Usama El Shamy

Microscale Energy Dissipation Mechanisms in Cyclically-Loaded Granular Soils

This paper utilizes the Discrete Element Method to characterize energy dissipation mechanisms in cyclically loaded soils based on micromechanical considerations. Computational simulations of consolidated undrained cyclic triaxial tests were conducted at various relative densities and were subjected to cyclic loading of different frequencies and shear strain amplitudes. The different components of microscale energies were monitored during the course of the simulations and characterized into input and dissipated energies. A comparison is made between the dissipated energy computed from microscopic energy components and macroscopic energy calculated based on the area of the deviator stress-axial strain loops. These energies are then used to obtain the specific damping capacity defined as the ratio of dissipated energy during one cycle to the maximum stored elastic energy during the same cycle. The conducted simulations highlight the importance of calculating actual stored energy in the system as opposed to approximating it to be that calculated as the triangular area under the secant modulus. Finally, a series of simulations that resulted in liquefaction are discussed, and the amount of energy dissipated to liquefaction is examined based on these results.

Pore-Scale Modeling of Fine-Particle Migration in Granular Filters

AbstractFine-particle migration and associated internal erosion are major concerns for dam safety. Granular filters are used to prevent fine-particle migration, and several empirical models have been introduced for the design of these filters. Few computational techniques have tracked particle transport through the filters. This paper presents a three-dimensional transient fully coupled pore-scale model used to study the mechanism of fine-particle migration in granular filters. A pore-scale idealization of the fluid was achieved by using the lattice Boltzmann method, and the solid phase was modeled at a microscale using a discrete element method. The fluid forces applied on the particles were calculated on the basis of the momentum exchange between the fluid and particles. The proposed numerical technique was used to model the migration of base-soil particles through granular filters of different particle sizes. Results of conducted simulations provided the erosion percentages and flow rates during the si...

Direct Velocity and Shear-Stress Measurements in Shear-Induced Erosion of a Particle Bed

Understanding the complex fluid-particle interactions in shear-induced erosion and scour requires measurement of the interstitial fluid motion in both stationary and moving particle beds. These measurements are confounded by the differing material properties of the fluids and particles used in experiments. In the present investigation, full-field fluid-velocity measurements for flow over a bed of 3-mm-diameter monodisperse glass beads were obtained using digital particle image velocimetry (DPIV) in an aqueous solution of sodium iodide to match the refractive index of the particles. Fluorescent seeding particles were used in the fluid together with optical filters and image processing to provide both time-averaged and instantaneous measurements of the fluid velocity for both stationary and moving particle beds at channel Reynolds numbers (Re) in the range 4900–9000. Recent investigations have used refractive index matched fluids to obtain flow field measurements in the particle bed, but have been limited to time-averaged measurements, stationary particle beds, and/or low Re. The results indicate that particle motion was initiated at a critical Shields number of approximately 0.017, but some hysteresis in the Shields number was observed once motion was initiated. Velocity profiles were compared with power law and log-law models, showing reasonable agreement in certain flow regimes, but the particle-layer shear stress predicted by a log-law fit was found to be 10–30 times the actual shear stress measured from DPIV measurements, suggesting measurements of particle shear in channel flow based on a log-law model may not be sufficiently accurate. A correction to the log-law model utilized in porous wall flows was applied, and the adjusted value of von Karman constant increased with the shear Reynolds number for the cases without particle motion, but no clear trend was observed for the moving particle cases.

Discrete Element Method Study on Effect of Shear-Induced Anisotropy on Thermal Conductivity of Granular Soils

AbstractIn this study, a simple idealization of heat transfer by conduction at the interparticle contacts is utilized for the evaluation of thermal conductivity of granular soils when subjected to external loading conditions. Discrete element method simulations employing this idealization were performed to examine the impact of loading the soil in a consolidated drained triaxial test environment on soil thermal conductivity. Results of conducted simulations show that shear-induced anisotropy results in an anisotropic thermal conductivity tensor. The results also indicate that contact density affects the average thermal conductivity of granular materials. The larger the average number of contacts per particle, or the coordination number, the larger the thermal conductivity. During shearing, the coordination number tends to decrease, resulting in a reduction in soil thermal conductivity with dense soils, showing a larger decrease in thermal conductivity upon shearing.

Discrete-Element Method Simulations of the Response of Soil-Foundation-Structure Systems to Multidirectional Seismic Motion

AbstractIn this study, a three-dimensional microscale framework utilizing the discrete-element method (DEM) is presented to analyze the seismic response of soil-foundation-structure systems subjected to three-directional base motion. The proposed approach is employed to investigate the response of a single lumped mass on a square spread footing founded on a dry granular deposit. The soil is idealized as a collection of spherical particles using DEM. The spread footing is modeled as a rigid block composed of clumped particles, and its motion is described by the resultant forces and moments acting upon it. The structure is modeled as a column made of clumped particles with a concentrated mass specified for the particle at the top. Analysis is done in a fully coupled scheme in time domain while taking into account the effects of soil nonlinear behavior, possible separation between the foundation base and soil because of rocking, possible sliding of the footing, and dynamic soil-foundation interactions. A tec...

A Microscale Approach for the Seismic Response of MDOF Structures Including Soil-Foundation-Structure Interaction

In this article, a three-dimensional microscale computational framework utilizing the discrete element method is presented to analyze the seismic response of soil-foundation- MDOF structure systems. The proposed approach is used to explore the response of MDOF structures on a square embedded footing founded on a dry granular deposit. Computational simulations were conducted to investigate the response of the system to several base excitations. The impact of replacing a MDOF structure with its equivalent single degree of freedom (ESDOF) structure while accounting for soil-foundation-structure interaction (SFSI) is investigated. Detrimental or beneficial effect of SFSI on the response is also examined.

Integration of Centrifuge Testing in Undergraduate Geotechnical Engineering Education at Remote Campuses.

We report the results of a pilot study aimed at developing, implementing, and assessing an educational module that integrates remote major research instrumentation into undergraduate classes. Specifically, this study employs Internet Web-based technologies to allow for real-time video monitoring and execution of cutting-edge experiments. The students’ activities within the module are centred on building a model of a shallow foundation on a sand deposit utilising a centrifuge facility and using this model for: (1) visual observation of the response of soil-foundation systems, (2) learning the use of instrumentation, (3) interpretation of acquired data, and (4) comparing experimental results to theoretical predictions. Testing a soil-foundation system helped the students identify the lab experiments needed to analyse and design the system. A survey was used to gauge students’ perceptions of learning as a result of introducing the module, which were found to be positive.

Micromechanical Modeling of the Seismic Response of Gravity Retaining Walls

In this study, the analysis of the seismic response of a soil-retaining wall system is performed based on a three-dimensional microscale framework utilizing the discrete element method (DEM). The proposed method is employed to investigate the seismic response of gravity-type retaining walls with three degrees of freedom. In the simulation, the granular soil deposit is idealized as a collection of spherical soil particles; the retaining wall is simulated as a rigid block composed of clumped particles to yield the physical characteristics of a real-life retaining wall. The model is processed under the gravitational acceleration of 50 g to reduce the total duration of the simulation and dimensions of the model.

Multiscale Modeling of Fine Particles Migration in Granular Filters

We present early results of a fully coupled transient three-dimensional model employed to study fundamentals of fine particles migration in granular filters. Filtration process and internal erosion are a particle level phenomenon. A numerical model that is capable of tracking the migration of the base soil particles through the filter pores at a microscale is therefore needed. The discrete element method is used to model the granular material. The pore fluid is modeled at a mesoscale and solved using the lattice Boltzmann method. The proposed approach is used to model the migration of base soil particles through a granular filter. Results of conducted simulation provide the erosion percentage and flow rate during the simulation.

Liquefaction of Granular Soils at the Microscale

In this study, we introduce results of a novel coupled pore-scale model of pore-fluid interacting with discrete particles for soil liquefaction. A mircoscale idealization of the solid phase is achieved using the discrete element method (DEM) while the fluid phase is modeled at a mesoscale using the lattice Boltzmann method (LBM). The fluid forces applied on the particles are calculated based on the momentum exchange between the fluid and particles. The proposed approach is used to model liquefaction of a saturated granular soil deposit subjected to seismic excitations. Results of conducted simulations suggest that liquefaction is due to reduction in void space during shaking that leads to buildup in pore-fluid pressure.