Evaluation of Phosphatic Clay Disposal and Reclamation Methods. Volume 5: Shear Strength Characteristics of Phosphatic Clays

摘要

In this report, the undrained stress-strain-strength characteristics of six normally consolidated phosphatic clays are investigated. Clays selected for the evaluation of anisotropic and stress history effects and for the sand-clay mix study generally bracketed the range of plasticity and settling, consolidation and strength behavior reported for phosphatic clays. A total of 44 tests were performed as part of this investigation. The phosphatic clays are moderately sensitive with sensitivities ranging from about 1.5 to 3.5 depending on the plasticity of the clay. Phosphatic clays appear to be less sensitive at lower liquidity indices (higher solids contents). The shear strength of phosphatic clays was determined to be strain rate sensitive although to a lesser extent than anticipated based on the high plasticity of these clays. This finding implies that phosphatic clays are not as susceptible to undrained creep deformations as originally anticipated. The undrained Young's secant modulus of phosphatic clays was determined to be highly stress-level dependent as expected. However, all phosphatic clays seem to exhibit approximately the same magnitude of normalized modulus, irrespective of plasticity. Although the data are consistent, the undrained modulus is higher than one would expect for such plastic materials implying smaller undrained deformations upon loading than would occur with high plasticity natural sedimentary clay deposits consolidated to the same effective stress. The effect of stress history and overconsolidation due to desiccation and/or pre-loading produced significant changes in the stress-strain-strength behavior of phosphatic clays. Trends were consistent for all three phosphatic clays investigated.