| This study investigated the tip behavior of large diameter drilled shafts embedded short distances in Florida Limestone (i.e., L/D = 1 & 3). This dissertation focused on the modeling of axial, shear, and moment responses for tips of shafts subject to combined axial and lateral loading in homogeneous and heterogeneous limestone.;For this study, laboratory (centrifuge) tests, Finite Element Analysis (FEA), and field investigation (from two sites: 17th Street and Fuller Warren Bridges) were conducted.;The study from the centrifuge test found that the tip stiffness could be estimated within serviceability condition using the linear stiffness relation proposed by Misra and Doherty. Furthermore, O'Neil's method of assessing tip resistance versus displacement was accurate if the harmonic mass modulus of the heterogeneous rock (two-layer system) was assessed within two diameters (D) below the tip of the shaft. In the case of tip shear, it was found that a bilinear elastic plastic model with failure assessed using 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.;To evaluate the effect of spatial variability of rock material properties on the tip resistance of drilled shafts subjected to axial load, field investigation and two dimensional FEA were conducted. In addition, with regard to the vertical spatial variability of the rock (using harmonic mass modulus with the influence zone at 2D below the tip of a shaft), a relationship was derived between deterministic and probabilistic end-bearing resistances in homogeneous and heterogeneous rock formations. The FEA was conducted at 1 in tip displacement, defined as the serviceability condition.;An important finding from the field investigation was the variability of the LRFD resistance factors (o) from the spatial variability of the rock at the two sites. Using standard geostatistics measures (e.g., variogram, covariance, etc.), the variance of the harmonic mass modulus with the influence zone at 2D below the tip of 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. |
