Chung R. Song

Computation of Elastic Properties of Portland Cement Using Molecular Dynamics

There is a growing interest in relating nanostructures to the macro properties of engineering materials such as composites and cement materials. Better understanding of structure and elastic properties of nanoparticles in concrete by modeling and experiment could lead to nanoengineered concrete with much better performance and energy efficiency. In this study, the molecular dynamics (MD) atomistic simulation technique was applied to study the elastic properties of major portland cement compounds (i.e., alite, belite, and aluminate). Applicability of three commonly used force fields: COMPASS, Universal force field (UFF), and Dreiding were evaluated in the MD simulation. The combination of different simulation cell sizes and force fields was investigated. MD simulation results of cement were comparable to the experimental data. The results could be used as nanoparticle properties for multiscale modeling of concrete, cementitious composites, and aggregate.

Microstructure consideration with plastic spin and multiple back-stresses for large strain problems in soils

Abstract The effect of sub-structure changes in geo-materials is investigated here by incorporating the plastic spin in the anisotropic modified Cam Clay Model. The plastic spin is formulated as a function of internal variables in the constitutive equation. Dual back-stresses are incorporated as internal variables. The short range back-stress is used for the intragrain back-stress while the long range back-stress is used for the inter-grain back-stress. A linear evolution equation is used for the short range back-stress, and a non linear evolution equation is used for the long range back-stress. Individual evolution equations are used for the plastic spins for the short range back-stresses and for the long range back-stresses, respectively. To simulate the behavior of saturated soils at large strains, the coupled theory of mixtures with elasto-plasticity in an updated Lagrangian reference frame is adopted. The results showed that the effects of the long range back-stress are more significant for the large strain region while that of short range back-stress are more affective for the small strain region.

Reducing Erosion of Earthen Levees Using Engineered Flood Wall Surface

Erosion was one of the major causes for the failure of New Orleans levee system during Hurricane Katrina. Protection of flood walls from erosion failure can be achieved in many different ways. This study consisted of experimental research to develop engineered flood wall surfaces that can reduce the erosion energy of the plunging water before the water hits the levee surface, so that the flood protection system becomes more resilient. Test results were compared to hydrodynamics simulation results by using FLOW3D. The results revealed that the erosion resistance of levees can be substantially reduced by providing protective structures at the surface of the flood walls. An effectively designed protective structure could reduce the erosion depth as much as 40% and extend the erosion time as much as 400%.

Erosion Study of New Orleans Levee Materials Subjected to Plunging Water

During Hurricane Katrina, overtopping water caused erosion and subsequent failure of several sections of I-type flood walls in New Orleans. Erosion stemmed from the kinetic energy of water falling from the top of the flood wall, unlike the typical surface erosion caused by shear flow. This study evaluated the effects of important parameters of levee soils—fines content, degree of compaction (DOC), clay mineralogy, and water content in relation to the erosion behavior of New Orleans levees subjected to the plunging water. In general, test results showed that a higher fines content contributed to greater erosion resistance. The trend became unclear when fines content exceeded 20–25%. A higher degree of compaction did not necessarily contribute to greater erosion resistance. Underwater soaked soils showed much less erosion resistance than nonsoaked soils. Soils containing expansive clay minerals showed less erosion resistance than soils containing nonexpansive clay minerals.

Evaluation of a T-Wall Section in New Orleans Considering 3D Soil-Structure Interaction

T-walls in New Orleans survived Hurricane Katrina whereas I-walls obviously failed in several sections. However, it is still unclear whether these T-walls truly survived the hurricane with a fair amount of safety margins or barely survived it with undetected damages. The initial design of T-walls was based on simplified loading conditions with limited consideration of soil-structure interaction. In this study, three-dimensional (3D) numerical analyses were conducted, incorporating realistic loading conditions and soil-structure interactions but with time-saving techniques to evaluate the detailed behavior of T-walls. This paper addressed the procedure of innovative 3D numerical analyses and important findings by using special structural elements in FLAC3D. From this study, T-walls were found to have adequate stress levels in H-piles and concrete walls. However, it showed that the major factor that may cause the instability of the T-wall was the slope instability-type unbalanced force. This unbalanced forc...

Multi-scale non-local approach for geomaterials

Abstract A thermodynamically consistent multi-scale, rate dependent, non local approach is developed in this work for geo-materials in conjunction with the anisotropic modified Cam Clay model. The gradient for the micro-structure is incorporated through the micro level gradient of the back-stress and volumetric strain while the gradient for macro-structure is incorporated through the macro level gradient of back-stress and volumetric plastic strain. Gradient results in the regularization of the local behavior. Visco-plasticity is also incorporated for an additional regularization of the local behavior. Therefore, the effects of two separate regularizations are naturally separated. The plastic spin is incorporated to separate the effect of micro-structural rotation from the gradient effect. The flow characteristics of the soil is also incorporated in order to separate the viscosity effect from the flow effect. Through this multi scale non local approach, a more realistic simulation of large strain problems such as shear band formation can be achieved.

Effects of Force Field in Molecular Mechanics Simulation of Geo-Materials

Molecular mechanics simulation of geo-materials provides enhanced understanding of fine details of material properties down to nano- or sub nano- scales. The molecular mechanics simulation of geo-materials however, is based on the non- deterministic mechanics due to the fact that the position and time of the electrons are not known simultaneously - so called Einsteins principle of uncertainty. To address this uncertainty, several (empirical) force fields are proposed. Each force field has a unique background and assumptions, and consequently presents different computational results. This study investigated the effects of different force fields (COMPASS, CVFF, PCFF, and Universal) on the predicted elastic properties of quartz sand grains.

Anisotropic Analysis of I-Walls in New Orleans

Many numerical analyses of New Orleans levee and floodwall sections adopted mostly isotropic soil models partly due to the inherent simplicity of isotropic models. However, the isotropic models may not be sufficient to properly address the anisotropic behavior of soils. To overcome this imperfection, this study incorporated an anisotropic Modified Cam Clay model in a commercially available finite difference code, FLAC3D, and analyzed a floodwall and levee section in the 17th St. Canal in New Orleans. The analysis showed that the anisotropic model resulted in a similar overall deformation to the Mohr-Coulomb isotropic model. However, the anisotropic model showed more widely spread yielded elements and higher shear strain gradient in the Lacustrine clay layer, reconfirming that the Lacustrine clay layer played a major role for the failure in the 17th St. Canal. This result also signified that the isotropic Mohr-Coulomb model might be good for evaluating overall behavior in moderate deformation problems, while the anisotropic Modified Cam Clay model was good even for large strain problems where more accurate evaluation of yielding is needed.

Reevaluation of the Gap Formation in the New Orleans Levee System

AbstractIn the aftermath of Hurricane Katrina, studies pointed out that gap formation was related to breaches of many floodwalls in New Orleans. However, it is still necessary to discern whether those gaps were the cause or the result of the failure to justify the application of retrofitting techniques designed to correct gap formation problems. For that purpose, this study conducted numerical and analytical evaluations to investigate the in-depth mechanism of gap formation using an effective stress approach. To this end, it was found that gap was the cause of failure for the London Ave. canal because of the seepage effect; however, it could be the result of failure for the 17th St. canal because of the subsidence of the soft soil layer underneath the levee. When the stiffness and strength of the soft layer were increased considering the geotechnical variability in the numerical analysis for the 17th St. canal, however, it showed that the failure mode changed and gap became the cause of failure, meaning t...

Self-sealing bentonite strip: an effective method to prevent gap development for floodwalls in New Orleans

Many sections of floodwalls in New Orleans, LA were damaged or suffered catastrophic failure as a result of Hurricane Katrina. One of the key triggering mechanisms of failure reported is the gap development between the floodwall and soil. This study developed an effective retrofitting technique to prevent this gap development by introducing a buried layer of self-sealing sand and bentonite mixture. This self-sealing layer was expected to swell fast enough so that it could seal the gap without any time delay and to exert insignificant swelling pressure to the levee so that it did not affect the stability of the levee. Among several mixtures of sand and bentonite, the mixture of 70% sand and 30% bentonite (by dry weight) proved to be a more effective one among laboratory and large scale model tests conducted in this study by swelling fast enough to seal the gap (approximately 10% swelling strain in 2 days), but exerting insignificant swelling pressure (162.03 kPa in 2 days) to the levee. From the numerical analysis using FLAC3D, it was confirmed that the stability of the levee was actually increased by sealing the gap even with the minor swelling pressure from the self-sealing layer. From 1/64th scale centrifuge tests, the wall with the self-sealing layer did not fail at 64 g, while the one without the self-sealing layer failed at 25 g acceleration.