VTechWorks Repository :: Browsing by Author "Dove, Joseph E."

Browsing by Author "Dove, Joseph E."

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Analysis of a Lateral Spreading Case History from the 2007 Pisco, Peru Earthquake
Gangrade, Rajat Mukesh (Virginia Tech, 2013-06-21)
On August 15, 2007, Pisco, Peru was hit by an earthquake of Magnitude (Mw) = 8.0 which triggered multiple liquefaction induced lateral spreads. The subduction earthquake lasted for approximately 100 seconds and showed a complex rupture. From the geotechnical perspective, the Pisco earthquake was significant for the amount of soil liquefaction observed. A massive liquefaction induced seaward displacement of a marine terrace was observed in the Canchamana complex. Later analysis using the pre- and post-earthquake images showed that the lateral displacements were concentrated only on some regions. Despite the lateral homogeneity of the marine terrace, some cross-sections showed large displacements while others had minimal displacements. The detailed documentation of this case-history makes it an ideal case-study for the determination of the undrained strength of the liquefied soils; hence, the main objective of this research is to use the extensive data from the Canchamana Slide to estimate the shear strength of the liquefied soils. In engineering practice, the undrained strength of liquefied soil is typically estimated by correlating SPT-N values to: 1) absolute value of residual strength, or 2) residual strength ratio. Our research aims to contribute an important data point that will add to the current understanding of the residual strength of liquefied soils.
Analyzing internal shearing in compound landslides using MPM
Nissar, Nahmed (Virginia Tech, 2020-06-25)
Landslides cause significant damage worldwide and therefore epitomize the most important problems in geotechnical engineering. Hence, perceiving the mechanics involved in the deformation process of landslides is necessary for risk assessment. In addition to the resistance offered by basal shear surfaces, internal shearing also influences the stability and kinematics of compound landslides. For compound landslides, internal shearing is essential to develop feasible sliding mechanisms. The internal distortion is caused by the formation of shear bands that develop within the sliding mass. The strain localization is generally attributed to slope changes along the basal sliding surface (or topography) that constrain the strain field of the landslide. The development of these internal shear bands also controls the energy dissipation, and its distribution determines the final degradation of the material. This work focuses on the study of internal failure mechanisms that develop in compound landslides. A theoretical model of a compound landslide is numerically analyzed using the Material Point Method (MPM), a state-of-the-art numerical technique appropriate to model large deformation problems. The internal failure pattern is identified for different basal sliding geometries. Based on that, a generalized method is proposed to estimate the internal failure mechanism of bi-planar compound geometries. The material degradation and energy dissipation are evaluated in terms of the accumulated deviatoric strain and the reaction forces exerted by the landslide on a vertical wall. Moreover, preliminary studies are conducted to analyze the use of barriers as a mitigation strategy to counter landslide damage, and their efficiencies are investigated.
Assessment of the Cyclic Strain Approach for the Evaluation of Initial Liquefaction
Rodriguez Arriaga, Eduardo (Virginia Tech, 2017-06-30)
Field-based liquefaction evaluation procedures include the stress-based, strain-based, and energybased based approaches. The existence of a volumetric threshold shear strain, γtv, under which there is no development of excess pore pressures, and the unique relationship between pore pressure ratio and cyclic shear strain, γc, make a compelling argument for using a strain-based approach. However, the cyclic strain approach has not yet been standardized for field evaluations. The primary objective of this thesis is to use published databases of 415 shear-wave velocity and 230 Standard Penetration Test liquefaction field case histories to investigate the performance of the cyclic strain approach for the evaluation of initial liquefaction relative to the cyclic stress approach. Additionally, the concept of the γtv is expressed in terms of the peak ground surface acceleration and defined as the threshold amax. Computing (amax)t could provide a fast and simple evaluation for initial liquefaction, where no liquefaction is expected for a minimum computed (amax)t determined from the case histories. The variant of the strain-based procedure proposed herein avoids the direct need for laboratory cyclic testing by employing pore pressure generation models that are functions of cyclic shear strain, number of equivalent cycles, and relative density to predict initial liquefaction. The results from the proposed procedure are compared with those of the stress-based approach to determine which better matches the field observations of the case histories. It was found that the cyclic strain approach resulted in 70% to 77% correct predictions. In contrast, the cyclic stress approach yielded 87% to 90% correct predictions. The reasons why the predictions were not always correct with the cyclic strain approach are due to inherent limitations of the cyclic strain approach. Most significantly, an inherent and potentially fatal limitation of the strain-based procedure is it ignoring the softening of the soil stiffness due to excess pore pressure in representing the earthquake loading in terms of γc and neqγ.
The Atomic-scale Finite Element Method for Analyzing Mechanical Behavior of Carbon Nanotube and Quartz
Kim, Kyusang (Virginia Tech, 2006-08-22)
The mechanical behavior of discrete atoms has been studied with molecular dynamics whose computational time is proportional to the square of the number of atoms, O(N²). Recently, a faster algorithm, Atomic-scale Finite Element Method (AFEM) with computational time proportional to the number of atoms, O(N), had been developed. The main idea of AFEM, compared with conventional finite element method is to replace nodes with atoms and elements with electric forces between atoms. When interpreting a non-linear system, it is necessary to use an iteration scheme. A simulation of molecular dynamics based on the Verlet's method was conducted in order to validate AFEM in one dimension. The speed of AFEM was investigated in one and two dimensional atomic systems. The results showed that the computational time of AFEM is approximately proportional to the number of atoms, and the absolute computation time appears to be small. The frameworks of AFEM not only for multi-body potential but also pair potential are presented. Finally, AFEM was applied to analyze and interpret the mechanical behavior of a carbon nanotube and a quartz. The buckling behavior of carbon nanotube showed a good agreement with the results illustrated in the original literature.
Back-analysis methods for optimal tunnel design
Vardakos, Sotirios (Virginia Tech, 2007-01-24)
A fundamental element of the observational method in geotechnical engineering practice is the utilization of a carefully laid out performance monitoring system which provides rapid insight of critical behavioral trends of the work. Especially in tunnels, this is of paramount importance when the contractual arrangements allow an adaptive tunnel support design during construction such as the NATM approach. Utilization of measurements can reveal important aspects of the ground-support interaction, warning of potential problems, and design optimization and forecasting of future behavior of the underground work. The term back-analysis involves all the necessary procedures so that a predicted simulation yields results as close as possible to the observed behavior. This research aims in a better understanding of the back-analysis methodologies by examining both simplified approaches of tunnel response prediction but also more complex numerical methods. Today a wealth of monitoring techniques is available for tunnel monitoring. Progress has also been recorded in the area of back-analysis in geotechnical engineering by various researchers. One of the most frequently encountered questions in this reverse engineering type of work is the uniqueness of the final solution. When possible errors are incorporated during data acquisition, the back analysis problem becomes formidable. Up to the present, various researchers have presented back-analysis schemes, often coupled with numerical methods such as the Finite Element Method, and in some cases the more general approach of neural networks has been applied. The present research focuses on the application of back-analysis techniques that are applicable to various conditions and are directly coupled with a widely available numerical program. Different methods are discussed and examples are given. The strength and importance of global optimization is introduced for geotechnical engineering applications along with the novel implementation of two global optimization algorithms in geotechnical parameter identification. The techniques developed are applied to the back-analysis of a modern NATM highway tunnel in China and the results are discussed.
Building, Updating and Verifying Fracture Models in Real Time for Hard Rock Tunneling
Decker, Jeramy Bruyn (Virginia Tech, 2007-04-20)
Fractures and fracture networks govern the mechanical and fluid flow behavior of rock masses. Tunneling and other rock mechanics applications therefore require the characterization of rock fractures based on geological data. Field investigations produce only a limited amount of data from boreholes, outcrops, cut slopes, and geophysical surveys. In tunneling, the process of excavation creates a priceless opportunity to gather more data during construction. Typically, however, these data are not utilized due to the impedance of sampling and analysis on the flow of construction, and safety concerns with sampling within unlined tunnel sections. However, the use of this additional data would increase the overall safety, quality, and cost savings of tunneling. This study deals with several aspects of the above, with the goal of creating methods and tools to allow engineers and geologists to gather and analysis fracture data in tunnels without interrupting the excavation and without compromising safety. Distribution-independent trace density and mean trace length estimators are developed using principles of stereology. An optimization technique is developed utilizing Differential Evolution to infer fracture size and shape from trace data obtained on two or more nonparallel sampling planes. A method of producing nearly bias free empirical trace length CDF's is also introduced. These new methods and tools were validated using Monte Carlo simulations. A field study was conducted in an existing tunnel allowing the above methods and tools to be further validated and tested. A relational database was developed to aid in storage, retrieval, and analysis of field data. Fracture models were built and updated using fracture data from within the tunnel. Utilization of state of the art imaging techniques allowed for remote sampling and analysis, which were enhanced by the use of 3d visualization techniques.
Characterization of High Porosity Drainage Layer Materials for M-E Pavement Design
Zhang, Yinning (Virginia Tech, 2015-02-12)
The objective of this study is to characterize the properties of typically adopted drainage layer materials in VA, OK, and ID. A series of laboratory tests have been conducted to quantify the volumetric properties, permeability and mechanical properties of the laboratory-compacted asphalt treated and cement treated permeable base specimens. The modified test protocols to determine the dynamic modulus of the drainage layer materials have been provided, which can be followed to determine the dynamic modulus of the drainage layers as level 1 input in Mechanistic-Empirical (M-E) pavement design. The measured dynamic moduli have been used to calibrate the original NCHRP 1-37A model to facilitate its application on drainage layer materials for prediction of the dynamic modulus as level 2 input. The compressive strength of the cement treated permeable base mixture of different air void contents has also been quantified in laboratory. Numerical simulations are conducted to investigate the location effects and the contribution of the drainage layer as a structural component within pavement. The optimal air void content of the drainage layer is recommended for Virginia, Oklahoma and Idaho based on the laboratory-determined permeability and the predicted pavement performances during 20-year service life.
Characterization of Soft Clay and Clay-tire Interaction for the Prediction of Ground Mobility
Pandit, Rashna (Virginia Tech, 2023-08-22)
Predicting tire performance on soft, fine-grained soils is required for many off-road explorations in the military, mining, agricultural, and earth-moving sectors. However, the mobility in deformable material is extremely challenging, especially in the presence of water. Although there is a significant amount of research on terrains such as sands, there is a lack of research on fine-grained soils. This research is part of a bigger project that presents a novel approach to improve the mobility of off-road vehicles on wet deformable soils. The approach integrates experimental data from small-scale soil testing, large-scale soil-tire interaction testing, and advanced physics-based numerical simulation techniques. In particular, this thesis attempts to characterize the clay-tire interface by conducting large-scale direct shear tests. In addition to clay-tire contact friction, the properties and strength parameters of the soft clay are determined by conducting various index properties and advanced tests. The testing program accounts for different stresses, loading conditions, and boundary conditions, decided taking into account real field conditions. The results from all these experiments will be used to calibrate and validate the material constitutive models required for the development of a mobility predictive numerical model. Overall, this study contributes to the development of more advanced and accurate terramechanics models that involve deformable terrains like soft clays.
Clay Mineral Image Collection for Education in Geotechnical Engineering and the Earth Sciences
Stone, Gordon; Dove, Joseph E.; Han, Nizhou; Dove, Patricia M. (2015-12-03)
This file contains a collection of scanning electron microscope images of Kaolinite and Bentonite pure clay minerals, and the fine portion of a natural soil.
Computational Analysis of Asphalt Binder based on Phase Field Method
Hou, Yue (Virginia Tech, 2014-04-29)
The mechanical performance evaluation of asphalt binder has always been a challenging issue for pavement engineers. Recently, the Phase Field Method (PFM) has emerged as a powerful computational tool to simulate the microstructure evolution of asphalt binder. PFM analyzes the structure from the free energy aspect and can provide a view of the whole microstructure evolution process. In this dissertation, asphalt binder performance is analyzed by PFM in three aspects: first, the relationship between asphalt chemistry and performance is investigated. The components of asphalt are simplified to three: asphaltene, resin and oil. Simulation results show that phase separation will occur under certain thermal conditions and result in an uneven distribution of residual thermal stress. Second, asphalt cracking is analyzed by PFM. The traditional approach to analyze crack propagation is Classic Fracture Mechanics first proposed by Griffith, which needs to clearly depict the crack front conditions and may cause complex cracking topologies. PFM describes the microstructure using a phase-field variable which assumes positive one in the intact solid and negative one in the crack void. The fracture toughness is modeled as the surface energy stored in the diffuse interface between the intact solid and crack void. To account for the growth of cracks, a non-conserved Allen-Cahn equation is adopted to evolve the phase-field variable. The energy based formulation of the phase-field method handles the competition between the growth of surface energy and release of elastic energy in a natural way: the crack propagation is a result of the energy minimization in the direction of the steepest descent. Both the linear elasticity and phase-field equation are solved in a unified finite element frame work, which is implemented in the commercial software COMSOL. Different crack mode simulations are performed for validation. It was discovered that the onset of crack propagation agrees very well with the Griffith criterion and experimental results. Third, asphalt self-healing phenomenon is studied based on the Atomic Force Microscopy (AFM) technology. The self-healing mechanism is simulated in two ways: thermodynamic approach and mechanical approach. Cahn-Hilliard dynamics and Allen-Cahn dynamics are adopted, respectively.
Deep Energy Foundations: Geotechnical Challenges and Design Considerations
Abdelaziz, Sherif Lotfy Abdel Motaleb (Virginia Tech, 2013-05-07)
Traditionally, geothermal boreholes have utilized the ground energy for space heating and cooling. In this system, a circulation loop is placed in a small-diameter borehole typically extending to a depth of 200-300 ft. The hole is then backfilled with a mixture of sand, bentonite and/or cement. The loop is connected to a geothermal heat pump and the fluid inside the loop is circulated. The heat energy is fed into the ground for cooling in the summer and withdrawn from the ground for heating in the winter. Geothermal heat pumps work more efficiently for space heating and cooling compared to air-source heat pumps. The reason is ground-source systems use the ground as a constant temperature source which serves as a more favorable baseline compared to the ambient air temperature. A significant cost associated with any deep geothermal borehole is the drilling required for installation. Because Energy Piles perform the dual function of exchanging heat and providing structural support, and are only installed at sites where pile foundations are already required, these systems provide the thermal performance of deep geothermal systems without the additional drilling costs. Low maintenance, long lifetime, less variation in energy supply compared to solar and wind power, and environmental friendliness have been cited as additional Energy Pile advantages. Case studies show that they can significantly lower heating/cooling costs and reduce the carbon footprint. Energy cost savings for typical buildings outfitted with Energy Piles could be as much as 70 percent. The use of Energy Piles has rapidly increased over the last decade, especially in Europe where more than 500 applications are reported. Primary installations have been in Germany, Austria, Switzerland and United Kingdom. Notable projects include the 56-story high Frankfurt Main Tower in Germany, Dock E Terminal Extension at Zurich International Airport in Switzerland and the One New Change building complex in London U.K. Energy piles have seen very little use in the North America, only a handful of completed projects are known; Marine Discovery Center in Ontario, Canada, Lakefront Hotel in Geneva, New York and the Art Stable building in Seattle, Washington. Energy Piles are typically installed with cast-in-place technology (i.e. drilled shafts, continuous flight auger piles, micropiles etc.) while some driven pile applications are also reported. Other types of geotechnical structures in contact with the ground, such as shallow foundations, retaining walls, basement walls, tunnel linings and earth anchors, also offer significant potential for harnessing near-surface geothermal energy. Energy Pile design needs to integrate geotechnical, structural and heat exchange considerations. Geotechnical characteristics of the foundation soils and the level of the structural loads are typically the deciding factors for the selection and dimensioning of the pile foundations. The geothermal heat exchange capacity of an Energy Pile is a key parameter to be considered in design. Thermal characteristics of the ground as well as the heating and cooling loads from the structure need to be considered for the number of piles that will be utilized as heat exchangers. Therefore, the thermal properties of the site need to be evaluated for an Energy Pile application in addition to the traditional geotechnical characterization for foundation design. Energy Piles bring new challenges to geotechnical pile design. During a heat exchange operation, the pile will expand and contract relative to the soil as heat is injected and extracted, respectively. These relative movements have the potential to alter the shear transfer mechanism at the pile-soil interface. Furthermore, the range of temperature increases near the pile surface, though limited by practical operational guidelines, can have a significant effect on pore pressures generation and soil strength. This dissertation provides answers for several research questions including the long-term performance of Energy Piles, the applicability of the thermal conductivity tests to Energy Piles. Furthermore, it presents the results and a detailed discussion about the full scale in-situ thermo-mechanical pile load test conducted at Virginia Tech.
The design and construction of a large plane strain apparatus for testing reinforced soil specimens
Dove, Joseph E. (Virginia Polytechnic Institute and State University, 1986)
The increased popularity of using reinforcement in embankment fills and slopes has created a need for studies of the interaction between the reinforcement and surrounding soil to provide an improved basis for limit equilibrium design methods. The purpose of this study was to develop a plane strain triaxial apparatus and associated equipment to study the interaction of soil and reinforcement. The plane strain apparatus was designed to model an element of soil situated along a slip surface in a reinforced embankment. The device constructed is capable of testing a sample 9.4 inches square and 23 inches high with full-size reinforcing materials included. A 30,000-pound capacity load frame and a 20,000-pound capacity electric load cell were also constructed during this study. Drawings of the equipment constructed and a design method for the load cell are included. Preliminary tests on unreinforced and reinforced samples under a confining pressure of 10 psi were performed to check the equipment operation. These tests show that the equipment functions as planned. Modifications desirable to improve the performance of the equipment are given.
Design of Bridging Layers in Geosynthetic-Reinforced Column-Supported Embankments
Smith, Miriam E. (Virginia Tech, 2005-07-08)
Column-supported geosynthetic-reinforced embankments have great potential for application in soft ground conditions when there is a need to accelerate construction and/or protect adjacent facilities from the settlement that would otherwise be induced by the new embankment load. The columns in column-supported embankments can be driven piles, vibro-concrete columns, deep-mixing-method columns, stone columns, or any other suitable type of column. A bridging layer consisting of several feet of sand or sand and gravel is also used to help transfer the embankment load to the columns. Geosynthetic reinforcement is often employed in bridging layers to enhance load transfer to the columns and increase the spacing between columns. Several methods have been developed to calculate the load on the geosynthetic reinforcement, but the calculated loads differ by over an order of magnitude in some cases, and there is not agreement on which method is correct. In this research, a new method was developed for calculating the load on the geosynthetic reinforcement. The new method employs one of the existing mechanistically-based approaches, and combines it with consideration of the stiffnesses of the embankment, geosynthetic, column, and subgrade soil. The new method was verified against the results of a large numerical parameter study, for which the numerical procedures themselves were verified against closed-form solutions for membranes, pilot-scale experiments, and instrumented field case histories. The results of the numerical analyses and the new calculation procedure indicate that the net vertical load on the portion of the geosynthetic reinforcement between columns increases with increasing clear spacing between columns and increasing geosynthetic stiffness. The net vertical load on the geosynthetic decreases with increasing stiffness and strength of the foundation and embankment soils and with increasing elevation of the geosynthetic above the top of the columns or pile caps. A key finding of the research is that, if the subgrade support is good, geosynthetic reinforcement does not have a significant effect on system performance. The new calculation procedure is implemented in an easy-to-use spreadsheet, and recommendations for designing geosynthetic-reinforced bridging layers are provided.
Development of a Decision Support Tool for Identifying Appropriate Means and Methods for Locating Underground Utility Assets
Hutchins, Lewis Anthony (Virginia Tech, 2009-12-15)
The location of utilities buried beneath the built environment has always been a concern for those conducting work that involves excavation or the placement of items into the ground. Whether physically removing material or driving piles, posts, and more, the potential for accidental utility strikes is increasing with the movement of more traditional utilities from above ground to below. Also, the addition of utilities and new technology in underground spaces that hasn't existed in the past, such as fiber optics and more highspeed telecommunication lines, is occupying more space. Traditional methods of surveying, in combination with surface geophysics and the development and improvement of processes and technologies to track the location of buried assets led to the engineering services category termed Subsurface Utility Engineering (SUE). In order to aid utility engineers and consultants who are responsible for the collection of utility data this research aimed to help identify a way to compare the various technologies and incorporate information about the individual project in order to choose the most appropriate locating method for a project with a defined set of parameters. The result was the development of standard evaluation forms that can be sent to technology vendors and consultants to evaluate the performance and limitations of a technology. This data can then be compiled into a database located within an Excel-based program created to compare the technologies. The program, VT PALMS (Virginia Tech Program for Asset Locating Method Selection), consists of the performance and economic databases, a project information Page sheet, and the results of the comparison of each technology in the database to the information on the project information sheet. The results are presented in three ways; 1) a speedometer chart with a needle that indicates the percentage of the parameters used in the database that are compatible - also referred to as the Reliability Factor, 2) a matrix view that indicates the parameters where a potential conflict may exist, and 3) an economic indicator that shows the comparable cost of using each technology listed in the database.
Development of an Energy-based Liquefaction Evaluation Procedure
Ulmer, Kristin Jane (Virginia Tech, 2020-01-20)
Soil liquefaction during earthquakes is a phenomenon that can cause tremendous damage to structures such as bridges, roads, buildings, and pipelines. The objective of this research is to develop an energy-based approach for evaluating the potential for liquefaction triggering. The current state-of-practice for the evaluation of liquefaction triggering is the "simplified" stressbased framework where resistance to liquefaction is correlated to an in situ test metric (e.g., normalized standard penetration test N-value, N1,60cs, normalized cone penetration tip resistance, qc1Ncs, or normalized small strain shear wave velocity, Vs1). Although rarely used in practice, the strain-based procedure is commonly cited as an attractive alternative to the stress-based framework because excess pore pressure generation (and, in turn, liquefaction triggering) is more directly related to strains than stresses. However, the method has some inherent and potentially fatal limitations in not being able to appropriately define both the amplitude and duration of the induced loading in a total stress framework. The energy-based method proposed herein builds on the merits of both the stress- and strain-based procedures, while circumventing their inherent limitations. The basis of the proposed energy-based approach is a macro-level, low cycle fatigue theory in which dissipated energy (or work) per unit volume is used as the damage metric. Because dissipated energy is defined by both stress and strain, this energy-based method brings together stress- and strain-based concepts. To develop this approach, a database of liquefaction and nonliquefaction case histories was assembled for multiple in situ test metrics. Dissipated energy per unit volume associated with each case history was estimated and a family of limit-state curves were developed using maximum likelihood regression for different in situ test metrics defining the amount of dissipated energy required to trigger liquefaction. To ensure consistency between these limit-state curves and laboratory data, a series of cyclic tests were performed on samples of sand. These laboratory-based limit-state curves were reconciled with the field-based limit-state curves using a consistent definition of liquefaction.
Digital Mix Design for Performance Optimization of Asphalt Mixture
Li, Ying (Virginia Tech, 2015-03-27)
Asphalt mix design includes the determination of a gradation, asphalt content, other volumetric properties, the evaluation of mechanical properties and moisture damage potentials. In this study, a computational method is developed to aid mix design. Discrete element method (DEM) was used to simulate the formation of skeleton and voids structures of asphalt concrete of different gradations of aggregates. The optimum gradation could be determined by manipulating the particle locations and orientations and placing smaller particles in the voids among larger particles. This method aims at an optimum gradation, which has been achieved through experimental methods. However, this method takes the mechanical properties or performance of the mixture into consideration, such as inter-aggregate contacts and local stability. A simple visco-elastic model was applied to model the contacts between asphalt binder and aggregates. The surface texture of an aggregate particle can be taken into consideration in the inter-particle contact model. The void content before compactions was used to judge the relative merits of a gradation. Once a gradation is selected, the Voids in Mineral Aggregate (VMA) can be determined. For a certain air void content, the mastics volume or the binder volume or the asphalt content can be determined via a digital compression test. The surface area of all the aggregates and the film thickness can be then calculated. The asphalt content can also be determined using an alternative approach that is based on modeling the inter-particle contact with an asphalt binder layer. In this study, considering the necessity of preservation of the compaction temperature, the effect of various temperatures on Hot Mix Asphalt (HMA) samples properties has been evaluated. As well, to evaluate the effect of this parameter on different grading, two different grading have been used and samples were compacted at various temperatures. Air voids also influence pore water pressure and shrinkage of asphalt binder and mixture significantly. The shrinkage is measured on a digital model that represents beams in a steel mold and is defined as the linear autogenous deformation at horizontal direction.
Digital Simulative Test of Asphalt Mixtures Using Finite Element Method and X-Ray Tomography Images
Wang, Yongping (Virginia Tech, 2007-05-10)
Simulative tests, such as asphalt pavement analyzer (APA), Hamberg rut tester etc. have been widely used to evaluate the performance of asphalt mixtures. However, simulative tests to evaluate the performance of the mixtures cannot give fundamental properties of Asphalt Concrete (AC) due to the complex stress and strain fields. On the other hand, due to the availability of high-performance computing systems and software, numerical techniques are gaining popularity. This dissertation presents a computational simulation method of the APA tests in order to evaluate the rutting potential of asphalt mixtures based on actual microstructure reconstructed from X-ray tomography images. In the study, the microstructure of AC is obtained through the analysis of X-ray images, which included the digital information of the microstructure for the scanned specimen. In the simulations the three phases, mastic (asphalt binder with mineral filler), aggregates, and voids are assigned with different material properties. Aggregates are modeled as an elastic material, and air voids are removed during the loading steps. The adopted two-layer model is only used to represent the rate and temperature dependent behavior of the mastics. The parameters are obtained with inverse methods. Based on the sensitivity analysis of the parameters, an iterative procedure is performed to optimize the parameters using the experimental measurement and results of the model simulations. A parametric study is also conducted to study the effect of major parameters such as the stiffness ratio of the networks on the macro response of the model. The simulation results obtained shows a good agreement with the experimental results. The dissertation also presents a method to measure micro strains in asphalt mixture. An automated procedure using tomography images to reconstruct three-dimensional particles is developed. The translations of the particles are obtained from the coordinate differences of particles' mass centers before and after the APA testing. The micro and macro strains in the mixture are calculated based on the particle translations. A good correlation is found between measured strains and experimental result.
Effects of Coarse Aggregate Morphological Characteristics on Mechanical Performance of Stone Matrix Asphalt
Liu, Yufeng (Virginia Tech, 2017-07-26)
This research focused on three main objectives: (1) quantify coarse aggregate morphological characteristics using an improved FTI (Fourier Transform Interferometry) image analysis system, (2) evaluate the effects of morphological characteristics of coarse aggregates of various mineral compositions on the mechanical performances of stone matrix asphalt (SMA) mixtures constituted; (3) investigate the relationship between the uncompacted void content of coarse aggregates and morphological characteristics. To achieve the first research objective, a Fourier Transform Interferometry (FTI) system was adopted to capture three-dimensional high-resolution images of aggregates. Based on these digital images, the FTI system uses the two-dimensional Fast Fourier Transform (FFT2) method to rapidly measure aggregate morphological characteristics, including sphericity, flatness ratio, elongation ratio, angularity, and surface texture. The computed shape characteristics of all aggregates were in good agreement with manual measurement results, demonstrating the accuracy and reliability of this image analysis system. For the second objective, a series of simple performance laboratory tests were performed on eight types of SMA mixtures with different morphological characteristics. Test included wheel-track loading, dynamic modulus, flow number, and beam fatigue. The wheel tracking test included asphalt pavement analyzer (APA) and Model mobile load simulator (MMLS). In the APA test, samples included eight types of SMA mixtures that consisted of aggregates of 22 fractions. In the MMLS test, six types of SMA mixture samples that consist of coarse aggregate of 15 fractions were tested. Regression analyses were then conducted between weighted mean morphological characteristics and performance parameters. The fatigue performance parameters include |E*|sin φ, where |E*| is complex modulus obtained from dynamic modulus test, the number of loading cycles to failure, and the seismic modulus difference. The rutting performance parameters include |E*|/sin φ, flow number, flow number slope, rut depth and creep slope. For the third objective, different coarse aggregate fractions from different quarries in Virginia were analyzed using the improved FTI system. Regression analyses were performed to investigate the correlation between morphological characteristics and uncompacted void content of coarse aggregates at the size ranges of 4.75-9.5mm and 9.5-12.5 mm, respectively. Aggregate morphological characteristics were found to play an important role in the mechanical performance of stone matrix asphalt mixture and the uncompacted air void content of the coarse aggregates. Both the experimental results and simulation results demonstrated that using more of equi-dimensional, less flaky and elongated coarse aggregates with angular and rougher-textured aggregates is favorable to the mechanical performances of SMA mixtures. Recommended values for each morphological characteristic are provided.
Effects of Site Response on the Correlation Structure of Ground Motion Residuals
Motamed, Maryam (Virginia Tech, 2014-02-06)
Seismic hazard analyses require an estimate of earthquake ground motions from future events. These predictions are achieved through Ground Motion Prediction Equations, which include a prediction of the median and the standard deviation of ground motion parameters. The differences between observed and predicted ground motions, when normalized by the standard deviation, are referred to as epsilon (𝜖). For spectral accelerations, the correlation structure of normalized residuals across oscillator periods is important for guiding ground motion selection. Correlation structures for large global datasets have been studied extensively. These correlation structures reflect effects that are averaged over the entire dataset underlying the analyses. This paper considers the effects of site response, at given sites, on the correlation structure of normalized residuals. This is achieved by performing site response analyses for two hypothetical soil profiles using a set of 85 rock input motions. Results show that there is no significant difference between correlation coefficients for rock ground motions and correlation coefficients after considering the effects of site response for the chosen sites.
Engineering Characteristics of Coal Combustion Residuals and a Reconstitution Technique for Triaxial Samples
Lacour, Nicholas Alexander (Virginia Tech, 2012-06-19)
Traditionally, coal combustion residuals (CCRs) were disposed of with little engineering consideration. Initially, common practice was to use a wet-scrubbing system to cut down on emissions of fly ash from the combustion facilities, where the ash materials were sluiced to the disposal facility and allowed to sediment out, forming deep deposits of meta-stable ash. As the life of the disposal facility progressed, new phases of the impoundment were constructed, often using the upstream method. One such facility experienced a massive slope stability failure on December 22, 2008 in Kingston, Tennessee, releasing millions of cubic yards of impounded ash material into the Watts Bar reservoir and damaging surrounding property. This failure led to the call for new federal regulations on CCR disposal areas and led coal burning facilities to seek out geotechnical consultants to review and help in the future design of their disposal facilities. CCRs are not a natural soil, nor a material that many geotechnical engineers deal with on a regular basis, so this thesis focuses on compiling engineering characteristics of CCRs determined by different researchers, while also reviewing current engineering practice when dealing with CCR disposal facilities. Since the majority of coal-burning facilities used the sluicing method to dispose of CCRs at one point, many times it is desirable to construct new "dry-disposal" phases above the retired ash impoundments; since in-situ sampling of CCRs is difficult and likely produces highly disturbed samples, a sample reconstitution technique is also presented for use in triaxial testing of surface impounded CCRs.

Browsing by Author "Dove, Joseph E."

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