible. This option would
also require the least amount of operational maintenance,” Jordaan explains. A DN 1 000 mm steel pipe bridge was also constructed across the Plankenbrug River to maintain gravity flow. For the river crossing, a hydraulic computer model was generated to ensure the pre- and post-construction river capacity remained unchanged. A new stormwater diversion structure was also designed and constructed downstream of the Enkanini informal settlement to divert polluted stormwater run-off into the sewer network.
Innovative solutions The existing sewer network had to remain live during construction. Due to the limited working space for the Phase 2 works, this required over-pumping of sewage past the works for a total length of over 1 km at a maximum flow rate of 300 ℓ/s. To limit pumping lengths, the sewer was designed to allow for this section of the works to be constructed in two phases. Each required its own temporary pump station. A temporary screening facility was also designed to be constructed upstream of the first pump station, to remove solids and items that would otherwise compromise the pump operation. For Phase 3, over-pumping was avoided by constructing manholes on top of the existing sewers wherever the new sewer alignment clashed with the existing system. In this way, the existing sewers could remain live while construction of the new sewer was completed.
Service crossings Along the way, various road and railway crossings were required. By using the following techniques, disruption to the public was kept to a minimum:
