Undrained Cyclic Shear Behavior of Sensitive Saprolite Soil

This study presents an investigation of the cyclic and post-cyclic behavior of a sensitive saprolite soil. Saprolite soils are formed by the chemical decomposition of rock to soil particles that remain in place. Sensitive saprolites have an open boxwork structure that when worked by means of excavation, placement, or compaction lose this structure and become very wet. The authors were not able to find studies discussing cyclic and post-cyclic behavior of these unusual soils. The soils in this study classify as high-plastic silt (MH) based on their USCS classification with a plasticity index ranging between 18 and 36, a fines content between 52% and 97%, and a saturated water content of between 50% and 65%. Laboratory tests include stress-controlled cyclic direct simple shear (DSS), post-cyclic reconsolidation, static and post-cyclic monotonic DSS, and consolidation tests. The characterized stress history shows these soils have an overconsolidation ratio (OCR) between 1.5 and 2.0. The results of this testing program indicated that the cyclic resistance ratio (defined as shear stress normalized by initial vertical effective stress) to reach 3% single amplitude shear strain in 15 cycles was 0.29. Post-cyclic shear tests showed a strength reduction between 12% and 22% interpreted at 10% shear strain. Post-cyclic reconsolidation tests showed volumetric strains between 1.0% and 1.5% for specimens with residual excess pore pressures between 70% and 85%. The results of the laboratory cyclic tests in this study are compared to other fined-grained soils reported in literature.