Residual strength based on CPT sleeve friction and a constant volume ring shear device

Determination of an acceptable lower bound, large strain undrained shear strength of tailings is fundamental for evaluating the stability tailings storage facilities comprising loosely deposited tailings. Currently, techniques used include laboratory shear strength testing and in-situ measurements including the cone penetration test (CPT) and vane shear test (VST). A common technique in clayey soils is to assume that CPT sleeve friction (f_s) represents the remoulded (i.e., lower bound, large strain) undrained strength of clay soil. This paper evaluates the correlation between the large strain undrained shear strength ratio obtained from f_s measurements and the value measured using constant volume ring shear tests (CVRS) using clayey iron ore tailings samples and a laboratory-standard kaolin. Slurry samples of the two materials considered were prepared in beam centrifuge strongboxes, underwent self-weight consolidation, and then were tested “in-flight” using miniature CPTs that included f_s measurement. CVRS tests were carried out on identical materials for comparison to enable an independent measurement of large strain undrained shear strength, along with slurry consolidometer tests to provide consolidation data. A comparison of the results of the CVRS and CPT sleeve measurements are then made, along with discussion on the implications of these comparisons. Potential future useful testing is suggested to elucidate these issues.