Unsaturated Shear Strength Characteristics of Filtered Tailings through Modified Direct Simple Shear (MDSS) Apparatus
Authors: Udeshika Dilani Thenuwara1, Dareeju Biyanvilage2, Les Dawes1, Chaminda Gallage1
Conference: 21st International Conference on Soil Mechanics and Geotechnical Engineering
Date: June 14-19, 2026
1 School of Civil and Environmental Engineering, Faculty of Engineering, Queensland University of Technology, Brisbane, Australia
2 Knight Piésold Pty Ltd., Brisbane, Australia
ABSTRACT
Filtered Tailings Storage Facilities (FTSFs) are increasingly recognised as a sustainable alternative to conventional slurry tailings storage facilities within the mining industry, owing to their enhanced stability and environmental benefits. However, the geotechnical design of FTSFs presents unique challenges due to the unsaturated state of filtered tailings as their design is hampered by a fundamental gap in understanding unsaturated shear strength, as empirical models fail to capture the unique response of tailings. A significant knowledge gap exists in applying Critical State Soil Mechanics (CSSM) principles, which are mature for saturated soils, to unsaturated materials such as tailings. Consequently, this study attempted to evaluate the critical state behaviour of filtered tailings. This study investigates the unsaturated shear strength tailings with a dry density of 1.9g/cm³ under a low suction level (30 kPa) and saturated conditions. The tests were conducted using a Modified Direct Simple Shear (MDSS) apparatus, specifically designed to incorporate a throughout the test-suction control feature, to circumvent the sophisticated and time-consuming triaxial testing program. The study demonstrates that higher density (1.9g/cm³) and matric suction (30kPa) significantly enhance tailings' shear strength. Critically, comparison of the results from the study with literature indicate that a unique critical state line (CSL) does not exist for this material; instead, a family of parallel CSLs can be observed, governed by the initial particle size distribution and compaction fabric, demonstrating transitional behaviour where the end-state retains a memory of the initial structure. A key limitation of this research is the focus on a single density and suction level; the metastable nature of suction-derived strength underscores that hydraulic management is paramount for long-term stability. Future research must expand testing to a wider range of suctions and densities to develop predictive models that integrate the effects of suction and fabric, enabling reliable design standards for FTSFs.
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