UNIQUE RELATIONSHIP BETWEEN SMALL STRAIN SHEAR MODULUS AND EFFECTIVE STRESSES AT FAILURE

Abstract

This dissertation is comprised of three manuscripts developed from different topics of geotechnical and earthquake engineering. The first topic investigates a link between small and large strain behavior of dilatant soils. The second topic deals with the use of a reduced density in the calculation of small strain shear modulus from shear wave velocity due to the occurrence of relative motion between the water and soil-skeleton as a shear wave passes through the soil. The third and final topic investigates ground motion selection and scaling procedures from various methods found in the literature for seismic hazard analyses in the northeastern United States. Current geotechnical practice relies on empirical relationships with in situ tests to determine the effective stress strength parameters for dense cohesionless soils. Although these methods work reasonably well in practice, they cannot account for in situ effects related to time, fabric, and cementation. These factors are especially important for brittle or sensitive soils, such as loess and cemented sands. To develop methods that can predict strength in these types of soils, a better understanding of the link between small and large strain behavior is needed. The objective of the first manuscript and Appendices A and B is to evaluate the hypothesis of a unique relationship between the small strain shear modulus (G0) and the effective stresses at failure (σ'1f) for dilatant soils (i.e., G0/σ'1f = constant). This is accomplished by a laboratory testing program consisting of isotropically consolidated triaxial compression tests with shear wave velocity measurements throughout the test. The soils tested in this study include a quartz sand, calcareous sand, non-plastic silt, reconstituted high plasticity clay, and undisturbed sensitive clay, and the results are compared to previous studies by the authors on weakly cemented sands. The results from these tests showed that the ratio G0/σ'1f was approximately 200 ± 20 for the quartz sand and non-plastic silt, 130 ± 6 for the clays, and 128 for the calcareous sand and was independent of void ratio, degree of cementation, and confining stress. If true for other soils, this finding could have important implications for evaluating staged construction on sensitive soils and estimating the strength of dilative soils in situ. Small strain shear modulus (G0) is an important dynamic soil property used in different aspect of geotechnical and earthquake engineering such as seismic site response analysis, liquefaction potential, soil-structure interaction, foundation vibrations, etc. Typically, G0 is obtained in-situ or