The 1988 edition of the FHWA drilled shaft manual provides equations for drilled shafts in eompression in clay and in sand. The equations used, proposed by Reese and O'Neill, were based on data from load tests and are empirical. The design equations appear to do well in clays and in loose to medium dense sand. Practitioners who are designing drilled shafts for soils of relatively density of 50% to 100%, often consider other design methods including the one proposed by Mayne and Harris. The 1999 edition of the FHWA drilled shaft manual added to the earlier edition an equation for designing for compression in gravelly sand or gravel and also adopted the Mayne and Harris equation for "intermediate geomalerials". The authors of this paper started working on a new design equation for cohesionless soil materials of all relative densities around 1998. The development of thai equation was based in part on results of the full-scale fully instrumented tests. A nonlinear large-deformation capable finite element program, GEOT2D, was also. The design equation was published in 2000. Subsequently that design equation was advanced to include cohesive soil materials and is the unified design equation presented here. The unified equation predicts side resistance of a cylindrical drilled shaft in compression. A procedure for estimating the end-bearing capacity is also presented. The end-bearing equation is related to the side resistance near the tip of the shaft. The proposed unified equation is rational and simple to use and involves lateral earth pressure coefficients. These earth pressure coefficients depend only on the cohesion, the angle of internal resistance, and the overburden pressure. The unified equation complements the FHWA procedure and enhances the understanding of the FHWA equations. It is shown that by using specific values of cohesion and friction angle in the unified equation, one could obtain predictions similar to that of the FHWA equations for clay, for sand, and for gravelly sand or gravel.
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