Difference between load-transfer relationships for laterally loaded pile groups: Active p-y or passive p-delta

Two-dimensional (2D) finite-element analyses were carried out to study undrained soil deformation around piles displaced laterally through soil. The load-transfer p-delta curves produced were found to be applicable for design during passive lateral loading but not for active lateral loading of pile groups. The p-delta curves characterize the local soil-shear deformation around the pile, whereas p-y curves used in the subgrade-reaction method of active lateral-pile-loading design also include the effects of global soil displacement. Hence, determination of p-y curves from basic soil parameters requires consideration of both the local and global soil behavior and the pile-group geometry and loading. Because p-delta curves stiffen with reducing pile spacing, whereas active load-transfer curves soften, there is a significant stiffness disparity between the two forms of load-transfer curves for closely spaced pile groups. Care should be taken in choosing the appropriate form of reaction curve for design.