Factors affecting the initial stiffness of cohesive soils

The paper presents the results of an extensive laboratory program conducted to establish the factors affecting the initial stiffness (Young's modulus) of cohesive soils in undrained triaxial compression. The testing program, conducted on resedimented Boston blue clay (RBBC) comprised more than 100 phases of undrained shear and isolated the role of: overconsolidation ratio (OCR), consolidation stress level, void ratio, lateral stress ratio, preshear consolidation path, strain rate and duration of laboratory aging. On-specimen measurement of the axial strain was performed employing a novel LVDT-based device capable of resolving displacements of less than 0.1 mu m, corresponding to approximately 0.0001% axial strain for a 80 mm height triaxial specimen. Results indicate that for all testing conditions, at small strains the stress strain behavior of RBBC (OCR=1-8) is linear. Provided that the strain rate is sufficiently (by 25-30 times) greater than the preshear creep rate, the Young's modulus is rate independent and, for the same duration of laboratory aging, is controlled solely by consolidation stress level and void ratio, or OCR, through two exponential functions. Prolonged laboratory aging leads to an increase in the initial modulus, equal to 20-25% per log cycle of time for NC RBBC. The stiffness of OC RBBC is slightly affected by load reversals.