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11 <br />Figure 14: Vane shear test apparatus. <br />Both the pocket penetrometer and torvane tests are <br />quick field tests which provide approximate <br />measurements of undrained shear strength, subject to <br />appropriate limitations. The pocket penetrometer test <br />provides an approximate measure of the unconfined <br />compressive strength of soils through a small scale <br />bearing capacity test. The piston at the end of a <br />pocket pentrometer, shown in Figure 15, is pressed <br />into the soil to get a measurement of the unconfined <br />compressive strength. It can give misleading results <br />because it only tests a small area of soil and can easily <br />be affected by disturbance. <br />Figure 15: Pocket penetrometer. <br />The torvane test provides an approximate measure of <br />undrained shear strength rather than the unconfined <br />compressive strength; remember that undrained shear <br />strength (Su) is equal to one-half of the unconfined <br />compressive strength (qu). In a torvane test, a soil <br />sample is removed from an area of interest and the <br />vane end of a torvane, shown in Figure 16, is pushed <br />into the soil sample. The torvane is twisted until it <br />breaks free from the sample. The dial gauge is used to <br />measure the shear strength. <br />Figure 16: Torvane. <br />Field testing is a crude method of measuring shear <br />strengths in comparison to laboratory testing. Field <br />testing should be used only as an approximate first <br />estimate of undrained shear strength, with the <br />strength estimates further refined by laboratory tests <br />for purposes of analysis and design. <br />The Standard Penetration Test (SPT), Cone <br />Penetrometer Test (CPT), and shear wave velocity <br />measurements can be used to estimate shear strength <br />parameters through the application of empirical <br />correlations. These methods of investigation are <br />described in numerous geotechnical engineering <br />reference books and will be discussed in the follow-on <br />(Part 3) to this article. Several recommended <br />references are listed at the end of this article. <br />Conclusion <br />Shear strength characterization is a fine art that <br />requires experienced and knowledge to assign tests <br />relevant for the soil/sample type and loading <br />conditions needing to be analyzed. Interpreting the <br />results also requires an educated eye to evaluate all <br />pertinent test parameters to determine the confidence <br />level in the results. A subsequent article will discuss <br />utilizing laboratory and field testing results for <br />selection and development of shear strength <br />parameters to be used in analyses. <br />Su