• Operational risk: High clay content may require blending, pre-drying, or different filtration technologies to achieve acceptable moisture contents. 2. Geotechnical Design Considerations for Filtered Tailings Stacks Filtered stacks are formed by placing dewatered tailings in compacted lifts over an engineered foundation, and, where applicable, a starter embankment. Key design elements include foundation improvement (if needed), lift thickness and compaction energy, trafficability, drainage layers, and erosion control. Unsaturated conditions introduce suction-driven strength that is sensitive to moisture ingress from rainfall, capillary rise, or freeze–thaw cycles. This section summarizes the key factors to consider in the design of the filtered tailings stack. 2.1. Compaction and Density Control Compaction reduces permeability and increases shear strength, but is challenging to achieve consistently as moisture varies. Design specifications should define target dry density and moisture windows, lift thickness, and field QA/QC (e.g., nuclear density gauge, sand cone). While Proctor tests define theoretical moisture–density envelopes, achieving these densities consistently across large, active lift surfaces remains a challenge. Variations in particle size distribution, fines content, and mineralogy influence compactability and result in spatial variability in shear strength and hydraulic conductivity. This variability directly impacts slope stability, trafficability, and overall stack performance. 2.2. Moisture Infiltration and Suction Variability Rainfall and operational over‑wetting reduce matric suction and can markedly lower factors of safety. Defining an operational moisture window and corresponding mitigation actions (e.g., pre‑wetting or blending when the material becomes too dry; drying, limiting lift thickness, or delaying placement when it becomes too wet) is therefore critical for maintaining stability and trafficability of the filtered tailings stack. Laboratory testing often characterizes soil‑water characteristic curves (SWCCs) and the suction‑dependent shear strength parameters used to estimate unsaturated behaviour. However, field conditions rarely replicate laboratory states. Variability in lift thickness, compaction effort, infiltration patterns, and micro‑climatic exposure produces spatially and temporally heterogeneous suction profiles within a lift. These variations can evolve rapidly following rainfall, snowmelt, or operational disturbances, causing suction loss and localized softening that laboratory-derived curves may not predict. Better modelling tools and long‑term monitoring data are required to quantify how infiltration events, evaporation cycles, and operational moisture variability influence suction and strength throughout the stack. Similarly, continuous monitoring—using in situ sensors such as tensiometers, moisture probes, and automated weather stations—can provide real‑time feedback to operators, enabling them to adjust lift sequencing, compaction, and traffic to maintain the stack within the defined operational moisture window. Ultimately, improved integration of unsaturated soil mechanics, field monitoring, and operational controls is essential to manage moisture‑driven suction variability reliably. Doing so will
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