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.