Capping of Tailings Deposits: Analysis, Design and Implementation

Un-rehabilitated, large coal tailings deposits present a safety and environmental risk to current and ceased coal operations. Capping of tailings deposits, involving the spreading of granular mine waste over the surface of the tailings using heavy machinery is often undertaken to rehabilitate such sites. There is often uncertainty about the risks associated with the capping works due to a lack of understanding of the condition and strength of the tailings at depth, as well as likely failure mechanisms. This paper presents a design methodology for final capping of deep, unconsolidated soft coal tailings deposits. The approach outlines a testing, analysis and construction sequence to minimise risk during capping works. A case study of the method is presented for a coal mine in the Hunter Valley of New South Wales (NSW), Australia. The design process presented in this paper outlines the significance of appropriate site investigation works required to assess the undrained shear strength profile of the tailings, which often varies considerably with depth, location and time. The results of the investigation form the basis for analysing the stability of the capping works, where heavy machinery pushing material across the surface of the tailings creates a short-term unbalanced load. Global stability due to unbalanced loads as well as local bearing capacity are considered. The analysis is then used to develop limiting fill thickness and machinery loads for the works, with consideration given to subsequent capping layers. Implementation of the capping methodology is also discussed.