Shear strength of coal combustion product by vane shear
Date
2018
Authors
Herweynen, Wesley J., author
Bareither, Christopher A., advisor
Scalia, Joseph, advisor
Ridley, John, committee member
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Abstract
The objective of this study was to evaluate the shear strength of a coal combustion product (CCP) using the vane shear test. The CCP was obtained from a CCP evaporation pond in the Eastern United States, and consisted primarily of silt-sized particles. A series of small-scale vane shear (diameter = 12.5 mm and height = 25 mm) and large-scale vane shear (diameter = 25 mm and height = 50 mm) tests were conducted on CCP. Undrained and drained strength envelopes were determined for CCP using consolidated undrained (CU) triaxial compression tests. Triaxial results were verified via consolidated drained (CD) direct shear tests on similarly prepared CCP specimens and comparing the results with the drained strength envelope. In addition, effects of the following variables on the vane shear strength of CCP were evaluated using the small-scale vane: (i) rate of vane rotation, (ii) time delay between vane insertion and beginning rotation (td), and (iii) elapsed time under the final vertical effective stress prior to shearing (tc). A fine synthetic tailing (FST), which was 100% fine grained with approximately 40% clay-sized particles, was evaluated for comparison via small-scale vane shear. FST was selected as the higher clay content and lower permeability, relative to CCP, made the material more suited for evaluating vane shear with undrained conditions. All test specimens were prepared in the laboratory via the slurry deposition method and consolidated to the target vertical effective stress. Vane shear strength results were compared to drained and undrained strength envelopes for CCP and FST. Vane shear strength results were represented in terms of peak shear strength and the initial horizontal effective stress acting on the vertical-oriented failure surface during vane shear. Vane shear tests on CCP in small-scale vane shear and large-scale vane shear yielded shear strengths that plotted between the drained and undrained strength envelopes. This was explained by the small diameter of the vane and/or high permeability of CCP that allowed drainage to occur during testing. Small-scale vane shear tests on FST yielded shear strengths comparable to the undrained strength envelope, which was justified by the considerably lower permeability of FST relative to CCP. Additional evaluation of small-scale vane shear tests on CCP revealed that rate of rotation and td had no influence on measured peak shear strength. This was attributed to the small vane size and high permeability of CCP, which allowed excess pore pressure to dissipate regardless of how fast the material was sheared. Diagenesis was observed to occur in CCP, whereby time-dependent chemical reactions lead to an increase in strength with time. In small-scale vane shear tests on CCP, peak strength was reached after approximately 72 hr. These vane shear tests that accounted for diagenesis (i.e., were allowed to remain under vertical stress for ≥ 72 hr) were found to be most comparable to the drained strength envelope identified via triaxial and direct shear testing. Thus, accurate measures of peak shear strength in CCP must account for diagenesis to occur.
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Subject
drained strength
undrained strength
coal combustion product
vane shear
shear strength