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