Identification and management of dispersive mine spoils

Identification and management of dispersive mine spoils

This report details a project that has focussed heavily on soil properties and the impacts of infiltrating water on both soil properties and soil behaviour. Included with the report is a review of literature covering aspects of soil chemistry, structure, clay dispersion, and tunnel erosion. Readers not familiar with those aspects of soil science may find it helpful to read the literature review (Appendix 1) prior to working through the main body of the report.

Mine sites that have been involved in, and have supported the project, are:
• Coppabella Coal Mine (Australian Premium Coals Pty Ltd) – 10 km east of Coppabella and 39 km west of Nebo, Central Queensland.
• Higginsville Gold Mine (Resolute Mining Ltd) – 70 km north of Norseman and 110 km south-south east of Kalgoorlie, Western Australia
• St Ives Gold Mine (Gold Fields) – 80 km south of Kalgoorlie and 20 km south of Kambalda, Western Australia.
• Telfer Gold Mine (Newcrest Mining Ltd) – approximately 450 km East of Port Headland, Western Australia.
• Jundee Gold Mine (Newmont Australia) – approximately 193 km north of the township of Leinster, Western Australia.

This project has highlighted a number of important issues. Firstly, it has shown the importance of soluble salt content in some spoils, and the need to manage salt content to maintain stability. Secondly, the project has shown the existence of effectively two mechanisms for tunnel erosion (movement of dispersed clay and also movement of non-cohesive fine particles), where previously tunnel erosion was attributed solely to clay dispersion. This finding has been supported by considerable field observation, and means that the range of materials at risk from tunnel erosion is greater than initially believed. Irrespective of the method by which tunnels form, the project has indicated strong interactions between the design of constructed landforms and the development of tunnel erosion. Where water is ponded over saline sodic spoil, leaching of salt by the ponded water results in reduced soluble salt, increased dispersion, and development of tunnel erosion. For non-cohesive materials, long durations of ponding are also a major factor in developing tunnel erosion. Although retention of rainfall and runoff water on constructed landforms is widely considered to be highly desirable, in practice there is a range of situations where ponding of water is a recipe for disaster. Because of the range of mechanisms by which tunnel erosion develops, there is no single test that will provide optimal information across the entire range of materials considered. Rather, it appears that initial assessment of soil chemical and physical data is required, followed by specific tests to assess the specific tunnel erosion mechanism indicated by material properties. Initial soil/spoil parameters that provide information on tunnel erosion potential are:
i) EC (to assess potential salinity impacts on dispersion);
ii) Cations, with particular emphasis on exchangeable sodium percentage (ESP) to assess dispersion potential;
iii) Particle size distribution (to provide an indication of soil cohesion and liquefaction contributions to tunnel formation/failure), and
iv) Clay mineralogy (for swelling influence).

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