Rebuilding a piece of Dunedin’s history

The Ross Creek Reservoir, built in 1867, was one of Aotearoa New Zealand’s first major urban water supplies, and forms a key part of Aotearoa’s engineering history. While classed as a Heritage New Zealand Category 1 structure, the ageing municipal water supply facility had been superseded by newer, remote supply works and had sat unused for twenty years. When deep cracks were discovered in the reservoir in 2010, Dunedin City Council decided to upgrade the reservoir to put in back in use. Refurbishing Ross Creek would preserve an important heritage site in New Zealand’s water history, but also help mitigate any risks around the city’s dependence on the newer pipeline supply. Ross Creek would act as a key backup should an earthquake or other natural disaster impact Dunedin’s Deep Creek and Deep Stream reservoirs.

The primary objective was to improve the security of the high PIC body of water, and facilitate its incorporation into the city’s water supply resilience system. Delivered by the WSP team, the project included buttress stabilisation of the puddled clay core embankment dam, crest raising to increase freeboard along with structural modifications to the spillway geometry to increase flood discharge capacity, treatment of defects within the core, construction of a new syphonic offtake, and repairs to the original gravity offtake works and associated tunnel through the dam.

The Solution

WSP took an innovative approach to the complex challenges of the project, which prioritised health and safety, and robust research in to the unique features of the site despite incomplete historical records. Throughout, WSP used a simple-to-implement project construction hazard management plan that focused on mitigating any potential failure modes on the site.

First, the team implemented a special operating regime for the reservoir to mitigate the cracks found in the reservoir in 2010. This involved partially lowering the reservoir to minimise any potential for uncontrolled release, plus an intensified programme of surveillance monitoring and preparations for emergency intervention if required. WSP also undertook a rigorous forensic assessment of the cracks on the shoulder of the reservoir, including back analysis sensitivity assessment and stress strain modelling. This was part of broader heritage investigation of the facility, as the team built their understanding of the site conditions at the time the reservoir was built in 1867, to compensate for the lack of reliable construction era records. In addition, the team extended their understanding of the assets through researching its long service performance and repair history.

Research into the site’s heritage was in parallel with research completed by WSP with the University of Canterbury, into the seismic resilience performance and strain softening behaviour of the silt fill used in the original embankment, through a programme of cyclic testing on recovered undisturbed samples.

To stabilise the reservoir, WSP completed a 3D design of warped buttress geometry which maintained the left abutment spillway facility required to keep the reservoir operational. Seepage and groundwater was extensively drained from the downstream shoulder to bolster stability, while drains originally constructed in around 1880 were discovered beneath the earth embankment during the excavation of the infilled creek channel, and recommissioned to enhance the performance of the aquifer. Sand and gravel filter layers were also incorporated at the interface between the earth embankment and the rockfill buttress to prevent internal erosion of fines on potential seepage paths.

In addition, the project used a combination of modified soil at the shoulder head scarp, and a bentonite slurry cut off trench and plastic injection grouting in the upper core interface zone, to seal off various defects in the reservoir. A new structural crest wall and associated mechanically stabilised earth (MSE) on the raised crest worked to link the buttress and original embankment together under severe seismic loading. A new syphonic offtake and vacuum priming system, constructed through a shallow trench in the true left abutment crest zone, reduced reliance upon the masonry intake tower for emergency dewatering purposes and supplied a proposed transfer pump station.

Mitigating the impact of potential natural hazard events, like earthquakes, was also at the forefront of the project. A concrete isolation plug was constructed within the offtake tunnel and next to the masonry intake tower to reduce the possibility of uncontrolled release of water, if the tower damaged in an earthquake or other seismic event.

Finally, the programme of recommissioning the reservoir was progressively staged to suit unique, extremely low permeability of the earth embankment.

The Benefits

The successful refurbishment of the Ross Creek Reservoir means it has been integrated into the water supply system for Dunedin City Council, and now acts as a backup to the main Deep Creek and Deep Stream reservoirs. The repaired reservoir has also bolstered the city’s ability to withstand extreme weather events. 

As well as this, the process of restoring Ross Creek has highlighted to the community the important engineering and heritage features of the site, and ensured its continued maintenance and use. The reservoir has also increased community usage and recreational value of the side, and which has been further supported by a programme of pedestrian bridge replacements, track upgrading, landscaping and planting on walking and cycling tracks.