A hybrid method based on three-dimensional finite element idealization in the near field and a semi-analytic scheme using the principles of wave propagation in multilayered half space in the far field is proposed for dynamic soil-structure interaction analysis. It combines the advantages of both the numerical and analytical techniques. A structure resting on the surface or embedded in a multilayered soil-medium and subjected to vertically propagating plane waves is analyzed by using the method. An important aspect of the soil-structure interaction problem considered is the presence of waves scattered from the soil-structure interface and geometrical irregularities in the soil. The dynamic response of an embedded structure of rectangular cross section to a vertically propagating compressional pulse is solved as an example problem. The proposed method is verified by comparison of its predictions with those from a finite element procedure with absorbing boundaries, and from an analytical solution. The results from the hybrid method compare well with those from the other two, with closer correlation between the hybrid and analytical methods.

The most popular method in dynamic soil-structure interaction analysis is the finite element method. The versatility in problems involving different materials and complex geometries is its main advantage, yet FEM can not simulate unbounded domains completely. A hybrid method is proposed in this research, which models the near field (structure and surrounding soil) by finite elements and the far field by a continuum approach. The system is excited by monochromatic body waves (P and SV) propagating with oblique incidence and harmonic time dependence. The far field problem is solved using Thomson-Haskell formulation associated with the delta matrix technique. The soil profile does not contain any soft layer and the layers are assumed to be linearly elastic, isotropic, homogeneous and perfectly bonded at the interfaces. Two-dimensional (in-plane) formulation is considered and the analysis is performed on both k- and o-planes through time and spatial Fourier transforms of the field equations and boundary conditions. (Abstract shortened with permission of author.)

Growing concerns over climate change is driving research aimed at determining ways of retaining soil carbon (C) within managed northeastern forests. Earthworms are exotic to the state of Vermont and the current extent of earthworm community presence in the state's forests, as well as the long term impact these communities will have on soil C storage, is still unknown. Current research suggests that earthworms have conflicting effects on the C cycle of soils, simultaneously enhancing mineralization through soil mixing, while protecting C through the stabilization of microaggregate (mA) structures. The mA soil fraction represents a pool of physically stable structures capable of maintaining occluded C for long periods of time. To date, studies investigating earthworm effects on mA formation and occluded C have rarely been done in undisturbed forest soils. Earthworms were found in 10 of 18 forest sites utilized in a statewide Vermont earthworm survey, and community presence correlated with thinner forest floor depths. For 8 sites, the impact of earthworm presence on the quantity of C within water stable mA was investigated. Earthworm presence correlated with greater total C in the top 20 cm of mineral soil, highlighting the relocation of the forest floor noted in all 18 sites. A small, but significant, decrease was noted in the proportion of bulk soil mA, however through C enrichment from the forest floor, there was a significant increase in the pool of mA-associated C. A paired mesocosm study was also conducted, utilizing the endogeic earthworm species Aporrectodea tuberculata, placed in an earthworm-free, undisturbed forest soil. Findings from this study corroborated the correlations noted in the field with significant, though small, decreases in the proportion of bulk soil mA. The larger macroaggregate fraction was increased by about 4 times under earthworm influence. The C enrichment of mA structures occluded within the macroaggregate fraction accounted for approximately 95% of the total increase in mA-associated C, and 50% of the total C integrated into the mineral soil. It can be assumed that the C preferentially occluded within the mA structures by earthworm ingestion will experience longer mean residence time relative to bulk soil C. We conclude that, for the forest soils investigated, earthworm communities decreased the proportion of mA slightly but that the pool of physically stabilized C was increased through mA turnover. Forest soils usually experience low soil mixing and therefore typically contain high proportions of mA, though the quantity of C within these structures varies. Due to mA restructuring within the earthworm gut, it is unlikely that earthworm community expansions will alter the proportion of mA in forest soils, however the quantity of C present within these structures is likely to increase. The individual site investigated in the controlled study was particularly low in mineral soil C, and therefore the long-term presence of earthworms would likely result in an increase to mineral C storage. However, this result may not be applicable for forests with high levels of mineral soil C prior to earthworm invasion.

Ce travail de thèse s’articule autour de la problématique de l’évolution de la première fréquence propre d’une éolienne en mer au cours de sa durée d’exploitation. Ce type de structure élancée est soumis à des chargements cycliques et dynamiques tels que le vent, les vagues, les courants marins, la rotation des pales. Ces chargements ont des fréquences de sollicitation proches de la première fréquence propre de l’éolienne, celle-ci s’inscrivant dans un intervalle restreint entre la fréquence du rotor et celle des pales. Afin d’éviter tout phénomène de résonance, il convient d’évaluer précisément à la fois la première fréquence propre de l’éolienne juste après son installation ainsi que son évolution au cours de son exploitation. Ces deux problématiques ont été considérées dans cette étude.Dans ce contexte, un travail expérimental a été réalisé autour de deux modèles physiques 1g d’éolienne en mer fondée sur monopieu et installée dans un massif de sable de Fontainebleau. En se basant sur une modélisation de la fondation à l’aide d’un ensemble de ressorts, latéral et de torsion, une série d’essais a été réalisée afin d’évaluer à la fois la raideur de ces ressorts, mais aussi la fréquence propre du modèle physique considéré. Cette méthode a été comparée à des méthodes existantes et a aussi permis le développement de méthodes originales. Réaliser les essais sur deux modèles à échelle différente (1/60 et 1/120) a permis d’adapter les résultats obtenus aux cas d’une éolienne à échelle 1. Enfin, dans le cadre de l’étude de l’évolution de la structure au cours de son exploitation, des essais de sollicitations cycliques ont été réalisés. Une étude paramétrique se concentrant sur l’influence de la fréquence de la sollicitation et de la force globale appliquée a été proposée. Les résultats obtenus permettent d’analyser le comportement des modèles réduits, soumis à un maximum d’un million de cycles, en se concentrant sur l’évolution du déplacement, de la rotation et de la fréquence propre de ces structures. Ces résultats sont comparés aux limites imposées pour l’état limite de service d’une éolienne grandeur réelle / This PhD thesis deals with the main issue which is the evolution of the first natural frequency of an offshore wind turbine. These slender structures are submitted to cyclic and dynamic loads such as wind, waves, currents, and the blade rotations. The frequencies related to these loads are close to the first natural frequency of the turbine, which lay in a narrow interval between the frequencies of the rotor and the blades. In order to avoid any resonance phenomenon, one needs a precise evaluation of not only the natural frequency of the wind turbine after its installation but also its evolution during the operation of the turbine. These two issues are considered in this work.In this context, an experimental work was developed considering two 1g physical models of an offshore wind turbine with a monopile foundation installed in Fontainebleau sand. Based on the modelling of the foundation as a set of lateral and rotational springs, an experimental program was developed in order to evaluate the stiffness of these springs and the natural frequency of the scaled models. This method was first compared to the existing methods and has allowed to develop some original methods to evaluate the natural frequency of the considered scaled model. The tests, conducted on two physical models with different scales (1/60 and 1/120), also allowed us to adapt the obtained results to a real offshore wind turbine. As a part of the study of the turbine’s evolution during its operation, cyclic load tests were conducted. A parametric study is proposed with a focus on the influence of the load’s frequency and its amplitude. The obtained results allowed us to analyze the behavior of the scaled models submitted up to one million cycles considering, mainly, the evolution of the displacement, the rotation, and the natural frequency of the structures. These results were then compared to the limits imposed by the serviceability limit state of a real offshore wind turbine

Minimization of Construction Costs of Substation Grounding Grids: In every electrical installation, one of the most important aspects is adequate grounding; in particular, the grounding of high-voltage substations to protect people and equipment in the event of an electrical fault. Well-designed grounding systems ensure the performance of power systems and safety of personnel. It is desirable that the substation grounding provides a near zero resistance to remote earth. The prevailing practice of most utilities is to install a grid of horizontal ground electrodes (buried bare copper conductors) supplemented by a number of vertical ground rods connected to the grid, and by a number of equipment grounding mats and interconnecting cables. The grounding grid provides a common ground for the electrical equipment and for all metallic structures at the station. It also limits the surface potential gradient. Currently the IEEE 80-2000 standard for substations grounding limits the determination of the grounding parameters (namely step, touch and ground potential rise) to that of a uniform soil model unless the Sunde graphical method is used. With the Sunde graphical method, it relies on interpretation to obtain a two layer soil model. Without the use of the graphical method, the IEEE 81-1983 has several empirical equations that can be used for the two layer model; however, these equations rely on the use of images which retard the speed of calculations to the point where the overall optimization of the grounding grid (with respect to size and shape) has yet to be determined. The goal of the thesis was to improve upon the current restrictions for the grounding grid design, while minimizing the material (i.e., copper conductors) and installation costs of a grid. The first part of the research examined previous work through a combination of literature review, mathematical computations, and field measurements to validate the theoretical aspects of grid design. The thesis introduces an optimized uniform and two-layer soil with fast accurate calculations directly from soil measurements without the use of graphical methods or the use of complex image theory. Next, the thesis develops enhanced grounding parameter equations using Simpson’s Rule of integration. The final part of the thesis demonstrates how it is possible to optimize the configuration of the grounding grid itself, minimizing costs, and yet still achieving a safe installation. This is the first time such an optimization is possible, and it is made possible by the techniques developed in this thesis. The techniques are applied to existing real-world grid designs, and the results obtained show the effectiveness of the method in reducing construction costs. This thesis shows how these construction and material savings are realized by utilizing a process whereby the grounding design minimizes the overall cost. The overall contribution of this thesis is the optimization of the grounding grid design by eliminating the current restrictions found in the IEEE standards 80 and 81, respectively, and offering an optimized grounding system design, starting from the soil model to the actual grounding design itself.

Problems associated with shrink-swell soils are well known geotechnical problems that have been studied and researched by many geotechnical researchers for many decades. Potentially shrink-swell soils can be found almost anywhere in the world especially in the semi-arid regions of the tropical and temperate climate. Foundation slabs on grade on shrink-swell soils are one of the most efficient and inexpensive solutions for this kind of problematic soil. It is commonly used in residential foundations or any light weight structure on shrink-swell soils. Many design methods have been established for this specific problem such as Building Research Advisory Board (BRAB), Wire Reinforcement Institute (WRI), Post- Tensioning Institute (PTI), and Australian Standards (AS 2870) design methods. This research investigates most of these methods, and then, proposes a moisture diffusion soil volume change model, a soil-weather interaction model, and a soil-structure interaction model. The proposed moisture diffusion soil volume change model starts with proposing a new laboratory test to determine the coefficient of unsaturated diffusivity for intact soils. Then, it introduces the development of a cracked soil diffusion factor, provides a chart for it, and explains a large scale laboratory test that verifies the proposed moisture diffusion soil volume change model. The proposed soil-weather interaction model uses the FAO 56-PM method to simulate a weightless cover performance for six cities in the US that suffer significantly from shallow foundation problems on shrink-swell soils due to seasonal weather variations. These simulations provide more accurate weather site-specific parameters such as the range of surface suction variations. The proposed weather-site specific parameters will be input parameters to the soil structure models. The proposed soil-structure interaction model uses Mitchell (1979) equations for moisture diffusion under covered soil to develop a new closed form solution for the soil mound shape under the foundation slab. Then, it presents a parametric study by carrying out several 2D finite elements plane strain simulations for plates resting on a semiinfinite elastic continuum and resting on different soil mounds. The parametric study outcomes are then presented in design charts that end with a new design procedure for foundation slabs on shrink-swell soils. Finally, based on the developed weather-soil-structure interaction models, this research details two procedures of a proposed new design method for foundation slabs on grade on shrink-swell soils: a suction based design procedure and a water content based design procedure.

Thesis (Ph.D.) -- University of Texas at Arlington, 2008.

Thesis (M.S.)--West Virginia University, 2003. / Title from document title page. Document formatted into pages; contains vii, 137 p. : ill. (some col.), maps (some col.). Includes abstract. Includes bibliographical references (p. 71-76).

Thesis (M. Phil.)--University of Hong Kong, 1988.

published_or_final_version / Geography and Geology / Master / Master of Philosophy