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<br /> <br /> <br /> <br />15 <br />Soil Characterization (Part 3) - Shear <br />Strength Characterization for Slope <br />Stability Analyses <br />Introduction <br />Shear strength is one of the most important factors in <br />slope stability analyses, yet is also the most complex. <br />Developing relevant and representative shear strength <br />characterization of both the embankment and <br />foundation soils for various loading conditions is a task <br />that often gets muddled even by experienced <br />engineers. This article endeavors to clarify some of the <br />key considerations and methods used to develop shear <br />strength parameters for slope stability analyses of <br />embankment dams. Shear strengths can be measured <br />through laboratory and field testing or estimated using <br />empirical correlations. This article explains these <br />methods for shear strength characterization of various <br />soils under different loading conditions. <br />Previous Articles <br />The fundamentals of slope stability analyses were <br />presented in the November 2013 issue of the Western <br />Dam Engineering newsletter in an article titled <br />“Embankment Dam Slope Stability 101,” where the <br />topic of shear strength characterization for slope <br />stability analysis was introduced. Discussion was also <br />provided on slope stability modeling for the following <br />embankment loading conditions: end-of-construction, <br />steady-state drained, rapid drawdown, and seismic. <br />The fundamentals of soil characterization for dams, <br />including some introductory aspects of shear strength <br />characterization, were presented in the July 2014 <br />newsletter issue in an article titled “Soil <br />Characterization (Part 1) – Here’s the Dirt.” That article <br />presented a broad overview of properties pertinent to <br />the overall performance and analysis of dams. <br />A subsequent article presented in the October 2014 <br />newsletter issue titled “Soil Characterization (Part 2) – <br />Laboratory and Field Shear Strength Testing” defined <br />shear strength, undrained and drained conditions, <br />and total and effective stresses and discussed various <br />types of laboratory and field testing for evaluating the <br />shear strength of cohesionless (sands and gravels) and <br />cohesive (clays and silts) soils. This current article <br />elaborates on utilizing laboratory and field testing <br />results for selection and development of shear <br />strength parameters to be used in slope stability <br />analyses for embankment dams. <br />You are invited to revisit and review the above three <br />articles, as this article builds on many of the concepts <br />presented in the previous articles. <br />What this Article Does Not Cover <br />This article focuses on shear strength characterization <br />for various stability analyses. Considerations to <br />determine what loading conditions should be analyzed, <br />determining appropriate phreatic and pore pressure <br />conditions, selecting critical cross sections, and the <br />appropriate analysis methodology are outside the <br />scope of this article. This article does not discuss shear <br />strength characterization of rock or special soils such <br />as cemented sands, highly sensitive (“quick”) clays, and <br />organic soils; the discussion is limited to the most <br />common soils used in dam engineering and <br />construction. This article also does not cover seismic <br />shear strength characterization. <br />Typical Loading Conditions <br />Before developing shear strength parameters for use <br />in slope stability analyses, it is important to understand <br />the potential loading conditions by which an <br />embankment dam should be evaluated. Shear <br />strengths in embankment and foundation soils change <br />throughout various loading conditions during the life of <br />a dam. As a result, the stability of an embankment dam <br />varies depending on the particular loading condition <br />the dam experiences at a given time. Typical loading <br />conditions include end-of-construction, steady-state <br />drained, and rapid drawdown, and are discussed <br />further below. <br />End-of-Construction Loading Condition <br />The end-of-construction loading condition evaluates <br />the stability of an embankment dam during and at the <br />end of construction. Construction may include the <br />initial construction of a dam or additional construction <br />resulting from dam modifications. This loading case <br />often controls the design of new, or significant <br />enlargements, of embankments; especially those <br />founded on soft clays. This loading condition should be <br />analyzed for embankment dams consisting of fine- <br />grained (cohesive) embankment and/or foundation