Shear Strength Characterisation of In-pit Mud to Ensure Low Wall Stability

High wet season rainfall totals between 2008 and 2013 resulted in the extensive flooding of open cut coal mines, particularly in the Bowen Basin and the formation of in-pit “mud” up to 10 m deep.  To re-start mining and spoiling in-pit, it has proved necessary to remove the mud, which is incurring a very high cost.

This project addressed the high potential for the geotechnical instability and failure of low wall spoil dumped on in-pit, water-softened spoil or floor material (known in the industry as “mud”), specifically:

• Minimisation of geotechnical risk with a focus on deeper excavations and higher spoils; including the improved understanding, modelling, monitoring and management of principal hazards; and
• Improved assessment and evaluation of hydrogeology on mining, including impacts on slope stability and degradation of material parameters.

Not having to remove undegraded wet-up spoil and floor materials will produce very significant cost and time savings, particularly in situations where the cleaning out of pit bases presents logistical problems.  The results of the project will also lead to generally improved characterisation of spoil and floor materials subjected to wetting-up by rainfall runoff and groundwater inflow accumulated in the pit, and to improved estimates of the degraded shear strength parameters of spoil and floor materials susceptible to water-softening.

The objectives of this project were to:

• Sample and collect fresh spoil and degraded mud that had formed in-pit from mines within the Bowen Basin;
• Characterise the material, to determine its shear strength; and to
• Investigate the impact these results have on low wall design.

The laboratory testing involved physical, chemical, mineralogical, geochemical and geotechnical characterisation of all spoil and mud samples, tested as sampled and after degradation.  The testing included moisture content, total suction, Atterberg limits, specific gravity, electrical conductivity, pH and Emerson class testing.  Particle size distributions were determined dry and after 24 hours of soaking.  For spoil materials, the influence of prolonged saturation and wetting and drying cycles were also investigated.  The geotechnical testing included standard consolidation and direct shear testing as sampled and after 24 hours of saturation.  Select mud samples were also investigated using a large slurry consolidometer, simulating in-pit spoiling and the measuring the influence the loading rate had on pore water pressures and consolidation.

Results were used to identify relationships between the determined material characteristics and their geotechnical parameters, and to understand the degree of degradation.  A modified slake durability test was developed allowing for the rapid identification of degradable materials.  Comparisons of the shear strength results were made to the current BMA spoil strength framework using the program Slide 7.0 for four scenarios typical of Australia open pit coal mining operations.  A model was also developed using particle size distribution results to estimate the shear strength of in-pit mud.