Critical state, dilatancy and particle breakage of mine waste rock
Date
2011
Authors
Fox, Zachary P., author
Carraro, J. Antonio H., advisor
Shackelford, Charles D., committee member
Borch, Thomas, committee member
Overton, Daniel D., committee member
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Abstract
Critical state, dilatancy and particle breakage characteristics of two mine waste rock (MWR) materials were systematically studied in drained isotropic and axisymmetric compression. A specimen preparation technique that simulated material dumping in the field was adopted and the technique is shown to be suitable for reconstitution of uniform and repeatable specimens of MWR for element testing. The MWR types tested were unoxidized and oxidized sedimentary argillite taken from the Ordovician Vinini formation in northeastern Nevada. Acid-base accounting results indicate that the neutralization potential (NP) and acid-producing potential (AP) values decreased for the oxidized material. Static, monotonic, isotropically compressed drained triaxial tests were performed on 150-mm-diameter, 300-mm-tall cylindrical specimens with maximum particle size equal to 25.4 mm. Laboratory particle size distributions were modeled to be parallel to the collected field gradation in order to create specimens with appropriate maximum particle sizes for the testing apparatus. The intrinsic parameters that characterize critical-state, dilatancy and particle breakage of each MWR material tested were determined allowing analysis of constitutive behavior to be carried out using an appropriate theoretical framework for granular soils experiencing particle breakage during testing. While the critical state friction angles were very similar between the two MWR types (unoxidized = 38.3° and oxidized = 36.7°), dilatancy is much greater in the unoxidized specimens than in the oxidized specimens. Bolton's (1986) fitting parameters Q and R were determined and values agree well with those found in the literature for geomaterials with similar stress-dilatancy behavior and grain tensile strengths. Grain tensile strength was evaluated through point load strength index testing giving values for grain tensile strength for the unoxidized material that are 10 times greater than observed for the oxidized material. Particle size distributions were determined before and after testing to evaluate particle breakage due to the combined effects of isotropic and axisymmetric compression as well as evaluate the increase in surface area due to particle breakage. The fractal dimension (D) was evaluated before and after testing in order to assess the validity of the underlying assumptions of the modified work equation presented by McDowell et al. (1996). The surface energy of the materials tested was found to be in the range of 5-24 J/m2. All of these results indicate that in situ weathering may degrade the shear strength characteristics of a quarried sedimentary mine waste rockfill by weakening the intrinsic shear strength parameters of the MWR. The only rigorous way to properly assess the strength degradation of the MWR materials tested involves careful assessment of the critical state, dilatancy and particle breakage characteristics.
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Subject
critical state
dilatancy
mine waste rock
particle breakage
particle strength
triaxial