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