Mahdi Taiebat

Cyclic liquefaction of granular soils is influenced by particle-level parameters such as particle size distribution and particle shape. These parameters are difficult to isolate in real soils, whereas use of Discrete element method (DEM) can be an important step to resolve this problem. It can also be used to model these individual effects on the undrained cyclic response.Samples with increasing particle size distributions, denoted by an increase in coefficient of uniformity are prepared under isotropic compression at two different relative densities. Constant volume, cyclic simple shear simulations at a low relative density under various loading intensities show that increasing the coefficient of uniformity initially increases, then decreases, the cyclic liquefaction resistance. This observation is largely reversed when the relative density is increased, in which case the liquefaction resistance decreases initially and then increases as the coefficient of uniformity increases.Microscopic examinations of the samples at their initial state are conducted to provide insight into their liquefaction resistance. Microparameters based on inter-particle contacts and normal forces reveal reasonable correlations with the macroscopic response. Furthermore, the initial state-based state parameter is found to significantly correlate with the cyclic resistance ratio irrespective of coefficient of uniformity and relative density. Finally, the determinate structure of particle assemblies (isostaticity) at the advent of cyclic liquefaction is studied in terms of average contacts.A similar approach is used to study the effect of particle shape under constant volume cyclic loading. Samples with a variety of simplified particle shapes denoted by aspect ratio and blockiness are isotropically compressed under two relative densities. Samples with non-spherical particles have lower liquefaction resistance than those with spherical particles at low relative density. In contrast, at a medium relative density, the non-spherical samples show comparatively higher liquefaction resistance. It has been discovered that these macrolevel observations under the influence of particle shape can be adequately explained by microparameters previously used to study the particle size distribution. The determinate structure is also identified here and its variation is studied for the range of particle shape and relative densities.

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The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Cyclic liquefaction of granular soils during earthquakes often results in catastrophic damages to civil infrastructure. Understanding and modeling of this complex phenomenon are of crucial importance in geotechnical engineering. Motivated by its practical importance, this study focuses on modeling the granular materials response under constant volume cyclic shearing from both micromechanics and continuum mechanics. At the micromechanical level, discrete element method was used to carry out an extensive set of uni- and multidirectional cyclic shear simulations on idealized granular assemblies. Unidirectional simulations were analyzed to explore the microstructural evolution concerning particle connectivity, force transmission, and anisotropies. Liquefaction state was marked by a significant drop in coordination number, where the granular system became fluid-like, and deformed significantly to rebuild the contact network. Stress-force-fabric relationship was verified, revealing increasing and decreasing patterns, respectively, for the proportions of fabric and force anisotropies. The multidirectional analysis explored the effects of shear paths on the cyclic response of granular assembly. Multidirectional simulations presented lower cyclic liquefaction resistance than unidirectional ones. Microscopically, particle connectivity, particle-void fabric, and anisotropies were investigated to shed light on the stability, deformation, and load-bearing network of the granular assembly, respectively. At the continuum level, the study focused on constitutive modeling of sand response in both pre- and post-liquefaction stages. A new constitutive model is formulated by incorporating two new constitutive ingredients into the platform of a reference critical state compatible bounding surface plasticity model with kinematic hardening. The first ingredient is a memory surface for more precisely controlling stiffness affecting the plastic deviatoric and volumetric strains and ensuing pore pressure development in the pre-liquefaction stage. The second ingredient is the concept of semifluidized state and the related formulation of stiffness and dilatancy degradation, aiming at modeling large shear strain development in the post-liquefaction stage. The new model successfully simulates undrained cyclic torsional and triaxial tests with different CSRs, separately for the pre- and post-liquefaction stages, as well as liquefaction strength curves. The new model was also assessed in the simulation of several multidirectional cyclic shear tests. The development of this constitutive model contributes to future applications in seismic site response analysis.

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Cyclic shearing of granular materials under undrained conditions can induce a reduction of mean effective stress and increase of the pore water pressure. In extreme cases, the mean effective stress can temporarily vanish and lead to a “semifluidized state", in which large shear strains are developed and accumulated. Predicting the level of deformations developed during liquefaction and especially in the post-liquefaction stage using constitutive models is a changeling task, and yet important to evaluate the safety of geotechnical structures.A sand plasticity model, which is the precursor of the SANISAND family of models, was considered as the reference model in this study. The model has proven success in the simulation of monotonic and cyclic response of sand in the pre-liquefaction state. A series of modifications were introduced out to improve the predictability of the model for the post-liquefaction cyclic shear strain. The modifications were motivated by carrying out a number of constant-volume cyclic shear triaxial simulations using the discrete element method (DEM). The DEM simulation results revealed that a high number of floating particles with zero contact in a semifluidized state, which explained the vanishing of load-bearing structures and large shear strain accumulations. Thus, linking discrete and continuum modelings via the semifluidized state, inspired introducing a new state internal variable named strain liquefaction factor (SLF) to model the degradation of stiffness. The SLF evolves within the semifluidized range; its constitutive role is to reduce the values of parameters controlling the plastic modulus and dilatancy, maintaining the same plastic volumetric strain rate, in the semifluidized range. The evolution rate equation of the SLF includes a back-to-zero recovery term under drained loading. The extended model was validated against a series of undrained cyclic simple shear tests at the element level. Then this model was implemented in a finite difference platform and used in the benchmark study LEAP for simulating centrifuge experiments of a submerged slope subjected to dynamic excitations. Comparisons between experiments and simulations were satisfactory, and especially the simulated horizontal displacement was improved using the SLF. This work is expected to extensively benefit the numerical modeling of liquefaction-related problems in the future.

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One way of studying the soil-inclusion interaction in the pullout test is by numerical modeling. Several of the numerical studies available in the literature lack the integration of consistent material characterization as input for the numerical model, resulting in little phenomenological description of the soil-inclusion interface behavior. There is, therefore, a need for an improved evidence-based understanding of the factors influencing the pullout resistance of different inclusions. Accordingly, the main objective of this study was to capture the pullout response of different inclusions, for which extensive laboratory pullout test data existed, through a phenomenological numerical model that uses physically-based parameters. This numerical model is henceforth used in a parametric study to assess the adequacy of the laboratory test data in the literature and ASTM D6706-01 recommendations.The finite difference software FLAC was used to simulate the laboratory response of three sheet inclusions and three geogrids, embedded in a pullout box filled with a uniformly graded sand (Badger sand) and subjected to vertical stresses up to 17 kPa. In the numerical model, the inclusions were represented by an elastic continuum at the center of the pullout box. The sand was modeled using NorSand, a constitutive model that is able to capture the dilative behavior of dense sands. An alternative approach to the usual spring interface is proposed to model the soil-inclusion interaction, where a thin continuum layer following a NorSand behavior is used, and the friction angle changed according to the interface strength of each inclusion. The soil and interface parameters were obtained from a laboratory testing program on Badger sand including triaxial, direct shear and direct simple shear tests.The results of this dissertation yield three principal contributions: 1) plane strain conditions and a stress-dependency of the critical state friction angle prevail in the pullout box; 2) the use of a constitutive model that can simulate dilation to represent the soil-inclusion interface behavior is able to capture the complete pullout response of the different inclusions; and 3) different aspects of ASTM D6706-01 pullout recommendations deserve improvement for a correct interpretation of the soil-inclusion interaction factor.

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Natural clays are anisotropic in their in-situ state and have an undisturbed shear strength in excess of the remoulded strength. In addition, most of the structures founded on clay deposits must be designed to withstand cyclic loads such as seismic ground motions or ocean waves. When subjected to earthquake or wind induced cyclic loadings, clay exhibits a complex response. A realistic modeling of clay response under irregular cyclic loading requires an appropriate stress--strain relationship described by a constitutive model. This thesis extends the formulation of an existing constitutive model, namely Simple ANIsotropic CLAY plasticity (SANICLAY) model, by incorporation of a, well-established in geomechanics, bounding surface formulation for successful simulations of clay response under cyclic loading. The most important aspects of the proposed formulation are the position of a projection center and the ability to capture continuous stiffness degradation. The proposed projection center is established in the instant of any stress reversal, and it realistically reflects the experimentally observed plastic strains. A damage parameter is also adopted to better simulate the continuous stiffness reduction during the course of applied cyclic loading. The proposed model is developed with the aim of maintaining the simplicity, and yet including an adequate level of sophistication for successful modeling of the key features of clay response. The model formulation is presented in detail, followed by details of its implementation for applications in boundary value problems. Verification of the model implementation and validation of its performance are also presented. Verification of the model implementation is required in order to build confidence prior to its validation. Followed model validation demonstrates the capabilities of the model in capturing a number of important characteristic features of clay response in cyclic loading. Further exploration of model response in multi-directional cyclic shear is performed demonstrating its extension into more complex multi-directional cyclic shear. Development of the model, its implementation, verification of its implementation, validation of its performance, and exploration of model response in multi-directional cyclic shear provide a tool that can be used in modeling clay response under cyclic loadings. Limitations and recommendations for future work are discussed as well.

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When piles act in a group, soil–pile interaction reduces the lateral resistance of the individual piles. A practical approach to characterize the group behavior in different pile groups is using appropriate factors such as p–multiplier or group reduction factor. The experimental studies on pile groups are usually carried out on small pile groups with close spacings and free-head condition. These limitations are due to the difficulty and high cost of full scale testing particularly in larger pile groups. These limitations justify using three–dimensional numerical simulations to study lateral response of pile groups. This research focuses on group reduction factors and p–multipliers to characterize the group effects in a wide range of pile groups. In order to systematically study the group reduction factors, a numerically derived benchmark database is established using a continuum approach to simulate the response of the pile groups. The capability of the numerical model in predicting the pile group behavior is first evaluated by three–dimensional continuum modeling of three field tests on actual pile groups. Then the continuum model is used to generate benchmark database. The calculated group reduction factors compare well with available experimental data, which are typically extracted from small pile groups. Current study also covers a wide range of pile groups with different numbers of piles, various pile spacings and pile head condition for which there is no experimental data available in the literature. Furthermore, this study gives greater insight into the interaction between piles based on their row position in the pile groups with different layouts. To this end, carried load at the pile head and bending moment profiles for different piles are compared based on their row position in the group when they are pushed simultaneously. The p–multipliers are also calculated to quantify the contribution of different rows to the lateral resistance of the group.The study shows that design guidelines such as AASHTO and FEMA P-751 overestimate the group reduction factors and p–multipliers, hence the lateral resistance, in larger pile groups or pile groups with larger spacings, especially for fixed pile head conditions.

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Bridge designers have adopted simple approximate methods to take into account soil-structure-interaction (SSI) in dynamic analysis of bridge systems. The most popular one is the substructuring method in which the response of the foundation soil and its interaction with the pile foundation and the abutment system are represented by a set of one-dimensional springs and dashpots. While this method has been widely used in practice, it has never been validated by comparing the results with those obtained from full-scale analyses. This thesis aims to evaluate the substructuring method and to quantify the level of associated errors for the use in bridge engineering. To this end, the baseline data required for the evaluation process is provided by full-scale nonlinear dynamic analysis of the bridge systems subjected to earthquake shaking using continuum modeling method. This involves detailed modeling of the foundation soil, pile foundations, abutment system, and the whole bridge structure. Three representative bridge systems with two, three, and nine spans are simulated. In all models, nonlinear hysteretic response of the foundation soil and the bridge piers are accounted for in the analyses using advanced constitutive models. The numerical model of the bridge is validated by simulating the seismic response of the Meloland Road Overpass for which extensive measured data exist over past earthquake events. Subsequently each one of the three bridge systems is also simulated using the substructuring method. Comparing the obtained results with the baseline data indicates that the substructure model may not be sufficiently reliable in predicting the bridge response. In particular the method is shown to misrepresent the spectral responses of the bridge, pier deflections, shear forces and bending moments induced at the pier base, and longitudinal and transverse forces induced to the abutments. The substructuring method is shown to suffer from several fundamental drawbacks that cannot be simply resolved. Using the recent advances in constitutive modeling of geotechnical and structural materials, and in computational tools and high-performance parallel computing, this thesis shows that large-scale continuum models can gradually become a powerful and significantly more reliable alternative for proper modeling of seismic SSI in bridge engineering.

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