Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Xinbao Yu

Abstract

Low plasticity to non-plastic mine tailings are typically transported to a storage area through pipelines in form of slurry or thickened tailings. The main concern with such tailings deposits is the possibility of liquefaction and the consequent failure of the tailings retaining dams. Tailings can also be used as Cemented Paste Backfill (CPB) material to fill previously mined underground voids. The state of practice to improve the mechanical properties of tailings is to add a small quantity of cementitious materials or binder agents. Portland cement (PC) is the most common binder agent that is used to improve the mechanical properties of mine tailings in underground backfilling applications. To prepare a cheaper and more environment-friendly cemented tailings mix, the application of class C fly ash (FA) both as a single binder agent and supplementary cementitious materials (SCM) to replace PC was evaluated. In this study, laboratory tests indicated potential application for class C fly ash to be used as a single binder to improve the mechanical properties of tailings at the paste consistency. Additionally, class C fly ash can be used to replace a significant portion of Portland cement (PC) in CPB. Fairly similar UCS values were obtained for 3%PC and 1.5%PC:3%FA specimens. The reduction of PC to half and addition of class C fly ash increased the initial and final setting times by approximately 3.5 times. After developing a CPB mix design the liquefaction of early-age CPB through a comprehensive framework of critical state soils mechanics was studied. The critical state soil mechanics is a robust approach to evaluate the liquefaction behavior of sands, silts, and early-age CPB at a broad range of void ratios. Previous studies have indicated early-age CPB is not susceptible to static liquefaction. A critical state approach taken in this study shows why the CPB specimens in literature did not experience static liquefaction. Taking this comprehensive approach to explain the mechanics of CPB is critical as the field void ratios of CPB, and consequently its liquefaction behavior, may vary from those tested in the laboratory. The early-age behavior of CPB was evaluated when the samples reached the initial set time at 22-hour curing period, and the UCS values ranging from 78 kPa to 92 kPa. A critical state line of uncemented tailings and CPB specimens was developed using a set of drained and undrained triaxial tests. The triaxial confining pressures targeted common vertical effective stresses in stopes. The static liquefaction behavior of early-age CPB can be predicted using the state parameter (ѱ) of the tailings or early-age CPB. In addition to the static liquefaction, the cyclic mobility of tailings and early-age CPB were evaluated using a set of cyclic direst simple shear tests. The cyclic tests were performed using the cyclic stress ratios appropriate for most seismic regions. Based on the cyclic test results, the CPB prepared with 50% replacement of PC with fly ash indicated adequate resistance against liquefaction at its initial setting time for reasonably large earthquakes in most seismic regions. Finally, the applicability of a select existing constitutive model (NorSand) to predict the behavior of early-age CPB was evaluated. NorSand was found to be applicable to early-age CPB particularly post peak strength, approximately after 1% axial strain, due to breakage of cementitious bonds.

Keywords

Liquefaction, Critical state, Cemented paste backfill, CPB, Tailings amended with fly ash, Fly ash, NorSand, Tailings, Silt, Flow liquefaction, Static liquefaction, CSL, State parameter

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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