Mahdi Taiebat


Research Classification

Research Interests

theoretical and computational geomechanics
constitutive modeling of engineering materials
physics and mechanics of granular materials
geotechnical earthquake engineering
seismic soil-structure interaction

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I am available and interested in collaborations (e.g. clusters, grants).
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Continuum modeling (FEM, FDM, MPM)
Discrete element modeling


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Graduate Student Supervision

Doctoral Student Supervision

Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Effects of particle size distribution and particle shape on cyclic liquefaction response of granular materials (2022)

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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Modeling the degradation of clayey soils subjected to undrained cyclic shearing (2022)

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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Micro- and macromechanical modeling of granular material under constant volume cyclic shearing (2020)

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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Multi-scale modeling of cyclic shearing and liquefaction response of granular materials (2020)

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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Numerical modeling and analysis of pullout tests of sheet and geogrid inclusions in sand (2019)

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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Constitutive and numerical modeling of clay subjected to cyclic loading (2018)

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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Numerical study on the response of pile groups under lateral loading (2015)

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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Three-dimensional nonlinear analysis of dynamic soil-pile-structure interaction for bridge systems under earthquake shakings (2015)

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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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Modeling of flow liquefaction and large deformations in tailings dams using material point method (2021)

The release of tailings and impounded water after dam breaches has had catastrophic consequences for the population and the environment. These events have gained the attention of the geotechnical community because they occur without warning, causing massive damage. Flow liquefaction of tailings materials has been the cause of instability of recent tailings dam failures. These disasters were initiated by the triggering event that led the system to experience flow liquefaction and large deformations. To predict the evolution of flow-type instabilities, designers usually carry out dam-break analyses that rely on fluid mechanics methods since conventional slope stability methods such as limit equilibrium and finite elements only provide information until the onset of failure. Such decoupling of the geomechanics-based analysis for stability and fluid mechanics-based analysis for dam-break is not desirable, given the recent advances in critical state soil mechanics, strain-softening material models, and large deformation analysis methods. Hence, there is a need to develop analyses that can simulate the failure initiation and the post-failure stage observed during tailings dam disasters.In the present study, the triggering and the subsequent motion have been analyzed in a unified framework for the case of a saturated upstream tailings dam. Two different triggers are considered: the increment of pore water pressure and the sudden loss of strength in the foundation. A critical state bounding surface plasticity model is used for the tailings. This model can reproduce the contractive brittle behavior of loose saturated tailings. A two-phase single-point formulation of the material point method (MPM) has been considered to simulate the whole deformation process. This method has recently increased its popularity as a tool to deal with large deformation problems in geotechnical engineering. The influence of the initial void ratio on system performance is evaluated. The current study shows the capabilities of MPM along with an advanced constitutive model in simulating the complex stress-deformation mechanism observed in tailings dam failures.

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Impact of Bidirectional Seismic Shearing on Volumetric Response of Sand Deposits (2019)

Ground motion-induced waves have a multidirectional nature when traveling through soil deposits. It is the interaction between their three components, one of vertical compression and two of horizontal shear, what drives the response of soils in the field. Experimental studies have shown that, for the case of soil liquefaction, neglecting one of the two horizontal shear components can potentially lead to an underestimation of seismic demand. However, while the deleterious effects of considering only one shearing direction for the seismic response of sand deposits are acknowledged, they are not properly addressed in engineering practice. A numerical study conducted here provides insight into the potential increased response of level ground dry and saturated sand deposits when subjected to unidirectional and bidirectional shear earthquake loading. The simulations utilized a three-dimensional finite difference computational platform which was verified using several analytical solutions of wave propagation through single and double phase medium. In addition, the analyses made use of an anisotropic bounding surface plasticity model with validated capabilities for capturing the volumetric response of sand deposits subjected to bidirectional cyclic shearing. Therefore, the results of the analyses were evaluated in terms of surface settlement for the dry cases and excess pore pressure for the saturated cases. The comparison of the response of bidirectional against the unidirectional seismic shearing analysis showed the importance of accounting for two horizontal rather than one ground motion component, as it was determined that the volumetric response under bidirectional shearing was always higher. Specifically, the dry models exhibited 80\% increase of surface settlement and the saturated models indicated up to 60\% rise of the mean values of peak excess pore water pressure ratio along the depth of the deposit due to bidirectional shearing. Moreover, in the saturated deposits studied, the bidirectional seismic shearing induced about 20\% increase on the thickness of the liquefied sand layer.

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Stress-deformation analysis of Denis-Perron dam: verification and validation for better prediction of rockfill response (2017)

Rockfill dams present a challenge for engineers due to the many uncertainties revolving aroundthe behaviour of rockfill. A governing factor in the behaviour of rockfill is the particle breakagedue to change of moisture, which was observed in laboratory and field conditions. Alonso andOldecop have proposed a rockfill model (RM), where the suction inside the cracks of the rockfillis a state variable that controls the breakage mechanism. This research focuses on verification andvalidation of stress-deformation analysis methodologies, for better prediction of rockfill response.It involves application of the RM in numerical simulation of a benchmark case study on the wellinstrumented Denis-Perron dam (SM3). Denis-Perron dam is a rockfill dam with a central till core,171 metres high and 378 metres long, located on the Sainte-Marquerite river in northern Quebec,Canada. The instrumentation data was made available by Hydro-Qu´ebec, for a period of six yearsof construction, impoundment, and operation of the dam. Numerical simulations are conducted usingCode Bright – a fully coupled three phase finite element program for unsaturated porous media.A validation stage was first carried out through modelling of Beliche dam – a well studied case byAlonso et al. The numerical model of the SM3 dam captures the staged construction, reservoir impoundmentand rainfall history recorded. Model parameters for the till core and rockfill shoulderswere either calibrated using limited available laboratory and field data, adopted from literature, orassumed with some rationale. Deformations measured by the inclinometers during constructionand impoundment, both upstream and downstream, are simulated successfully. Piezometer andpressure cell measurements are replicated to a very good extent. Post-construction deformationsare reproduced with reasonable success, given the limited data for detailed characterization of thevarious zones in the dam. Some important challenges around characterization of the rockfill compressibilityand the related scaling issues for model calibration are presented and discussed. Anattempt is made to quantify the amount of scaling observed through a back analysis of field measurements.Finally, the effect of permeability on rockfill in the development of deformations isdiscussed.

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Nonlinear Seismic Ground Response of Shallow Sand Sites (2015)

Predicting the ground response to the propagation of seismic waves is one of the most important aspects of geotechnical engineering. Advanced soil constitutive models provide significant opportunity to improve the understanding of nonlinear ground response during a seismic event, and offer the capability of simulating complex nonlinear soil behaviour which is not captured by means of traditional ground response analyses in geotechnical engineering. Moreover, observations of distinctive nonlinear soil behaviour during recent large earthquake events such as the 2011 Tohoku earthquake point towards the need to more reliably simulate realistic soil behaviour in order to understand the complex dynamic response of soils. The intent of this thesis is to utilize the SANISAND bounding surface plasticity model based on the work of Dafalias and Manzari (2004) to simulate the response of shallow sand deposits to a number of earthquake motions, with the aim of evaluating the ability of the model to simulate relatively complex nonlinear soil behaviour. Furthermore, both total and effective stress analysis techniques are carried out in order to highlight the importance of modeling the interaction between the pore fluid phase and the soil solid. For this purpose, two sites are analyzed, including a case history of a real downhole seismograph array and a generic site. The capability of the SANISAND model to simulate the phenomenon of high frequency dilation pulses is also explored. The SANISAND constitutive model is shown to adequately simulate the seismic ground response of a shallow sand soil column at a real downhole seismic array in Sendai, Japan by comparison to surface seismograph recordings for several earthquake events on the east coast of Japan. Soil permeability in the effective stress analyses is influential in the dynamic response of the soil to earthquake motions. Furthermore, modeling the pore fluid – soil solid interaction in an effective stress analysis is shown to be important for shallow medium dense sand sites subjected to cyclic mobility and strain stiffening. High frequency ground motion during the seismic response of a generic 10 m deep sand site is suggested to be caused by acceleration pulses as a result of soil dilation.

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A Simple Anisotropic Bounding Surface Plasticity Model for Cyclic Response of Clays (2012)

Natural clays are anisotropic in their in-situ state and have an undisturbed shear strengthin excess of the remoulded strength. In addition, most of the structures founded on claydeposits must be designed to withstand cyclic loads such as seismic ground motions or oceanwaves. When subjected to earthquake or wind induced cyclic loadings, clay exhibits complexresponse. A realistic modeling of clay response under irregular cyclic loading requires an appropriatedescription of the stress{strain relationship. This thesis extends the formulation ofa Simple ANIsotropic CLAY plasticity (SANICLAY) model by incorporation of a boundingsurface formulation for successful simulations of clay response under cyclic loading. The mostimportant aspects of the proposed formulation are the position of a projection center andthe ability to capture continuous sti ness degradation. The proposed projection center isestablished in the instant of any stress reversal and it realistically reects the experimentallyobserved plastic strains. A damage parameter is also adopted to better simulate the continuoussti ness reduction during the course of applied cyclic loading. The proposed modelis developed with the aim of maintaining the simplicity and yet including an adequate levelof sophistication for successful modeling of the key features of clay response. The modelformulation is presented in detail followed by its qualitative and quantitative comparisonwith experimentally observed clay response. The presented model validation demonstratesthe capabilities of the model in capturing a number of important characteristic features ofclay response in both monotonic and cyclic loadings. Limitations and recommendations forfuture work are discussed as well.ii

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