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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. Ph. D.

Column-supported embankments (CSE) enable accelerated construction on soft soils, high performance, and protection of adjacent facilities. The foundation columns transfer embankment and service loading to a competent stratum at depth such that loading on the soft soil can be reduced. This has the beneficial effects of reducing settlement and lateral displacement, and improving stability. Selection of column type depends on the design load, cost, constructability, etc., although unreinforced concrete columns are commonly used. A load transfer platform (LTP) is often included at the embankment base. This is a layer of coarse-grained fill that may include one or more layers of geosynthetic reinforcement. The LTP improves vertical load transfer to columns by mobilizing the shear strength of the LTP fill and the membrane effect of the geosynthetic. The geosynthetic reinforcement also responds in tension to lateral spreading. Herein, lateral spreading is defined as the lateral displacements occurring in response to lateral earth pressures in the embankment and foundation. Excessive lateral spreading can lead to bending failure of the concrete columns, tensile failure of the geosynthetic reinforcement, and instability of the system. Design procedures recommend inclusion of geosynthetic reinforcement to mitigate lateral spreading, with assumptions for the lateral thrust distribution, failure mode, and calculation of geosynthetic tensile capacity. The necessity and sufficiency of these assumptions have not been fully validated. In addition, unreinforced concrete columns have low tensile strength and can fail in bending, but recommendations for calculating column bending moments are not available. This research examines the limitations in CSE lateral spreading design with the goal of advancing fundamental understanding of lateral spreading mechanics. The research was performed using three-dimensional finite difference analyses. Limiting conditions for lateral spreading analysis were identified using case history records, and an undrained-dissipated approach was validated for the numerical analysis of limiting conditions (i.e., undrained end-of-construction and long-term excess pore pressure dissipated). The numerical model was calibrated using a well-documented case history. Additional analyses of the case history were performed to examine the lateral earth pressures in the foundation, column bending moments, and geosynthetic contribution to resisting lateral spreading. A parametric study was conducted to examine the lateral thrust distribution in 128 CSE scenarios. A refined substructure model was adopted for analyzing peak geosynthetic tensions and strains. Lastly, failure analyses were performed to examine the effect of different CSE design parameters on embankment failure height, failure mode, and deformations. The research produced qualitative and quantitative information about the following: (1) the percent thrust resistance provided by the geosynthetic as a function of its stiffness; (2) the geosynthetic contribution to ultimate and serviceability limit states; (3) the change in lateral thrust distribution throughout the embankment system before and after dissipation of excess pore water pressures; (4) the column-soil interactions involved in embankment failure; and (5) identification of two failure modes in the undrained condition. Design guidance based on these findings is provided. Doctor of Philosophy Column-supported embankments (CSEs) have been designated by the Federal Highway Administration as a critical technology for new highway alignment projects and widening of existing highways. CSEs enable accelerated construction and high performance in weak soils, which are factors critical to project success. In a CSE, columns are installed in the weak soil, followed by rapid construction of the soil embankment that provides the necessary elevation and foundation for the roadway. The columns transfer most of the embankment and traffic loading to a competent soil stratum at depth. Concrete without steel reinforcement is commonly used to construct the columns, although material selection depends on cost, constructability, expected load, etc. Layers of geosynthetic reinforcement can also be included at the embankment base. The geosynthetics help to transfer loads to the columns and resist excessive movement that could lead to instability. The entire embankment system should be designed for safety and economy. This research was motivated by uncertainties in design to mitigate lateral spreading. Lateral spreading refers to lateral displacements occurring in response to lateral earth pressures in the embankment and foundation. Excessive lateral spreading can lead to failure of the columns, geosynthetic reinforcement, and the entire embankment system. This research aims to advance fundamental understanding of lateral spreading in CSEs and to re-evaluate current design assumptions. Corresponding design guidance is provided.

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. Master of Science Organic soils are formed continuously as matter from deceased organisms – mainly plants – is deposited in wet environs and decomposes. Organic soils are most commonly found in swamps, marshes, and coastal areas. These soils make poor foundation materials due to their low strengths. Deep mixing, or soil mixing, involves introducing a binder like Portland cement or lime into soil and blending the soil and binder together to form columns or blocks. Upon mixing, cementitious reactions occur, and the soil-binder mixture gains strength as it cures. Deep mixing may be performed using either a dry binder, known as dry mixing, or a binder-water slurry, referred to as wet mixing. Deep mixing may be used to treat either inorganic or organic soils to depths of 30 meters or greater. Contractor experience has shown that deep mixing is one of the most effective methods of improving the strength of organic soils. Lab-scale studies (by previous researchers) of wet mixing of inorganic soils have found that the strength of soil-binder mixtures can be expressed as a function of mixture curing time and curing temperature, as well as the quantity of binder used, or binder factor, and the consistency of the binder slurry. No corresponding expression has been generated for wet mixing of organic soils, although many studies on the subject have been performed by previous researchers. The goal of this research was to generate such an expression for one organic soil. The soil used was made of sphagnum peat moss, an organic material commonly found in nature, and an inorganic clay used by previous researchers in studies of deep mixing in inorganic soils. The binder used in this research was a Portland cement. For this research, 43 unique soil-binder mixtures were manufactured. Each mixture involved a unique combination of soil organic matter content, binder factor, and binder slurry consistency. After a soil-binder mixture was made, it was divided, placed into cylindrical molds, and allowed to cure. The temperature of the curing environment of the mixture was monitored. Mixture compressive strength was assessed after 7, 14, and 28 days of curing using two cylindrically molded specimens of the mixture. Data on mixture strength was then evaluated to assess whether it could be expressed as a function of the variables tested. iv This research determined that the strength of at least some organic soils improved with wet mixing can be expressed as a function of soil organic matter content, binder factor, binder slurry consistency, and mixture curing time and curing temperature. The function will likely prove useful to deep mixing contractors, who routinely perform lab-scale deep mixing trials on samples of the soils to be improved in the field. Assuming wet mixing is used, the results of the trials are used to select values of binder factor and binder slurry consistency for the project. The function generated from this research will allow deep mixing contractors to select these values more reliably during the lab-scale phase of their work.

The Deep Mixing Method (DMM) is a widely used, in-situ ground improvement technique that modifies and improves the engineering properties of soil by blending the soil with a cementitious binder. Laboratory specimens were prepared to represent soil improved by the wet method of deep mixing, in which the binder is delivered in the form of a cement-water slurry. To study the influence of curing temperature on the strength of the treated soil, specimens were cured in temperature-controlled water baths for the desired curing time. After curing, unconfined compressive strength (UCS) tests were conducted on the specimens. To investigate the optimum mix design for the wet method of deep mixing, UCS tests were performed to measure the strength of cured specimens, and laboratory miniature vane shear tests were conducted on uncured specimens to measure the undrained shear strength (su), which is used to represent the consistency of the mixture right after mixing. The consistency is important for field mixing because a softer mixture is easier to mix thoroughly. Based on the UCS test results, an equation that can provide a good fit to the strength data of the cured binder-treated soil is proposed. When the curing temperature was changed during curing, the UCS of the specimen cured at a low temperature and then cured at a high temperature was greater than the UCS of the specimen cured at a high temperature first. This seems to be due to different effects of elevated curing temperatures at early and late curing times on the cement reaction rates, such that elevating the curing temperature later produces a more constant reaction rate, which contributes to the reaction efficiency. An optimum mix design that minimizes the amount of binder while satisfying both a target strength of the cured mixture and a target consistency of the uncured mixture can be established by using the fitted equations for UCS and su. The amount of binder required for the optimum mix design increases as the plasticity of the base soil increases and the water content of the base soil (wbase soil) decreases. Master of Science The Deep Mixing Method (DMM) is a ground improvement technique widely used to improve the strength and stiffness of loose sands, soft clays, and organic soils. The DMM is useful for both inland and coastal construction. There are two types of deep mixing. The dry method of deep mixing involves adding the binder in the form of dry powder, and the wet method of deep mixing involves mixing binder-water slurry with the soil. The strength of the cured mixture is significantly influenced by the amount of added cement and water, the curing time, and the curing temperature. This research evaluates the influence of curing temperature on the strength of cured cement-treated soil mixture. Mixture proportions and curing conditions also influence the consistency of the mixture right after mixing, which is important because it affects the amount of mixing energy necessary to thoroughly mix the binder slurry with the soil. This research developed and evaluated fitting equations that correlate the cured mixture strength and the uncured mixture consistency with mixture proportions and curing conditions. These fitting equations can then be used to select an economical and practical mix design method that minimizes the amount of binder needed to achieve both the desired cured strength and uncured consistency. The amount of binder required for the optimum mix design increases as the plasticity of the base soil increases and the water content of the base soil (wbase soil) decreases.

A laboratory testing program was conducted on cement-treated soil mixtures fabricated to represent materials produced by the wet method of deep mixing. The testing program focused on investigating the influences that variations in laboratory testing conditions and in the mix design have on measured property values. A base soil was fabricated from commercially available soil components to produce a very soft lean clay that is relatively easy to mix and can be replicated for future research. The mix designs included a range of water-to-cement ratios of the slurries and a range of cement factors to produce a range of mixture consistencies and a range of unconfined compressive strengths after curing. Unconfined compressive strength (UCS) tests and unconsolidated-undrained (UU) triaxial compression tests were conducted. Secant modulus of elasticity were determined from bottom platen displacements, deformations between bottom platen and cross bar, and from LVDT's placed directly on the cement-treated soil specimens. Five end-face treatment methods were used for the specimens: sawing-and-hand-trimming, machine grinding, sulfur capping, neoprene pads, and gypsum capping. Key findings of this research include the following: (1) The end-face treatment method does not have a significant effect on the unconfined compressive strength and secant modulus; (2) a relationship of UCS with curing time, total-water-to-cement ratio, and dry density of the mixture; (3) the secant modulus determined by bottom platen displacements is significantly affected by slack and deformations in the load frame; (4) the secant modulus determined by local strain measurements was about 630 time the UCS; (5) typical values of Poisson's ratio range from about 0.05 to 0.25 for stress levels equal to half the UCS and about 0.15 to 0.35 at the UCS; (6) Confinement increased the strength at high strains from less than 20% the UCS to about 60% the UCS. In addition to testing the cured mixtures, the consistency of the mixtures were measured right after mixing using a laboratory miniature vane. A combination of the UCS relationship along with the mixture consistency may provide useful information for deep mixing contractors. MS

Geosynthetic reinforced soil (GRS) is a soil improvement technology in which closely spaced horizontal layers of geosynthetic are embedded in a soil mass to provide lateral support and increase strength. GRS is popular due to a relatively new application for bridge support, as well as long-standing application in mechanically stabilized earth walls. Several different GRS design methods have been used, and some are application-specific and not based on fundamental principles of mechanics. Because consensus regarding fundamental behavior of GRS is lacking, numerical and mathematical analyses were performed for laboratory tests obtained from the published literature of GRS under triaxial compression in consolidated-drained conditions. A three-dimensional numerical model was developed using FLAC3D. An existing constitutive model for the soil component was modified to incorporate confining pressure dependency of friction angle and dilation parameters, while retaining the constitutive model's ability to represent nonlinear stress-strain response and plastic yield. Procedures to obtain the parameter values from drained triaxial compression tests on soil specimens were developed. A method to estimate the parameter values from particle size distribution and relative compaction was also developed. The geosynthetic reinforcement was represented by two-dimensional orthotropic elements with soil-geosynthetic interfaces on each side. Comparisons between the numerical analyses and laboratory tests exhibited good agreement for strains from zero to 3% for tests with 1 to 3 layers of reinforcement. As failure is approached at larger strains, agreement was good for specimens that had 1 or 2 layers of reinforcement and soil friction angle less than 40 degrees. For other conditions, the numerical model experienced convergence problems that could not be overcome by mesh refinement or reducing the applied loading rate; however, it appears that, if convergence problems can be solved, the numerical model may provide a mechanics-based representation of GRS behavior, at least for triaxial test conditions. Three mathematical theories of GRS failure available in published literature were applied to the laboratory triaxial tests. Comparisons between the theories and the tests results demonstrated that all three theories have important limitations. These numerical and mathematical evaluations of laboratory GRS tests provided a basis for recommending further research. Ph. D.

Many offshore energy infrastructures are built on carbonate sands which are skeletal remains of marine organisms. Carbonate sands have a porous grain structure and are more compressible compared to quartzitic sand grains which are abundant in alluvial depositional environments. Consequently, there is a stark difference in material behavior of carbonate sands and it is difficult to characterize this distinct behavior with conventional methods. This study focuses on micro-scale characterization of carbonate and quartzitic sands to overcome this challenge. Experimental studies consist of nanoindentation tests performed on 17 different sands; 7 quartzitic and 10 carbonate sand samples. Mechanical properties of individual sand grains with different mineralogies are determined using nanoindentation. A force is applied by the nanoindenter on the grain surface and the load-displacement curve is developed. Modulus and hardness of individual sand grains are evaluated. Nanoindentation test results show that modulus and hardness of carbonate sands are significantly lower than quartzitic sands. For quartzitic grains, mechanical properties are relatively independent of indentation depth; whereas, for carbonate grains there is a considerable decrease in both Young's modulus and hardness values with increasing indentation depth. Results from this study can further be used for the evaluation of compressibility and strength characteristics of these two types of sands as part of a multi-scale analysis framework. Master of Science

With rising awareness that future generations may not have access to the resources and quality of life that exist today, sustainable development has become a priority within civil engineering. One important component of sustainable development is environmental stewardship, which concerns both the resources taken from the environment, and the wastes and byproducts emitted to the environment. To facilitate more sustainable development, environmental accounting is necessary within civil and geotechnical engineering design and construction. Historically, geotechnical practice has focused on maximizing design performance while minimizing monetary costs, and well established methods exist for quantifying these factors. Quantitative consideration of environmental consequences has seldom played a large role in geotechnical design and construction, and clear guidelines and a methodology for such an assessment are not available within the geotechnical profession. Therefore, this research has focused on establishing a method for quantitative streamlined environmental Life Cycle Analysis of energy and carbon dioxide (CO2) emissions for geotechnical ground improvement works, known as the Streamlined Energy and Emissions Assessment Model (SEEAM). The boundaries for the SEEAM extend from raw material extraction through the completion of construction, including the energy and CO2 emissions associated with construction materials, construction site operations, and the transportation of construction materials and wastes. The methodology relies on energy and CO2 emissions coefficients, which represent typical industry average values and not necessarily the specific processes contributing to a project. Therefore, there is uncertainty in SEEAM analyses, which is addressed via a Monte Carlo simulation framework that assumes the energy and CO2 emissions coefficients each follow a lognormal distribution. Data sets of total energy and CO2 emissions generated by the Monte Carlo simulation framework with the SEEAM may be used to statistically compare the energy and CO2 emissions of different geotechnical design alternatives. Such comparisons can help facilitate designing for minimum environmental consequences, thus advancing sustainable development within geotechnical engineering. For clarity, the development and application of the SEEAM is illustrated using two different geotechnical case history projects, including rehabilitation of levee LPV 111 in New Orleans, LA, and the construction of foundations for a replacement dormitory on the Virginia Tech campus. Ph. D.

Pile-supported T-walls, which are concrete floodwalls that are shaped like an inverted "T" and supported by batter piles, are commonly used by the United States Army Corps of Engineers (USACE) to protect low-lying portions of New Orleans and other areas. The design of a T-wall in southern Louisiana is complex, as the structure needs to resist both 1) large settlements caused by fill placed beneath and beside the T-wall before T-wall construction or by fill placed beside the T-wall after T-wall construction, and 2) large lateral flood loads that are imposed during a hurricane. As a result of these loading conditions, large bending moments can develop in the batter piles and these moments need to be accounted for as part of the T wall design. The goal of this research is to develop a more complete understanding of the pile bending moments in T wall systems, specifically for cross sections where large settlements may occur. As a first step towards this goal, Rensselaer Polytechnic Institute (RPI) performed a series of eight centrifuge tests to investigate and physically model the effects of settlement-induced bending moments on pile-supported T-walls. The centrifuge tests were evaluated and interpreted, and in order to better capture uncertainty, upper and lower bounds were estimated for the interpreted data. The centrifuge results offered some valuable insights on their own, but were ultimately used as the basis for validating and calibrating corresponding numerical models. The numerical models were developed following a rigorous modeling approach and using rational and reasonable assumptions based on widely-accepted and well-justified procedures. The numerical model results were in good agreement with the centrifuge results without the need for significant calibration or modifications. This good agreement indicates that similar numerical models can be developed to reliably analyze actual T-wall cross sections. Detailed recommendations were developed for using numerical models to analyze pile-supported T walls, and an example problem is presented herein that illustrates the application of this approach. These same techniques were then used to perform a parametric study to analyze the combined and separate effects of flood loading for a wide range of different T-wall cross sections. The range was selected in collaboration with the USACE in order to reasonably cover cross sections and conditions that 1) are typically encountered in practice, and 2) were expected to generate both upper and lower bound pile bending moments. In total, 3,648 cross sections were analyzed, and 29,184 sets of analysis results were generated since each cross section was analyzed for eight different loading conditions. Summary results are provided to show the influence of the loading conditions and parameters on T-wall response, including the influence of flood loading, new fill symmetry, pile fixity, number of piles, subsurface profile, pile batter, pile type, levee slope, T-wall elevation, and the presence of existing levee fill. In addition, the key results for all of the analyses are provided in the appendices and in an electronic database. Based on the parametric study results, a simplified analysis procedure was developed that can be used to calculated maximum pile bending moments for T walls installed directly on foundation soils due to settlements. In this procedure, the loads from new fill placed during or after T-wall construction are distributed onto the pile, and the pile response is analyzed using traditional p-y curves and a beam on elastic foundation formulation. This procedure shows good agreement with the numerical model results for a range of conditions. To demonstrate the application of the procedure, the same example problem that is analyzed numerically is reanalyzed using the simplified analysis procedure. Due to the complexity of the problem, it was not possible to modify this procedure or develop a similar procedure for T-walls installed on top of new or existing levees. Overall, this research demonstrates that numerical models can be used to calculate the bending moments that can develop in pile-supported T-walls due to settlements and flood loading, provides valuable insights into the behavior of T-walls and the influence of various parameters on T-wall response, presents a large database of T-wall analysis results, and recommends a simplified analysis procedure that can be used in some cases to calculate pile bending moments due to settlements. Ph. D.

Geosynthetic Reinforced Soil (GRS) is a promising technology that can be implemented in walls, culverts, rock fall barriers, and bridge abutments. Its use in walls and abutments is similar to Mechanically Stabilized Earth Walls (MSEW) reinforced with geosynthetics. Both GRS and MSEW are reinforced soil technologies that use reinforcement to provide tensile capacity within soil masses. However, the soil theories behind each method and the design methodologies associated with GRS and MSEW technologies are quite different. Therefore, a study was undertaken to compare the required tensile strength predicted by these various reinforced soil design methodologies. For the purposes of this study, the required ultimate tensile strength was defined as the ultimate tensile strength needed in the reinforcement after all applicable factors of safety, load factors, and reduction factors were applied. The investigation explored both MSEW and GRS. GRS has been made an FHWA "Every Day Counts" initiative. Due to the push to implement GRS technology, it is critical to understand how GRS design methods differs from classic MSEW design methods, specifically in the prediction of ultimate tensile strength required. A parametric study was performed comparing five different reinforced soil analysis methods. Two are current MSEW design methods and one was a proposed revision to an existing MSEW design method. The final two were GRS design methods. These design methods are among the most current and/or widely used design references in the United States regarding reinforced soil technology. There are significant differences between the methods in the governing soil theory particularly between GRS and MSEW design methods. The goal of the study was to understand which design parameters had the most influence on calculated values of the required ultimate tensile strength and nominal "unfactored" tensile strength. A base case was established and a reasonable set of parameter variations was determined. Two loading conditions were imposed, a roadway loading scenario and a bridge loading scenario. Based on parametric study findings, conclusions were drawn about which design parameters had the most influence for different design methods. Additionally, the difference in overall predicted required tensile strength was assessed between the various methods. Finally, the underlying soil theory and assumptions employed by the different methods and their influence on predicted required tensile strength values was interpreted. Master of Science

Rapid drawdown (RDD) occurs when the water level adjacent to a slope or embankment is lowered quickly after a long period of being elevated either at the normal operating level for a dam or in the case of levees, during a prolonged flood. The current state of practice for RDD analysis is a multi-stage undrained strength method based on limit equilibrium. The primary objective of this research was to develop a new method for rapid drawdown based on the finite element method. The new method estimates undrained strengths based on effective consolidation stresses from finite element analysis and the results of isotropically consolidated undrained triaxial compression (ICU-TC) tests. The field strengths appropriate for use with this rapid drawdown method were found to be on average 70% of the strength measured in ICU-TC tests based on back analysis of rapid drawdown failures. For rapid drawdown, anisotropic consolidation, plane strain deformation, and principal stress rotation were shown to produce field undrained strengths in the range of 60 to 80% of the strengths measured in isotropically consolidated undrained (ICU) triaxial compression. The current limit equilibrium method for rapid drawdown was shown to produce a similar reduction in ICU-TC strength. This study also investigated other issues related to RDD. Effective stress analysis of RDD, especially using uncoupled transient seepage analysis, was shown to be inappropriate because important aspects of soil behavior are ignored. Consolidated-undrained strength tests on compacted clay specimens highlighted the importance of relative compaction on undrained strength. Anisotropic consolidation was shown to produce lower undrained strengths in triaxial compression than isotropic consolidation, especially at higher consolidation stresses. The behavior of compacted specimens under principal stress rotation was investigated using triaxial and direct simple shear tests. Finally probabilistic methods were applied to RDD to assess the probability that the factor of safety is less than one, assuming RDD occurs. Ph. D.

The fully softened shear strength was defined by Skempton (1970) as the peak drained shear strength of a clay in a normally consolidated state. All the experience available on the applicability of the fully softened shear strength for slopes is based on back-analyses. Back-analyses of first-time failures in cuts in stiff-fissured clays and embankments constructed of fat clays have shown that, over a long period of time, the shear strength gets reduced from what is measured in the laboratory using undisturbed samples to the fully softened shear strength. These back-analyses require knowledge or assumption of pore pressures in the slope, which will have a significant influence on the shear strength obtained. Karl Terzaghi, in 1936, was the first person that qualitatively explained the behavior of cut slopes in stiff-fissured clays. According to Terzaghi (1936), a softening process is initiated by the water percolating into the fissures causing swelling and decreasing the overall shear strength of the clay mass. Investigations presented later by Skempton and his colleagues showed that the controlling shear strength for cuts in stiff-fissured clays was equal to the fully softened shear strength and recommended this shear strength to be used for design (Skempton 1970; Chandler and Skempton 1974; Chandler 1974; Skempton 1977). Skempton (1977) concluded that displacements caused by progressive failure decrease the shear strength of stiff clays toward the fully softened shear strength. At first, it was believed that only stiff-fissured clays were subjected to softening and that intact clays should be designed using the peak shear strength measured using undisturbed samples (Skempton and Brown 1961; Skempton 1964, 1970). Recent publications have showed that the likelihood of a clay experiencing softening is more dependent on the plasticity of the clay rather than the fissures (Bjerrum 1967; Chandler 1984a; Mesri and Abdel-Ghaffar 1993). Fat clays, when compared to lean clays, tend to be more brittle. This means that fat clays have a more pronounced decrease in shear strength after the peak shear strength is achieved and for this reason are more susceptible to progressive failure. First-time failures in stiff clays usually occur a long period of time after construction. For this reason, steady state seepage was used in the back-analyses of the case histories presented by Skempton and his colleagues. They found that a pore pressure ratio of 0.3 was applicable to first-time failures in cuts in stiff-fissured clays (James 1970; Vaughan and Walbancke 1973; Chandler 1974; Skempton 1977). Investigations presented by Professor Steve Wright and his colleagues of the University of Texas at Austin showed, based on back-analyses, that the fully softened shear strength is also the controlling shear strength of compacted embankments constructed of highly plastic clays (Green and Wright 1986; Kayyal and Wright 1991; Wright 2005; Wright et al. 2007). Steve Wright and his colleagues concluded that weathering, expressed in cycles of wetting and drying, was the main mechanism decreasing the shear strength of compacted clay embankments toward the fully softened shear strength. Failures in this type of projects were found to be shallow (less than 10 ft deep) and to occur numerous years after construction (USACE 1983; Stauffer and Wright 1984; Kayyal and Wright 1991; Wright et al. 2007). A pore pressure ratio ranging from 0.4 to 0.6 was found to be applicable for the case histories analyzed by Wright and his colleagues. Day and Axten (1989) recommended the use of the infinite slope method with seepage parallel to the slope face for slope stability analyses. This same recommendation was presented by Lade (2010). A seepage parallel to the slope face corresponds to a pore pressure ratio ranging from 0.4 to 0.5 for slopes with ratios of 2H:1V to 5H:1V. Failures on compacted clay embankments related to softening have been reported in Texas (Stauffer and Wright 1984; Kayyal and Wright 1991; Wright 2005; Wright et al. 2007), and Mississippi (USACE 1983). According to McCook (2012), softening of this type of structures also occur in Louisiana To perform slope stability analyses using fully softened shear strength parameter, the type of soils, type of projects, and depths where this shear strength is applicable, and the pore pressures and factor of safety to be used in design should be determined. As stated above, the fully softened shear strength has been found to be the controlling shear strength of cuts in stiff clays and compacted embankments constructed of highly plastic clays. Steady state seepage conditions should be used to design cuts in stiff clays, and a pore pressure ratio ranging from 0.4 to 0.6 or a phreatic surface at the surface of the slope should be used to design compacted embankments made of fat clays. In cuts in stiff clays, both shallow and deep failures related to fully softened shear strength have been observed. For this type of project, the recommended methodology for design is to assign a curved fully softened failure envelope to the whole slope, search for the critical failure surface, and obtain the factor of safety. This approach will provide the correct factor of safety but the critical surface obtained might not be what is expected to occur in situ. Pore pressures corresponding to steady state seepage should be used for design. It should be emphasized that the recommendation to use fully softened shear strength in first-time failures in stiff clays is based on the back-analyses of case histories. Research is required to better understand progressive failure and its influence on the shear strength mobilized in situ. In compacted embankments constructed of fat clays, only shallow failures related to fully softened shear strength have been observed. For this type of projects, the recommended methodology for design is to assign a curved fully softened failure envelope to the whole embankment, search for the critical failure surface, and obtain the factor of safety. If for any reason deep failures are to be considered in designing compacted embankments constructed of fat clays, based on the fact that failures in this type of projects are usually shallow, the first 10 ft below the surface of the slope should be assumed to have a shear strength equal to the fully softened shear strength. Pore pressures should be calculated based on a water table coincident with the slope face. The fully softened shear strength should not be used in the foundation soil. If any softening occurred in the foundation soil, this should be reflected in the shear strength measured using undisturbed samples. Softening of the foundation soil is not expected to occur after the embankment is constructed. The consequences of shallow and a deep failures are usually not the same. For this reason, is reasonable that the same factor of safety should not be required for both cases. A shallow failure may be considered by some agencies solely as a maintenance issue. The factor of safety should be based on the uncertainties in the parameters being used for design and the consequences of failure of the structure (Duncan and Wright 2005). The parameters that have more impact on the factor of safety obtained for slope stability are shear strength and pore pressures. The fully softened shear strength is the lowest shear strength expected to be mobilized in first-time slides. This shear strength, coupled with a conservative assumption of pore pressure gives a low uncertainty in the parameters that have the most influence in the factor of safety. For shallow failures, the consequences of failure are very low. For this reason, if the fully softened shear strength is used, coupled with a water table corresponding to the worst case scenario possible, a factor of safety as low as 1.25 can be used. For deep failures, the consequences of failure will vary depending on the structure. The pore pressure for this type of analyses should be based on the worst seepage condition expected throughout the life of the project. In this case, for structures with low to mid consequences of failure, a factor of safety of 1.35 can be used. For structures with a high consequence of failure, a factor of safety of 1.50 can be used. These factors of safety are based on the recommendations presented by Duncan and Wright (2005) for factors of safety based on uncertainties in the parameters and consequences of failures. The fully softened shear strength should be measured using normally consolidated remolded specimens as recommended by Skempton (1977). Soil samples should be hydrated for two days using distilled or site-specific water. The soil sample should then be washed or pushed through a No. 40 (425 µm) sieve. To achieve the desired water content, the soil sample cab be air-dried or more water should be added. Water contents equal to or higher than the liquid limit should be used to prepare test specimens for fully softened shear strength measurements. The direct shear device is recommended for fully softened shear strength measurements. The Bromhead ring shear device does not provide accurate values of fully softened shear strength. The triaxial device requires more time and effort to measure the fully softened shear strength and provides about the same fully softened shear strength as the direct shear device. The fully softened shear strength failure envelope can be estimated using the correlation presented in Figure 6.59 for the parameters required for Equation 4.1. This correlation is only intended to be used in preliminary design or if better information is not available. Laboratory determination of fully softened shear strength is always recommended for final designs. If this is not possible, the confidence limits presented in Figure 6.59 should be used to determine the fully softened shear strength parameters. Ph. D.

Granular soils exhibit a generally beneficial change in engineering properties with time after deposition or densification, during a process commonly known as aging. Soil properties reported to change during aging include the small strain modulus and stiffness, penetration resistance, liquefaction resistance, and pile setup. Different hypotheses have been proposed to explain the occurrence of aging in granular soils, including cementation induced by dissolution of silica and precipitation at the particle contacts, cementation due to microbiological activity, internal stress redistribution caused by particle crushing, and buckling of the load chains due to particle slippage. New evidence points out that internal and time-dependent changes in the soil structure caused by particle slippage and rearrangement as the source of the time-dependent variations in soil properties. This investigation is focused on the study of aging at the particle scale to determine its main driving mechanism and define the factors which affect it. Results from an extensive laboratory testing program and computer simulations based on the discrete element method provide insight into the causes of aging and its effects on the macroscopic properties of sands based on the analysis of the interaction between sand grains. Ph. D.

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. Ph. D.

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. Ph. D.

Increasing interest, from the U.S. Army Corps of Engineers (USACE) and other agencies, in using deep-mixing methods (DMM) to improve the stability of levees constructed on soft ground is driven by the need to reduce levee footprints and environmental impacts and to allow for more rapid construction. Suitable methods for analysis and design of these systems are needed to ensure that the DMM technology is properly applied. DMM shear walls oriented perpendicular to the levee alignment are an effective arrangement for supporting unbalanced lateral loads. Shear walls constructed by overlapping individual DMM columns installed with single-axis or multiple axis equipment include vertical joints caused by the reduced width of the wall at the overlap between adjacent columns. These joints can be made weaker by misalignment during construction, which reduces the efficiency of the overlap. Depending on the prevalence and strength of these joints, complex failure mechanisms, such as racking due to slipping along vertical joints between adjacent installations in the shear walls, can occur. Ordinary limit equilibrium analyses only account for a composite shearing failure mode; whereas, numerical stress-strain analyses can account for other failure modes. Five case studies provided by the USACE were analyzed to evaluate the behavior of levee and floodwall systems founded on soft ground stabilized with DMM shear walls. These identified and illustrated potential failure mechanisms of these types of systems. Two-dimensional numerical stability and settlement analyses were performed for the case studies using the FLAC computer program. The key findings and conclusions for the individual case studies were assessed and integrated into general conclusions about design of deep-mixing support for levees and floodwalls. One of the significant findings from this research was to identify the potential for a partial depth racking failure, which can control design when the DMM shear walls are socketted into a relatively strong bearing layer. The potential for partial depth racking failure is not discussed in the literature and represents a new failure mode identified by this research. This discovery also highlights the importance of adapting suitable methods for analysis and design of these systems to address all potential failure modes. Ph. D.

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. Ph. D.

Clay consolidation has generally been considered from a macro-scale perspective by measuring the macro-scale compression of a clay soil over time. Clay particles in consolidation tests experience shear and normal forces at the inter-particle level due to force applied to the soil at the macro-scale. These shear and normal forces cause the particles to slide at the micro-scale and produce macro-scale changes in soil volume and shape. By considering the inter-particle interactions at the micro-scale, the shear force - normal force - velocity relationship can be described by the Rate Process Theory (RPT). This research investigated the use of the RPT for analyzing sliding at individual clay particle contacts during secondary compression to describe macro-scale clay behavior. The novel micro-scale friction experiments conducted in this research demonstrated that an Atomic Force Microscope (AFM) can be used to obtain coefficient of friction (μ) measurements for montmorillonite. This method allows for the measurements to be performed over spatial scales of a few microns, can be done under dry conditions or a wide range of aqueous solutions, and requires no calibration beyond making a few microscopic measurements of the probe. Control tests of silica on mica (μ = 0.29 ± 0.02) agree with literature values where limits indicate one standard deviation.μ values for wet and dry sodium montmorillonite were determined to be 0.20 ± 0.03 and 0.72 ± 0.03, respectively. The micro-scale AFM and macro-scale triaxial shear, ring shear, and direct shear experimental data ofμ as a function of sliding velocity were found to match well with those calculated using common RPT parameter values. The activation energy for the macro-scale triaxial shear and corresponding micro-scale friction regime experiments fall within the expected range for pure montmorillonite of 84–109 kJ/mol. Additionally, the micro-scale and macro-scale experimental results fall within the expected range for the number of bonds per unit of normal force of 10^7–10^9 bonds/N. A discrete element method (DEM) model was developed to calculate thin, disk-shaped clay particle movement in three dimensions during compression using the RPT as a contact model. The DEM compression results were compared to macro-scale consolidation experiments conducted on the same reference clay as the micro-scale AFM experiments. The influences on the compression of the number of bonds at each clay contact per unit of normal contact force and the activation energy were quantified. Increasing the activation energy decreased the compression, as expected. Similarly, increasing the number of bonds per unit of normal force at the contacts decreased the compression, as expected. Realistic clay fabrics with varying particle sizes, particle size distributions, and aspect ratios led to a compression model with behavior similar to the macro-scale laboratory compression tests. This research provides evidence of the close correspondence between macro-scale and micro-scaleμ measurements and contributes to multi-disciplinary understanding of factors that control friction between clay particles and deformation of clay masses. The results from this work can be applied to a wide range of time-dependent phenomena such as clay secondary compression, shear deformation, and fault dynamics behavior. Ph. D.