For many years, sewage treatment in the town of Opua was primarily by way of on-property septic tank systems and drainage fields. As the population grew and treatment facilities aged, there were ongoing issues with systems failing to meet environmental discharge standards.
Challenging topographical and geological conditions made the construction of a gravity sewer reticulation system prohibitively expensive. This led to the Far North District Council requesting design and construction of a low pressure sewer scheme. The scheme would service 160 properties and provide facility for the connection of a further 30 properties.
United Civil were faced with overcoming several significant challenges, to deliver the project successfully. This included accepting a high level of design risk and working with extremely challenging topography. Additional challenges were inflexible timeframes, funding constraints and working in an urban area that required extensive consultation with often absent owners.
Key features included:
- an 81 metre difference in elevation required a network design incorporating innovative engineering to deliver a cost effective solution
- challenging topographical and geological conditions, including narrow and winding roadways on ridge lines, limited machinery access (requiring extensive works by hand) and the available room to construct trunk rising mains
- highly challenging design due to the large difference in elevation and the need to limit the number of booster pump stations plus minimise sewage retention times
- meeting the objective of improving water quality and reducing any risk to public health, while minimising the risk of overflow, even in cases of extended power outage
- informing and gaining agreement from absent landowners (who were renting out their homes for much of the year) about works on their property, while accommodating the requirements of temporary residents.
The Opua Sanitary Sewer Scheme was completed within tight and inflexible deadlines, on budget and without incident. This serves as a testament to the technical expertise, project management and risk management skills of our staff. The project went on to win at the New Zealand Contractors Federation Hirepool Construction Awards, in 2013.