Cadia Gold and Copper Mine

Detection of possible collapse hazard

The Cadia Gold and Copper Mine is located about 25 kilometers south of the city of Orange in the province of New South Wales in Australia. The mine has been operated by Newcrest Mining since 1998, and produces more than 25,000 kg of gold and 90,000 tons of copper per year. The Cadia complex consists of a large open pit mine, two underground mines, and two tailings storage facilities.

1. Collapse tailings dam

On March 9, 2018, disaster struck Cadia. A section of the northern reservoir’s tailings dam collapsed, partially draining into the southern storage facility. Fortunately, there were no deaths or injuries in this incident and no environmental damage occurred. However, Newcrest was forced to temporarily cease extraction and to repair the dam. Planning the reconstruction of the dam alone would take about two years. Because of an expected 25% drop in revenue, the value of Newcrest shares additionally decreased by more than $1.4 billion in the first weeks after the collapse.

Satellite image after the collapse of the northern tailings dam.

2. The need for monitoring

Health, Safety, Security and Environment play an important role in the mining industry; after all, small mistakes can have major consequences. An important aspect of this is monitoring the stability of infrastructure and objects in the mine, such as slopes or tailings dams. Since individual tailings storage facilities can hold more than 10 million tons of mine tailings, a collapse can have major effects. Therefore, it is essential to monitor the stability of these objects. However, conventional techniques have several limitations; spatial measurement coverage is often low, there is a need for on-site measurement personnel, and measurement is relatively expensive in the case of frequent monitoring. Modern satellite-based monitoring techniques can thus add great value to HSSE in mining.

“A strong acceleration in the rate of subsidence was found about 3 months before the collapse.”

Deformations of the northern tailings dam between 2015 and 2018, measured with InSAR.

3. InSAR on the Cadia tailings dam.

InSAR is one such technique. It allows remote weekly to monthly measurements with millimeter precision of large areas. This allows close monitoring of deformations or subsidence in and around the mine. After the Cadia tailings dam incident in 2018, we used satellite data to look at the deformations that occurred before the dam collapse. With some 70 measurements taken roughly once every 12 days, this dataset provides a strong basis for historical analysis. It was found that the inner portion of the dam had subsided by more than 20 mm/year on average between 2015 and 2018. However, the dam did not move uniformly. Whereas the eastern section subsided relatively linearly, the stretch around the collapse showed a very different signal. There, a strong acceleration in the subsidence rate was found around November/December 2017, some 3 months before the collapse occurred! This acceleration was also seen in the stretch northwest of the collapse.

4. Portent of collapse

With this, it can be concluded that an anomalous movement could be seen at the Cadia tailings dam about 3 months before it collapsed. This was confirmed in Newcrest’s analysis of the collapse. Using these InSAR measurements, the Cadia team could possibly have intervened earlier by doing local inspections or taking precautionary measures. InSAR data thus provides a powerful tool for pinpointing necessary precautions or for detecting potential future collapses. All at lower cost and better HSSE in and around the mine.

Left: a time series of the deformations of the eastern section of the dam. Right: a time series of the part of the dam that collapsed.