In the Hunter Valley (and in other coal mining regions of Australia) coal tailings typically comprise slurries of low to medium plasticity clay, with the more intractable cases ranging into high plasticity. A combination of high compressibility and low permeability results in deposits having a very low coefficient of consolidation. When combined with a relatively high rate of deposition (rise rate) the result is a significantly under-consolidated deposit. This has major implications for design and management including considerations of density (storage capacity), strength (dam break assessment and capping), and settlement (post closure). This paper presents the results of a detailed evaluation of one coal tailings deposit. The storage had been active for 15 years, and is in excess of 80 m deep. Deposition is ongoing. For the purpose of integrated mine site planning the owner required a detailed evaluation of the condition of the tailings in the storage. The work was carried out in stages: a compilation of existing laboratory testing of coal tailings from the site and the region (focused on Rowe cell consolidation, but including other parameters), followed by predictive consolidation modelling of the tailings deposit using finite-strain methods field-work comprising CPTu probing (from a barge mounted rig), sampling and laboratory testing to confirm the theoretical predictions.
The cone penetrometer test probes provided excellent confirmation of the numerical predictions, and showed that the tailings are significantly under-consolidated over a large part of the thickness of the deposit. These results are presented and discussed. The results of the finite strain modelling are also compared to simplified results using conventional consolidation theory, and the closed form solution of Gibson (1958).