Soil spring stiffnesses for laterally loaded large diameter rigid caissons

Laterally loaded large diameter caissons that exhibit rigid body motion are often analyzed by modeling the soil as equivalent springs. Equivalent soil spring stiffness is usually developed empirically. In this study, the soil spring stiffnesses are developed rigorously considering three-dimensional caisson-soil interaction based on a continuum-based analytical framework. A laterally loaded rigid circular caisson embedded in elastic soil is analyzed using the principle of virtual work. The soil displacement field is chosen based on the geometry and kinematics of the caisson, and the vertical displacement of the soil caused by the large-diameter caisson rotation is explicitly considered in the analysis, which has been neglected in previous studies. An iterative algorithm is used to obtain the solutions. It is shown that the caisson-soil interaction can be represented by a five-spring model along the shaft and at the base of the caisson. The equations of these spring stiffnesses are mathematically derived from the analysis without recourse to empiricism. Similarly, the equivalent spring stiffnesses of the caisson-soil system replacing the caisson and soil continuum are derived analytically from the solution. Based on the detailed parametric studies, fitted algebraic equations for these spring stiffnesses are obtained, which can be readily used by practicing engineers.