Unique relationship between small strain shear modulus and effective stresses at failure

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. 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\u27 1f) for dilatant soils (i.e., G0/σ\u27 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\u27 1f was approximately 200 ± 20 for the quartz sand and non-plastic silt, 150 ± 4 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. The objective of the second manuscript is to investigate the concept of an effective (reduced) density required to obtain the correct small strain shear modulus. This was accomplished by measuring the shear wave velocity of three different materials of different sizes including 6-mm glass beads, coarse grained sand, and medium-to-fine grained sand, under dry and saturated conditions at different confining stresses. The results showed that using the total density overestimates G 0 by up to 20% in coarse materials (i.e., 6-mm glass beads and coarse sand) and therefore the effective density must be used. Results for the fine-to-medium grained sand were inconclusive. The objective of this study is to evaluate the appropriateness procedure of different methods for choosing and scaling ground motions in the NEUS. Six different methods were selected to critically compare and analyze them by performing a site response analysis at two sites in NEUS. A uniform hazard spectrum (UHS) was used as the target spectrum. The results showed that the bedrock ground motions representing the NEUS have high response between periods of 0 and 0.1 seconds. The UHS that can be obtained from the USGS is not defined between 0 and 0.1 seconds and thus should not be used for spectral matching or scaling within this period range. A single spectrally-matched synthetic motion showed spikes in the response around the natural period of the site and was likely to be overconservative. For structures having a natural period at or near the PGA, a simple scaling of a suite of 7 pairs of records by the median of the suite is recommended. The method proposed by Kottke and Rathje (2008) is recommended for period ranges above the PGA only when the suite of records is fit to the UHS above a period of 0.1 seconds. (Abstract shortened by UMI.