Determination of peak and ultimate shear strength parameters of compacted clay

Shear strength parameters of compacted clay liners (CCLs) are needed for the safety design and stability analyses of mine waste dumps. The shear strengths of soils are commonly determined through laboratory direct shear or triaxial tests. However, for the same soil sample, different testing methods may produce large sets of scattered shear strength data, leading to difficulty in selecting the appropriate design parameters. In this study, extensive laboratory tests were carried out to investigate the shear strength parameters of the compacted clay for the geotechnical stability design in the McArthur River Mine (MRM). Both single-stage and multi-stage reversal direct shear tests were carried out under dry (as-sampled) and wet (in a water bath) conditions, and under applied normal stresses of 50 kPa, 100 kPa and 200 kPa. Three shearing rates of 1 mm/min (rapid), 0.1 mm/min (intermediate) and 0.01 mm/min (slow) were attempted to study the influence of shearing rate on shear strength of the compacted clay. Furthermore, one unconfined compression (UC) test, three unconsolidated undrained (UU) triaxial tests and three consolidated undrained (CU) triaxial tests were also carried out under the equivalent effective confining pressures of 50 kPa, 100 kPa and 200 kPa. Subsequently, the peak and ultimate shear strength parameters of the compacted clay are recommended in a format that allows for a more flexible use in design in the MRM.