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Effect of tailings composition on the shear strength behavior of mine waste rock and tailings mixtures

Date

2014

Authors

Jehring, Megan M., author
Bareither, Christopher A., advisor
Shackelford, Charles D., committee member
Sutton, Sally J., committee member

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Abstract

The objective of this study was to evaluate the effect of mine tailings composition on the shear behavior and shear strength of co-mixed mine waste rock and tailings (WR&T). Crushed gravel was used as a synthetic waste rock and mixed with four types of tailings: (1) fine-grained garnet, (2) coarse-grained garnet, (3) copper, and (4) soda ash. Co-mixed WR&T specimens were prepared to target mixture ratios of mass of waste rock to mass of tailings (R) such that tailings "just filled" inter-particle void space of the waste rock (Ropt) prepared at the maximum void ratio of waste rock alone. Triaxial compression tests were conducted on waste rock, tailings, and co-mixed specimens at effective confining stresses (σʹc) of approximately 5, 10, 20, and 40 kPa. Low σʹTcT were selected to assess performance of co-mixed WR&T in final earthen cover applications for waste containment facilities. Waste rock and co-mixed WR&T specimens were 150-mm in diameter by 300-mm tall, whereas tailings specimens were 38-mm in diameter by 76-mm tall. Waste rock was tested with drained and undrained conditions, whereas undrained conditions were used for tailings and co-mixed specimens to reduce testing duration. Shear strength of the WR&T mixtures was comparable to that of waste rock alone. The effective stress friction angle (φʹ) of waste rock was 41°, whereas φʹ of the tailings ranged from 34° (copper) to 41° (soda ash). The WR&T mixtures had an average φʹ = 40° for fine-garnet mixtures and 39° for coarse-garnet and copper mixtures, which are similar to waste rock alone and suggests that the waste rock skeleton controlled shear strength of these mixtures. The soda ash mixtures had a slightly lower φʹ of 38° compared to waste rock alone, which was attributed to clay-sized tailings particles lubricating contacts between waste rock particles. Shear behavior of co-mixed WR&T was controlled by the tailings fraction when tailings were composed of silt and mixed to a ratio of R < Ropt. Waste rock controlled shear behavior of co-mixed WR&T when tailings were composed of sand or clay and mixed to a ratio of R ≥ Ropt. At σʹTcT = 5 kPa, the waste rock was entirely dilative, and transitioned to entirely contractive behavior at σʹTcT = 40 kPa. In WR&T mixtures, potential contraction of the waste rock skeleton will transfer normal and shear stress to the tailings fraction within the waste rock void space. Thus, shear behavior of co-mixed WR&T specimens were dependent on composition of the tailings and the overall soil structure, which is a function of R. The actual R for fine-garnet, copper, and soda ash mixtures was lower than the target ratio (R < Ropt) and corresponded to higher tailings content. An increase in tailings content creates a soil structure where tailings exist between inter-particle waste rock contacts and cause waste rock particles to "float" in a tailings matrix. Shear behavior of this co-mixed WR&T structure was dependent on composition of the tailings. Fine-garnet and copper mixtures expressed stronger dilative tendencies compared to tailings alone at all σʹTcT, which was attributed to interlocking between waste rock and tailings particles. Soda ash tailings alone were purely contractive, and combining two contractive materials resulted in a contractive WR&T mixture. The coarse-garnet tailings alone expressed strong dilative tendencies for all σʹTcT, whereas coarse-garnet mixtures exhibited similar shear behavior to waste rock alone. The contractive tendencies of coarse-garnet mixtures was attributed to specimens prepared at R > Ropt, which likely prevented involvement of the tailings fraction in transferring normal and shear stresses. The equivalent granular void ratio (e*), based on the global void ratio (eg) and tailings content, accurately characterized the soil structure of co-mixed WR&T by accounting for the contribution of tailings particles in transferring stress. The equivalent granular state parameter (Ψ*), determined using e*, was able to capture the shear behavior of all waste mixtures. Shear strength behavior of co-mixed WR&T can be predicted using Ψ* provided R, eg, and the steady state line of the WR&T mixture are known.

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Subject

co-mixing
tailings
shear strength
mine waste
cover systems

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