Distribution of End Bearing and Tip Shear on Drilled Shafts Founded in Florida Limestone

This study investigated the behavior of large diameter drilled shafts embedded short distances in Florida Limestone (i.e. L/D = 1 & 3). The work was performed via laboratory (centrifuge) tests and the two field sites (17th Street and Fuller Warren Bridges). The work focused on modeling the axial, shear and moment response of shafts subject to combined axial and lateral loading in uniform and variable strength/modulus limestone. The study found that O’Neill’s method of assessing tip resistance vs. displacement was accurate if the Harmonic or Geometric Mass Modulus of the rock is assessed within three diameters below the shaft’s tip. In the case of tip shear, it was found that a bilinear elastic plastic model with failure assessed from Mohr-Coulomb provided good results. For tip rotation the model proposed by Bell (1991) gave very reasonable results even though it requires assessment of tip shear and lateral tip displacement. An important finding from the field investigation was the variability of the LRFD resistance factors from the spatial variability of the rock at the two sites. Using standard Geostatistics measures, Variogram, covariance, etc., the Variance of the Geometric Mass Modulus 3D below a shaft was assessed, along with tip resistance, and its associated variability. LRFD resistance factors based on FOSM were calculated with reliability values of 2.5 and 3.0. An Excel spreadsheet is provided for LRFD assessment and design for other sites.