Drained and Undrained Softening and Post-Reconsolidation Strength Recovery

The effect of clay fraction on drained and undrained strain-weakening and subsequent recovery in strength through reconsolidation was investigated using a series of constant volume (CV) direct simple shear (DSS) tests and drained ring shear (DRS) tests. This study examined three different materials with a range of plasticity index and fines content values: a mixture of 80% silica fine sand and 20% kaolin by dry weight (20K80SFS), iron ore tailings (IOT), and kaolin. The specimens were monotonically sheared in multiple stages or alternatively cyclically sheared under stress-controlled conditions to various values of shear strain in DSS tests and reconsolidated to three different vertical effective stresses, and monotonic shearing was recommenced. The lowest fines content 20K80SFS mixture exhibited strength recovery post undrained shearing. The initial degree of shear strain and reconsolidated vertical effective stress was found to influence the post-reconsolidation undrained strength of soils with a higher plasticity index. The shear strength of kaolin specimens after significant undrained shearing showed a reduction that was consistent with that seen in the DRS tests at high shear strain, suggesting a similitude between the process of undrained and drained strain-weakening in predominantly clay soils. This study highlights (i) the potential for undrained strength recovery for pre-sheared specimens and (ii) the undrained frictional-weakening process depending on the clay fraction of the material, consistent with previous studies of drained frictional weakening.