Browsing by Author "Meehan, Christopher L. (orcid.org/0000-0002-9721-6275)"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Analytical Approach for Modeling Axisymmetric Levee Underseepage
    (American Society of Civil Engineers., 2014) Benjasupattananan, Sittinan; Meehan, Christopher L.; Meehan, Christopher L., Benjasupattananan, Sittinan; Meehan, Christopher L. (orcid.org/0000-0002-9721-6275); Benjasupattananan, Sittinan
    A variety of real-world levee underseepage problems can best be modeled using an axisymmetric analysis approach. In current practice, axisymmetric levee underseepage analyses are performed using numerical modeling approaches, such as the FEM. In contrast, levee underseepage analyses of planar cross sections are often analyzed using a blanket theory analytical approach, which is quicker and more convenient than numerical modeling. To address this problem, this paper provides a derivation of a series of closed-form blanket theory analytical equations that can be used to perform an axisymmetric levee underseepage analysis. This derivation begins from the governing equation of semiconfined uid ow beneath a levee in a shallow semiconfined aquifer. The equations that result from this derivation can be used to calculate the total head in the pervious foundation layer and the seepage quantity that passes through the levee foundation over time. Different equations are presented for different seepage directions relative to the axis of rotation and different model boundary conditions. A typical example problem is used to compare results from the axisymmetric analytical equations that are derived with those from axisymmetric finite-element analyses. For this example problem, the analytical equations yield results that are equal to or more conservative than those from the finite-element analysis, where conservative results correspond to greater seepage under the levee and higher heads at the levee toe.
  • Thumbnail Image
    Item
    Column Supported Embankments with Geosynthetic Encased Columns: Parametric Study
    (Springer International Publishing AG, 2014) Khabbazian, Majid; Meehan, Christopher L.; Kaliakin, Victor N.; Khabbazian, Majid, Meehan, Christopher L., Kaliakin, Victor N.; Meehan, Christopher L. (orcid.org/0000-0002-9721-6275); Kaliakin, Victor N.
    Three-dimensional finite element analyses were performed to investigate different factors that affect the behavior of column supported embankments (CSEs) that are constructed using geosynthetic encased columns (GECs) as the deep foundation elements. Analyses were performed to study the influence of the geosynthetic encasement on the behavior of granular columns (GCs) in CSEs. Stress reduction ratios (SRRs) obtained from finite element analyses were compared to those calculated from ten different analytical solutions. Parametric analyses were also carried out to study the effect of variations in the stiffness of the encasement, the area replacement ratio, and the length of the geosynthetic encasement on the performance of CSEs. Finally, the sensitivity of the numerical results, particularly the lateral displacement of GECs, to the constitutive model that was used to simulate the behavior of the granular column material was examined. Numerical results showed that encasing a GC in a CSE not only improves the performance of the CSE but also enhances the behavior of the GC. No agreement was found between the calculated values of the SRR from finite element analyses and those from existing analytical solutions.
  • Thumbnail Image
    Item
    Displacement-Based Internal Design of Geosynthetic-Reinforced Earth Structures Subjected to Seismic Loading Conditions
    (ICE Publishing Ltd., 2013) Vahedifard, F.; Leshchinsky, Dov; Meehan, Christopher L.; Vahedifard, F., Leshchinsky, D., Meehan, C. L.; Leshchinsky, Dov.; Meehan, Christopher L. (orcid.org/0000-0002-9721-6275)
    A majority of the existing analytical approaches for displacement-based seismic design of geosynthetic-reinforced earth structures (GRESs) have been developed by considering only a translational mode of failure (external sliding stability), and consequently do not provide a means for assessing the seismic displacement of GRESs due to rotational movements (internal stability). Internal rotational failure can degenerate to a translational one should it be more critical; however, the reverse is not true, which makes rotational failure a more generic mechanism. To address this issue, this paper presents a new analytical-numerical framework for the displacement-based design of GRESs, which assesses the potential for earthquake-induced displacements via an internal stability (rotational) failure mechanism. For design purposes, in order to determine the superimposed force in the reinforcement due to seismicity and its associated displacement, the proposed approach examines two limiting conditions: (a) the upper-bound force that can be mobilised in the reinforcement, as determined by pseudo-static limit equilibrium; and (b) the force that can be induced in the reinforcement by a given earthquake acceleration applied over a finite time increment. Either condition satisfies equilibrium. The prevailing seismically induced force and displacement in the reinforcement for each time increment are determined by selecting the smaller value that results from these two conditions. As an auxiliary tool, a set of pullout simulations was performed using finite-element analysis in order to relate the force and displacement in the geosynthetic reinforcement for various geosynthetic stiffnesses. To illustrate the application of the proposed method, a design example using a Kobe earthquake record is presented. For this example, the superimposed force in the reinforcement due to seismicity, the seismic displacement, and the seismic rotation are calculated. The required unfactored geosynthetic strength is then determined using a uniform distribution function.
  • Thumbnail Image
    Item
    Measuring "Fast" Shear Strengths Along Slickensided Surfaces in the Bromhead Ring Shear
    (ASTM International, 2008) Meehan, Christopher L.; Brandon, Thomas L.; Duncan, J. M. (James Michael); Meehan, C. L., Brandon, T. L., Duncan, J. M.; Meehan, Christopher L. (orcid.org/0000-0002-9721-6275)
    Fast and slow ring shear tests were performed in the Bromhead ring shear device to examine the effect of the displacement rate on the shear strength measured along slickensided discontinuities in Rancho Solano Fat Clay. For each test, initial drained shearing was performed at a displacement rate of 0.018 mm/min, fast shearing was performed at a rate of 44.5 mm/min, and drained shearing was recommenced at a displacement rate of 0:018 mm/min. Significant variations in measured post-peak shear strengths were observed, and problems with the pore pressure response in the soil surrounding the slickensided plane are discussed. As a result of these problems, it was concluded that fast Bromhead ring shear tests may not be suitable for evaluating the effects of fast shearing on the strengths of slickensided surfaces.
  • Thumbnail Image
    Item
    Numerical Study of the Effect of Geosynthetic Encasement on the Behaviour of Granular Columns
    (ICE Publishing, 2010) Khabbazian, Majid; Kaliakin, Victor N.; Meehan, Christopher L.; Khabbazian, M., Kaliakin, V. N., Meehan, C. L.; Meehan, Christopher L. (orcid.org/0000-0002-9721-6275)
    In very soft soils, the use of granular columns can be restricted due to the lack of adequate lateral confining pressure. In these conditions, the columns can be encased by a suitable geosynthetic to provide the required confining pressure and to increase their bearing capacity. Using a high-strength geosynthetic for confinement not only increases the strength of a granular column, but also prevents lateral displacement of the column into the very soft surrounding soil. This paper describes three-dimensional finite element analyses carried out to simulate the behaviour of a single granular column with and without encasement in a very soft clay using the computer program ABAQUS. Comprehensive numerical analyses were performed to study the influence of the geosynthetic stiffness, the friction and dilation angle of the column material, the length of geosynthetic encasement, the diameter of the column, the length of the column and the coefficient of in situ lateral earth pressure. Model results show that the stress–settlement behaviour of granular columns can be significantly improved by encasing them. The stiffness of the encasement was found to have a major effect on the stress–settlement response of encased columns and their associated load-carrying capacity. For partially encased columns, the optimum length of encasement was found to be a function of the stress that is applied to the column.
  • Thumbnail Image
    Item
    Using a Complex-Impedance Measuring Instrument to Determine In Situ Soil Unit Weight and Moisture Content
    (ASTM International, 2013) Meehan, Christopher L.; Hertz, Jason S.; Meehan, C. L., Hertz, Jason S.; Meehan, Christopher L. (orcid.org/0000-0002-9721-6275)
    In situ measurements of soil unit weight and moisture content play a critical role in conventional compaction quality assurance and quality control procedures. Recently, there have been a number of attempts to develop alternative electrically-based test devices that can be used to measure the in situ unit weight and/or moisture content of a compacted soil; these devices are intended to serve as alternatives to more traditional tests such as sand cone, rubber balloon, drive cylinder, or nuclear density gauge tests. The study described in this paper focuses on the use of a relatively new electrically-based in situ soil test device that uses measurements of soil complex impedance, soil capacitance, and soil resistance to infer in situ soil unit weight and moisture content; this device is typically referred to as a complex-impedance measuring instrument (CIMI). This paper provides a detailed explanation of current CIMI operating principles and also describes the utilization of a CIMI for field- and laboratory-based testing. The CIMI used in this study was calibrated and assessed in two field compaction projects in which different silty sands were used for construction. A mold-based calibration approach was developed for building an electrically-based soil model using the CIMI; this approach provides an alternative to field calibration of the device. In order to perform a more complete assessment of the CIMI in a controlled environment, a series of CIMI tests were conducted in a large “field box,” and the resulting in situ measurements of soil unit weight and moisture content made using the CIMI are compared with the results from nuclear density gauge, sand cone, and drive cylinder tests. The advantages and disadvantages of field versus mold calibration of the CIMI are discussed, and side-by-side assessment of the CIMI relative to other conventionally used compaction control tests allows the reader to assess the accuracy of this device.