The Christies Beach Waste Water Treatment Plant
Commissioned in the early 1970s, the Christies Beach Waste Water Treatment Plant receives and treats wastewater from surrounding communities and businesses.
Located in Adelaide’s southern suburbs, SA Water upgraded the Plant to increase capacity and meet the expected growth of the area.
Part of the overall Plant upgrade was the construction of a new outfall to discharge excess treated effuent in line with new environmental protection guidelines. The new 1,200mm diameter outfall pipeline was partially buried on the sea floor and extends some 650m from the shore into the Gulf of St Vincent.
Reflecting best practice construction, the delivery of the offshore pipe was executed through a combination of innovative approaches including a pipe launch methodology that involved a 180 metre temporary bridge, a sheetpile cofferdam installed through the beach crossing, and a shore-based winch pulling via an offshore anchor.
The 305m long, 12m wide bridge consists of a post-tensioned concrete box girder, erected by the balanced cantilever method using match-cast segments.
Key deliverables of the project included:
- 670m DN 1,200mm outfall pipe including 100 diffusers distributed over 250m.
- De-aeration structure for new Outfall.
- Rosette type diffuser on the discharge of the existing outfall.
- Cathodic protection system for new outfall.
- High flow control chamber for new and existing outfalls.
- Connections of the upgraded system to the existing Willunga Basin Diversion chamber.
- All associated pipework, concrete and electrical works.
The final pipeline profile required a trench through the beach and surf zone to bury the pipe up to four metres below the existing sand levels. This necessitated the use of shoring to hold open the trench during the excavation, launch and backfilling processes. Limited offshore geotechnical investigation was carried out prior to tenders, so trial piles were installed and extracted prior to full site mobilisation to prove that the required sheet pile toe levels could be reached.
Special Features
A ‘J’ curve configuration was chosen for the launch as an innovative alternative to the traditional ‘S’ curve. This choice created the opportunity to ‘self launch’ the first third of the pipe strings – the gradient of the launch ramp was sufficient for the self-weight of the pipe to overcome the friction of the launch rollers such that the pipe launched (actually had to be substantially held back) without needing to be pulled. This also enabled the site team to get through the learning curve associated with the launch sequence without having the time pressure of a weather window. It also enabled launching to commence prior to completion of offshore dredging, required to enable the pull rope to be laid out and fitted.
The contract included the casting of a 120mm thick reinforced concrete weight coating to the client-supplied steel pipe sections. We sourced formwork for this coating from a project in Western Australia where a similar pipe size had recently been completed and the steel forms were no longer required – great reuse and recycling implementation!
Special Features
Substantial reuse of materials and components used in the construction of the works was achieved. The sheet-pile waling system, for example was purchased second hand from the Perth Metro rail project and subsequently sold to the Alkimos project north of Perth on project completion.
Constructing the works progressively in short cofferdams required an innovative solution at the junction between cofferdams. Issues to consider such as retaining backfill behind the pipe starter length from the previous cofferdam and avoiding damage during subsequent excavation required careful thought. The solution adopted fulfilled all of these criteria and successfully completed the joints 14 times along the length of the pipeline.
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