Construction World January 2018

The Herrenknecht AVN 800 XC TBM, with a 1 190-mm-diameter extension kit being prepared for launch from the jacking shaft. (Photo: Terry February)

In the short term, this would provide sufficient capacity to convey all effluent from the Bridgetown Pump Station, while CF1 and CF2 are being rehabilitated. In the long-term, the flow can be split between all three Cape Flats bulk sewers in future peak conditions. Furthermore, the maximum flow rate of 1 300ℓ/s will leave sufficient capacity in the downstream CF3 gravity sewer (Phase 1) to accommodate other future inflows from potential development. Pressure analysis Air valves were designed to provide sufficient surge relief. Therefore the air valves were modelled carefully and selected with the aid of the ARIavCAD air-valve selection software from ARI. A surge analysis was then undertaken to identify the possible surge pressures that may be experienced during transient conditions. The CF3 hydraulic system was modelled in detail using the KYPipe 2012-Surge simulation package. Barometric loop and vortex drop Constraints associated with the pipeline vertical alignment and air-valve configura- tion required a customised barometric loop design for the rising main discharge cham- ber. The barometric loop would comprise an elevated outlet discharging into the down- stream gravity sewer via a drop structure. Given the final sewer alignment, the raised outlet would increase pipeline efficiency, while also eliminating the risk of leaks and foul odours at the air valves located along the pipeline, thereby reducing long-term maintenance for the client. A number of drop-structure options were investigated for the 6,8 m drop at the discharge chamber, taking into account the risk of odour problems, and the highly- corrosive and toxic conditions associated with turbulent flow. The final design uses the Vortex Flow concept developed by Dr Eugene Natarius, and was optimised

Jacking a ductile iron pipe behind the TBM (Photo: Terry February).

for the CF3-2, with a Computational Fluid Dynamics analysis undertaken by international experts at IPEX Inc. This specialised drop structure causes the effluent to flow in a spiral down a vertical shaft in a way that creates a downdraft that traps airborne gases, and forces air into the sewage flow to oxidise the hydrogen sulphide gas, thereby minimising odours and the release of toxic gases from the drop. On time The contract commenced in July 2015, however, physical construction work only started in January 2016 following a period during which the pipework and micro-tunnelling equipment was procured, manufactured and delivered to site. The Contract Due Completion Date was scheduled for 25 October 2017. However, the pipeline has already been commissioned and has been operational for a number of months. Work at the Bridgetown Pump Station is complete and the commencement of a 28-day Trial Operating Period was scheduled for 18 September 2017. All works have been constructed to a high quality, based on a strictly enforced specification. The aim was to provide the CCT with at least 50 years of low- maintenance service, however, considering the high-durability materials used for the project and the quality of the construction work, it is anticipated that this sewer will continue offering uninterrupted service to

the CCT for the next 80 to 100 years. Innovation technology The contract included 5 050 m of 1 000 mm diameter ductile iron rising main, and 250 m of 1 200 mm diameter concrete gravity sewer. In addition to the new bulk sewer, the contract included five air-valve chambers, four isolating/scour combination valve chambers, one discharge chamber, and two manholes. All chambers were constructed in situ, and made to withstand groundwater pressures without seepage. The Bridge -town Pump Station, which delivers flow into the CF3-2, was upgraded with the addition of a fourth pump, and new pump motors with associated electrical and mechanical upgrades. Open-Trench Installation Due to the limited work space available along much of the route, and depths ranging between 2,5 m and 4,5 m, in sandy ground conditions with a high water table, the design and contract specifications took into account that a large portion of the pipe-trench excavation would need to be undertaken using a suitable shoring system. CSV Construction’s preferred method was to use interlocking steel sheet piles, since the system also acts as a coffer dam, thereby limiting the dewatering requirements during construction. It can also be installed and shifted along the

The CF3-2 starts at the Bridgetown Pump Station as a rising main and ultimately discharges into the existing section of the Cape Flats 3 Bulk Sewer (CF3-1) which was constructed in 1996 and currently links the Philippi Interceptor Sewer to the Cape Flats WWTW.

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CONSTRUCTION WORLD JANUARY 2018