Impact of Desiccation on Monotonic and Cyclic Shear Strength of Thickened Gold Tailings

Abstract

Various forms of thickened or high-density tailings, characterized by exhibition of a yield stress that facilitates deposition in gently sloped stacks, are increasingly used for a variety of mining operations. Although this technology reduces reliance on containment by dams and the associated risk of catastrophic failure, regulators are concerned with remobilization of such unbounded deposits by earthquakes or extreme rainfalls. Therefore, the monotonic and cyclic shear strength of thickened tailings layers are of significant interest. Thickened tailings experience a particular stress history during deposition, which may include desiccation, rewetting, and subsequent consolidation by burial under new tailings. The effect of this stress history on their shear response is investigated using a Norwegian Geotechnical Institute (NGI)-type simple shear device. Samples are alternately mechanically overconsolidated or subjected to desiccation and subsequent rewetting because of burial by fresh tailings before consolidation. The two sets of data, one overconsolidated by load, the other by matric suction, were compared using the suction stress concept to quantify an equivalent overconsolidation ratio (OCR) caused by desiccation. However, the effects of mechanical overconsolidation and desiccation were found to be qualitatively different for both the monotonic and cyclic responses; therefore, it is not possible to quantify the effects of desiccation using mechanical overconsolidation and one stress-state variable. Desiccated samples exhibited much weaker strength at phase transformation but more strain hardening compared with mechanically overconsolidated samples at the same OCR. For the desiccated samples, both monotonic strength at phase transformation and cyclic resistance continuously increased with increasing desiccation to air entry value (AEV). Past the AEV, a much smaller increase in strength was observed. Even a small amount of desiccation changed the monotonic response of the tailings from contractive to dilative behavior.