Project Overview

Horowhenua District Council has committed to upgrading the Levin Wastewater Treatment Plant, with an estimated investment of $110 million over the next 10 years. The programme will increase treatment capacity, support future growth and improve the long-term resilience of Levin’s wastewater infrastructure.

The first major treatment plant upgrade is the construction of a new Headworks facility, which is planned to be completed in early 2028. To enable the new Headworks and provide the additional capacity required, the first stage of works involved constructing a new trunk wastewater pipeline to the treatment plant while maintaining the operation of the existing live wastewater main running along the same corridor.

The Challenge 

The existing wastewater main was more than 70 years old and had shown signs of deterioration associated with age and previous seismic activity. Located on a stop bank, any damage to the asset during construction would have introduced operational risks and potential environmental consequences. 

Construction was undertaken through winter across farmland with limited access. Saturated ground conditions and a high groundwater table added complexity to excavation and installation activities, requiring extensive groundwater management. 

We developed the detailed pipeline design for the council using the available geotechnical information and loading requirements. The design included embankment works and trench installations to depths of up to five metres to invert.  

Outcomes 

The new 1,500 m wastewater pipeline comprises 1,000 mm and 900 mm internal diameter pipe connected using a proprietary electrofusion jointing system. To reduce construction risk, the design incorporated access turrets for inspection and cleaning in place of traditional large-diameter concrete manholes at every access point. Concrete manholes were used only at wider intervals to maintain personnel access where required. 

Stages 1 and 2 have been successfully constructed by Construction Contracts Ltd over the past 12 months. The completed works provide increased network capacity and improved operational access while reducing the risks associated with constructing and maintaining large manhole structures adjacent to the existing asset.