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21 used in stability analyses because the negative pore water pressures cannot be relied upon in the field. Typical drained Mohr-Coulomb failure envelopes for clays are presented in Figure 8. For normally consolidated clays, the Mohr-Coulomb strength envelope goes through the origin of stress and the effective stress cohesion (c’) is equal to zero. For overconsolidated clays, the Mohr-Coulomb strength envelope is generally curved in the low stress range, but still goes through the origin such that the effective stress cohesion is equal to zero. Similar to coarse- grained soils, an analyst may approximate the strength envelope as linear over the normal stress range of interest for the analysis, which may result in an “apparent” effective stress cohesion. Again, strength envelopes with intercepts (shown in Figure 8) are a mathematical convenience. Figure 8: Mohr-Coulomb strength envelope for clays. Stiff-fissured clays that are heavily overconsolidated should be carefully characterized. The undisturbed peak shear strength is generally not used to evaluate the stability of slopes composed of these soils. Shear strengths that can be mobilized in the field are generally less than in the laboratory since more softening and swelling occurs in these soils over long periods of time. Therefore, fully softened and/or residual shear strengths are generally more appropriate. If you are dealing with fissured clays, reference [2] should be consulted for its thorough discussion on that topic. Other factors affecting clay strengths include anisotropy and strain rate. The undrained shear strength of clays varies with the orientation of the principal stresses at failure and with the orientation of the failure plane. Isotropically consolidated soils generally yield higher shear strength than anisotropically consolidated soils where the vertical stress is higher than the horizontal. Common laboratory tests (i.e., CU) typically use isotropic consolidation, yet it should be recognized that soils in the field often exhibit anisotropic stress conditions. Furthermore, undrained shear strengths evaluated through laboratory testing can sometimes be overestimated due to the higher strain rates used to fail the specimen compared to those in the field. Figure 9 shows the undrained shear strength of saturated clay increases with increase in strain rate. Figure 9: Effect of strain rate on undrained shear strength of clay. Laboratory tests used to measure the undrained shear strength (c, φ or Su) of clays include the UC test (Su); UU, CU, CU’ triaxial shear test (c, φ or Su); and the DSS test (Su). Sample disturbance can reduce the undrained shear strength measured in laboratory tests. This effect may be reduced if the sample is consolidated to the same confining pressure it was consolidated to in the field. The SHANSEP (Stress History and Normalized Soil Engineering Properties) method can also compensate for sample disturbance and is a common approach used to estimate the undrained shear strength of clays. As described by Ladd and Foot (1974) and Ladd et al. (1977), the method involves consolidating clay samples to effective stresses that are greater than the in-situ stresses and interpreting Reference: Mitchell and Soga (2005)