Geotechnical systems involve a number of elements such as the porous media, composed of soil grains and pore-fluids, interacting with a structural system such as a foundation or a retaining structure. The resulting system is thus very complicated and becomes increasingly complex if it is subjected to extreme loading conditions such as those encountered during earthquakes or hurricanes. A prime example of failure of geotechnical systems that includes a structural element is the collapse of the flood-protection levee I-wall in the aftermath of Hurricane Katrina in New Orleans, LA. Current engineering tools and procedures are very limited in modeling the dynamics of the interactions between the flowing water, the soil media and the levee structure. To address this issue, the present work will develop a high-fidelity multiscale approach to provide as much information as reasonably feasible about the possible separation between the structure and the soil as floodwater rises, the impact of such separation on the distribution of water pressure surrounding the structure, and the potential occurrence of piping and scour in the soil media near the structure.
The activities of this interdisciplinary research effort are centered around the following goals: to provide an experimentally tuned and validated, and thus predictive, computational research tool for the analysis and design of geotechnical systems subjected to extreme loading conditions; to resolve the fluid-particle interaction coupling term needed to model the mechanism of flood-induced erosion of levees; and to explore the complex response of flood-induced failure of levee systems under representative flooding scenarios. A novel experimental program will investigate the interaction between a fluid and embedded particles. Using transparent particles embedded in a matched-index-of-refraction fluid, the combined motion of the fluid and densely-packed particles will be measured using a combination of digital particle image velocimetry (DPIV) and direct imaging of the embedded particles. Taking the particles as surrogates for sand and/or soil particles, the experimental results will serve to construct a model for the lift force on particles - which is necessary for accurate simulations - and validate the computational method. A multi-scale computational approach that combines the best features of continuum-based techniques and microscale models will be constructed to idealize the soil-fluid-structure system that represents a levee. Floodwater will be modeled using a computational fluid dynamics (CFD) approach that utilizes the Arbitrary Lagrangian Eulerian (ALE) moving mesh technique for proper modeling of the fluid free-surface and fluid-structure interaction, far-field soil media will be idealized using the finite element method, and near-field soil media will be modeled using the discrete element method coupled with a CFD technique. The different scale subdomains will be coupled through proper transfer of contact information at their interfaces.
The outcome of the proposed work will improve the understanding of the complex behavior of geotechnical systems and contribute to hurricane risk assessment. The proposed computational framework will equally benefit other important geotechnical engineering applications such as the seismic response of quay walls, impact of storm surge on offshore structures, and the dynamic response of retaining structures and deep foundations. The outreach activities in this project utilize existing programs at Southern Methodist University such as Visioneering and SMU Engineering Summer Camps for Girls to promote interdisciplinary engineering to Middle and High School students from the local Dallas-Fort Worth Metroplex.
Geotechnical systems involve a number of elements such as the porous media, composed of soil grains and pore-fluids, interacting with a structural system such as a foundation or a retaining structure. The resulting system is thus very complicated and becomes increasingly complex if it is subjected to extreme loading conditions such as those encountered during earthquakes or hurricanes. In this research, a universal transient fully-coupled numerical technique that is capable of modeling saturated granular soils under any loading conditions with no or minimal assumptions and empiricism was presented. Recognizing that the solid phase behavior is dictated by a particle scale, a microscale idealization of the solid phase using the discrete element method (DEM) was adopted. A pore-scale idealization of the fluid was achieved by using the lattice Boltzmann method (LBM). LBM is capable of modeling different fluid regimes and capturing the fluid behavior around the particles as well as obtaining the fluid forces on the particles without any hypotheses or empirical relations. A fully-coupled transient LBM-DEM model was developed and employed to study fundamentals of flood-induced surface erosion in a particle bed. The proposed approach was used to model both single particles and particle beds subjected to Couette flow conditions. The behavior of both the single particle and the particle bed depended on particle diameter and surface shear fluid velocity. The conducted simulations showed that the fluid flow profile penetrates the bed for a small distance. This penetration initiates sheetflow and surface erosion as the fluid interacts with particles. The same LBM-DEM model was then used to study the migration of base soil particles through granular filters of different particle sizes. Results of conducted simulations provided the erosion percentage and flow rate during the simulations. The results obtained from the simulations agreed with behavior of the filters reported in the literature. In the experimental component of this project, a flow loop was developed and tested for measuring particle and fluid flow behavior under shear experienced in pressure driven channel flow. Using seeding particles in the fluid, digital particle image velocimetry (DPIV) was used to measure the fluid motion. Matching the index of refraction of the fluid and particles allowed for full field measurements of the flow. Later experiments added dye to the fluid and a separate camera to image the dye, which allowed independent measurement of the particle bed motion by tracking voids in the imaged dye. The flow loop allowed for direct measurements of the flow field and particle motion, providing unique capabilities for measuring the overall flow behavior in a global fashion. Using this system, experiments were performed with particle beds of several different initial heights and different average flow rates to assess the influence of shear and particle bed height on flow evolution, particle migration, particle fluidization, and particle velocity. Measurement of the fluid velocity field allowed direct measurement of the fluid shear stress applied to the particle bed to indicate the influence of this important parameter on the particle bed evolution. Likewise, measurement of the instantaneous motion of the particles in the particle bed allowed for correlation of the particle bed behavior with the associated velocity field. Finally, the full-field, time varying nature of the results allowed for comparison with and validation of computational results produced for this project. The ability to directly measure the fluid velocity field with both stationary and moving particles together with the ability to measure instantaneous particle motion provided unprecedented fidelity of experimental results and an ability to quantify the fundamental physics driving the fluid particle interactions. Considering the fluid velocity measurements, a number of interesting observations were made including (1) fluid shear and penetration of the moving fluid layer into the particle bed can change when particles begin moving (i.e., shear stress hysteresis), which can make determination of the critical Shields number challenging, and (2) the shear observed at the particle bed surface is generally much different than that estimated from the fluid velocity profile in the log layer away from the particle bed. Additionally, the experimental results allowed comparison of global flow features such as the fluid velocity profile and fluctuating velocity component profiles with computational results. Generally, computational results showed good agreement with experimental measurements. Selected Publications: Abdelhamid, Y., and El Shamy, U. (2014). "Multiscale Modeling of Soil-Fluid-Structure Interaction," Geo-Congress 2014, GSP 234, Atlanta, GA. Abdelhamid, Y. and El Shamy, U. (2014). "Pore-scale Modeling of Surface Erosion in a Particle Bed." International Journal for Numerical and Analytical Methods in Geomechanics, 38(2), pp. 142-166. Abdelhamid, Y., and El Shamy, U. (2013). "A Fundamental Study on Surface Erosion in Levee Systems," Geo-Congress 2013, GSP 231, San Diego, CA. El Shamy, U., Abdelhamid, Y., Krueger, P., and Zhongfeng, A. (2012). "A Particle-Based Model of Flow-Induced Scour," Proceedings, the 6th International Conference on Scour and Erosion, Paris, France.