Diepsloot Outfall Sewer Pipeline and Bridges Repairs


The Diepsloot concrete outfall sewer over the bridges is subjected to damaging elements comprising mechanical and chemical erosion internally due to the effluent load and external wear of concrete due to cyclic thermal and moisture variation damage as well as outright vandalism by unauthorised intruders. The wear and tear on the pipeline reduces its functional life and could result in a disastrous collapse if necessary interventions are not instituted to repair and protect the sewer timeously. A collapse of this sewer, which handles a colossal amount of sewage for almost half the city of Johannesburg, would be a huge and damaging setback to the City’s function, reputation, immediate and downstream environment.

The current project is to execute a planned major rehabilitation of the sewer in order to preserve and prolong the functional life of the pipeline and supporting bridges. The external condition of the pipeline can be assessed and measured easily by visual inspections. The pipeline requires minor structural repairs externally and protection from temperature variations. The original choice protection comprising insulation and aluminium cladding has largely been stolen for its scrap value. The replacement protection invariably needs to have no scrap value. It is not common to clad the sewer pipeline size and so precedents of such are scarce. Trial applications of different products will be employed to determine fitness for purpose.  The biggest challenge is to conduct a comprehensive condition assessment of the sewer internally as the prevailing high sewage flows present hazardous conditions for human access and locally available CCTV technology cannot operate satisfactorily within the swift sewage current. A practical solution to facilitate access to conduct internal sewer inspections is to divert the sewage out of the affected pipe section in order to facilitate safe access for such inspections. The by-pass sewer has to be robust enough to convey the expected sewage flows to facilitate both the inspections and subsequent internal repairs as necessary. Another pressing need for the by-pass is to provide an alternative routing for the effluent in the event of damage on the exposed pipe section that would result in a major sewage spill into the surroundings. A previous disaster assessment study concluded that the disaster management plan in the event of a sewage spill requires a rapid deployment of such a bypass. The scale of the by-pass is such that rapid deployment is not possible to match the timelines demanded to deal with a spillage to sufficiently mitigate resultant disastrous environmental damage. The solution proposed is to provide the by-pass as a permanent installation that can be activated to divert the sewage in a short period.

The technical interventions will entail detailed hydraulic study and modelling, security evaluations, environmental study, project specifications, construction procurement, construction and close-out.