Vertical static and dynamic pile-to-pile interaction in non-linear soil

A vertically loaded floating pile in clay affects a neighbouring pile by increasing the latter's displacement due to its own load. As a result, a group of rigidly capped piles exhibits a force/settlement ratio (‘vertical stiffness’) that is smaller than the sum of the individual stiffnesses of each pile – ‘efficiency’ in static stiffness less than 1. However, under dynamic steady-state loading the response of the pile group is an oscillatory function of frequency, and at certain frequencies a complete reversal of the static trend occurs, with the elastic dynamic group ‘efficiency’ exceeding not only the static ‘efficiency’, but also unity. To assess the realism of such behaviour, finite-element inelastic soil models were utilised to explore the influence of soil non-linearity on pile-to-pile interaction factors, under both static and dynamic loading. It is found that, with realistically inelastic undrained clay behaviour, the influence of a loaded pile on its neighbour diminishes radically with increasing amplitude of imposed displacement. The presence of a number of in-between piles, as well as the neighbouring pile's own rigidity, has no substantial effect on the interaction. The observed trends are explained by recourse to simple physical arguments. The diagrams provided for the pile-to-pile interaction factor are utilised to obtain the vertical dynamic impedance (i.e. stiffness and damping) of a 2 × 2 and a 3 × 3 rigidly capped pile group. It is found that these impedances are in accord with those resulting from three-dimensional analysis of the complete pile group. The difference between elastic and inelastic efficiency factors is shown to be substantial. The validity of the numerical results is strictly limited to piles in soft clays, whose resisting stress on the pile shaft equals their undrained shear strength.