Factors affecting the initial stiffness and stiffness degradation of cohesive soils

Abstract

This research investigated the pre-failure stress-strain behavior of cohesive soils through an ex.tensive testing program conducted on Resedimented Boston Blue Clay (RBBC). The uniformity and reproducibility in the behavior of this soil permitted a detailed study of the role of the following factors on both initial stiffness and small strain (Ea<O.1%) non-linearity in undrained triaxial compression: consolidation stress level (0.5-12 ksc), overconsolidation ratio ( 1,2,4,8), pre-shear consolidation stress path (loading vs. unloading), pre-shear lateral stress ratio (Ko- I), rate of shear (0.05-4%/hr), and duration of laboratory aging ( 1-2 cycles). Pre-failure stress-strain behavior was evaluated using a novel device specifically designed to perform onspecimen measurements of axial strains. The device makes use of two miniature submersible LVDT's mounted on a pair of yokes which clamp onto the soil specimen. This measuring system is capable of resolving displacements of less than 0.1 microns, corresponding to less than 0.0001% strain, over a 10% strain range. A multi-stage (i.e., a series of undrained shear and consolidation phases) testing approach was developed to minimize specimen variability, reduce testing time and conserve soil. The testing program, which comprises more than IOO phases of undrained shear, isolated parameters (initial stiffness, linear threshold (ea at 0.98 Eu MAXJ, coefficient of non-linearity [E0.00k/Eo.01k]. plastic strain at 0.1 % total strain, axial strain since most recent load reversal) that provide a consistent and concise description or the small strain behavior or cohesive soils. These parameters were used to develop a framework describing the dependence of the small strain ( 10-1 -10-1 %) behavior of soft clays on the above six factors. Experimental results indicate that the initial stiffness, ... is governed by both the vertical consolidation and the void ratio through a double exponential function ... Laboratory aging leads Lo an increase or the initial stiffness by approximately 20-25% per log cycle of time. The stiffness of OC RBBC is slightly affected by load reversals. The small strain non-linearity of RBBC is virtually independent of void ratio or stress level and is controlled primarily by the direction of loading prior to shear. Normally consolidated RBBC presents lhc smallest linear region and the most rapid stiffness degradation. Unloading to the OC state results in the most dmmutic increase in the linear threshold and decrease in the non-linearity. Subsequent reloading leads to an increase in the non-linearity that is proportional to the reloading strain. Increases in K, strain rate and aging all contribute to a reduction in the small strain non-linearity of the soil. These effects can be interpreted in terms or the level of shear stress mobilized at any given strain level, and the capacity to override/reduce viscous deformation of the soil.