Assessing the undrained strength cross-anisotropy of three tailings types

Abstract

The cross-anisotropic nature of soil strength has been studied and documented for decades, including the increased propensity for cross-anisotropy in layered materials. However, current engineering practice for tailings storage facilities (TSFs) does not appear to generally include cross-anisotropy considerations in the development of shear strengths. This being despite the very common layering profile seen in subaerially-deposited tailings. To provide additional data to highlight the strength cross-anisotropy of tailings, high quality block samples from three TSFs were obtained and trimmed to enable Hollow Cylinder Torsional Shear tests to be sheared at principal stress angles of 0 and 45 degrees during undrained shearing. Consolidation procedures were carried out such that the drained rotation of principal stress angle that would precede potential undrained shear events for below-slope tailings was reasonably simulated. The results indicated the significant effects of cross-anisotropy on the undrained strength, instability stress ratio, contractive tendency and brittleness of each of the three tailings types. The magnitude of cross-anisotropy effects seen was generally consistent with previous published data on sands.

Discussion of “Evaluation of flow liquefaction and liquefied strength using the cone penetration test: an update”

“…The better agreement seen for the torsional simple shear testsplane strain tests, with rotation of principal stress angle during shearingis consistent with the well-documented cross-anisotropic nature of sands (Sivathayalan and Vaid 2002;Reid 2020), clays (Seah 1990), and tailings (Reid et al 2022) along with the lower values of instability stress ratio and critical friction ratio expected in plane strain conditions (Wanatowski and Chu 2007;Jefferies and Shuttle 2011;Shuttle et al 2022). Indeed, it has been observed in the past that torsional simple shear tests generally provide better agreement with back-analysis-inferred liquefied strengths (e.g., Yoshimine et al 1999) than triaxial compression.…”

Unsaturated soil behaviour in thickened and filtered tailings

“…Additionally, for hard rock tailings, desiccation has been shown to increase the stiffness and dilatancy, resulting, for a given test, stronger and stronger samples at higher and higher void ratios (Figure 12). The samples in Figure 12 are air-dried, rewetted, and then consolidated before shear; results are reported in Daliri et al (2014Daliri et al ( , 2016 and Simms (2017) on gold tailings, with some replication on a different gold tailings by Reid et al (2022). It may be useful to note that it does not take drying all the way to the shrinkage limit (in the case of Figure 12, the shrinkage limit is at w = 18%) to induce dilative behaviour or, as shown in Figure 13, to induce a sizeable increase in shear strength measured by a vane.…”

“…Here we define desiccation as the past inducement of matric suction in the tailings (as opposed to simply an acceleration of consolidation by drying). This impression of the author is gathered from conversations with consultants and mine operators, but also some 'facts', such as the recent proposal from The University of Western Australia to use air-drying as part of a standard sample preparation technique for element testing (Reid et al 2022), discussion of CPT through desiccated tailings (Robertson et al 2017), and some statements made by authors of public reports on tailings dam failures that allude to the positive impact of, e.g. rendering hard rock tailings dilative that are contractive before desiccation.…”