Incrementally Nonlinear Responses of Soft Chicago Glacial Clays

As many designs in geotechnical projects are controlled by serviceability rather than stability requirements, control of deformation becomes crucial, especially in urban area. In well-designed structures, working strain levels in soils typically are less than 0.1%, emphasizing the need to investigate soil response at small strains. Where the ground experiences applied stress other than that from one-dimensional deposition, one of the important factors affecting the small strain behavior, "recent stress history," becomes critical. In addition, the importance of soil anisotropy and its change under various loading conditions have been illustrated when predicting ground movements. This dissertation presents the results and analysis of a comprehensive laboratory investigation of the very small, small, large strain response and anisotropy of soft Chicago glacial clays.;Triaxial testing programs were conducted with bender element sets and local LVDTs on high quality block samples obtained from the Block 37 in Chicago, IL. Drained and undrained stress probe tests were conducted after KC recompression to the in situ vertical effective stress of the each sample to investigate the stress-strain response representative of the in situ conditions and anisotropy evolution. Undrained compression and extension tests with pre-shear stress paths selected to isolate the effects of recent stress history such that the only difference between the stress paths was the direction of loading to a common effective stress condition were conducted. Results are investigated in terms of stress-strain behavior, modulus degradation, stress path rotation angle, yield surface, Strain Response Envelopes (SRE) and anisotropy.;The significant variability of the shear, bulk, and cross-coupling moduli with stress probe direction and strain level shown in the SREs provide experimental evidence that the soft Chicago clays are incrementally nonlinear.;Based on the comparison of the shear modulus degradation curves obtained from undrained compression and extension tests with three different pre-shear paths, it was found that recent stress history and proximity to a yield surface affects shear moduli degradation, but recent stress history had a larger impact. Softer responses occurred for stress paths close to the same direction as the pre-shear path and stiffer responses occurred for stress paths in a direction opposite to the pre-shear stress path.;Results of tests conducted with three sets of bender elements show that soft Chicago clays are cross-anisotropic. It also was found that bender element data can be used to define the beginning point of irrecoverable deformation. Stiffness anisotropy measured from bender elements changes from its initial value at the onset of irrecoverable deformation. This implies structural change occurs at the onset of irrecoverable deformation.;The specimen quality cut from the Osterberg fixed piston samples is slightly lower than that from the hand-cut block samples. Although the Osterberg fixed piston samples have slightly lower quality than the hand-cut block samples, bender elements results are equivalent to those of the hand-cut block samples. The recent stress history effects on stress-strain responses at small strains also were found from the Osterberg fixed piston specimens.