Evaluation of a Load Test and Finite Element Analysis of Drilled Shafts in Stiff Soils

Abstract

It is generally believed that the current FHWA guideline for designing drilled shafts is overly conservative when applied to stiff soils. Practitioners who are designing drilled shafts in soils with relative densities of 50% to above 95% have to look elsewhere for design guidance. Various researchers have proposed solutions that are peculiar to still soils. Another solution is to perform load tests on pilot drilled shafts prior to installing production shafts. To date, the New Mexico State Highway and Transportation Department had performed full-scale load tests on more than a dozen drilled shafts. Most of them were installed in stiff soils. This paper presents an interim comprehensive drilled shaft design equation that is applicable to soils of all relative densities including those near or at 100%. The solution extends the design approach described in the FHWA drilled shaft design manual to include dense cohesionless soils. The development of this equation was based in part on the results of the most recent full-scale fully instrumented test in which Osterberg cells were used. A nonlinear large-deformation capable finite element program, GEOT2D, was used to match the test data and afterwards generated the information needed to develop the design equations. The interim design equation is shown to be comparable with predictions using the FHWA equations if the soil relative density is assumed to be about 35%.