Browsing by Author "Castellanos, Bernardo Antonio"

Now showing 1 - 13 of 13
  • Thumbnail Image
    Advancement of Using Portable Free Fall Penetrometers for Geotechnical Site Characterization of Energetic Sandy Nearshore Areas
    Albatal, Ali Hefdhallah Ali (Virginia Tech, 2018-04-24)
    Portable Free Fall Penetrometers (PFFPs) are lightweight tools used for rapid and economic characterization of surficial subaqueous sediments. PFFPs vary in weight, shape and size with options for using add-on units. The different configurations enable deployments in various environments and water depths, including the nearshore zone where conventional methods are challenged by energetic hydrodynamics and limited navigable depth. Moreover, PFFPs offer an opportunity to reduce the high site investigation costs associated with conventional offshore geotechnical site investigation methods. These costs are often a major obstacle for small projects serving remote communities or testing novel renewable energy harvesting machines. However, PFFPs still face issues regarding data analysis and interpretation, particularly in energetic sandy nearshore areas. This includes a lack of data and accepted analysis methods for such environments. Therefore, the goal of this research was to advance data interpretation and sediments characterization methods using PFFPs with emphasis on deployments in energetic nearshore environments. PFFP tests were conducted in the nearshore areas of: Yakutat Bay, AK; Cannon Beach, AK; and the U.S. Army Corps of Engineers' Field Research Facility's beach, Duck, NC. From the measurements, the research goal was addressed by: (1) introducing a methodology to create a regional sediment classification scheme utilizing the PFFP deceleration and pore pressure measurements, sediment traces on the probe upon retrieval, and previous literature; (2) investigating the effect of wave forcing on the sediments' behavior through correlating variations in sediment strength to wave climate, sandbar migration, and depth of closure, as well as identifying areas of significant sediment mobilization processes; and (3) estimating the relative density and friction angle of sand in energetic nearshore areas from PFFP measurements. For the latter, the field data was supported by vacuum triaxial tests and PFFP deployments under controlled laboratory conditions on sand samples prepared at different relative densities. The research outcomes address gaps in knowledge with regard to the limited studies available that investigate the sand geotechnical properties in energetic nearshore areas. More specifically, the research contributes to the understanding of surficial sediment geotechnical properties in energetic nearshore areas and the enhancement of sediment characterization and interpretation methods.
  • Thumbnail Image
    Characterization of Soft Clay and Clay-tire Interaction for the Prediction of Ground Mobility
    Pandit, Rashna (Virginia Tech, 2023-08-22)
    Predicting tire performance on soft, fine-grained soils is required for many off-road explorations in the military, mining, agricultural, and earth-moving sectors. However, the mobility in deformable material is extremely challenging, especially in the presence of water. Although there is a significant amount of research on terrains such as sands, there is a lack of research on fine-grained soils. This research is part of a bigger project that presents a novel approach to improve the mobility of off-road vehicles on wet deformable soils. The approach integrates experimental data from small-scale soil testing, large-scale soil-tire interaction testing, and advanced physics-based numerical simulation techniques. In particular, this thesis attempts to characterize the clay-tire interface by conducting large-scale direct shear tests. In addition to clay-tire contact friction, the properties and strength parameters of the soft clay are determined by conducting various index properties and advanced tests. The testing program accounts for different stresses, loading conditions, and boundary conditions, decided taking into account real field conditions. The results from all these experiments will be used to calibrate and validate the material constitutive models required for the development of a mobility predictive numerical model. Overall, this study contributes to the development of more advanced and accurate terramechanics models that involve deformable terrains like soft clays.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Expanded Byrne Model for Evaluating Seismic Compression
    Jiang, Yusheng (Virginia Tech, 2019-09-18)
    The Byrne (1991) model was developed to predict excess pore water pressure for saturated sands under cyclic loading. However, the model can also be used to predict seismic compression in dry or partially saturated clean sands, which is the focus of this research. The original Byrne (1991) model has two primary limitations. One limitation is that calibration coefficients for the model have only been developed for clean sand, while seismic compression is a concern for a variety of soil types in engineering practice. Another limitation is that the existing calibration coefficients are solely correlated with soil relative density. This is in contrast to findings from studies performed over the last two decades that show various environmental and compositional factors, in addition to relative density, influence seismic compression behavior. To overcome these shortcomings and others the model was transformed to allow it to be implemented in "simplified" and "non-simplified" manners and systematic model calibration procedures were developed by means of MATLAB code. Both "simplified" and "non-simplified" variants of the model are used to analyze a site in Japan impacted by the 2007, Mw6.6 Niigata-ken Chuetsu-oki earthquake. The results from the analyses are in general accord with the post-earthquake field observations and highlight the utility and versatility of the models.
  • Thumbnail Image
    Geotechnical Investigation and Characterization of Bivalve-Sediment Interactions
    Consolvo, Samuel Thomas (Virginia Tech, 2020-06-24)
    Scour around important foundation elements for bridges and other coastal infrastructure is the leading cause of failure and instability of those structures. Traditional scour mitigation methods, such as the placement of riprap, the use of collars or slots, embedding foundations deeper, or a combination thereof can be costly, require long-term maintenance, and can potentially have detrimental environmental effects downstream. These difficulties with traditional methods are potentially alleviated with the implementation of self-sustaining bivalve (e.g., mussel, oyster, scallop) farms that could act as mats of interconnected living barriers, protecting the seabed from scour. The mats would help to attract larval settlement by making the substrate a more suitable habitat, contributing to the sustainability of the bivalve farms. Colonies of bivalves are already being used as living shorelines for retreatment mitigation, embankment stabilization, and supporting habitat for other marine life. These applications are accomplished, in part, by bivalves' strong attachment capabilities from the bioadhesives they secrete that act as a strong underwater glue, adhering their shells to granular substrate. Some species of mussels have been shown to withstand water flow velocities greater than 6 m/s without detaching. For reference, riprap with a median grain size of about 655 mm has been shown to require a flow velocity of at least 1.7 m/s for incipient motion of the boulder-sized riprap. In addition to the contiguous living bivalve mat offering scour protection, the whole or fragmented shells (i.e., shell hash) that are left behind from dead bivalves are hypothesized to reduce erosion potential. Shell hash-laden sediments should be able to better withstand shearing, thereby increasing the critical shear stress required to erode material, compared to sediment without shell hash. Habitat suitability for bivalve colonies is also an important consideration to evaluate what surface enhancements may be needed for a site to be selected for implementation of bivalve scour mats. Bed surfaces that consist of unconsolidated fine-grained sediment are unlikely to be able to support bivalve species as the organisms could sink into the sediment, not allowing solid anchoring points. In contrast, harder substrates typically found in granular sediments offer much more suitable habitats. Along with testing the influence of shell hash and bioadhesive on sediment behavior, this thesis aims to establish a methodology to evaluate whether a section of seafloor can support bivalves or enhancement materials (e.g., shell, shale, or slag fragments) without them sinking, thereby depriving them of oxygen. Together, the examining of geotechnical aspects of bivalve habitat enhancement through seabed soil alteration and the influence of shell hash and bioadhesives on sediment shear behavior are part of a novel multidisciplinary approach toward this proposed bioengineered scour solution. Consequently, the research objectives explored in this thesis are as follows: (1) characterize morphology of existing bivalve colonies through acoustic and direct field measurements; (2) evaluate the spatial variation of the sediment shear strength in terms of proximity to bivalve colonies; (3) expand the domain of confining pressures and shell hash weight fractions used in sediment strength testing; (4) quantify the changes in shear strength and erodibility from laboratory tests on sampled material with and without the presence of bioadhesives, as well as shell fragments mixed in with the sediment; and, (5) develop a methodology ranking system for the suitability of a surficial sediments to support seeding material to improve benthic life habitat substrates. Three exploratory field surveys were conducted where colonies of oysters and other benthic life were present: in the Piankatank River in Virginia, in the Northwest Arm of the Sydney Harbour in Nova Scotia, Canada, and at the Rachel Carson Reserve in North Carolina. Field sampling techniques included Ponar grab samples, hand-dug samples, X-ray rectangular prism cores, and cylindrical push cores, which were all pivotal to understanding sediment composition, size and shape of particle distributions, as well as in-situ depth profiles of shells. Remote sensing and intrusive instrumentation included a rotary scanning sonar, acoustic Doppler current profilers, CTD (Conductivity, Temperature, Depth) probes, underwater cameras, a portable free-fall penetrometer, and in-situ jet erosion testing which helped to characterize the morphology of the bivalve colonies and the spatial variability of sediment strength. Subsequent laboratory experiments included grain size distribution analyses, vacuum triaxial tests to measure changes in shear strength with and without shell hash, and miniature vane and pocket erodometer tests on bioadhesive-treated sediments. The results showed: (1) a significant increase in the standard deviation of the backscatter intensity where the oyster reef was located; (2) the in-situ sediment shear strength increased slightly closer to the oyster reef at the Piankatank River site; (3) samples with a higher oyster density exhibited less uniform particle size distributions; (4) the presence of less than approximately 4% (by weight) of shell fragments increased the secant friction angle by approximately 6° relative to samples with no shell fragments; and, (5) the harbor bed of the Northwest Arm of the Sydney Harbour is a suitable stiffness for enhancement with shell hash over about 23% of its area. Preliminary testing showed a subtle increase in the torsional shear resistance and a decrease in erodibility for bioadhesive-treated samples; however, further testing is needed for confidence to be achieved in the results due to bioadhesive supply issues.
  • Thumbnail Image
    In-Situ Geotechnical Characterization of Soft Estuarine Surficial Sediments Using a Portable Free Fall Penetrometer
    Kiptoo, Dennis Kipngetich (Virginia Tech, 2020-07-02)
    Knowledge of geotechnical soil properties in the upper meter of the seabed is important for challenges such as scour around submerged structures, management of unexploded ordnances, and generally issues associated with active sediment transport and deposition. Portable free fall penetrometers have been previously used to provide initial information on sediment type, strength, and stratification, but challenges with the calibration of empirical parameters such as the cone factor and strain rate factor hampered the derivation of geotechnical design parameters such as undrained shear strength. This challenge applies particularly in areas of more rare seabed soil conditions such as very soft estuarine sediments. This study aims to advance the analysis procedure of portable free fall penetrometers (PFFP) in soft subaquatic fine-grained soils with natural water contents greater than the liquid limit by estimating the undrained shear strength (su). The logarithmic and power law methods for strain rate correction were investigated at sites in the York River Estuary and yielded a match to vane shear results at a logarithmic multiplier of k=0.1-0.3 and a power law rate exponent of β=0.01-0.03, indicating minimal strain rate effects. Resulting representative cone factors based on sediment strength and profile groupings ranged from 7 to 12 for logarithmic, power law, and no strain correction, and were tested at sites in the Potomac River with similar sediment properties. The PFFP su compared well with mini-vane shear measurements with differences of less than ± 0.5 kPa. Additionally, the PFFP su showed inappreciable differences in strength with or without strain rate application. Therefore, these high water content soils that exhibit little strain rate effects within a soil behavior context, can be better understood through rheological studies. Rheological studies were conducted, and the storage and loss modulus were observed to remain constant when the soil is tested over a range of frequencies. This indicates that the sediment strength is not affected by the rate of soil testing. The outcome of this study is the advanced the use of the PFFP by quantifying the strain rate effects and defining the applicable cone factors for use in estimating the undrained shear strength of soft estuarine marine soils. Furthermore, the understanding of soil behavior of these soils has been explored from rheological context.
  • Thumbnail Image
    Influences of Curing Conditions and Organic Matter on Characteristics of Cement-treated Soil for the Wet Method of Deep Mixing
    Ju, Hwanik (Virginia Tech, 2023-07-14)
    The wet method of deep mixing constructs binder-treated soil columns by mixing a binder-water slurry with soft soil in-situ to improve the engineering properties of the soil. The strength of binder-treated soil is affected by characteristics of the in-situ soil and binder, mixing conditions, and curing conditions.The study presented herein aims to investigate the influences of curing time, curing temperature, mix design proportion, organic matter in the soil, and curing stress on the strength of cement-treated soil. Fabricated and natural soft soils were mixed with a cement-water slurry to mimic soil improved by the wet method of deep mixing. Laboratory-size samples were cured under various curing conditions and tested for unconfined compressive strength (UCS).The experimental test results showed that (1) a higher curing temperature and longer curing time generally increase the strength; (2) organic matter in cement-treated soil decrease and/or delay the strength development; and (3) curing stress affects the strength but its effect is influenced by drainage conditions. Based on the test results, strength-predicting correlations for cement-treated soil that account for various curing conditions and organic contents were proposed and validated.This research contributes to advancing the knowledge about the effects of strength-controlling factors of soil improved by cement and to improving the reliability of strength predictions with the proposed correlations. Therefore, the number of sample batches that need to be prepared and tested in a deep mixing project can be reduced, thereby saving the project's time and costs while achieving the target strength of the improved soil.
  • Thumbnail Image
    Influences of Test Conditions and Mixture Proportions on Property Values of Soil Treated with Cement to Represent the Wet Method of Deep Mixing
    Nevarez Garibaldi, Roberto (Virginia Tech, 2017-09-19)
    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.
  • Thumbnail Image
    Investigation of Pore Pressures During High-Velocity Impact by a Free Fall Penetrometer
    Mumtaz, Muhammad Bilal (Virginia Tech, 2018-06-28)
    Free-fall penetrometers (FFPs) are an attractive tool for the rapid characterization of sediments in the nearshore and coastal areas. To improve their measurement capabilities, modern FFPs can be equipped with pore pressure sensors. Pore pressure measurements are extensively used in traditional cone penetration testing, but their usage and interpretation is still limited for FFP testing. This thesis represents an effort to advance the interpretation of pore pressure measurements from FFP testing. Data was collected using the torpedo-shaped FFP BlueDrop during surveys at Herschel Island, YT, Yakutat, AK, Clay Bank, VA, and Yorktown, VA. Additionally, test deployments in the laboratory were performed in kaolin clay. Data analysis was focused on pore pressure measurements during these deployments. Two major advancements regarding current data analysis of FFP pore pressure measurements were explored: 1) a method based on fluid dynamic principles was proposed to correct the pressure recordings for the dynamic flow effects due to the high-velocity fall and impact. The results show that using Bernoulli’s theorem coupled with the concept of pressure coefficients results in good agreement between measured and hydrostatic pressures during the free-fall and initial penetration stage. 2) Pore pressure dissipation curves measured by the penetrometer at rest at maximum penetration depth were also studied. The mechanisms behind the non-standard dissipation curves were explored. The results suggest that non-standard dissipation curves can be interpreted by correcting according to Sully et al.’s (1999) extrapolation technique. The technique can also be used with data from an unsaturated or clogged filter.
  • Thumbnail Image
    A Method for Interpreting the In-Situ Consolidation State of Surficial Seabed Sediments using a Free-Fall Penetrometer
    Dorvinen, Jared Ian (Virginia Tech, 2016-10-13)
    Free-fall penetrometers (FFP) are useful instruments for the rapid characterization of seabed sediments. However, the interpretation of FFP data remains largely a skilled task. In order to increase the reliability of results obtained using these instruments, in both expert and non-expert hands, it is advantageous to establish well defined and repeatable procedures for instrument use and data interpretation. The purpose of this research was therefore to develop and refine methods for the interpretation of FFP data. Data were gathered with the FFP Nimrod during two surveys following dredging in Sydney Harbour, Nova Scotia. The challenge of interpreting the data from these two surveys in an efficient and consistent manner was the basis of this work and led to the development of new techniques for improving resolution of the mud-line, identifying areas of erosion and deposition, and qualitatively evaluating the consolidation state of cohesive marine sediments. The method developed for improving the resolution of the mud-line simply describes a procedure of combining the data from different accelerometers with different accuracies and ranges to more clearly define the point of impact with the sea-floor. The method developed to evaluate in-situ sediment consolidation state combines theories of self-weight consolidation and ultimate bearing capacity to predict a range of potential bearing capacities for normally consolidated cohesive sediments. Finally, by combining the previous two methods a third method is proposed for locating areas of potential erosion and deposition.
  • Thumbnail Image
    Numerical Analysis of RAP Elements under Dynamic Loading
    Saade, Angela Charbel (Virginia Tech, 2019-01-24)
    The 2010-2011 Canterbury, New Zealand, Earthquake Sequence (CES) resulted in 185 fatalities and approximately $NZ40 billion in damage, much of which was due to liquefaction and related phenomena. As a result, an extensive soil improvement field testing program was initiated and Rammed Aggregate Piers� (RAP) were shown to be a feasible method to mitigate the risk from liquefaction during future events. To better design and more fully assess the efficacy of reinforcement techniques against liquefaction, pre- and post-treatment in-situ test data are compiled, to include results from cone penetration tests (CPT), direct-push crosshole tests, and vibroseis (T-Rex) shaking tests. The data are used to evaluate the capabilities of numerical tools to predict the liquefaction response of unimproved and improved sites. A finite difference (FD) numerical model is developed in a FLAC platform and a coupled analysis using the Finn model with Byrne (1991) formulation is conducted. The FD model calibrated for top-down shakings similar to the vibroseis tests succeeded in qualitatively reproducing the general observed behavior without quantitatively matching the in-situ values for shear strains and excess pore pressure ratios. The introduction of the RAP elements to the FD model reduced the shear strain, but slightly overestimated that reduction. Considering more advanced constitutive models that better simulate the complexity of the soil behavior under dynamic loading would likely increase the accuracy of the predicted response.
  • Thumbnail Image
    Transient Seepage Analysis for Levees and Dams: Numerical and Monitoring Approaches
    Walshire, Lucas Adam (Virginia Tech, 2024-05-03)
    An investigation into the transient impacts of flood loadings on earthen embankments was conducted. Two embankments were instrumented and monitored over a period of four years. One of these embankments was a levee located along the Mississippi River just north of Cairo, Illinois. The other embankment was part of a catchment basin at the Engineer Research and Development Center located in Vicksburg, MS. Tensiometer and porous block sensors were used to monitor the pore water pressures in the embankments. It was found that when measuring the field soil water retention, tensiometers were more responsive than porous block sensors at low suctions; although, at shallower depths, the tensiometer performance was limited during periods of extended drying. It was shown that during the start of flooding, pore water pressures in the embankment soils were near −10 kPa at depths less than 2 m, which was greater than the normally assumed hydrostatic conditions. An investigation into flood hydrographs collected from across the United States showed that flood durations could be hundreds of days long. These hydrographs were collected over a period of 10 years. It was found that the recorded peak flood stage exceeded the major flood stage 11% of the time. An uncouple transient seepage model of a 2015 Mississippi River flood event that occurred at the Cairo levee showed that an uncoupled model could simulate the field measurements; however, the material properties that resulted in the most accurate simulation differed from those measured in the laboratory. Soil water retention characteristics of the embankment soils were assessed, and it was found that laboratory measured soil water retention curves could be used to bracket field measurements. Slope stability analyses were performed as a proxy to assessing the progression of the wetting front in the levees. Accounting for the increase in shear strength due to the presence of matric suction resulted in minimal impacts to stability factors of safety for levee embankments during flood loadings. The results of this investigation will help to improve the reliability of transient seepage analyses and provides guidance for future embankment monitoring investigations.
  • Thumbnail Image
    Use and Measurement of Fully Softened Shear Strength
    Castellanos, Bernardo Antonio (Virginia Tech, 2014-03-17)
    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.