The recently completed Silverstream wastewater storage facility for the Hutt City and Upper Hutt City Councils may at first glance appear to be a relatively simple concrete structure. In reality, it posed many complex design and construction challenges that required the coordinated involvement of numerous engineering disciplines.

Making significant use of precast concrete elements, the result is New Zealand’s largest capacity wastewater storage facility, which during its short period of operation has considerably reduced the frequency of sewage overflows into the Hutt River. 

Upper Hutt wastewater has previously been discharged into the Hutt River about seven times a year during periods of intense wet weather. This was necessary to avoid overloading the downstream sewer capacity and pumping stations. The problem is caused by the impact on the sewerage network of rainwater and gully trap stormwater inflow, as well as high ground water table infiltration during heavy rain, says MWH Project Manager Steve Hutchison. This problem is common to most wastewater systems.

Situated at the southern end of Upper Hutt, the new Silverstream wastewater storage facility is designed to not only reduce the number of overflows into the river, but to also regulate the forward flow of wastewater to maximise downstream capacity. In addition, the facility will settle and screen any infrequent overflows to the river which occur during extreme wet weather. Remote monitoring and control of this facility is undertaken through a SCADA system at the Seaview wastewater treatment plant.

CH2M Beca was appointed as design consultant, and completed a detailed hydraulic analysis and a world’s best practice study in collaboration with operator Hutt Valley Water Services and MWH. The initial concept design of two circular tanks at the northern end of the Silverstream site was revised to a single rectangular tank for enhanced self-cleaning. The tank has a capacity of 10,000m3, and is located at the southern end of the site to minimise pumping and pipeline lengths.

Under the direction of Beca Design Manager Doug Stirrat, a range of design tasks were carried out. These included a site investigation, geotechnical and topographical surveys and a hydraulic analysis. Various structural, mechanical and civil designs were investigated, along with acoustic and odour modelling studies, and landscaping concepts.

In terms of its structural design, the tank is 72m x 31m in plan, with walls varying in height from 5.5-6.6m due to a 2% fall in the tank floor to aid the self-cleaning process.

The design team selected a structural system consisting of precast reinforced concrete wall panels, spanning vertically as propped cantilevers between the foundations and the roof. The wall panels are tapered in thickness to save on concrete volume, varying from around 500-600mm thick at the base to 300mm thick at the top. Brian Perry Civil cast the precast wall panels on site, using a 40MPa concrete mix supplied by Firth. The wall panels were joined together using cast-in-place concrete, which contained a shrinkage-reducing additive.

Precast concrete was chosen partly for the superior crack control possible by allowing a significant proportion of the concrete’s shrinkage to occur prior to the wall panels being erected. The pouring of precast panels in a horizontal position also allowed for a more consistent and high quality concrete finish, and meant the formwork requirements of in-situ concrete construction were not necessary.

The tank was designed in accordance with standard water retaining principles, including a series of stress limitations to control crack widths and large in-situ slab pours to minimise joints. The interior part of the floor slab was poured first, with the perimeter infill poured at a later stage after any floor slab shrinkage had taken place. Water stops were incorporated into the detailing, providing two lines of protection against leakage. The tank roof is comprised of double-T roof panels, supplied by R & C Precast Ltd, supported on longitudinal beams and in-situ columns.

An innovative feature of the wastewater storage tank is the low concrete partition walls that divide the tank into six longitudinal channels, utilised during the cleaning of the tank. The rectangular shape and 2% fall in the tank’s floor assist the automated cleaning action generated by the sudden release of a large amount of water within the confined space.

The project also involved the construction of a 1200 litre-a-second inlet pumping station. This adjoining structure consists of an in-situ reinforced concrete wet well, header chamber and associated electrics, housed within an 8m deep excavation.

Architectural features considered during the design included minimising the tank’s height, and surrounding the structure with landscape planting. Care was also taken with the structural detailing to include vertical in-situ joints that were proud of the wall surface to break up the lines of the exterior walls of the tank.

Brian Perry Civil began construction of the $8.3 million wastewater tank in August 2005, and completed work in August 2006, when the tank became fully operational. Since that time the tank has met the design criteria and has been called into action on seven occasions.