Response of laterally loaded group shafts for bridge foundations in cohesionless soils using a 3D FE soil-structure model

The lateral stiffness of bridge foundations has a major effect on its response to lateral loads. Response values such as maximum moments, maximum displacements, and plastic hinge location below ground need to be accurately predicted for a safe and sound design. Many approaches have been used to model foundation lateral stiffness such as the Winkler beam (spring models) and the concept of length of fixity to simplify the design. This paper discusses the response of laterally loaded group shafts using three-dimensional finite element (FE) soil-structure models for single shaft-soil system and group shaft-soil system in cohesionless soils. Parameters investigated include soil modulus, shaft slenderness, shaft-soil interface, soil effective zone, shear interaction, and support conditions at the bottom of the shaft. Results from this investigation provide criteria for the effective volume of soil that needs to be included in the FE analysis for single and group shaft analysis. It also showed that shaft spacing, soil weight, soil modulus, interface conditions, and support conditions are important factors that need to be included when modelling group shafts in cohesionless soils. Results from the group shaft analysis confirmed the effect of spacing on group shaft moments and displacements and the results were compared to existing group displacement amplification factors.