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7 The tests are generally performed in a controlled-strain manner (specimen strained axially at a predetermined rate); usually performed at a strain rate between 0.5 and 1.25 mm/hr. A minimum of three specimens, each under a different confining pressure, are generally tested to establish the relationship between shear stress and normal stress, which allows construction of a Mohr-Coulomb failure envelope and estimation of the shear strength parameters (c,I, c’,I’). Confining pressures should bracket the range of normal stresses expected in the field. Often, confining pressures representative of the in-situ stress state in addition to two larger stress states are specified. Four types of triaxial tests are typically conducted: 1.Unconsolidated-Undrained (UU or Q [quick]) Test 2.Consolidated-Drained (CD or S [slow]) Test 3.Consolidated-Undrained (CU or R [rapid]) Test 4.Consolidated-Undrained (CU’ or R-bar) Test with Pore Pressure Measurements Unconsolidated-Undrained (UU or Q) Test In the UU test, drainage is prevented and, although a chamber pressure is applied, the soil specimen is not consolidated under a confining pressure prior to axial loading. The drainage valve remains closed during application of the confining pressure and axial loading and shearing of the specimen. The water content of the soil prior to testing remains the same during testing. Pore water pressures are not measured during the test. The UU test therefore measures total stress strength parameters (c,I) and is applicable for cohesive soils (clays and silts). If the UU specimen is fully saturated, increasing the chamber pressure to larger total stresses will induce an equivalent increase in pore water pressure, in which case, the effective stress of the sample remains unchanged. Therefore, the measured shear strength, which is referred to as the I=0 strength characterization, will be the same for all chamber pressures. Some variability may occur in the resulting shear strengths due to sample variation, sample disturbance, and testing imperfections. Figure 8 illustrates idealized UU test results and the I=0 strength characterization. In reality, the specimens are typically not fully saturated, and there is a slight increase in shear strength with increasing chamber pressure. This is because the effective confining stresses actually increase since the pore pressure response is less than the confining pressure. Figure 8: UU test results on saturated soil. Examples of when to perform a UU test include the following conditions: rapid loading of a slowly permeable soil, rapid construction of an embankment (or other type of loading) over a soft clay, and excavation in a soft clay. Consolidated-Drained (CD or S) Test In the CD test, drainage is permitted throughout the test. The specimen is first saturated and consolidated under chamber pressure and back pressure resulting in a specified effective confining pressure. Pore water pressures generated by application of the confining pressure are allowed to dissipate until the specimen reaches its state of consolidation under the specified effective confining pressure. The CD test can be performed using either isotropically consolidated (IC- D) samples or anisotropically consolidated (AC-D) samples, in which load is applied to the piston so that the vertical consolidation load is generally higher than the lateral load. During axial loading, the specimen is sheared to failure with an open drainage valve at a slow enough rate such that excess pore water pressures dissipate during the test. The pore water volume change (either a decrease or increase) is measured during the shear stage of the test. The CD test therefore evaluates drained effective stress strength parameters (c’,I’). Effective stress strength parameters for moist samples are often assumed to be the same as that for saturated Reference:Head (1986)