Nonlinear subgrade modulus of sandy soils for analysis of piled raft foundations

Abstract Continuum-based three-dimensional (3D) elasto-plastic finite element (FE) analyses of piled raft foundations (PRFs) resting on sandy soils and subjected to uniformly distributed vertical loads are performed using the software Abaqus, and equations are developed for estimating the nonlinear soil subgrade moduli (spring stiffnesses) beneath the raft, beneath the pile base, and along the pile shaft. The unified clay and sand model (CASM) is used as the soil constitutive model in the continuum-based 3D FE analysis. Four different types of sand with three different states (loose, medium and dense) and twenty different configurations of PRFs with rectangular, square, and strip rafts are considered in this study. The developed equations of the raft and pile spring stiffnesses are subsequently used in a soil spring-based nonlinear FE analysis of PRFs performed using a structural engineering software. It is shown through verification and validation studies that the developed equations of the raft and pile spring stiffnesses can be reliably used in spring-based FE analysis of PRFs, instead of the computationally expensive continuum-based 3D FE analysis, for quick and accurate estimate of PRF responses. The developed equations of spring stiffnesses are applicable to PRFs that are similar in dimensions considered in this study.