Laserfiche WebLink
4 <br />a frictional component (I,I') and a non-frictional <br />component, or cohesion (c, c’). Figure 1 below <br />graphically depicts the Mohr-Coulomb failure criterion. <br />Figure 1: Mohr-Coulomb failure criterion. <br />Mohr-Coulomb failure envelopes can be developed <br />through the use of Mohr’s circles of stress representing <br />the stress states at failure for a series of tests (often <br />three tests). As seen in Figure 1, both total and <br />effective stress failure envelopes can be developed. <br />Failure envelopes for soils are often curved. However, <br />for mathematical simplicity, an analyst will <br />approximate the failure envelope as linear over the <br />normal stress range of interest for the analysis. The <br />Mohr-Coulomb failure criterion is, therefore, <br />represented by a straight line (failure envelope) with a <br />slope designated as the friction angle (I,I') and an <br />intercept called the cohesion (c, c’). The normal (or <br />vertical) stress (ı,ı’) acting on soil in an embankment <br />at a given depth, is represented by the horizontal axis, <br />and can be either total stress or effective stress. Note <br />that the greater the normal stress, the greater the <br />frictional component and overall shear strength. <br />Shear strength tests are performed on soils using a <br />range of consolidation pressures to develop the <br />strength envelope from Mohr-Coulomb plots. The <br />following section will discuss various laboratory tests <br />used to evaluate the shear strength of soils, and which <br />laboratory tests are appropriate for drained and <br />undrained loading conditions. <br />Laboratory Shear Strength Tests <br />The five types of laboratory tests most widely used to <br />estimate shear strength in soils are:direct shear, <br />unconfined compression,triaxial shear,direct simple <br />shear, and torsional ring shear. These five tests are <br />described in detail below and are performed in <br />accordance with ASTM standards. The ASTM standard <br />for each test outlines sample preparation, failure <br />criterion, and procedures for saturation, consolidation, <br />loading, and pore pressure measurements (where <br />applicable). <br />Direct Shear Test <br />The oldest and simplest shear strength test is the <br />direct shear (DS) test. DS testing is performed as <br />described in ASTM D3080. In the DS test, a thin soil <br />sample is placed in a shear box that is split horizontally <br />into halves. A normal force (Pv) is applied to the top of <br />the loading head. The normal force typically ranges <br />from 0 to 150 pounds per square inch (psi). The lower <br />half of the box is fixed, while a shear force (Ph) is <br />applied to the upper half, thereby moving the upper <br />half parallel to the lower half and forcing the soil <br />specimen to fail along a horizontal shear plane. A <br />schematic of the test apparatus is shown in Figure 2. <br />Figure 2: Direct shear test apparatus. <br />The DS test is performed in a strain-controlled <br />(deformation-controlled) manner per ASTM D3080. In <br />the strain-controlled test, a constant rate of shear <br />displacement is applied to the top half of the box by a <br />motor that acts through gears. Shear displacement <br />('H) of the top half of the box is measured by a <br />horizontal dial gauge or displacement transducer. The <br />resisting shear force of the soil can be measured by a <br />horizontal proving ring or load cell. A dial gauge or <br />displacement transducer on the upper loading plate <br />Reference: Bowles (1988)