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3 Hence, whether a particular loading should be considered undrained or drained is dependent on rate of loading, soil permeability, and the distance over which drainage must occur to prevent pore water pressure changes. One method for estimating whether a soil will behave in a drained or undrained manner during loading is presented in the article “Embankment Dam Slope Stability 101” from the November 2013 newsletter. Alternatively, soils having a coefficient of permeability greater than approximately 1 x 10-3 centimeters per second (cm/s) can be considered to be free-draining under static loading, as a general rule of thumb (Duncan and Wright, 2005). Although conditions can be intermediate between undrained and drained, loading conditions are almost always modeled as either one or the other. In some cases, when it is not clear whether the loading conditions are undrained or drained, both cases are considered in the analysis. Total stresses within a soil mass include both stresses resulting from forces transmitted through interparticle contacts and pore water pressures.Effective stresses within a soil mass include only stresses resulting from the forces transmitted through interparticle contacts. At any given location, the effective stress equals the total stress minus the pore water pressure. Soil strengths can be defined as a function of either total stresses or effective stresses. When strengths defined in terms of total stress are used in stability analysis, the approach is commonly called the total stress method, while the term effective stress method is used when strengths defined in terms of effective stress are used in stability analysis. Effective stress methods should always be used for drained loading conditions. For undrained loading, one needs to choose between total stress methods and effective stress methods. Total stress methods are used when it is easier to predict the strength during undrained loading than it is to predict the pore water pressures during undrained loading, which is almost always the case. Soil strengths are always governed by effective stresses, or interparticle forces, regardless of loading condition. Total stress strength characterizations are simply used in those cases where we cannot easily predict pore water pressure responses and we can more easily predict the undrained strength. The pore water pressure is implicit in the selected total stress strength; the pore pressure is whatever value is necessary to produce an effective stress state that results in the predicted strength. A future article (Part 3 of the series) will provide guidance on when undrained conditions, drained conditions, total stress methods, and effective stress methods are generally used for shear strength characterization and slope stability analysis. But first, shear strength parameters and strength testing to evaluate the parameters will be discussed. Shear Strength Explained Shear strength can be defined as the ability of soil to resist failure (rupture or sliding) under shear loading. The shear load is the result of gravity forces from the soil mass and any external loads (e.g. reservoir loads, equipment loads, seismic loads). Soil shear strength depends on: x Types of soil particles and mineralogy x Consolidation pressure x Drainage allowed x Stress history, including overconsolidation x Stress paths The most common way of representing or characterizing shear strength of soils is the Mohr- Coulomb failure criterion using the following equations: s = c +ı tanࢥ (total stress) s = c’ +ı’tanࢥ’(effective stress) Where s = shear strength; c = cohesion;V = effective or total stress; and I = internal friction angle. As represented by the Mohr-Coulomb failure criterion, the shear strength characterization for a soil consists of Shear strength of a soil is controlled by effective stresses, whether failure occurs under drained or undrained conditions.