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20 Figure 6: Relationship among CPT cone resistance, overburden pressure, and relative density for sands The drained shear strength of coarse-grained soils is strongly affected by relative density. Values of φ’ corresponding to confining pressures of about 1 atmosphere are related to relative density for sands in Figure 7. Figure 7: Correlation between friction angle and relative density for sands Fine-Grained Soils (Clays and Silts) Fine-grained soils (clays and silts) are generally defined by undrained shear strengths for short-term loading conditions and drained shear strengths for long-term loading conditions. These soils generally have low permeabilities and can develop excess pore water pressures during some static loading conditions. Given adequate drainage and time, pore water pressures will eventually dissipate in fine-grained soils. Clays Characterizations of shear strengths of clays are complex and the characterizations can be quite different for the different loading conditions, as discussed above. The drained shear strength of clays can be expressed in terms of effective stress (c’, φ’) strength parameters. The undrained shear strength can be expressed in terms of total stress (c, φ) strength parameters or in terms of undrained strength, Su. Further, there are different forms of characterization that can be used for Su – for example, constant Su, Su as a function of effective confining stress, and Su as a function of depth. The overconsolidation ratio (OCR) of clays also has an impact on strength and is defined as the ratio of the maximum preconsolidation pressure of a soil mass to the current consolidation pressure the soil mass experiences. For normally consolidated to lightly overconsolidated clays, both undrained and drained shear strengths are of interest. When normally consolidated to lightly overconsolidated clays are loaded in shear, they tend to compress and generate positive pore water pressures, thereby resulting in an undrained shear strength that is generally less than the drained shear strength. Hence, the lower undrained shear strength must be used when analyzing undrained loading conditions. In contrast, for heavily overconsolidated, or compacted, clays, drained shear strengths are of most interest because these clays tend to expand when loaded in shear, and therefore, generate negative pore water pressures. The negative pore water pressures result in an undrained shear strength that is generally greater than the drained shear strength. As discussed previously with respect to rapid drawdown analyses, higher strengths resulting from negative pore water pressures are not normally Reference: Duncan, Wright, and Brandon (2014) Reference: Duncan, Wright, and Brandon (2014)