An innovative design solution

Wellington City’s population is expected to grow by around 65,000 people over the coming 30 years, with over 7,000 new dwellings predicted to be added in the Victoria Street area of the CBD. With the wastewater infrastructure significantly undersized to cope with such growth, our client Wellington Water Limited (WWL) identified a further significant risk of wastewater overflows to the harbour.

WWL engaged GHD consulting engineers to design a combined drywell/wetwell pump station and rising main pipeline. The project involved constructing a new 6.5m diameter, 7m deep underground pumping station, including mechanical fit-out and 175m of new rising main on a contaminated site. Brian Perry Civil (BPC) won the contract by impressing WWL with its innovative solution, based on the segmental precast caisson system.

The project involved working from the top-down with a pre-made segmented ‘jigsaw’ of concrete panels—the first time such an approach is believed to have been completed in New Zealand. By opting to use this approach there was no requirement for large scale temporary works (shoring). The caisson method uses the absolute minimum footprint of the excavation, without over-digging for working space, reducing the volume of material to be excavated. Another benefit was minimising the volume of contaminated material—from the high levels of petro carbon contaminants in the groundwater—that had to be removed.

The central city site created many challenges, which the segmental caisson system helped overcome. This system is constructed in one metre increments at ground level, and then pushed into the ground using hydraulic jacks. With no vibration effects from this method, the danger to adjacent structures (including a three storey unreinforced masonry structure) was overcome. The system also helped overcome the risk of excessive noise for residents and businesses.

A vulnerable tree within one metre of the site was saved by moving the location of the pump station by 300mm. Careful planning also helped retain two lighting columns within one metre of the new structure.

Another challenge came when installing the rising main, discovering significant differences from the records provided, including much deeper banks of cable than had been anticipated in the original design. A directional drilled solution that minimised risk to underground services and traffic management was employed.

Using innovative solutions and out of the box thinking turned difficult challenges into a real success story. Brian Perry Civil’s innovative approach was recognised with a CCNZ Wairarapa Branch Award in May 2019.

Awards

1. Winner of 2019 Civil Contractors New Zealand (CCNZ) Wellington Wairarapa Branch Regional Awards - projects over $2M
2. Winner of 2019 IPWEA Award for Best Public Works Project under $5M