The Static And Dynamic Analysis Of Embankment Dams Using An Endochronic Soil Model

Author : Nicholas Aaron Paull
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.5/5 (825 download)

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Download or read book Modeling Spatial Variability of Liquefiable Deposits for Nonlinear Dynamic Analyses of Embankment Dams written by Nicholas Aaron Paull and published by . This book was released on 2020 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Evaluating the potential for seismic deformation of an embankment on a liquefiable deposit requires consideration of the deposit's spatial variability, the scale of any potential deformation mechanisms, and the quality of the site characterization information, among other factors. This dissertation is completed to aid geotechnical engineers in the consideration of these factors for the design and reevaluation of embankment dams founded on liquefiable deposits subjected to seismic ground shaking events. It is comprised primarily of three studies that examine different methods which can be used to model the spatial variability of soil properties for nonlinear dynamic analyses (NDAs) of embankment dams on liquefiable deposits. The first study (chapters 2 and 3) examines the use of unconditional random fields to model the soil properties of a liquefiable deposit consisting only of sand and compares the deformations obtained to those obtained using uniform properties in the sand stratum as is commonly done in current practice. The second study (chapters 4 and 5) examines the use of random fields conditioned on known data at specific locations, obtained during site investigation, to model the soil properties of a liquefiable deposit consisting only of sand and compares the deformations obtained to those obtained using unconditional random fields in the sand stratum. The third study (chapter 6) examines the use of 3D transition probability to model the soil categories of a multi-soil type deposit to assess potential differences in deformations that may arise from different assumptions of soil type used in NDAs. All three studies are used to assess different ways in which spatial variability of soil properties in a liquefiable foundation layer can me modeled to estimate the deformations that may occur when an embankment dam is subjected to a seismic ground motion event. All analyses herein are carried out using the finite difference software FLAC 8.0 with the user-defined nonlinear constitutive model PM4Sand v3.1 for the liquefiable soils. For the purposes of this dissertation, nonlinear deformation analysis and nonlinear dynamic analysis are interchangeable terms. Chapters 2 and 3 examine different size embankments on a spatially variable (stochastic) liquefiable alluvial deposit to determine the effect of embankment size on the potential variability in seismic deformations and the selection of equivalent representative uniform properties. Simulations are presented for embankments ranging from 5 m to 45 m high on stochastic and uniform alluvial layers subjected to a range of input motions, with sensitivity cases including the effects of various parameters describing the alluvium and embankments. Crest settlements and slope displacements obtained from the analyses with stochastic and uniform alluvial layers are compared to obtain equivalent uniform or representative percentile properties for which a uniform model produces the same deformation as a stochastic model. The representative percentile properties to estimate median deformations from a set of stochastic realizations are generally between the 40th and 60th percentile, whereas the representative percentile properties to estimate deformations conservatively (i.e., exceeded in less than 16% of the analysis cases) are generally closer to the 30th percentile. The variability in deformation patterns obtained with the stochastic models increases as the alluvium's scale of fluctuation in the horizontal direction increases relative to the embankment base width. Recommendations regarding factors to consider in selecting representative properties for spatially variable alluvial foundations in NDAs of embankment dams and the corresponding variability in deformations are presented. Chapters 4 and 5 examine the potential benefits of incorporating the spatial variability of liquefiable soils captured in site investigation data into NDAs of embankment dams using conditional random fields. Two-dimensional NDAs are performed for a series of hypothetical embankment dams on a spatially variable liquefiable foundation layer to evaluate the utility of representing the foundation layer with random fields conditioned on different levels of site characterization information. A set of two-dimensional parent models, each representing a "true" foundation condition, were generated using unconditional random fields of equivalent clean sand, corrected Standard Penetration Test (N1)[subscript 60cs] values. Different levels of site characterization were then represented by combining different numbers of "local borings" (i.e., columns of data from the parent model) with the optional inclusion of constraints on the geostatistical properties that might come from "site-wide explorations." NDAs were performed using the same input motions for the parent model (which represents perfect knowledge of soil conditions), a set of realizations conditioned on the local borings alone, and a set of realizations conditioned on the local borings with site-wide statistics. Embankment deformations obtained for the conditional realizations are compared to those for the parent model to evaluate the impacts of different conditioning borings and how increasing levels of site characterization might improve the accuracy of deformation predictions. Parametric analyses include varying the embankment size, scales of fluctuation in the foundation stratum, number of conditioning borings, and ground motions. The results of these comparisons illustrate potential benefits and limitations in using conditional random fields for representing spatial variable liquefiable foundation layers in NDAs for levees or embankment dams. Chapter 6 presents a case study on the use of conditional transition probability as a method of separating soil categories for interbedded alluvial foundation strata in NDAs of Perris Dam. The challenges involved in synthesizing information from different sources (i.e., geologic conditions, different site investigation tools, lab data, field classifications) into soil categories for interbedded alluvium are examined. The application of conditional transition probability methods for developing three-dimensional (3D) realizations of the upper Holocene and lower Pleistocene alluvial strata over a 305 m wide interval along the dam alignment is described including the challenges with insufficient data and nonstationary geologic conditions. Two-dimensional (2D) NDA models of Perris Dam are created by slicing the 3D transition probability realizations into five 2D cross sections. The constitutive models PM4Sand and PM4Silt were used to model the sand and clay soil categories in the alluvial strata, as well as the different zones in the embankment. The deformations and variability in deformations for each cross section are compared and sensitivity studies are completed to examine the impact of several factors including the impacts of the small strain shear modulus for the alluvium, the mean lengths and sills for the alluvium categories, the strengths for each alluvium soil category, and different ground motions. NDA cross sections of Perris Dam with uniformly (non-categorical) distributed properties are completed with and without additional deterministic imbedded soil lenses and the deformations are compared with the transition probability models and deterministic models previously completed by others. The utility of the conditional transition probability models for NDAs of Perris Dam, along with the implications and lessons for practice, are discussed.