On the 8th of February Hydro Tasmania hosted TasWater and Tasmanian Irrigation engineers for a site visit to Catagunya Dam, located on the Lower River Derwent catchment. Catagunya is a 50m high post-tensioned dam which utilises an array of 200-tonne capacity steel cables to provide the necessary structural stability against the reservoir water load. The dam was originally designed as a gravity structure, but the design was modified to incorporate the relatively new 1960s era post tensioning technology, where the use of post tensioning achieved an estimated reduction in concrete in the order of 20% compared with a conventional gravity dam. At the time of construction, Catagunya was the highest post-tensioned dam in Australia, with an adopted 100-year design life for the anchors.
The design life was reduced to 50 years shortly after installation following investigations of a test anchor at nearby Meadowbank Dam, where geotechnical engineers cored a 1200mm diameter hole to inspect the integrity of the test anchor and found voids in the grout column around the wires. The implication was that with unknown encapsulation of the anchors by the grout, ingress of water and potential for corrosion of the steel anchor wires could dramatically reduce the tensile capacity of the original 412 post-tensioned anchors. In 2009, 92 modern, large diameter, multiple barrier corrosion protected, post-tensioned anchors were installed, making the original anchors obsolete and ensuring ongoing stability of the dam. See below an excellent video on this process posted on Youtube.
The site visit was to observe the five-yearly integrity test on one of the 92 anchors, utilising specialist equipment and technicians from SRG Global. The anchor we observed was accessed via a travelling gantry, located on the right-hand side of the dam spillway. Each anchor must be accessed by removing the protective capping, recording the condition of the corrosion protection system, and then fitting a custom hydraulic jack arrangement before applying a target lift-off test force in the order of 1700kN. Results are monitored over time to assess the on-going performance of each anchor. Once tested, and the hydraulic jack removed, the corrosion protection system is methodically reinstated, and the cap replaced. The diligence in properly applying the corrosion protection and cap has been found to be critical for the on-going integrity and ease of re-testing of the anchor head.
The site visit was a great opportunity to view the periodic testing of a major dam asset in Tasmania, with plenty of information and learnings to take away from the experience. Thanks to Hydro Tasmania for providing the opportunity and to Oliver Giudici for providing technical background on the dam history and anchor testing program.
YP State Rep Tasmania