Flow slides and the undrained brittleness index of some mine tailings

Abstract

This paper discusses material parameters related to flow slides in saturated mine tailings embankments. Several studies of tailings embankment failures indicated that the failures took place very quickly and were characterized by extensive toe disturbance - both features typify flow slides in granular soils which are associated with large pore pressures and reduced (residual) shear strength. The brittleness index proposed by Bishop (1967, 1973) has been shown to relate postrupture behaviour of slopes to the peak and residual strength parameters of soils. In this study the undrained brittleness index was determined from stress-controlled isotropically consolidated undrained triaxial tests on tailings from three mining operations which produced garnet, iron ore and zinc. Test variables that influenced the brittleness index and thus peak and residual shear parameters included varying initial (before consolidation) void ratio, i.e., as placed relative density, and consolidation pressure. It was found that the initial void ratio was the most important parameter governing the undrained brittleness index. At a given initial void ratio the brittleness index generally increased with increasing consolidation pressures but the magnitude of this effect varied with the type of tailings. For the three types of tailings tested, it was found that when the initial dry unit weight was less than 70–75% of the maximum dry unit weight obtained from the modified Proctor test (ASTM D-1557), the undrained brittleness index was greater than 50% and further reduction in the initial dry unit weight had little effect on the index. However, the index decreased sharply to zero for all tailings over the range of consolidation pressures used when the initial percent dry unit weight increased from 75% to about 80%. This indicates that for the tailings tested and over the range of variables (initial unit weight and consolidation pressure) considered, the possibility of a flow slide due to a static load would be precluded if the initial compaction exceeded 80% of maximum.