Mahdi Taiebat


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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2020)
Micro- and macromechanical modeling of granular material under constant volume cyclic shearing (2020)

No abstract available.

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 (2010 - 2018)
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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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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