Grey Eagle Mine

At the Grey Eagle Mine in northern California, gold was mined from an open pit, milled and leached in a conventional cyanide leach circuit, and tailings were disposed of in a small (15 acre) but high (420 ft toe to crest) tailings impoundment, constructed from the mine waste rock. Cyanide and metals contaminated seepage was exhibited from the tailings impoundment prior to mine closure. Dr. Robertson was the Project Principal for SRK for the investigation into the cause of the seepage and the evaluation and design of remediation measures.

Geotechnical and geochemical investigations, including drilling and testing were performed to define the contaminant source terms and plume pathway. Of particular interest was the need to determine the long term consolidation and settlement characteristics of the tailings in order to model the long term expulsion of contaminated tailings pore water. Specialized laboratory testing was performed to enable the consolidation settlement to be determined using non-linear finite strain consolidation theory. Both self weight consolidation and consolidation under cover loads were estimated together with the long term contaminated water expulsion to both the foundation and surface discharge pathways.

Remediation measures implemented include the design and placement of a multiple layered cover over extremely soft tailings. Methods of advancing this cover over the very soft tailings were developed, tested and finally implemented.

• Geotechnical and geochemical investigations
• Laboratory testing to assess the consolidation settlement using non-linear finite strain consolidation theory
• Modelling of the long-term expulsion of contaminated tailings porewater through self weight consolidation and consolidation under cover loads
• Design and implementation of a multi-layer cover over soft tailings

Noranda Inc. (now Xstrata)