Finite element modeling of soil-structure interaction for drilled shaft foundations

The stiffness of bridge foundations has a major effect on its response to lateral loads. Response values such as maximum moments and displacements, frequency, and plastic hinge locations need to be accurately predicted for a safe and sound design. Many approaches have been used to obtain foundations stiffness such as the Winkler Beam model (spring model) and the concept of length of fixity to simplify the design. These models do not adequately describe the behavior of the soil-structure system and their interaction. Other models do not include the three-dimensional effects of the superstructure. This dissertation presents an analytical investigation of drilled shaft response under lateral loads using three-dimensional finite element model for column-drilled shaft-soil system. The developed model takes into account the soil stiffness and material properties, shaft slenderness, interfacial properties, soil mass and interlayer shear coupling, effective soil zone around the shaft, different support conditions, as well as mesh density and distribution. Results from this investigation showed that soil layers close to ground surface, support conditions, and shaft slenderness are important factors that need to be considered in the lateral load analysis of drilled shafts. The 3-D model also showed that simplified models such as the Winkler Beam (spring) models, overestimate moments and deflections. A parametric analysis was conducted to evaluate the effect of the various parameters that influence the lateral response of drilled shaft. The developed model was verified by comparing it to existing simplified models with the proper correlation, and to existing experimental results. The model was also used to predict the lateral response of group shafts with variable center-to-center spacing and compare it to that of a single shaft. A table was proposed to estimate the maximum displacements and moments of group shafts by relating them to those of a single shaft. The 3-D finite element continuum model for the shaft-soil system is an improved model that takes into account all the factors that influence soil-shaft system, including the superstructure and can accurately predict the lateral response of drilled shafts. The model can be used to generate design response curves for maximum moments and deflections of drilled shafts.