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<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.