Particle size segregation and excess pore water pressure with respect to rate of rise of tailings dams

Abstract

Gold tailings were deposited in thick lifts at full depth where tailings slurry was deposited to fill up the settling column in one lift within the shortest time possible (2 minutes) and thin lifts (660mm to 580mm) under simulated rates of rise of 20m/yr and 10m/yr in the laboratory using instrumented settling column apparatus. Rate of rise (RoR) was taken as the vertical increase in height of settled tailings over a given period of time expressed in metres per year. The deposited tailings segregated along the height of settling columns depicting a height profile ranging from fine grained to coarse grained particles from the top to the bottom of the settling columns. Tailings permeability values of the segregated tailings profiles increased with decreasing settling column height. Scanning electron microscope micrographs based fabric, particle size gradations, saturated vertical permeability and excess pore water pressures correlated well with segregated tailings profiles. Low magnitude excess pore water pressures in the range of 6kPa maximum were recorded. Full depth deposition yielded the highest values of excess pore water pressures followed by the excess pore water pressures for 20m/yr RoR at 50% of the full depth deposition magnitudes, with the least excess pore water pressures generated for the 10m/yr RoR which were 30% of the full depth deposition excess pore water pressure magnitudes. 90% of the excess pore water pressures dissipated during the sedimentation phase and coincided with the occurrence of the bulk of the tailings settlement. The remaining 10% of excess pore water pressures took a disproportionately longer period of time to dissipate and were resurgent with any disturbances.