To satisfy the ever increasing global and domestic demand for coal, many coal mine operators in Australia, with thick coal seams at depth, are investigating the feasibility of deep open pit mining. Pit depths of up to 500 m have been proposed, with projected spoil pile heights of up to 600 m. The increased thickness of overburden becomes a significant issue, as this increases the amount of spoil produced. Mine operators then have to decide where to place spoil in the early stages of mine planning. Therefore, the parameters on which decisions are made have to be determined up front, to accurately estimate the net present value costs necessary to prove the financial viability of deep open pit mining at a given site.
The mechanisms that cause spoil volume change are: bulking, self-weight settlement, saturation-induced collapse and degradation, all of which act together to alter the shape and height of spoil piles. Thus, the understanding of these volume change mechanisms is critical to the design, sizing, geotechnical stability and cost estimation of an open pit coal mine.
This thesis documents the research, funded by Australian Coal Association Research Program (ACARP) Project C19022, aimed at investigating the effects of settlement and strength of coal mine spoil. As part of the research, several mine sites were visited. Representative spoil samples were collected and were subjected to a comprehensive laboratory testing programme.
Laboratory direct shear testing undertaken in two sizes, 60 mm square and 300 mm square, found that coal mine spoil materials are best characterised as having low cohesion (generally less than 10 kPa) and reasonably high friction angles (generally greater than 30°). On testing under dry conditions, the friction angle is typically in the range from 27 to 35°, while on testing under wet conditions, the friction angle decreases to the range 20 to 34°. Wetting up is seen to cause some reduction in strength, especially for the weakly-cemented and highly plastic spoils.
Laboratory compression tests were performed on two sizes: the standard 76 mm diameter oedometer, capable of stresses up to 1 MPa, and the 150 mm diameter high stress oedometer, capable of stresses up to 10 MPa. Self-weight settlement of initially dry spoil was found to be up to 45% of initial height at 10 MPa stress. Of this, about 80% is expected to occur during placement and is thus not ‘seen’. Collapse settlement on wetting up was found to cause additional settlements of up to 15% at stresses of about 500 kPa, decreasing to about 5% at 10 MPa stress. Negligible further collapse is expected after a collapse event, since water merely fills the voids between particles. In addition, test results indicated that the void ratio of coal mine spoil approached values of 0.3 to 0.35, corresponding to dry densities of between 1.95 and 2.1 t/m3, at 10 MPa applied stress, regardless of moisture state, maximum particle size, scalping method and magnitude of initial loading.
As part of the research, a degradation test method was developed to investigate the effects of wetting and drying on exposure to the weather. Test results showed settlements of the order of 1 to 26% of height, although this is expected to be limited to the top few metres of the spoil pile. Sieving analysis undertaken during the degradation tests have shown that weather effects can reduce particle sizes by up to an order of magnitude.
The compression and degradation test results were used as input in the development of a spoil settlement prediction tool. The prediction tool shows that weaker spoil types tend to initially bulk up slightly more and also settle more when compared to the more durable spoil types. Based on the prediction tool, net bulking factors of typical coal mine spoil are expected to range from 1.27 to 1.38 in dry conditions and 1.12 to 1.25 in wet conditions. Net bulking factors for spoil from the Ipswich Coalfields and Hunter Valley Coalfields have been calculated as 1.11 and 1.14, respectively. These values were validated using data from literature and mine site data. Additional loading due to spoil piles up to 600 m in height is expected to lower the net bulking values by a further 2 to 4% of height.
The laboratory testing data, along with the spoil settlement prediction tool, is expected to assist coal mine operators in determining the settlement and net bulking of spoil with mixed compositions at their mine site. This is expected to lead to more accurate and robust volume estimates for mine planning purposes.