Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Direct Estimate of Unit Shaft Resistance, f s , of Steel Round Shaft Piles and Grouted Helical Micropiles Suggestions for estimating the unit side resistance, f s , of deep foundations in a variety of soils have been presented. This approach is convenient for helical piles/anchors and reduces assumptions in first estimating shear strength and then estimating other factors to obtain f s . Poulos (1989) summarized a number of reported correlations between pile unit side resistance and SPT N-value and suggested that most of these correlations could be expressed using the general equation: f s = β + α N Equation 4-29 Lutenegger (2011) presented a summary of more-or-less “global” reported correlations between SPT N-values and unit side resistance friction for both driven and bored piles in a number of different soil materials and shown in Table 4-12.

DESIGN METHODOLOGY

Figure 4-12. Relationship between Bearing Capacity Number and Friction Angle from Normalized CPT Tip Resistance – from Robertson and Campanella (1983)

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