VTechWorks Repository :: Browsing by Author "Filz, George M."

Browsing by Author "Filz, George M."

Now showing 1 - 20 of 73

Analysis of Transient Seepage Through Levees
Sleep, Matthew David (Virginia Tech, 2011-10-25)
Levees are a significant part of the United States flood protection infrastructure. It is estimated that over 100,000 miles of levees exist in the United States. Most of these levees were designed many years ago to protect farmland and rural areas. As growth continues in the United States, many of these levees are now protecting homes and other important structures. The American Society of Civil Engineers gave the levees in the United States a grade of D- in 2009. To bring flood protection up to modern standards there requires adequate methods of evaluating levees with respect to seepage, erosion, piping and slope instability. Transient seepage analyses provide an effective method of evaluating seepage through levees and its potentially destabilizing effects. Floods against levees usually last for days or weeks. In response to a flood, pore pressures within the levee will change from negative (suction) to positive as the phreatic surface progresses through the levee. These changes can be calculated by finite element transient seepage analyses. In order for the transient seepage analysis to be valid, appropriate soil properties and initial conditions must be used. The research investigation described here provides simple and practical methods for estimating the initial conditions and soil properties required for transient seepage analyses, and illustrates their use through a number of examples.
Atomistic Characterization and Modeling of the Deformation and Failure Properties of Asphalt-Aggregate Interface
Lu, Yang (Virginia Tech, 2010-04-20)
This dissertation is dedicated to develop models and methods to bridge atomistic and continuum scales of deformation processes in asphalt-aggregate interfacial composite materials systems. The deformation and failure behaviors, e.g. nanoscale strength, deformation, stiffness, and adhesion/cohesion at asphalt-aggregate interfaces are all evaluated by means of atomistic simulations. The atomistic modeling approach is employed to simulate mechanical properties, which is connected by their common dependence on the nanoscale bonding and their sensitive dependences on mechanics and moisture sensitivity. Specifically, CVFF-aug forcefield is employed in the atomistic calculations to study the fundamental failure processes that appear at the interface as a result of a mechanical deformation. There are five primary aspects to this dissertation. First, the multiscale features of asphalt concrete materials are characterized by using nanoscale characterization & fabrication devices, e.g. High Resolution Optical Microscope (HROM), Environmental Scanning Electron Microscope (ESEM), Transmission Electron Microscope (TEM), Focused Ion Beam (FIB), and Atomistic Force Microscope (AFM). Second, based on the multiscale devices characterization of the interfaces, a 2-layer atomistic bitumen-rock interface structure is constructed. Interface structure evolution under uniaxial tension is performed with various deformation rates. Comparison is made between both theoretical and experimental characterizations of interface configuration. Molecular dynamics (MD) simulations are used to investigate potential relationships between interface structure and morphology. Influences of deformation rate and temperature factors are discussed in terms of interface region stress-strain relation and loading time duration. Third, molecular dynamics simulations are also performed to provide a characterization of atomic scale mechanical behaviors for a 3-layer confined shear structure which leads to interfacial shear failure. In addition, atomistic static simulation approach is employed to calculate a couple of mineral crystals' elastic constants. Furthermore, molecular dynamics simulations are also used to predict the static, thermodynamic, and mechanical properties of three asphalt molecular models. Fourth, the high performance parallel computing technology is extensively employed throughout this dissertation. In addition to use the large-scale MD program, LAMMPS, the author developed a high performance parallel distributive computing program, MPI_multistress, to implement the multiscale understanding/predicting of materials mechanical behaviors. Finally, this research also focuses on the evaluation of the susceptibility of aggregates and asphalts to moisture damage through understanding the nano-mechanisms that influence adhesive bond between aggregates and asphalt, as well as the cohesive strength and moisture susceptibility of the specific asphalt-aggregate interfaces. Surface energy theory and pull-out simulation are used to compute the adhesive bond strength between the aggregates and asphalt, as well as the cohesive bond strength within the binder. In general, this dissertation has focused on the development of nanoscale modeling methods to assess asphalt-aggregate interfacial atomistic deformation and failure behaviors, as well as moisture effects on asphalt mixture strength. Simulation results provide valuable insights into mechanistic details of nanoscale interactions, particularly under conditions of various deformation rates and different temperatures. The results obtained show that a reasonable agreement between the theoretical and pavement industry observations is satisfactory. We conclude that the theoretical calculations presented here are useful in asphalt concrete industry for predicting the mechanical properties of asphalt-aggregate interfaces, which are difficult to obtain experimentally because of their small size.
Column-Supported Embankments: Full-Scale Tests and Design Recommendations
Sloan, Joel Andrew (Virginia Tech, 2011-05-26)
When an embankment is to be constructed over ground that is too soft or compressible to adequately support the embankment, columns of strong material can be placed in the soft ground to provide the necessary support by transferring the embankment load to a firm stratum. This technology is known as column-supported embankments (CSEs). A geosynthetic-reinforced load transfer platform (LTP) or bridging layer may be constructed immediately above the columns to help transfer the load from the embankment to the columns. There are two principal reasons to use CSEs: 1) accelerated construction compared to more conventional construction methods such as prefabricated vertical drains (PVDs) or staged construction, and 2) protection of adjacent facilities from distress, such as settlement of existing pavements when a roadway is being widened. One of the most significant obstacles limiting the use of CSEs is the lack of a standard design procedure which has been properly validated. This report and the testing described herein were undertaken to help resolve some of the uncertainty regarding CSE design procedures in light of the advantages of the CSE technology and potential for significant contributions to the Strategic Highway Research Program, which include accelerated construction and long-lived facilities. Twelve design/analysis procedures are described in this report, and ratings are assigned based on information available in the literature. A test facility was constructed and the facility, instrumentation, materials, equipment, and test procedures are described. A total of 5 CSE tests were conducted with 2 ft diameter columns in a square array. The first test had a column center-to-center spacing of 10 ft and the remaining four tests had center-to-center spacings of 6 ft. The Adapted Terzaghi Method of determining the vertical stress on the geosynthetic reinforcement and the Parabolic Method of determining the tension in the geosynthetic reinforcement provide the best agreement with the test results. The tests also illustrate the importance of soft soil support in CSE performance and behavior. A generalized formulation of the Adapted Terzaghi Method for any column/unit cell geometry and two layers of embankment fill is presented, and two new formulations of the Parabolic Method for triangular arrangements is described. A recommended design procedure is presented which includes use of the GeogridBridge Excel workbook described by Filz and Smith (2006, 2007), which was adapted for both square and triangular column arrangements. GeogridBridge uses the Adapted Terzaghi Method and the Parabolic Method in a load-displacement compatibility design approach. For completeness, recommended quality control and quality assurance procedures are also provided, and a new guide specification is presented.
Computational Techniques for Efficient Solution of Discretized Biot's Theory for Fluid Flow in Deformable Porous Media
Lee, Im Soo (Virginia Tech, 2008-06-24)
In soil and rock mechanics, coupling effects between geomechanics field and fluid-flow field are important to understand many physical phenomena. Coupling effects in fluid-saturated porous media comes from the interaction between the geomechanics field and the fluid flow. Stresses subjected on the porous material result volumetric strains and fluid diffusion in the pores. In turn, pore pressure change cause effective stresses change that leads to the deformation of the geomechanics field. Coupling effects have been neglected in traditional geotechnical engineering and petroleum engineering however, it should not be ignored or simplified to increases reliability of the results. The coupling effect in porous media was theoretically established in the poroelasticity theory developed by Biot, and it has become a powerful theory for modeling three-dimensional consolidation type of problem. The analysis of the porous media with fully-coupled simulations based on the Biot's theory requires intensive computational effort due to the large number of interacting fields. Therefore, advanced computational techniques need to be exploited to reduce computational time. In order to solve the coupled problem, several techniques are currently available such as one-way coupling, partial-coupling, and full-coupling. The fully-coupled approach is the most rigorous approach and produces the most correct results. However, it needs large computational efforts because it solves the geomechanics and the fluid-flow unknowns simultaneously and monolithically. In order to overcome this limitation, staggered solution based on the Biot's theory is proposed and implemented using a modular approach. In this thesis, Biot's equations are implemented using a Finite Element method and/or Finite Difference method with expansion of nonlinear stress-strain constitutive relation and multi-phase fluid flow. Fully-coupled effects are achieved by updating the compressibility matrix and by using an additional source term in the conventional fluid flow equation. The proposed method is tested in multi-phase FE and FD fluid flow codes coupled with a FE geomechanical code and numerical results are compared with analytical solutions and published results.
Critical height and surface deformation of column-supported embankments
McGuire, Michael Patrick (Virginia Tech, 2011-11-01)
Column-supported embankments with or without basal geosynthetic reinforcement can be used in soft ground conditions to reduce settlement by transferring the embankment load to the columns through stress redistribution above and below the foundation subgrade level. Column-supported embankments are typically used to accelerate construction and/or protect adjacent facilities from additional settlement. The column elements consist of driven piles or formed-in-place columns that are installed in an array to support a bridging layer or load transfer platform. The bridging layer is constructed to enhance load transfer using several feet of compacted sand or sand and gravel that may include one or more layers of high-strength geotextile or geogrid reinforcement. Mobilization of the mechanisms of load transfer in a column-supported embankment requires some amount of differential settlement between the columns and the embankment as well as between the columns and the foundation soil. When the embankment height is low relative to the clear spacing between columns, there is the risk of poor ride quality due to the reflection of the differential foundation settlement at the surface of the embankment. The minimum embankment height where differential surface settlement does not occur for a particular width and spacing of column is the critical height. The conventional approach is to express critical height as a fixed ratio of the clear span between adjacent columns; however, there is no consensus on what ratio to use and whether a single ratio is applicable to all realistic column arrangements. The primary objective of this research is to improve the understanding of how column-supported embankments deform in response to differential foundation settlement. A bench-scale experimental apparatus was constructed and the equipment, materials, instrumentation, and test procedures are described. The apparatus was able to precisely measure the deformation occurring at the sample surface in response to differential settlement at the base of the sample. Critical heights were determined for five combinations of column diameter and spacing representing a wide range of possible column arrangements. In addition, tests were performed using four different column diameters in a single column configuration with ability to measure the load acting on the column and apply a surcharge pressure to the sample. In total, 183 bench-scale tests were performed over a range of sample heights, sample densities, and reinforcement stiffnesses. Three-dimensional numerical analyses were conducted to model the experiments. The critical heights calculated using the numerical model agreed with the experimental results. The results of the laboratory tests and numerical analyses indicate that critical height depends on the width and spacing of the columns and is not significantly influenced by the density of the embankment fill or the presence of reinforcement. A new method to estimate critical height was developed and validated against extensive case histories as well as experimental studies and numerical analyses performed by others.
Design Methodology for Permeable Reactive Barriers Combined With Monitored Natural Attenuation
Hafsi, Amine (Virginia Tech, 2008-04-23)
Permeable reactive barrier (PRB) technology is increasingly considered for in situ treatment of contaminated groundwater; however, current design formulas for PRBs are limited and do not properly account for all major physical and attenuation processes driving remediation. This study focused on developing a simple methodology to design PRBs that is easy to implement while improving accuracy and being more conservative than the available design methodologies. An empirical design equation and a simple analytical design equation were obtained to calculate the thickness of a PRB capable of degrading a contaminant from a source contaminant concentration to a maximum contaminant level at a Point of compliance . Both equations integrate the fundamental components that drive the natural attenuation process of the aquifer and the reactive capacity of the PRB.The empirical design equation was derived from a dataset of random hypothetical cases that used the solutions of the PRB conceptual model (Solution I). The analytical design equation was derived from particular solutions of the model (Solution II) which the study showed fit the complex solutions of the model well. Using the hypothetical cases, the analytical equation has shown that it gives an estimated thickness of the PRB just 15 % lower or higher than the real thickness of the PRB 95 percent of the time. To calculate the design thickness of a PRB, Natural attenuation capacity of the aquifer can be estimated from the observed contaminant concentration changes along aquifer flowpaths prior to the installation of a PRB. Bench-scale or pilot testing can provide good estimates of the required residence times ( Gavaskar et al. 2000) , which will provide the reactive capacity of the PRB needed for the calculation. The results of this study suggest also that the installation location downgradient from the source of contaminant is flexible. If a PRB is installed in two different locations, it will achieve the same remediation goals. This important finding gives engineers and scientists the choice to adjust the location of their PRBs so that the overall project can be the most feasible and cost effective.
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.
Design of Bridging Layers in Geosynthetic-Reinforced, Column-Supported Embankments
Filz, George M.; Miriam E. Smith (Virginia Center for Transportation Innovation and Research, 2006-04-01)
The cost of column-supported embankments depends, in part, on the spacing between the columns and the size of the columns and pile caps. Geosynthetic reinforcement is often employed in bridging layers to enhance load transfer to the columns and to increase the column spacing. The number, stiffness, and strength of geosynthetic layers are selected based on considerations of load transfer and deformation. 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 reinforcement, columns, and existing site 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 field case histories. The new method for calculating load on the geosynthetic was integrated into a 10-step design procedure for geosynthetic-reinforced bridging layers in column-supported embankments. The design procedure addresses such details as the thickness and type of the bridging layer soil, selection of the geosynthetic reinforcement, if needed, and the embankment settlement. The necessary calculations have been programmed into a Microsoft Excel workbook. The workbook may be accessed at www.virginiadot.org/vtrc/main/online%5Freports/pdf/geogridbridge.pdf
Design, performance, and analysis of a multi-level air permeability test
Haney, Orrick Rowland (Virginia Tech, 1994-02-05)
The design, performance, and analysis of a soil vapor extraction system to identify zones most conductive to air transport and quantify kair in sequential soil layers is presented. A multi-level extraction well, with alternating solid and screened sections, was utilized to characterize multi-layered media. The field site, located in the Carolina Slate Belt within the physiographic region known as the Piedmont, is comprised of alternating layers of different soil types of varying kair, including thin bands of clay, silt, and sand. The pneumatic test consisted of one multi-level extraction well and four multi-level pressure monitoring wells. Screen locations were based on previous site characterization. Vapors were extracted at one screen while pressure, temperature, and volumetric flow rate were monitored using a computer data acquisition system. Data was analyzed by both steady-state and transient solution techniques using pressure drawdown versus time data collected at various locations. Results from vapor extraction tests indicate that the multi-level approach is advantageous when dealing with heterogenous media, since the most permeable layer was identified. Transient and steady-state solutions indicate that a k_air= 2.0 X 10⁻⁷ cm² is representative of the located permeable layer within the subsurface. Vacuum system, formation, and extraction well characteristics are evaluated to determine pressure as a function of volumetric flow rate.
The development of a modular finite element program for analysis of soil-structure interaction
Morrison, Clark Stephen (Virginia Tech, 1995)
The development of SAGE, a modular finite element program for analysis of soil structure interaction, is described. The modular structure of the program makes it easy to validate, easy to understand, easy to modify, and easy to extend. Issues affecting the development of the program are discussed. Newton-Raphson iteration, and its application to finite element analysis is described. Methods for improving the convergence behavior of Newton-Raphson iteration are discussed. The methods include two global convergence algorithms: the line search and the dogleg search. Use of a consistent tangent stress-strain matrix for formulating the stiffness matrix, and its influence on convergence, is discussed. Approximate methods for calculating the consistent tangent stress-strain matrix are presented. Numerical procedures for simulating point loads, distributed loads, gravity loads, excavation, and fill placement are given. It is shown that Newton-Raphson iteration will correct numerical errors associated with the use of very stiff interface elements adjacent to relatively soft soil elements. The results of the use of Gauss integration and Newton-Cotes integration for interface elements are compared. A modification of the hyperbolic model incorporating Mohr-Coulomb plasticity is described. It is shown that use of this model substantially reduces "overshoot", or instances of elements carrying stresses that exceed the strength of the element. Implementation of the Cam clay model into SAGE is described. Several simple example problems are presented that illustrate the stress-strain behavior calculated using this model. Analyses of a footing subjected to combined vertical and horizontal loads are described. The problem was chosen to illustrate the capacity of SAGE to calculate stresses and deformations in soil-structure systems subjected to unusual loading conditions.
Development of an extended hyperbolic model for concrete-to-soil interfaces
Gómez, Jesús Emilio (Virginia Tech, 2000-07-19)
Placement and compaction of the backfill behind an earth retaining wall may induce a vertical shear force at the soil-to-wall interface. This vertical shear force, or downdrag, is beneficial for the stability of the structure. A significant reduction in construction costs may result if the downdrag is accounted for during design. This potential reduction in costs is particularly interesting in the case of U.S. Army Corps of Engineers lock walls. A simplified procedure is available in the literature for estimating the downdrag force developed at the wall-backfill interface during backfilling of a retaining wall. However, finite element analyses of typical U.S. Army Corps of Engineers lock walls have shown that the magnitude of the downdrag force may decrease during operation of the lock with a rise in the water table in the backfill. They have also shown that pre- and post-construction stress paths followed by interface elements often involve simultaneous changes in shear and normal stresses and unloading-reloading. The hyperbolic formulation for interfaces (Clough and Duncan 1971) is accurate for modeling the interface response in the primary loading stage under constant normal stress. However, it has not been extended to model simultaneous changes in shear and normal stresses or unloading-reloading of the interface. The purpose of this research was to develop an interface model capable of giving accurate predictions of the interface response under field loading conditions, and to implement this model in a finite element program. In order to develop the necessary experimental data, a series of tests were performed on interfaces between concrete and two different types of sand. The tests included initial loading, staged shear, unloading-reloading, and shearing along complex stress paths. An extended hyperbolic model for interfaces was developed based on the results of the tests. The model is based on Clough and Duncan (1971) hyperbolic formulation, which has been extended to model the interface response to a variety of stress paths. Comparisons between model calculations and tests results showed that the model provides accurate estimates of the response of interfaces along complex stress paths. The extended hyperbolic model was implemented in the finite element program SOILSTRUCT-ALPHA, used by the U.S. Army Corps of Engineers for analyses of lock walls. A pilot-scale test was performed in the Instrumented Retaining Wall (IRW) at Virginia Tech that simulated construction and operation of a lock wall. SOILSTRUCT-ALPHA analyses of the IRW provided accurate estimates of the downdrag magnitude throughout inundation of the backfill. It is concluded that the extended hyperbolic model as implemented in SOILSTRUCT-ALPHA is adequate for routine analyses of lock walls.
Effect of Concentration of Sphagnum Peat Moss on Strength of Binder-Treated Soil
Bennett, Michael Dever (Virginia Tech, 2019-08-21)
Organic soils are formed as deceased plant and animal wildlife is deposited and decomposed in wet environs. These soils have loose structures, low undrained strengths, and high natural water contents, and require improvement before they can be used as foundation materials. Previous researchers have found that the deep mixing method effectively improves organic soils. This study presents a quantitative and reliable method for predicting the strength of one organic soil treated with deep mixing. For this thesis, organic soils were manufactured from commercially available components. Soil-binder mixture specimens with different values of organic matter content, OM, binder content, water-to-binder ratio, and curing time were tested for unconfined compressive strength (UCS). Least-squares regression was used to fit a predictive equation, modified from the findings of previous researchers, to this data. The equation estimates the UCS of a deep-mixed organic soil specimen using its total water-to-binder ratio and mixture dry unit weight. Soil OM is incorporated into the equation as a threshold binder content, aT, required to improve a soil with a given OM; the aT term is used to calculate an effective total water-to-binder ratio. This thesis reached several important conclusions. The modified equation was successfully fitted to the data, meaning that the UCS of some organic soil-binder mixtures may be predicted in the same manner as that of inorganic soil-binder mixtures. The fitting coefficients from the predictive equations indicated that for the soils and binder tested, specimens of organic soil-binder mixtures have a greater relative gain of UCS immediately after mixing compared to specimens of inorganic soil-binder mixtures. However, the inorganic mixtures generally have a greater relative gain of UCS during the curing period. The influence of curing temperature was found to be similar for organic and inorganic mixtures. For the organic soils and binder tested in this research, aT may be expressed as a linear or power function of OM. For both functions, the value of aT was negligible at values of OM below 45%, which reflects the chemistry of the organic matter in the peat moss. For projects involving deep mixing of organic soils, the predictive equation will be used most effectively by fitting it to the results of bench-scale testing and then checking it against the results of field-scale testing.
EPOLLS: An Empirical Method for Prediciting Surface Displacements Due to Liquefaction-Induced Lateral Spreading in Earthquakes
Rauch, Alan F. (Virginia Tech, 1997-05-05)
In historical, large-magnitude earthquakes, lateral spreading has been a very damaging type of ground failure. When a subsurface soil deposit liquefies, intact blocks of surficial soil can move downslope, or toward a vertical free face, even when the ground surface is nearly level. A lateral spread is defined as the mostly horizontal movement of gently sloping ground (less than 5% surface slope) due to elevated pore pressures or liquefaction in undelying, saturated soils. Here, lateral spreading is defined specifically to exclude liquefaction failures of steeper embankments and retaining walls, which can also produce lateral surface deformations. Lateral spreads commonly occur at waterfront sites underlain by saturated, recent sediments and are particularly threatening to buried utilities and transportation networks. While the occurrence of soil liquefaction and lateral spreading can be predicted at a given site, methods are needed to estimate the magnitude of the resulting deformations. In this research effort, an empirical model was developed for predicting horizontal and vertical surface displacements due to liquefaction-induced lateral spreading. The resulting model is called "EPOLLS" for Empirical Prediction Of Liquefaction-induced Lateral Spreading. Multiple linear regression analyses were used to develop model equations from a compiled database of historical lateral spreads. The complete EPOLLS model is comprised of four components: (1) Regional-EPOLLS for predicting horizontal displacements based on the seismic source and local severity of shaking, (2) Site-EPOLLS for improved predictions with the addition of data on the site topography, (3) Geotechnical-EPOLLS using additional data from soil borings at the site, and (4) Vertical-EPOLLS for predicting vertical displacements. The EPOLLS model is useful in phased liquefaction risk studies: starting with regional risk assessments and minimal site information, more precise predictions of displacements can be made with the addition of detailed site-specific data. In each component of the EPOLLS model, equations are given for predicting the average and standard deviation of displacements. Maximum displacements can be estimated using probabilities and the gamma distribution for horizontal displacements or the normal distribution for vertical displacements.
Erosion Protection for Soil Slopes Along Virginia's Highways
Scarborough, Jessee A.; Filz, George M.; Mitchell, James K.; Brandon, Thomas L. (Virginia Center for Transportation Innovation and Research, 2000-10-01)
A survey of the state of practice for designing slope erosion control measures within VDOT's nine districts has been conducted. On the basis of the survey, it is clear that there are no specific design procedures currently in use within VDOT for dealing with slope erosion. VDOT designers generally try to limit erosion by diverting runoff from adjacent areas, controlling concentrated flows on slopes, and establishing vegetation on slopes as quickly as possible. In addition, the Federal Highway Administration (FHWA) and the Departments of Transportation in states surrounding Virginia (Maryland, West Virginia, Kentucky, Tennessee, and North Carolina) were contacted. The state of practice for the FHWA and for these states appears to be similar to that used by VDOT. A review of the literature for soil erosion was performed. The universal soil loss equation (USLE), an empirical equation developed by the U.S. Department of Agriculture, was found to provide the best available quantitative tool for evaluating factors controlling the erosion process and determining what level of protection is appropriate. The authors recommend that the USLE be used to supplement VDOT's current principle-based design practices.
Evaluation of Analysis Methods used for the Assessment of I-walls Stability
Vega-Cortes, Liselle (Virginia Tech, 2007-12-04)
On Monday, 29 August 2005, Hurricane Katrina struck the U.S. gulf coast. The storm caused damage to 169 miles of the 284 miles that compose the Hurricane Protection System (HPS) of the area. The system suffered 46 breaches due to water levels overtopping and another four caused by instability due to soil foundation failure. The Interagency Performance Evaluation Task Force (IPET) conducted a study to analyze what happened on the I-wall breach of the various New Orleans flood control structures and looked for solutions to improve the design of these floodwalls. The purpose of the investigation, describe in this document, is to evaluate different methods to improve the analysis model created by IPET, select the best possible analysis techniques, and apply them to a current cross-section that did not fail during Hurrican Katrina. The use of Finite Element (FE) analysis to obtain the vertical total stress distribution in the vicinity of the I-wall and to calculate pore pressures proved to be an effective enhancement. The influence of overconsolidation on the shear strength distribution of the foundation soils was examined as well.
An Examination of the Validity of Steady State Shear Strength Determination Using Isotropically Consolidated Undrained Triaxial Tests
Porter, Jonathan R. (Virginia Tech, 1998-07-07)
The assessment of the shear strength of soil deposits after the occurrence of large strains is an important issue for geotechnical engineers. One method for doing so, the steady state approach, is based on the assumption that the steady state undrained shear strength is a unique function of the in situ void ratio and effective stress. This method, which has been applied to liquefaction and flow failures, has been criticized because it may overestimate the in situ shear strength. The key to the steady state approach is accurate determination of the relationship between void ratio and effective stress at steady state. This is typically accomplished using conventional isotropically consolidated undrained (ICU) triaxial tests. The triaxial test was developed for measuring peak strengths, which typically occur at small strains, but steady state conditions typically occur at much larger strains. At large strain levels, the suitability of conventional triaxial testing procedures and error corrections is uncertain. The measured response at large strains may be inaccurate due to the influence of various testing errors. Furthermore, the true material response in the test specimen at large strains may not accurately represent in situ material behavior at large strains. This research effort consisted of an experimental and analytical study to examine the validity of steady state undrained shear strength determination using conventional ICU triaxial tests. The analytical study addressed triaxial testing errors and conventional corrections that are applied to test data and their influence on the measured steady state parameters. Finite element analyses were conducted to investigate the influence of variations in restraint at the end platens on stress distributions in the sample and measured stress-strain response. The finite element analyses incorporated axisymmetric interface elements to model the friction characteristics between the end platens and the specimen ends. The experimental study focused on several sands that are susceptible to liquefaction. An interface direct shear test program was conducted in order to evaluate various schemes for reducing end platen friction. ICU triaxial tests were conducted on each material using both conventional and lubricated end platens.
An experimental and analytic study of earth loads on rigid retaining walls
Filz, George M. (Virginia Tech, 1992-04-01)
Experimental and analytic investigations were performed to examine the influences of wall height, backfill behavior, and compaction on the magnitudes of backfill loads on rigid retaining walls. Measurements of lateral and vertical backfill loads were made during tests using the Virginia Tech instrumented retaining wall facility. The tests were performed with two soils, moist Yatesville silty sand and dry Light Castle sand. Two hand-operated compactors, a vibrating plate compactor and a rammer compactor, were used to compact the backfill. The backfill height was 6.5 feet in all of the tests. Analyses of backfill loads were made using a compaction- induced lateral earth pressure theory and a vertical shear force theory. The compaction-induced lateral earth pressure theory was revised from a previous theory. The revisions improved the accuracy with which the theory models the hysteretic stress behavior of the backfill during compaction. The theory was also extended to include the pore pressure response of moist backfill in a rational manner. A vertical shear force theory was also developed during this research. The theory is based on consideration of backfill compressibility and mobilization of interface shear strength at the contact between the backfill and the wall. The theory provides a useful basis for understanding how wall height, backfill compressibility, wall-backfill interface behavior, and compaction-induced lateral pressures affect the vertical shear forces on rigid walls. Studies were also made to investigate the cause of erratic pressure cell readings. An important cause of drift in pressure cell readings was found to be moisture changes in the concrete in which the pressure cells were mounted. It was found that this problem could be mitigated by applying a water-seal treatment to the face of the wall. Both the vibrating plate compactor and the rammer compactor were instrumented to measure dynamic forces and energy transfer during compaction. The force applied by the vibrating plate compactor was about one-quarter of the manufacturer’s rated force. The force applied by the rammer compactor was about twice the manufacturer’s rated force. The transferred energy measurements provided a basis for relating laboratory and field compaction procedures.
Experimental and Analytical Investigations of Piles and Abutments of Integral Bridges
Arsoy, Sami (Virginia Tech, 2000-12-15)
Bridges without expansion joints are called "integral bridges." Eliminating joints from bridges crates concerns for the piles and the abutments of integral bridges because the abutments and the piles are subjected to temperature-induced cyclic lateral loads. As temperatures change daily and seasonally, the lengths of integral bridges increase and decrease, pushing the abutment against the approach fill and pulling it away. As a result the bridge superstructure, the abutment, the approach fill, the foundation piles and the foundation soil are all subjected to cyclic loading, and understanding their interactions is important for effective design and satisfactory performance of integral bridges. The ability of piles to accommodate lateral displacements is a significant factor in determining the maximum possible length of integral bridges. In order to build longer integral bridges, pile stresses should be kept low. This research project investigated the complex interactions that take place between the structural components of the integral bridge and the soil through experimental and analytical studies. A literature review was conducted to gain insight into the integral bridge/soil interactions, and to synthesize the information available about the cyclic loading damage to piles of integral bridges. The ability of the piles and the abutments to withstand cyclic loads was investigated by conducting large-scale cyclic load tests. Three pile types and three semi-integral abutments were tested in the laboratory. Experiments simulated 75 years of bridge life for each specimen by applying over 27,000 displacement cycles. Numerical analyses were conducted to investigate the interactions among the abutment, the approach fill, the foundation soil, and the piles. The original VDOT semi-integral abutment hinge experienced shear key failure as observed in two large-scale laboratory tests. The revised hinge detail did not exhibit any sign of damage. Both abutments tolerated 75-year worth of displacement cycles without any appreciable change in their behavior. Semi-integral abutments are recommended for longer integral bridges because they can reduce pile stresses. As the need to build longer integral bridges grows, the role of the semi-integral abutments is expected to become more important. The data from the experimental program indicates that steel H-piles are the best pile type for support of integral abutment bridges. Concrete piles are not recommended because under repeated lateral loads, tension cracks progressively worsen and significantly reduce vertical load carrying capacity of these piles. Pipe piles have high flexural stiffness, which results in an undesired condition for the shear stresses in the abutment. For this reason, stiff pipe piles are not recommended for support of integral bridges. Numerical analyses indicate that the interactions between the approach fill and the foundation soils create favorable conditions for stresses in piles supporting integral bridges. Because of these interactions, the foundation soil acts as if it were softer, resulting in reduction in pile stresses compared to a single pile in the same soil without the approach fill above it.
An Experimental Study on the Aging of Sands
Baxter, Christopher David Price (Virginia Tech, 1999-07-15)
There are numerous examples in the literature of time-dependent changes in the proper-ties of sands, or aging effects. Most of these aging effects are of increases in the cone penetration resistance. Time-dependent increases in penetration resistance have been measured in hydraulically placed fills and freshly densified deposits, with the largest in-creases following the use of ground modification techniques such as vibrocompaction, dynamic compaction, and blast densification. It is not known what causes these increases in penetration resistance to occur. The objective of this research was to gain an understanding of the possible mechanisms responsible for aging effects in sands. Current hypotheses to explain what causes aging effects in sands include increased interlocking of particles, internal stress arching, and precipitation of silica or carbonate minerals at the contacts between grains. To date, no unambiguous evidence has been presented to support these hypotheses. A laboratory testing program was developed to study the influence of different variables on the pres-ence and magnitude of aging effects. Three different sands were tested in rigid wall cells and buckets. Samples were aged under different effective stresses, densities, tempera-tures, and pore fluids. In every rigid wall cell, three independent measurements were made to monitor property changes during the aging process: small strain shear modulus using bender elements, electrical conductivity, and mini-cone penetration resistance. At the end of each test, detailed mineralogical tests were performed to assess changes in the chemistry of the samples and pore fluids. A total of 22 tests in rigid wall cells were per-formed with periods of aging ranging from 30 to 118 days. Mini-cone penetration resis-tances were measured in the buckets before and at various times during the aging process. Increases in the small strain shear modulus were measured with time. It was found that sand type and pore fluid composition greatly influenced the amount of increase in small strain shear modulus. Density was also found to influence the amount of increase in small strain shear modulus. Temperature was found to have little influence on the in-crease in small strain shear modulus with time. Changes in the chemistry of the samples were also measured with time. The dissolution and precipitation of minerals in solution was monitored with electrical conductivity measurements. In most of the tests, there was continual dissolution of minerals with time. Mineralogical studies and conductivity measurements indicated precipitation of carbonates and silica in two of the tests; however, scanning electron micrographs showed no visible evidence of precipitation. Despite the measured increases in small strain shear modulus and evidence of mineral precipitation, there were no increases in the mini-cone penetration resistance with time. This finding is significant and suggests that small-scale laboratory experiments do not capture the mechanism(s) that are responsible for time-dependent increases in penetration resistance in the field.
Experiments and Analysis of Water-filled Tubes Used as Temporary Flood Barriers
Freeman, Marcos (Virginia Tech, 2002-04-26)
Geosynthetic tubes filled with water are considered. The tubes can be used in applications to resist rising floodwaters. They can also be used to form breakwaters and protect shores from erosion. This thesis considers single and stacked tubes resting on a rigid and deformable foundation resisting rising hydrostatic headwater. Experiments were carried out to determine the behavior of a three-tube stacked configuration resting on a sand foundation. This study was a continuation of previous work on unstacked tubes. Many tests were performed to determine the deformation and stability of the system. A geosynthetic drain was placed beneath the tubes to prevent piping. The objective was to cause failure of the system in a sliding manner and formulate a hypothesis according to the placement of the drain beneath the tubes. In order to cause a sliding failure, a strapping system was developed to try and prevent the tubes from rolling. A single tube at rest, filled with water but with no external hydrostatic pressure, was considered for analysis first. The tube rested on a rigid foundation and was assumed to be infinitely long. The friction between the tube and the foundation was neglected, and the bending stiffness of the tube was assumed to be negligible. The tube material was assumed to be inextensible. Mathematica was used to solve the system of equations and compute the unknowns. Excel was used to plot the data and observe the behavior of the tubes. An analysis was also performed on a single tube with an apron attached, resting on a rigid foundation. The apron was attached on the rising headwater side to increase stability. The assumptions for the tube at rest were also applied in this analysis. Two cases were derived and analyzed: a case where the internal hydrostatic pressure remains constant, and a case where the cross-sectional area remains constant. For the second case, the internal pressure changes as the floodwater level rises. The results from this study demonstrated that water-filled tubes, stacked or with an apron attached, can be an effective alternative method to sandbags in resisting floodwaters.

Browsing by Author "Filz, George M."

Results Per Page

Sort Options