Geotechnical challenges during design of Aokautere Reservoir – Palmerston North

Palmerston North City Council (PNCC) is currently constructing one of two planned new 2,500m3 reservoirs and a new 1.2km long access road on a site at 291 Turitea Road, southeast of Palmerston North city. The reservoir site is located on a hill with slopes ranging from 150 to 300. The reservoir platform is to be created by cutting the hill top down by 7 m. The reservoir will be designed to be serviceable after a major earthquake event. The reservoir, and therefore its platform is designed as an Importance Level 4 (IL4) structure with special post-disaster function requirements. Site investigation and slope stability analysis was carried out for the reservoir platform to confirm the setback distance of the reservoir from the edge of the slope. The 1.2km access road alignment, which provides the alignment for the pumped rising main supplying the reservoirs, traverses steep valley sides with high sidling cuts. The challenges encountered during the design of the high cuts included restricted availability of information and limited scope for localised site investigation necessitating the extrapolation of surface geological mapping data. Conservative cut slope batters would have required large earthwork volumes at high cost. A risk based approach to the batter design was undertaken, accepting that steeper cut batter angles may result in failures along the access route, but that these failures could be managed for a lower overall cost. Lessons from the experience are drawn for application to similar situations in the future.