Civil Engineering October 2021 | Vol 29 No 9

34 October 2021 Civil Engineering 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 also performed a geotech- nical investigation along the sewer route, as well as conducted a cost evaluation on the different route options, finding that a gravity-flow pipeline along the river embankment was the most feasible. This option also required the least amount of operational maintenance. A DN 1 000 mm steel pipe bridge across the Plankenbrug River had to be constructed to maintain a gravity flow. For the river crossing, a hydraulic com- puter model was generated to ensure the pre- and post-construction river capacity remained unchanged. The pipe bridge was designed to withstand the force of water against it during flooding. The design withstood its first test shortly after construction when, during a storm, a large tree fell on top of the pipe bridge. The pipe bridge remained intact, with minor scratches to its coating, which could be easily repaired. Under this project, one new stormwater diversion structure was designed and constructed downstream of the Enkanini informal settlement to divert polluted stormwater runoff into the sewer network. AECOM generated a computer model to verify that the structure would be able to accommodate the typical flow in the informal stream, but that it would also be able to cope with large storm events without flooding. The sewer had to be installed along- side a river located in a highly built-up area, which in some locations offered less than 7.5 m working widths. This required careful and intrepid decision-making, as well as careful execution. In addition, seasonal river flooding needed to be managed, particularly during the new river retaining wall construction. Several existing services were accommodated to allow the gravity sewer to be built. This was a challenging project due to the intricate and dedicated safety precau- tions on site, as well as the many benefits concerning environmental, social and governance aspects. AECOM is driven by its refreshed Sustainable Legacies strategy that strives to improve social outcomes for the communities in which it serves, while also supporting and sustaining the environment. Limited working space available to install the bulk sewer resulted in challenging conditions The project created local employment opportunities