Mine tailings and waste rock co-disposal: Centrifuge modeling of consolidation and dynamic loading

Several factors have led to the extended need for investigation of the static and dynamic behavior of mine tailings and include the rising generation and surface storage of mine waste, its frequent use as a construction material for the retaining structures and the relatively high rate of associated failures, with seismic liquefaction being a rather common cause. In this experimental study, fine copper-gold tailings from a mining project located in a seismically active region were used to construct and test a layered deposit on the geotechnical centrifuge at the Center for Earthquake Engineering Simulation (CEES) at Rensselaer Polytechnic Institute (RPI). A set of six centrifuge tests was conducted to evaluate consolidation rate, settlement accumulation, shear strength evolution and response to seismic loading. The material was liquid at disposal and instrumentation of every layer proved to be a challenge. A total height of about 30 m of mine tailings was deposited in prototype scale (80 g). Brief consolidation of each layer took place at a lower centrifugal acceleration, thus enabling the material to gain some strength and facilitating instrumentation, while simultaneously modeling a few days between disposal of subsequent layers. Miniature pore pressure transducers were used to measure water pressure and settlement, bender elements for shear wave velocity, LVDTs for displacement and finally miniature accelerometers for dynamic loading. The complete deposits consolidated at 80 g for up to more than 2 years in prototype scale, but most settlement, excess pore pressure dissipation and stiffness increase occurred within the first 0.5 years. A mild slope was excavated at the surface after consolidation and the deposits were then subjected to a harmonic motion consisting of 50 cycles. The varying parameters were base acceleration amplitude and average degree of consolidation prior to dynamic loading. The material was found to be liquefiable and its performance in terms of acceleration, shear strain, slope stability, residual lateral deformation, pore pressure build-up and liquefied depth was somewhat dependent on the aforementioned parameters. The response of the deposits changed with increasing depth and effective stress, which made centrifuge testing highly advantageous.;Even fully consolidated, the material proved to be highly liquefiable and alternative methods of disposal were considered to improve its physical stability. Co-disposal of tailings and coarse waste rock is an emerging approach with potentially sizable advantages. Most mining operations produce both waste streams and dispose of them separately. Co-disposal can add to the shear strength of the tailings while keeping the rock submerged and reducing acid rock drainage (ARD). In this study, uniform co-mixing of the two materials prior to disposal was adopted and evaluated by means of additional centrifuge tests. Mixture ratio of waste rock to tailings by dry mass varied in the region of a rock skeleton just-filled with tailings. The performance of three mixtures was compared to tailings in terms of pore pressure dissipation rate, shear wave velocity, soil acceleration, lateral displacement, pore pressure build-up, liquefied depth, settlement and slope stability. Co-disposal of mine tailings and waste rock proved to be a promising alternative to conventional separate disposal. Two mixtures were further tested under more critical conditions. They were deposited at their angle of repose and subjected to dynamic loading soon after disposal. The main objective was to evaluate the possibility to eliminate or shorten the retaining structure. That possibility was found to be highly dependent on the mixture ratio. A total of eleven centrifuge tests were hence performed, not including an additional three calibration tests.