Factors affecting the shear strength of mine tailings/clay mixtures with varying clay content and clay mineralogy

Artificial mine tailings/clay mixtures were used to prepare concentrated slurries, from which beds with different composition, thickness and age were obtained through sedimentation. Those beds were sheared in a specially built Tilting Tank capable of simulating drained and partially drained conditions by varying the tilting rate. The stress range of interest was below 1.2 kPa, which was much lower that the stresses utilized by conventional geotechnical equipment, but of the same order of magnitude as those measured in the tailings management facilities. Negligible excess pore water pressure developed in the deposited beds when the rate of shearing was sufficiently slow, whereas a rapid shearing rate caused a significant excess pore pressure buildup that reduced the shear strength of the beds. Linear drained (effective stress) and partially drained (total stress) failure envelopes were defined for beds, prepared from various mixtures. The effective friction angle was found to vary between 35.2° and 40.4° depending on the percentage of clay in the mixtures and the type of the clay additive. Shearing under partially drained conditions yielded a total friction angle of the mixtures that was always lower than the effective friction angle and varied between 15.1° and 23.3°. It was found that adding clay to mine tailings generally caused a decrease in the frictional strength of the latter; however, the magnitude of this decrease was greater when the clay was bentonite and lower when it was kaolinite. The time for consolidation had little effect on the shear strength of the tailings/kaolinite mixtures, but led to an approximate increase of 2° in the frictional resistance of the tailings/bentonite mixtures.