Efficiency of pile groups in tension

Groups of piles provided to support tall structures such as transmission towers, chimneys, stacks, etc. are subjected to pullout forces. The uplift capacity of a single pile in clay or sand can be estimated rationally and reliably by methods available. In this note, the interaction between two identical piles in tension is studied by modelling the soil as a homogenous, linearly elastic medium and by using the boundary integral technique. The reduction in individual pile capacity due to the existence of another pile is quantified and found to depend on the spacing and length-to-diameter ratio of the pile, and the type of variation with depth of pile-soil interface strength. Efficiencies of typical pile groups are computed. The predictions compare well with model and full-scale test results. Key words: axial capacity, boundary element method, efficiency, groups, interaction, piles, uplift.