CONSULTING ENGINEERS
RETREAT LOW LIFT SEWER PUMP STATION AND REFURBISHMENT OF EXISTING BULK OUTFALL GRAVITY SEWER B Vi Consulting Engineers was appointed by the City PROJECT INFORMATION
of Cape Town to undertake the planning, design, tender documentation, project management and site supervision for the construction of the new Retreat Low Lift Sewer Pump Station. The pump station forms part of the Cape Flats Sewage Reticulation System. The proposed upgrades to the sewage infrastructure comprised of the following components: • Constructing the new Retreat low lift sewer pump station with increased capacity of 750 l/s flows. • Constructing grit chamber, installing automated screening equipment and odour control system. • Cured-in-place-pipe (CIPP) technology trenchless underground pipe refurbishment and rerouting of an existing 1 050 mm diameter gravity sewer. • Demolition of the old Retreat low lift sewer pump station, and construction of new multipurpose community sportsfield. The underground concrete structure reaches depths of up to 9 m below ground. With geotechnical investigation indicating the water table at only 1 m below natural ground level, groundwater presented a challenge to the deep excavation which required continuous and extensive dewatering. A cofferdam (12,85 m x 50 m) with wellpoint dewatering system was constructed from sheetpiles up to 12 m deep. The excavation process for the construction of the pump station below ground concrete structure entailed extensive dewatering of the cofferdam. This allowed for continuous access to the deep excavation. The dewatering was finally decommissioned after the below ground reinforced concrete base, walls and ground level slab had been constructed and backfilled. This was a critical point as the concrete structure needed to have sufficient weight before the water table could be allowed to rise without resulting in the structure floating due to buoyancy effect. The existing outfall gravity sewer was found to have deteriorated extensively and silted up above 50%, reducing the pipe capacity when construction commenced. This, along with regular local pipe collapses on the outfall gravity sewer, resulted in continuous surcharging of manholes and regular sewer flooding the Site. The overpumping scheme
• Company entering: BVi Consulting • Client: City of Cape Town • Consulting Engineer: BVi Consulting • Environmental: Silito Environmental • Civil Contractor: CSV/Mazarin JV • M&E Contractor: Tricom Africa
envisaged at design and local overpumping thus was regarded as a non-feasible approach as sewer could not be overpumped into a surcharged discharge chamber the and a silted outfall gravity sewer with reduced capacity. A revised overpumping scheme was implemented which would allow bypassing of the problematic outfall gravity sewer main between Retreat Low Lift Pump Station and Retreat Main Pump Station. This was to mitigate on-going risk of further collapses and surcharging leading to sewer flooding the new pump station construction site and ensure ready access for proposed refurbishment of the outfall gravity sewer. It negated the need for additional local overpumping during the CIPP stage of the project and mitigated risk of regular extensions of time due to flooding of the site. The new Retreat Low Lift sewer pump station was designed for energy efficiency, ease of operation and maintenance, high security and visual aesthetics. The incoming raw sewage has a high influx of rags and inorganic solids. Two automated mechanical raking screens (one duty, one standby) were installed to remove rags, reduce clogging and thus prolong the life span of new pumping equipment and downstream infrastructure. Three immersible sewage pumps (two duty, one standby) were installed with variable speed drives in a dry well configuration. Telemetry system and Scada system were installed for early detection and relaying of signals and faults to enable quick response as well as offsite monitoring of the pump station.
64 21 st Best Project Awards 2022
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