Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dr. Ernest K. Yanful

Abstract

Comprehensive geotechnical characterisation of mine tailings is required for the design, construction and safe operation of mine waste management facilities against the large number of potential failure risks. It is important that the characterization is carried out under the stress range and drainage conditions relevant to those encountered in the field.

The present research utilized mine tailings samples from a copper/nickel base-metal mine. Kaolinite and bentonite clays were added to the mine tailings in order to study the effect of clay percentage and clay mineralogy on the behaviour of saturated tailings/clay systems with varying composition. Slurries with moderate or high concentration of solids were prepared in the laboratory by mixing distilled water with mine tailings or artificial tailings/clay soils. Beds with different composition, thickness and age were sedimented from these slurries and tested under a stress range below 1 kPa, and under different degrees of drainage ranging from undrained to fully drained. The primary consolidation of the pure tailings beds was complete in approximately one hour and negligible volume changes occurred in the beds during secondary compression. The effect of adding kaolinite or bentonite to the tailings was to increase the time for primary consolidation of the mixed beds, but the rate of increase was greater when bentonite was used. The undrained shear strength of the beds was measured using an automated fall cone device at a depth interval of 1 cm below bed surface. It was found that the undrained strength increased, whereas the water content and void ratio decreased with depth within the beds. The factor controlling the undrained strength of the beds was the vertical effective stress, with the water content also having some secondary effect.

A specially built Tilting Tank was used to measure the shear strength of the beds under drained and partially drained conditions that were simulated by varying the loading rate. Bed failure within the Tank always occurred at a plane parallel to the surface of the bed and at a depth of 0.4 to 2.5 cm. Linear drained and partially drained shear strength envelopes with zero cohesion intercept were defined, with the partially drained (total) friction angle always remaining lower than the drained (effective) friction angle. The latter varied from 35.2° for the tailings/bentonite mixtures to 40.4° for the pure tailings, depending on the percentage and mineralogy of the clay fraction. It was found that adding clay to the mine tailings generally caused a decrease in the frictional resistance of the mixtures, with the effect being more pronounced for the bentonite additive. Time for consolidation did not influence the shear strength of the tailings and tailings/kaolinite mixtures, but produced an increase of 2.1° in the frictional resistance of the tailings/bentonite mixtures.

A critical stress for erosion as a function of depth was estimated for each bed using existing formulations for cohesive and noncohesive sediments and mixtures of both. A linear correlation between the undrained shear strength and the critical stress for erosion, with parameters dependent on the composition of the mixtures was proposed.


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