Contribution of matric suctions to the slope stability of tailings dams

Abstract

The strengthening effect of matric suctions is typically ignored in slope stability analyses. This is due to the uncertainty in the magnitude and reliability of in-situ matric suctions and the effect thereof on the shear strength of the material under consideration. Recent developments in field measurement probes allow for the long-term monitoring of in-situ matric suctions and volumetric water contents. The UP tensiometer was installed alongside volumetric water content sensors at various locations on a platinum and a gold tailings dam and monitored for a number of months.

The inclusion of matric suctions in slope stability analyses required the identification of a suitable model from the literature that can realistically predict the relationship between suction and shear strength. Shear strength vs suction data were generated by shearing various platinum tailings samples using a direct simple shear device. Matric suction was varied between tests by varying the sample water content to observe the effect of suction on shear strength. Limit equilibrium slope stability analyses of the monitored tailings dams allowed the effect of matric suctions on slope stability to be considered once a suitable unsaturated shear strength model was identified that best modelled the shear strength test results. The effect of matric suctions on slope stability was illustrated by comparing safety factors determined using the classical approach, which only relies on saturated soil mechanics and the Mohr-Coulomb (MC) strength model, and an approach which replaced the MC strength model with a suitable unsaturated shear strength model.

The results of unsaturated direct simple shear testing of platinum tailings showed that the model by Vanapalli et al. (1996) best predicts the relationship between matric suction and shear strength. This model, together with the observed unsaturated pore pressure regime observed in the tailings dams monitored, were used in limit equilibrium analyses to assess the contribution of suctions to the factor of safety against slope failure. It was found that the contribution of matric suction to the factor of safety against slope failure ranged between 1.6% and 3.8% for the platinum tailings dam and between 5.6% and 13% for the finer grained gold tailings dam considered in this study. It was thus concluded that the contribution of matric suction to the stability of the platinum and gold tailings dams investigated was small and that it would be both conservative and realistic to disregard.