Layering in tailings deposits: Characterization and implications for CPT interpretation

The cone penetration test (CPT) is widely used for tailings characterisation. Often, the CPT is used to infer the state parameter (ψ) using methods based on uniform soil specimens. This paper addresses a crucial question: To what extent do tailings deposits resemble uniform specimens? We quantitatively characterised layering in tube samples from three active gold tailings storage facilities (TSFs) and visually appraised a 3 m profile at a fourth TSF. Results reveal pronounced thin layering in all samples and the exposed profile. Our analysis suggests that the uniform layers documented herein are unlikely to exceed 10 cm. This is significantly less than the >0.6 m recommended for CPT interpretation. The PSD variability within a single 25 cm sample often matched or exceeded that observed across entire TSFs, highlighting the significance of the variability. Thin layering constitutes a significant deviation from the uniformity assumption underpinning widely used CPT interpretation methods and is likely to compromise the accuracy of CPT-based ψ estimates. Thin layering also raises fundamental questions about validating ψ estimates. We propose shifting research to include the approximate laboratory characterisation of layered specimens rather than focusing on the precise characterisation of uniform specimens that do not resemble in situ conditions.