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