Mahdi Taiebat

Professor

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Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
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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A Simple Anisotropic Bounding Surface Plasticity Model for Cyclic Response of Clays (2018)

No abstract available.

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)

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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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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