Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

First Advisor

Christopher D. P. Baxter

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 in the laboratory by measuring the shear wave velocity of the soil and knowing the bulk density of the soil (G0 = ρvs2). However, in a saturated media, and depending on the grain size (and thus, porosity and hydraulic conductivity) and frequency of the shear wave, this equation may be inaccurate and can lead to an overestimation of the small strain shear modulus. In some cases when the shear wave travels through the soil, a relative movement between the water and soil-skeleton occurs and a reduction of the density must be determined. 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 fine-to-medium grained sand, under dry and saturated conditions at different confining stresses. The results showed that using the total density overestimates G0 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.

An important aspect of a seismic site response analysis is the choice of appropriate ground motions and the methods for scaling ground motion records. In the case of northeastern United States (NEUS), available recorded ground motions are limited and earthquakes sources are not well defined. Additionally, ground motions from this region contain a distinctive high frequency content not present in ground motions from regions with more seismic activity. This makes the use of ground motions from high seismicity areas not suitable for seismic response analyses in the NEUS. These limitations make the selection and scaling of ground motions a challenge in this region. The objective of this study is to evaluate and compare different methods of selection and scaling of recorded ground motions for a site specific seismic response analysis to determine which methods are most appropriate for the northeastern United States. Five different criteria were defined to critically evaluate and compare the selected methods. These criteria were defined to evaluate (1) the ability of the method to produce a median response spectrum at bedrock that matches the UHS and its variability, (2).the ability of the method to characterize a response spectrum over a period range versus a single period, (3) the ability of the method to account for a range of magnitude and site-to-source distance earthquakes that are consistent with the UHS, (4) the Set-up time and run time required to obtain the response spectrum at bedrock, and (5) how the site response analysis result is affected by the method. Overall, the method proposed by Kottke and Rathje (2008) performed very well in most of the criteria compared to the other evaluated methods.

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