Construction World December 2022

CONSULTING ENGINEERS

RETREAT LOW LIFT SEWER PUMP STATION AND REFURBISHMENT OF EXISTING BULK OUTFALL GRAVITY SEWER B Vi Consul t ing Engineers was appointed by the Ci ty PROJECT INFORMATION

of Cape Town to undertake the planning, des ign, tender documentat ion, project management and s i te super vi s ion for the construct ion of the new Retreat Low Li f t Sewer Pump Stat ion. The pump stat ion forms part of the Cape Flats Sewage Ret iculat ion System. The proposed upgrades to the sewage inf rastructure compri sed of the fol lowing components : • Construct ing the new Retreat low l i f t sewer pump stat ion wi th increased capaci ty of 750 l /s flows . • Construct ing gri t chamber, instal l ing automated screening equipment and odour control system. • Cured- in-place-pipe (CIPP) technology trenchless underground pipe refurbi shment and rerout ing of an exi st ing 1 050 mm diameter gravi ty sewer. • Demol i t ion of the old Retreat low l i f t sewer pump stat ion, and construct ion of new mul t ipurpose communi ty sportsfield. The underground concrete structure reaches depths of up to 9 m below ground. Wi th geotechnical invest igat ion indicat ing the water table at only 1 m below natural ground level , groundwater presented a chal lenge to the deep excavat ion which requi red cont inuous and extens ive dewatering. A cof ferdam (12,85 m x 50 m) wi th wel lpoint dewatering system was constructed f rom sheetpi les up to 12 m deep. The excavat ion process for the construct ion of the pump stat ion below ground concrete structure entai led extens ive dewatering of the cof ferdam. Thi s al lowed for cont inuous access to the deep excavat ion. The dewatering was final ly decommi ss ioned af ter the below ground reinforced concrete base, wal ls and ground level slab had been constructed and backfi l led. Thi s was a cri t ical point as the concrete structure needed to have suf ficient weight before the water table could be al lowed to ri se wi thout resul t ing in the structure float ing due to buoyancy ef fect . The exi st ing out fal l gravi ty sewer was found to have deteriorated extens ively and s i l ted up above 50%, reducing the pipe capaci ty when construct ion commenced. Thi s , along wi th regular local pipe col lapses on the out fal l gravi ty sewer, resul ted in cont inuous surcharging of manholes and regular sewer flooding the Si te. 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

envi saged at des ign and local overpumping thus was regarded as a non- feas ible approach as sewer could not be overpumped into a surcharged di scharge chamber the and a s i l ted out fal l gravi ty sewer wi th reduced capaci ty. A revi sed overpumping scheme was implemented which would al low bypass ing of the problemat ic out fal l gravi ty sewer main between Retreat Low Li f t Pump Stat ion and Retreat Main Pump Stat ion. Thi s was to mi t igate on-going ri sk of further col lapses and surcharging leading to sewer flooding the new pump stat ion construct ion s i te and ensure ready access for proposed refurbi shment of the out fal l gravi ty sewer. I t negated the need for addi t ional local overpumping during the CIPP stage of the project and mi t igated ri sk of regular extens ions of t ime due to flooding of the s i te. The new Retreat Low Li f t sewer pump stat ion was des igned for energy ef ficiency, ease of operat ion and maintenance, high securi ty and vi sual aesthet ics . The incoming raw sewage has a high influx of rags and inorganic sol ids . Two automated mechanical raking screens (one duty, one standby) were instal led to remove rags , reduce clogging and thus prolong the l i fe span of new pumping equipment and downstream inf rastructure. Three immers ible sewage pumps ( two duty, one standby) were instal led wi th variable speed drives in a dr y wel l configurat ion. Telemetr y system and Scada system were instal led for early detect ion and relaying of s ignals and faul ts to enable quick response as wel l as of fs i te moni toring of the pump stat ion. 

64 21 st Best Project Awards 2022