State-owned Icon Water Ltd. is managing the Corin Dam rehabilitation located on the Cotter River in Namadgi National Park in the Australian Capital Territory (ACT), Australia. The projected cost is US$3 million and the work will ensure integrity of the dam’s spillway, crest of its upstream wall and the road atop the dam.
Corin Dam is 243 ft high and 925 ft long. The earthen dam wall is built on a rock foundation and impounds 71 billion liters forming the Corin Reservoir, which has a surface area of approximately 780 acres. Water from Corin Reservoir eventually reaches Australia’s capital city, Canberra.
Canberra sources major amounts of its water from three dams on the Cotter River -- Corin, Bendora (upstream) and Cotton (further upstream); and Googong Dam on the Queanbeyan River.
ACT is an enclave within New South Wales in southeast Australia and the government of ACT owns Icon Water Ltd.
During construction of a crest wall in 1998, Australian dam safety officials found a 50-mm-wide gap between Corin Dam’s clay core and the spillway bridge right abutment. Engineers placed cement-based grout in the gap and installed a settlement plate, then continued to monitor vertical settlement occurring adjacent to the bridge abutment.
According to published reports, during a dam assessment in 2007, Corin Dam officials determined conditions at the gap promoted a piping risk. In terms of dam walls, piping occurs when water seepage causes dam wall material to erode.
Icon Water hired Melbourne-based GHD to investigate and design improvements to substantially mitigate the identified risks.
GHD said its final design recommends temporarily removing the upstream rockfill and filter material to expose the core. To accomplish this engineers are in the process of transferring capacity from Corin Reservoir to Bendora catchment downstream. Reducing Corin Reservoir’s capacity by about 50% will release pressure on the dam wall in preparation for its rehabilitation.
GHD plans to widen the upstream side of the existing Corin Dam core by adding clay core that includes grader filters and rockfill.
Icon Water said it will begin work in October and it should take about four months to complete.