Browsing by Author "Dr. R. Wayne Skaggs, Committee Member"

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    Influence of Water Infiltration on the Deformation of Mechanically Stabilized Earth (MSE) Walls
    (2008-12-07) Kim, Wan Soo; Dr. Mohammed A. Gabr, Committee Member; Dr. M. Shamimur Rahman, Committee Member; Dr. R. Wayne Skaggs, Committee Member; Dr. Roy H. Borden, Committee Chair
    The use of locally available soils, often with significant fines contents, clearly has great economic impact on the cost of mechanically stabilized earth (MSE) wall construction. However, numerous problems associated with large deformations and some failures after surface water infiltration into these ?marginal soils? have been documented in the literature (e.g. Soong and Koerner, 1999). The documented problems can be better understood using the framework of unsaturated soil mechanics because compacted soils are typically found in an unsaturated state. This thesis presents the implementation of infiltration, seepage, volume change, and shear strength modeling of unsaturated conditions appropriate for compacted soils into the FLAC (Fast Lagrangian Analysis of Continua) code to study the behavior of an MSE wall constructed with marginal soils. The effect of matric suction (as well as net normal stress) on the soil properties such as elastic modulus and shear strength was considered in the developed FLAC model. The stress dependent elastic modulus was determined from the results of the one-dimensional oedometer test. The elastic moduli of specimens prepared under various compaction conditions were investigated for soaked specimens as well as specimens tested at as-compacted moisture contents while considering both drained and undrained loading conditions. A procedure to estimate the undrained modulus from the constitutive relation for drained loading and the predicted pore pressure due to loading was suggested. The prediction of pore pressure was computed by modifying Hilf?s equation. Comprehensive studies on the prediction on unsaturated shear strength were performed using the three most commonly used empirical procedures (Vanapalli et al.?s 1st and 2nd approaches and the Khalili and Khabbaz method). The influence of the shape of Soil Water Characteristic Curve (SWCC) on the predicted shear strength was investigated through a parametric study. Moreover, shear strength data published in the literature for fifteen soils were examined using these procedures. Comparisons between measured and predicted values of unsaturated shear strength were presented for different soil types. The effect of stress state on the prediction of shear strength was also discussed. A series of numerical simulations including transient seepage analyses and stress deformation analyses were performed in order to predict the behavior of a MSE wall subjected to surface-water infiltration. In this research, two mechanisms to cause the deformation due to wetting were considered in simulations; (1) the deformation induced by shear strength decreases and (2) the volumetric deformation (swell or collapse) due to wetting. The effect of a low-quality compaction zone behind the wall face and the resulting higher permeability on the wall behavior was investigated. As result of the simulations, the wall deformations (face deflections and settlements) and reinforcement tensions (average and maximum tensions) were presented during construction and after periods of infiltration.
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    Methods to Evaluate Normal Rainfall for Short-Term Wetland Hydrology Assessment.
    (2006-11-15) Sumner, Jaclyn Patricia; Dr. R. Wayne Skaggs, Committee Member; Dr. Randall K. Kolka, Committee Member; Dr. David L. Lindbo, Committee Member; Dr. Michael J. Vepraskas, Committee Chair
    It is assumed, but not proven, that wetland hydrology can be evaluated with single-season measurements of water table data, if antecedent rainfall is within a normal or drier than normal range. Four methods of rainfall analysis were compared to long-term records of water table levels to determine which method could be used to identify sites with wetland hydrology. Water tables were simulated by DRAINMOD for two sites in NC and collected manually for one site in MN. Single years from 40 to 45 years of long-term water table data records were evaluated and compared with results from long-term records. Plots with wetland hydrology had a water table within 30 cm of the surface, during the growing season, in at least half the years. Years meeting wetland hydrology were identified for each site. Normal rainfall was determined from using the 30th and 70th percentiles obtained from a WETS data set for the nearest available weather stations. The first two methods computed a 30-day moving rainfall total that was compared with a normal range of rainfall values obtained from the WETS data. The remaining methods used a 3-month period of antecedent rainfall approach along with the WETS data to determine through calculations whether a given period had a normal range of rainfall. Plots meeting wetland hydrology met it in over 90% of the years in the long-term water table data records at all three sites. For single-year data, the moving total methods provided the correct conclusion in less than 45% of the years that met wetland hydrology. The DAREM approaches, which used rainfall for the prior 3-month period, correctly identified plots with wetland hydrology in over 80% of the years. The results showed that when single-season data are used to identify wetland hydrology, then, in most cases, no more than two years of measurement will be needed to reach a correct conclusion, and in most cases a single season of data may be used. However, there may be plots that have significant periods of drought or wetness and it is imperative to study surrounding plots at the research site to obtain an overall wetland hydrology at the site.