Recent Stress-History Effects on Compressible Chicago Glacial Clays

This paper presents the results of an experimental program to investigate recent stress-history effects on small-strain stiffness of lightly overconsolidated compressible Chicago glacial clays. Stress-probe tests with different recent stress histories were conducted on high-quality block samples taken from an excavation in Evanston, Illinois. The stress histories applied prior to probing represent in situ greenfield conditions and an unloading path associated with a common situation encountered when building in congested urban areas. Results of the stress-probe tests are compared in terms of shear, volumetric, and coupled components and their stiffness degradation. Strain-response envelopes were constructed in shear and volumetric-strain space to graphically show the significant difference in these responses between the two sets of stress-probe tests. The results clearly indicate that the recent stress history affects shear, bulk, and the two cross-coupled moduli of the Chicago clays as a function of the angle change between the previous and current stress paths. Consequently, the overall characteristics of strain-response envelopes of the two sets of stress probes are different.