Undrained shear behavior and critical state analysis of mixed mine waste rock and tailings
Date
2019
Authors
Borja Castillo, Raquel N., author
Bareither, Christopher A., advisor
Scalia, Joseph, committee member
Gallen, Sean, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The objectives of this study were to (i) evaluate the undrained shear behavior of mine tailings and a tailings-dominated mixture of filtered tailings and waste rock (i.e. GeoWaste), (ii) identify the critical state of each material, and (iii) assess the impact of waste rock inclusions on the critical state of tailings. Mine tailings and waste rock were collected from an active mine where GeoWaste is being considered as a potential solution for mine waste management. GeoWaste was prepared at a mixture of 1.2 parts waste rock to 1 part tailings, by dry mass, which was a relevant mixture ratio for field implementation. Consolidated undrained (CU) triaxial compression tests were conducted on pure tailings and GeoWaste. Large-scale triaxial compression tests were conducted on 150-mm-diameter GeoWaste specimens, and 38-mm-diameter triaxial tests were conducted on tailings prepared to three initial conditions: filtered tailings that represented field conditions, dense filtered tailings, and paste tailings. Triaxial compression tests were conducted at effective confining pressures (σc') ranging between 20 and 500 kPa. Filtered tailings prepared to represent field conditions yielded contractive, strain-hardening behavior. Dense filtered tailings exhibited strain-hardening behavior, net positive pore pressure, and a transition from contractive to dilative tendencies. Paste tailings exhibited modest strain-hardening behavior. GeoWaste exhibited strain-hardening, contractive behavior, and a modest transition from contractive to dilative behavior was observed at σ'c = 500 kPa. The undrained shear behavior of GeoWaste was comparable to filtered tailings at σ'c = 50 kPa and 100 kPa. However, undrained shear behavior of GeoWaste at σʹc = 500 kPa changed related to tailings, which was characterized by a larger deviator stress and lower excess pore pressure. This GeoWaste behavior indicated improved shear resistance compared to filtered tailings, which was attributed to (i) inter-particle reinforcing effects between the waste rock particles within a tailings-dominated structure and (ii) densification of the GeoWaste structure. Shear strength parameters were calculated from the slope of a composite Kf Line for each material. Filtered tailings prepared to represent field conditions, and dense filtered tailings yielded effective tangent friction angle (φ't) = 33°, and paste tailings yielded φ't = 32°. Similarity in φ't between the three tailings prepared with different initial specimen characteristics was attributed to similar void ratios at the end of consolidation under a given σʹc. GeoWaste yielded φ't = 32°. Although composite φ't were similar between tailings and GeoWaste, the secant friction angles of GeoWaste increased with increasing σʹc, whereas the opposite trend was observed for tailings. The addition of waste rock particles to tailings in a fine-dominated structure to increase the shear resistance relative to tailings as effective consolidation stress increased. An assessment was conducted between the critical state lines for tailings and GeoWaste to determine if the critical state line for tailings can represent critical state conditions in GeoWaste. An equivalent tailings void ratio (e*t) that can represent the tailings fraction within GeoWaste correlated with the critical state line for tailings. In this study, the e*t for GeoWaste was determined via optimizing a fitting parameter in the e*t equation to correlated with the critical state line for tailings. Although this evaluation suggests that the critical state line for the tailings can be used to represent critical state conditions in GeoWaste, additional work is needed to determine e*t a priori.