Specification of Overburden for Use as Spoil Cover

This project searched for links between overburden data obtained during feasibility studies and characteristics of the subsequent spoil that would make it suitable as a cover layer over other spoil with properties hostile to rehabilitation. The study included:

• Definition of performance requirements for cover layers over acid, saline, dispersive and erodible spoil, and review of the properties that produce that performance;
• Inspection of cover spoil at three mines to verify the links between cover performance and properties;
• Subjection of overburden core samples from three projects to several standard laboratory tests related to weathering and durability to assess the value of those test methods in specifying suitability of overburden for cover spoil;
• Development of a simple soaking-drying test on larger core specimens that more closely replicates expected conditions for spoil at landform surfaces;
• Comparison of results from the various overburden tests and from geophysical logging to search for relationships that could be used to identify suitable overburden during feasibility studies.

Based on the performance review and cover inspections, a tentative specification for spoil cover is proposed using particle size grading and standard geochemical indices. Average results for each rock unit tested demonstrated correlations between results of the soaking-drying test and sonic and gamma log responses. Consequently, provisional criteria are suggested for screening overburden with the potential to produce cover spoil based on average geophysical responses. The specifications require verification by field trials but offer the ability to identify cover-suitable overburden before mining so that it can be won as a resource and selectively placed to improve  rehabilitation outcomes.