Shear strength characterisation of in-pit mud to ensure lowwall stability

Abstract

Weakened basal spoil and in-pit ‘mud’ caused by heavy rainfall events and flooding have been associated with open-cut strip mine lowwall failures, particularly in Australia’s Bowen Basin. Large-scale lowwall failures cause considerable disruptions to mining associated with a loss of production, damaged infrastructure, and the potential loss of life. The main objective of this research funded by the Australian Coal Associated Research Program (ACARP) was to characterise this in-pit mud to determine its parameters for use in design. An improved understanding of in-pit mud characteristics will reduce the mud removal requirements, or remove them entirely, resulting in enhanced mine safety and economics.Thirteen samples of mud and six samples of spoil were obtained from three mines within the Bowen Basin. The BMA Coal spoil shear strength framework was utilised for categorisation, with the selection of materials ranging from competent to incompetent. Each sample was thoroughly characterised with respect to its physical, chemical, mineralogical and geotechnical properties.Physical, chemical and mineralogical testing involved measurement of the materials in situ moisture content, specific gravity, pH, electrical conductivity, total suction, Emerson class, Atterberg limits, X-ray diffraction, exchangeable cations, and cation exchange capacity. The particle size distributions included dry sieving for material that did not agglomerate during drying at 60⁰, and wet sieving after 24 hours of soaking in a water bath without dispersant, followed by testing of the fine fraction without dispersant.Characterisation of the mud and spoil showed the majority of materials were dominated by Quartz, Kaolinite, Illite-Smectite and Albite. High levels of sodium Smectite were associated with materials having finer particle size distributions, higher liquid limits, and increased levels of degradation upon exposure to water. Typically, the geotechnical competency of the mud was related to the competency of the spoil it formed from.Accelerated degradation testing results showed that the majority of degradation occurs within the first 24 hours. Wetting and drying cycles produced a faster rate of degradation than prolonged saturation. From these results, the development of a modified slake durability testing methodology allowed for rapid identification of highly degradable spoil prone to slaking and dispersion.Geotechnical testing involved standard consolidation of the -2.36 mm fractions in a water bath, and large slurry consolidometer consolidation of select mud samples at -19 mm, and direct shear testing both as sampled, and after 24 hours of soaking in water. Spoil scalped to -19 mm was tested in a large direct shear box measuring 300 x 300 x 200 mm. Mud samples were tested in a standard direct shear box with dimensions of 60 x 60 x 30 mm, scalped to -6.7 mm.Consolidation testing revealed the least settlement associated with the coarse-grained muds, associated with stability in situ. Hydraulic conductivities calculated produced a range from 1.4x10-9 to 0.9x10-11 m/s, with mud formed from more competent spoil typically having higher conductivities, representative of the lower range of the material due to the effects of scalping. The large slurry consolidometer simulating truck and shovel loading conditions determined significant pore pressures develop in very fine-grained materials; however, negligible pressures developed within the coarse-grained mud, highlighting the potential for safe loading in situ if managed correctly.Shear strength testing indicated the majority of mud materials had significantly higher friction angles than the 18⁰ that is typically assumed, with results ranging between 25⁰ and 36⁰. In contrast to the tested spoil, the mud had lower average apparent cohesion measurements. Two mud materials with significant degrees of degradation had friction angles lower than 15⁰, but relatively high values of apparent cohesion. On average, dry material had higher shear strength values than wet material. The influence of wetting and drying cycles on shear strength showed that the majority of strength reduction occurs within the first cycle.Correlations between the material characterisation results and the geotechnical testing allowed for the development of a multivariate regression model to be developed, using the fractions of sand and gravel to predict the shear strength of in-pit mud with an r2 of 0.87. This model allows for quick estimations of mud friction angles using standardised, cheap testing methods.2D slope stability modelling using Slide 7.0 revealed that using the parameters obtained during the laboratory testing, there is potential for spoiling onto in-pit mud; typically found with material derived from Category 3, competent spoil. The results highlighted the potentially conservative design that results from the use of remoulded strength assumptions adopted from the BMA Coal spoil shear strength framework.This research has extensively characterised a range of in-pit muds, identifying material that has the potential to be safely spoiled upon in-pit. It has also allowed for the rapid identification of material prone to degradation and provided a model for predictions of shear strength with minimal testing requirements. Application of these results will improve handling techniques, mine safety, and treatment of in-pit mud.