927 Geotechnically, dry stacks: • eliminate the large supernatant pond, • operate without large conventional dams, and • rely on compacted, unsaturated tailings with high matric suction and improved shear strength. This configuration reduces the potential for catastrophic dam breach events, although it introduces its own challenges, including maintaining trafficability under wet conditions and managing surface runoff and erosion. 3.4.2 Cost And Operational Considerations From a cost perspective, filtered tailings are generally recognised as: • higher CAPEX, due to filter plant, structural, and materials handling infrastructure; and • higher direct OPEX, due to power, maintenance, and consumables associated with filtration and dry transport. However, integrated life-cycle analyses for conceptual Western Australian operations indicate that: • filtered tailings can become competitive or favourable when closure costs, water scarcity, land disturbance, and potential carbon pricing are fully internalised; • but in scenarios where energy costs are very high and carbon intensity is penalised, filtered options may rank less favourably unless conveyor-based haulage and efficient filter designs are adopted. Operationally, the main challenges include: • ensuring filter plant reliability at high throughput, • managing cake moisture variability, • designing robust materials handling and stacking systems, and • addressing dust and erosion at the stack surface. 3.5 Co-Disposal of Tailings and Waste Rock Co-disposal (or co-mixing) blends tailings with waste rock to generate a composite material with improved geotechnical and hydraulic properties. Two distinct but related concepts appear in the literature: 1. Co-mixing (engineered blends): laboratory-designed mixes of waste rock and tailings, calibrated to achieve targeted hydraulic conductivity and mechanical strength. 2. Co-disposal using filtered tailings and coarse rejects: filtered tailings cake blended with coarse waste rock or aggregates, producing a material suitable for stacked or valley-fill type facilities with enhanced stability and lower permeability. 3.5.1 Performance Characteristics Testing of co-mixed waste rock and tailings has shown that: • the addition of fine tailings can significantly reduce hydraulic conductivity compared with waste rock alone, • while maintaining adequate shear strength and resistance to erosion. Laboratory and pilot-scale work on co-disposing filtered cake with coarse rejects indicates that: • maintaining cake moisture near the optimum geotechnical range improves stability, • well-designed blends can reduce compaction and spreading effort, and • the resulting deposit can provide both mechanical support and barrier-like hydraulic performance. This makes co-disposal particularly attractive where acid rock drainage (ARD) risk is high, as the tailings component can limit oxygen ingress into sulphide-bearing waste rock.
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