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5 <br />measures the change in height of the specimen, or <br />vertical displacement ('V). Both peak and post-peak <br />shear strengths can be observed in the strain- <br />controlled test. <br />DS tests can only be used to accurately evaluate <br />drained shear strength parameters. The DS test cannot <br />accurately measure undrained strengths because <br />constant volume conditions are not achieved, since <br />water can be expelled from or drawn into the <br />specimen. Since the shear plane is relatively thin, <br />volume and moisture content changes can easily occur <br />on the shear plane, even if the volume of the total <br />specimen does not change. The DS test is generally <br />applicable to grained sands, clays, and silts, because <br />the typical DS box is too small to accommodate coarse <br />particles. However, the test is most appropriate for dry <br />or saturated sands. Sand has a relatively high <br />permeability whereby excess pore water pressures <br />generated due to loading dissipate quickly. Clays and <br />silts have a low permeability and therefore take longer <br />time to dissipate excess pore water pressures. As a <br />result, the shear force must be applied very slowly for <br />clays and silts, which can make DS testing of these soils <br />impractical. <br />Generally, a minimum of three specimens are tested to <br />establish the relation between shear stress and normal <br />stress at failure. Typical results for a DS test are <br />presented in Figure 3. <br />Figure 3: Typical direct shear test results. <br />Unconfined Compression Test <br />Of the tests mentioned in this article, the quickest and <br />least expensive is the unconfined compression (UC) <br />test. Its use is limited to evaluating the undrained <br />shear strength (Su) of saturated cohesive soils (clays <br />and silts). It is not suitable for dry or crumbly soil, <br />materials with fissures or lenses, or uncemented sands <br />and gravels. Unconfined compression testing is <br />performed as described in ASTM D2166. <br />In the UC test, a laterally unsupported specimen (no <br />horizontal confining pressure) is placed between two <br />end plates and loaded in axial compression until shear <br />failure occurs. The vertical load (Pv) is applied at a rate <br />that maintains a vertical strain of about 1 to 2 percent <br />per minute. A schematic of the test apparatus is shown <br />in Figure 4. <br />Figure 4: Unconfined compression test apparatus. <br />The unconfined compressive strength (qu) is defined as <br />the maximum axial compressive stress at which failure <br />occurs, or at which the axial strain reaches 15 percent <br />if there is no sudden failure. Since there is no <br />horizontal confining pressure, the total minor principal <br />stress at failure (V3) is zero and the total major <br />principal stress at failure is equal to the unconfined <br />compressive strength (V1 =qu). Thus, the undrained <br />shear strength (Su) is equal to one-half of the <br />unconfined compressive strength (1/2qu), as depicted <br />in Figure 5. <br />The UC test can be considered a special case of the <br />unconsolidated-undrained (UU) triaxial shear test, <br />described subsequently, in which the lateral stress is <br />set to zero. <br />Reference: <br />Pv