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22 measured strengths in terms of an undrained shear strength ratio, Su/σ’v. Shear strength ratios increase with higher OCRs; meaning overconsolidated clays have higher shear strength than normally consolidated. See reference [2] for further discussion. A field test used for direct measurement of the undrained shear strength (Su) of clays is the vane shear test, which has been successfully used for measuring the undrained shear strength of soft to medium stiff clays. Limitations are that the test can be affected by sand lenses and seams and the raw undrained shear strength measured from the test requires an empirical correction factor that varies with plasticity index and accounts for anisotropy and strain rate effects. The data on which the correction factor is based are widely scattered and therefore vane strengths should not be viewed as precise. The pocket penetrometer test and torvane test can be used to obtain very quick, approximate measurements of undrained shear strength in the field or laboratory. However, the pocket penetrometer and torvane tests are relatively crude and should be considered as only rough indications of shear strength. The laboratory test most commonly used to measure the drained shear strength (c’, φ‘) of clays is the CU’ triaxial shear test. The CU’ test is more practical than the CD test because the strain rates required for a CD test are typically extremely slow, requiring an impractically long test time. In addition, the CU’ test can be used to obtain both undrained (total stress) and drained (effective stress) shear strength parameters. However, the CD triaxial test and DS test can also be used, if the long test times can be accommodated. For stiff-fissured clays, laboratory tests are performed on remolded specimens to evaluate fully softened and/or residual drained shear strengths. The DS test is commonly used to measure the fully softened shear strength of stiff-fissured clays. Torsional ring shear tests are most suitable for estimating the residual shear strength of stiff-fissured clays since the test can measure shear stresses over any magnitude of displacement through continuous rotation. Laboratory tests provide the best strength data for clays, and reasonably undisturbed samples of these materials can typically be obtained. However, various empirical correlations developed to estimate strengths for clays may be sufficient in some cases. It is recommended these methods and correlations be used with caution, since the behavior, and hence strength characterization, for clays is typically more complex than for granular soils. A few of the more common shear strength correlations are presented below. These correlations, and others, are further described in reference [2]. It may also be useful to use these correlations as a check to validate the results of laboratory tests. Results of field tests including the SPT, CPT, and shear wave velocity measurements can be used for estimating the undrained shear strength (Su) of clays through the application of empirical correlations. SPT blow counts (N) in clay can be used to roughly estimate the variation of Su/N with plasticity index, as shown in Figure 10. Su can also be estimated from CPT cone resistance using the following relationship: 𝑆𝑢=𝑞𝑐−𝜎𝑣𝑣𝑁𝑘∗ Where: 𝑞𝑐= cone tip resistance, 𝜎𝑣𝑣= total overburden pressure at test depth, 𝑁𝑘∗= cone factor The variation of 𝑁𝑘∗ with plasticity index for a variety of clays is shown in Figure 11. An 𝑁𝑘∗ value of 14 is typically applied to clays for any value of plasticity index. Reference: Duncan, Wright, and Brandon (2014) Figure 10: Variation of the ratio of undrained shear strength (Su) divided by SPT blow count