“Seeking Relief” Rising Pore Pressures in Earth Dams – A Case Study

Patea Dam is owned by Trustpower Ltd. (Trustpower) and is a zoned earth dam. On commissioning, lake pressures were observed to be transferring into the underlying geology. This created an increased artesian pressure within the footprint of the structure, reducing the dam’s Factor of Safety (FoS) below what would be expected of a high Potential Impact Classification (PIC) dam. Remedial measures were investigated to correct long-term pressure trends at Patea Dam and ensure the long-term safety of the dam for its owner; the residents below the dam; and the recreational users of Lake Rotorangi. The approach of drilling relief wells into the underlying foundation is a method that had been historically used at Patea Dam, resulting in successful alleviation of excess artesian pressures. In April of 2015 a further two wells were commissioned to reduce the ongoing build-up of pore pressure. Numerous obstacles were overcome during the installation process, including challenging access and dangerous weather conditions to reach a satisfactory outcome for all parties involved. Initial results saw an approximate drop of 2m in the artesian pressures below the toe of the dam increasing the dams FoS and reducing artesian pressures to below identified alarm levels. This case study shows how relief wells can be successfully installed, even with numerous challenges, to safe-guard an important piece of New Zealand’s infrastructure for the future.