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2 <br />Soil Characterization (Part 2) – <br />Laboratory and Field Shear <br />Strength Testing <br />Introduction <br />This article presents discussions of the various types of <br />laboratory and field testing for evaluating the shear <br />strength of cohesionless (sands and gravels) and <br />cohesive (clays and silts) soils. <br />A subsequent article (Part 3) on shear strength <br />characterization will elaborate on utilizing laboratory <br />and field testing results to select and develop shear <br />strength parameters for use in embankment dam slope <br />stability analyses. <br />Previous Articles <br />The fundamentals of soil characterization for dams, <br />including some introductory aspects of shear strength <br />characterization, were presented in the July 2014 issue <br />of the Western Dam Engineering newsletter in an <br />article titled “Soil Characterization (Part 1) – Here’s the <br />Dirt.” That article presented a broad overview of <br />properties pertinent to the overall performance and <br />analysis of dams. <br />Additionally, the fundamentals of slope stability <br />analyses were presented in the November 2013 <br />newsletter issue in an article titled “Embankment Dam <br />Slope Stability 101,” where the topic of shear strength <br />characterization for slope stability analysis was <br />introduced. Discussion was also provided on slope <br />stability modeling for the following embankment <br />loading conditions:steady state,end of construction, <br />rapid drawdown, and seismic. <br />You are invited to revisit and review those two articles, <br />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 does not discuss shear strength testing of <br />rock or special soils such as cemented sands, stiff <br />fissured clays, 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. <br />Undrained vs. Drained Conditions and Total <br />vs. Effective Stresses <br />In this section, the concepts of undrained loading <br />conditions versus drained loading conditions and total <br />stress versus effective stress testing and analysis <br />methods are introduced. An understanding of these <br />concepts is important in evaluating soil behavior and <br />assigning appropriate shear strengths. <br />When saturated or partially saturated soils are loaded <br />in shear, they have a tendency to change in volume. <br />Loose sands or normally consolidated clays tend to <br />decease in volume, while dense sands or <br />overconsolidated clays tend to increase in volume. If <br />the loading is applied slowly enough, pore water will <br />flow into or out of the soil mass, the volume of the soil <br />mass will change, and pore pressures will not change. <br />However, if the loading is applied more quickly than <br />drainage can occur, pore water pressures will be <br />generated within the soil mass. Positive pore pressures <br />will generate in loose sands or normally consolidated <br />clays due to the tendency to compress, while negative <br />pore pressures will generate in dense or <br />overconsolidated clays due to the tendency to expand. <br />Coarse-grained soils (sands and gravels) have high <br />hydraulic conductivities (permeabilities) and sufficient <br />drainage capacity to prevent pore water pressures <br />from changing for most loadings (earthquake loading <br />being an exception that is beyond the scope of this <br />article), while fine-grained soils (clays and silts) have <br />low hydraulic conductivities and can develop excess <br />pore water pressures during some static loading <br />conditions. <br />Undrained conditions occur when loading is applied <br />more rapidly than soil drainage can occur. Under <br />undrained conditions, water cannot flow into or out of <br />the soil in the length of time the loading is applied. As <br />a result, pore water pressures increase or decrease in <br />response to changes in load, as described above. <br />Drained conditions occur when loading is applied <br />slowly enough relative to the permeability of the soil <br />that drainage of pore water can occur. Pore water <br />pressures do not change under drained loading <br />conditions, because water can move into or out of the <br />soil freely in response to changes in load.