Effects of geologic time on the initial stiffness of clays

The paper addresses the effect of geologic aging on the initial stiffness of clays through the analysis of in situ shear stiffness data obtained using a seismic cone penetrometer at three sites in Boston blue clay (BBC), a 12,000–14,000-year old post-glacial marine clay deposit. The effects of this aging period are estimated using as reference for the unaged behavior the results of over 90 triaxial shear phases conducted on specimens of resedimented Boston blue clay (RBBC), a soil prepared in the laboratory from natural BBC, and tested after 1 day of laboratory aging. same OCR and effective stress level, the natural clay displays an initial stiffness, which on the average, is approximately 50% greater than that of the laboratory resedimented clay. Based on the estimated age of the natural deposit, this corresponds to an increase of the stiffness per log cycle of time of 7%. This value of the aging coefficient, NG, is at the low end of values presented in the literature for clays, which are however all based on shorter aging durations (days to weeks).