The emergence of the novel bio-cementation techniques for liquefaction mitigation application has received more attention on the experimental level than the numerical level. Few studies have aimed at modeling the behavior of bio-cemented sands under various loading conditions, but their use can be restricted by the obtainability of their input parameters and the lack of a cumulative body of data to support their validation. This Dissertation presents the extension of a plasticity model for sands to bio-cemented sands (PM4SandC Version1). The motivation for this work is twofold: (1) to have a usable constitutive model which is applicable to bio-cemented sands (and by extension naturally cemented sands) under various initial and loading conditions, and (2) to advance the deployment of bio-cementation in the field as an alternative ground improvement method for liquefaction mitigation.The extension of the original constitutive model formulation to bio-cemented sands warrants an understanding of the main behaviors of interest. An extensive and critical review of the experimental studies on bio-cemented sands is performed first to collect the available knowledge on these geomaterials and identify the remaining gaps in this relatively new research field. This critical review of the mechanical testing is supplemented with one on the numerical studies on bio-cemented sands and other relevant cemented soils and weak rocks. The critical review provides insights into the mechanical behavior of bio-cemented sands and later informs the modifications to the original formulation of the constitutive model, and gathers the approaches used by other research to achieve this goal. In order to address the usability for the model by means of reasonable and attainable input parameters, a relationship between a field parameter, the cone tip resistance, and a constitutive parameter, the apparent cohesion, is developed to provide an estimation of the parameter most contributing to the cementation-induced changes. Readily available cone penetration measurements in a large tank experiment and a centrifuge test on bio-cemented sands in conjunction with synthesized cone penetration data from an axisymmetric penetration model using the Mohr-Coulomb constitutive model enable the development of such a relationship as a function of confining stress. The axisymmetric cone penetration model and its input parameters are described, simulations results are validated against experimental results, and simulations are extended to higher confining stresses before fitting a simple linear relationship. This relationship is later used in the estimation of cohesion for a system-level analysis using the PM4SandC model. Once the behaviors to be prioritized in the model extension and the input parameters to characterize cementation are known, the intended modifications are implemented in the original formulation of the sand model PM4Sand Version 3.2. Modifications in PM4SandC relative to the PM4Sand model include: (1) the introduction of input parameters quantifying the effect of cementation on the shear stiffness, the peak strength and the volumetric behavior, (2) the incorporation of an additional cementation-induced strength to the mean effective stress and hence the shift of the constitutive space to reflect the enhanced tensile strength of bio-cemented sands, (3) the adjustment of the dependence of the cemented shear modulus on confining stress, and (4) the addition of evolution laws to degrade/alter the cementation parameters as a function of damage accumulation. A generalized calibration demonstrating the performance of the model under various cementation levels, confining stresses, drainage, and loading conditions is performed in FLAC 8.1 and some guidance is provided to aid users in the calibration process. The extended model is then validated against a body of experimental data on bio-cemented sands. Overall, the model is able to qualitatively predict the trends seen in bio-cemented sands with minimal calibration effort by means of input parameters physically meaningful and obtainable from the field. The validation of the PM4SandC model at the element level is followed by its validation at the system level. Two one-dimensional site response analyses on bio-cemented columns are performed in FLAC 8.1 to (1) demonstrate the ability of the model to predict the free-field dynamic response of a treated site, (2) present a first-of-a-kind numerical effort to simulate site response analyses in bio-cemented sands. The PM4SandC constitutive model is assigned to the bio-cemented sand columns. The numerical analysis and its input parameters as well as the calibration process are described in detail. The dynamic response from the simulations in terms of accelerations, shear strains, cyclic demand, pore pressure generation, and response spectra is compared to the measured response in centrifuge models scaled to real site conditions. The constitutive model is found to reasonably approximate the dynamic responses from the centrifuge tests. A parametric investigation is also performed to provide insights into the effects of parameter uncertainties to PM4SandC on the overall dynamic response.

Well-graded coarse-grained soils encompass a broad range of materials that are typically found in natural deposits and man-made fills. Current standard-of-practice estimates the behavior of these soils using sand-based engineering methods in liquefaction evaluations, assuming transferable strength, stiffness, stress-dilatancy, and liquefaction triggering behaviors. This generalization does not explicitly consider the effects of median grain size (D50) and gradation (Cu) on soil behavior, even though these parameters alter the soil fabric, index, and physical properties of the soil, and by extension their response to monotonic and cyclic loading. The aim of this work is to develop an experimental database: (1) to examine the role of gradation and grain size on the monotonic and cyclic response of coarse-grained soils, and (2) to evaluate the appropriateness of commonly used sand-based relationships to capture the mechanisms of well-graded soils.

A series of constant-stress (CS) and constant-volume (CV) monotonic direct simple shear (DSS) tests and cyclic CV-DSS tests were performed on four poorly- to well-graded quartzite soils but composed of materials from a single natural deposit to minimize the effect of other grain properties (e.g., mineralogy, deposition, particle shape). This experimental program involves a wide range of relative densities (20 – 75%), initial consolidation stresses (50 – 600 kPa), and static shear stresses (to target CSR levels of 0.07 to 0.18). Experimental results are used to identify and describe trends regarding soil strength, stiffness, dilative tendencies, and critical state under monotonic loading, and to develop liquefaction triggering resistance curves under cyclic shearing. Conclusions will provide an initial basis to assess the appropriateness and potential re-evaluation of using sand-based relationships to capture the static and dynamic mechanisms of well-graded, coarse-grained soils, with future work geared towards numerical simulations and advanced constitutive models to compare with the laboratory results from these direct simple shear tests.

Liquefaction assessments require the ability to estimate triggering potential and possible consequences across a range of seismic hazard levels and in-situ soil conditions. This Dissertation aims to advance the fundamental understanding of the element level response of coarse-grained soils prior and after liquefaction triggering, and to evaluate tools currently used in engineering practice for the estimation of liquefaction effects on geosystems. This is achieved by (1) resolving experimental issues pertaining to performing Direct Simple Shear (DSS) testing, (2) establishing framework that facilitates the comparison of both triggering and post-triggering responses, and (3) performing extensive DSS investigations of coarse-grained materials encompassing a range of properties and conditions.

A series of constant-volume (CV) DSS tests show that applying a pre-conditioning sequence improves the engagement at the soil-platen interface when textured platens are used. Given the efficacy of these protocols to enhance the shear stress transfer to sand specimens, all the CV-DSS tests presented in this Dissertation consider a pre-conditioning sequence prior to constant-volume shearing. Although the pre-conditioning protocol presented herein is specific to the testing equipment and materials considered, recommendations are provided to develop pre-conditioning protocols for other soils, platens, and testing devices.

The factors and mechanisms controlling the accumulation of shear strains of clean uniform sands exhibiting cyclic mobility behavior under level-ground conditions are investigated from CV-DSS tests on Ottawa F-65 sand subjected to uniform and irregular cyclic loading conditions. Experimental data show that the rate of shear strain accumulation per loading cycle depends on the relative density, cyclic stress amplitude, and overburden stress. Mechanisms of shear strain accumulation are investigated by decoupling the shear strain developed in each loading cycle in two components: γ0, developed at near-zero effective stress, and γd, developed during dilation. Results show that γ0 mostly depends on the shear strain history, while γd depends on the cyclic stress amplitude and relative density.

The role of gradation and grain size on the liquefaction behavior of coarse-grained soils is investigated from a series of CV-DSS tests performed on seven soil mixtures with different coefficients of uniformity (Cu) and median grain sizes (D50). Empirical observations of liquefaction triggering, post-triggering shear strain accumulation, and post-liquefaction reconsolidation strains are synthesized to evaluate functional trends with Cu and D50. Results show that the effects of Cu and D50 on the liquefaction triggering resistance depend on the relative density (DR) at which they are compared, while the post-triggering shear strain accumulation decrease while increasing both Cu and D50. Results also show that an increase in Cu leads to smaller post-liquefaction reconsolidation volumetric strains, while changes in D50 have little effect on such strains.

The combined effect of gradation and sloping ground conditions on soil liquefaction is investigated from monotonic and cyclic CV-DSS tests performed on two sands with different gradations. Monotonic test results show that an increase in gradation slightly reduces the degree of strain-softening and peak shear stresses. Results from cyclic tests are synthesized to examine the effect of gradation on patterns of pore pressure generation, liquefaction triggering resistance, and sloping ground correction factors (Kα). Additional cyclic tests under irregular loading conditions are analyzed to evaluate the role of gradation in the post-triggering shear strain accumulation.

Finally, this Dissertation examines the capabilities and limitations of the PM4Sand constitutive model in capturing key features of the liquefaction behavior of two sands with different gradations under sloping ground conditions. Simulations with PM4sand successfully captured the monotonic response and liquefaction triggering resistance for both soils. Still, the calibration process required activating some of the secondary parameters of the model, and using alternative parameter sets depending on the target responses prioritized during calibration.