Sydney Desalination Drinking Water Tank
Project Name: Drinking Water Tank
Client: Blue Water (J.V.)
Principle: Sydney Water
Construction Type: Design and Construction
The Sydney Desalination Project was developed to provide 250 megalitres (ML) of treated drinking water per day with a future phase to increase the capability to 500 ML per day if required. The project supplies up to 15% of the drinking water to Sydney, Illawarra and the Blue Mountain region. This massive volume of water required the construction of a 40ML concrete water reservoir. Internally, this reservoir has a 75.43m diameter (over three quarters the length of a football field) and has a wall height of 10.2m (over 3 times the height of a basketball hoop).
Due to the nature of the project, and its significance, special design criteria were introduced to ensure the structure acheived a minimum design life of 100 years, devoting particular attention to both early age and long term shrinkage measures, crack control and future monitoring measures by Cathodic Protection. The design and construction system that we implemented not only complemented these strict requirements, they provided a solution far superior to any other offering. As a specialised contractor in the field of post-tensioning, our team made efficient use of both insitu and precast post-tensioning elements in the construction of this reservoir.
There was a need for the foundations to be piled as the site was on beach sands. We used a post-tensioned slab eliminating the need for movement joints which would have been required for a traditional reinforced concrete slab (movement joints would have raised further durability issues and raised further concerns about water permeability). The concrete slab needed to be a higher standard of finish and with concerns raised regarding shrinkage controls involved with a floor area of 5000 square metres, it was elected to provide only one construction joint in lieu of the need for multiple joints. Slab tendons were laid through the constuction joint and once stressed this was able to achieve a continuous slab.
The wall structure was constructed using full height precast panels. Simultaneous to the floor slab construction, our onsite team precast 72 wall panels and 6 stressing buttress units around the slabs perimeter. Each wall unit was 250mm thick with a width of 2.7m and weighed 16.9 tonnes. Each unit contained vertical post-tenstioned slab tendons as well as horizontal multi-strand ducts which were used to later facilitate the placing of 7 x 12mm diameter strand within each tendon. Buttress units were 600mm think with a width of 2500mm and each weighed in excess of 31 tonnes.
By providing a 300mm insitu gap between each of the precast units and having clear access internally, for the horizontal post-tensioning ducts were able to be joined together. Both an external and internal form was placed and through access windows at intervals no greater than 1.5m apart, the infill was poured using a concrete mix design developed particularly for this application with slumps in the order of 180mm, which allows a good workable mix to be vibrated around the post-tensioned ducts and reinforcement, tying the units together. Once completed the post-tension strands were placed and the wall was stressed.
Within the reservoir two internal structures needed to be built. The usual method of providing pipe work for the inlet and overflow was not suitable due to the large filling and emptying flow rates. The inlet structure consisted of two wall chambers, the outside chamber having a size of 4.4m x 5.0m and a height of 9.0m. The internal chamber has a size of 2.6m x 2.6m and a height of 7.5m. The overflow structure has a tower of 3m x 2.5m with an open top section of 9.1m long x 2.0m deep and having an overall height of 9.5m. As both structures were situated close to the reservoir wall, conventional means of forming and pouring the concrete in place was difficult and with the top section of the overflow weighting in the order of 41 tonnes imposed large construction loads for a formwork system. APS constructed both structures in precast tilt up wall panels, which were cast on the floor slab. The top section to the overflow was formed and cast outside the reservoir, then lifted over the reservoir wall and placed on top of the overflow shaft.