The role of unsaturated soil mechanics in unconventional tailings deposition

Abstract

Desiccation (water loss by drying or freeze-thaw sufficient to generate matric suction), can influence the performance of a tailings deposit both positively and negatively. The significance of desiccation is largest in tailings that have been dewatered prior to deposition, by thickening or filtration. Such tailings can be “stacked” or deposited with a significant slope, which usually implies that a substantial volume of tailings remain above water. Under such conditions the tailings, by accident or by design, may undergo desiccation before burial by fresh tailings. Desiccation can contribute substantially to strength, above and beyond the contribution arising from increase in density, through stress history effects. For some deposits, it is required practice that at least some tailings undergo desiccation to improve, particularly when those tailings for a structural part of a deposit. If, however, tailings remain exposed to the atmosphere in an unsaturated state for some period of time, this may have potential negative consequences through oxidation of sulphide minerals and the formation of acid drainage. This paper describes previous research on the strength gained through desiccation in tailings, and on modelling work that incorporates unsaturated soil phenomena into consolidation analysis. Both types of research are applied to a real field site, providing an example of how novel improvements to tailings management can arise out of application of principles of unsaturated soil mechanics.

Unsaturated soil behaviour in thickened and filtered tailings

“…Indeed, the largest increase in strength in the element tests occurs from w = 30% to 25%; w = 30% being the post-settling water content for these tailings. A comparison of similar data in Figure 13 with some vane tests from the field is shown in Simms (2021). A mechanism for the increase in dilatancy was postulated by Daliri et al (2014) based on SEM images of freeze-dried samples.…”

“…Fortunately, these somewhat complex interactions have been conceptualised by several models in the field of unsaturated soils, and many of these models (Basic Barcelona, or BBM, state surface or SSM, and Glasgow coupled or GCM) have been implemented in geotechnical codes, although for small strain conditions. Qi et al (2017aQi et al ( , 2020 reset these models into a large strain consolidation framework, the UNSATCON code, which has been applied to multilayer drying box tests (up to 1 m deep multilayer deposits, 1 by 1 m in plan), as well as field data from both hard rock and oil sands mines (Qi & Simms 2018a, Simms 2021). Here we do not focus on the details of the particular soil models (e.g.…”

Effects of low suction and wetting–drying cycles on filtered tailings shear strength

“…In recent years, developments of tailings dewatering technologies, and especially filtered tailings, have contributed to maximise water recovery and minimise geotechnical risk (Simms 2017(Simms , 2021. The objectives of filtered tailings in particular are to produce tailings with solid content greater than 80% using various dehydration processes (Lara et al 2013;Weatherwax & Kipara 2010).…”

“…The probability of failure in filtered TSFs is significantly reduced compared to conventional slurry TSF but the risk still exists (Wilson & Robertson 2015). Filtered TSFs are constructed on the surface and are exposed to climate conditions (Daliri et al 2016;Simms, 2017Simms, , 2021. Global warming will contribute to increase the frequency of extreme weather events such as droughts and heavy rains with direct consequences on mining operations (O'Gorman 2015).…”

“…Global warming will contribute to increase the frequency of extreme weather events such as droughts and heavy rains with direct consequences on mining operations (O'Gorman 2015). Therefore, filtered tailings are (and will continue to be) exposed to repeated drying (due to evaporation) and wetting (due to precipitation) cycles (Fredlund & Houston, 2013;Fredlund 2019;Simms 2021). Understanding the hydro-mechanical responses of filtered tailings to wetting and drying cycles is therefore crucial to ensure the short-and long-term the stability of filtered TSFs (Cacciuttolo Vargas & Pérez Campomanes 2022).…”