REASSESSMENT OF A TAILINGS FLOW SLIDE CALCULATOR

A widely-used tailings flow slide calculator for predicting the runout distance and time-evolution of the initial surge of mine tailings following the collapse of an earthen tailings dam is based upon an analysis by J. K. Jeyapalan et al. (1983). The analysis assumes (1) instantaneous disappearance of the tailings dam (2) laminar flow of tailings (3) behavior of tailings as a Bingham plastic (4) one-dimensional shallow flow (5) no incorporation of valley or stream bed materials into the tailings slide. The initial surge of tailings is predicted in terms of two dimensionless parameters, which are functions of the initial height of the dam, and the unit weight, plastic viscosity, and Bingham shear strength of the tailings. The objective of this study has been to reassess the tailings flow slide calculator in terms of its assumptions, mathematical analysis, and ability to predict the observed runouts of the initial surges following dam collapses. A database of 229 tailings dam failures from 1917 to 2010 was compiled, which included dam height, volume of stored tailings, volume of released tailings, and observed runout. Since the physical properties of mine tailings are not generally known, the tailings slide calculator was run with the greatest range of reasonable values for unit weight (14.1-17.3 kN/m³), plastic viscosity (0.096-4.8 kPa·s), and Bingham shear strength (0.96-7.2 kPa). Even using the values that maximized runout, the observed runout was underestimated in all but four cases. Moreover, the tailings slide calculator did not predict the strong correlations between runout and release volume (R² = 0.84) and storage volume (R² = 0.62). The mathematical analysis includes numerous algebraic errors and its numerical implementation is incorrect, even assuming the incorrect mathematics. The most critical assumption is the initial quiescent state of tailings and the instantaneous disappearance of the dam. The information that the dam has disappeared propagates upstream at the wave speed and ceases propagating when the toe of the tailings slide has stabilized, which limits the volume of tailings that can be involved in a slide. However, dam failure by seismic activity or flooding has the potential to mobilize the entire body of tailings simultaneously. An improved tailings slide calculator will be presented at the meeting.