The behaviour of piles undergoing a change in lateral loading direction

Pile foundations installed for offshore applications or in earthquake prone areas must be designed to withstand transient static and cyclic lateral loads that may occur in different lateral directions at different times. Research bas chiefly been devoted to studying the behaviour of a pile subjected to one dimensional lateral loading and the influence of changing the lateral direction of this loading has not been widely considered. This paper presents the results from a series of centrifuge tests which involve multi-stage loading of a pile, considering changes in loading direction. The behaviour of a pile subjected to an unload-reload loop in a single direction is compared to that of a pile reloaded at 90" to the initial loading direction. When piles are reloaded at 90'. the set of rules that apply to one dimensional reloading become ineffective because an 'effect of recent loading history' governs the behaviour. In particular, local hysteresis loops are widened and the global instantaneous stiffness varies. These experimental findings are in accord with predictions from a simple numerical model, which defines the soil surrounding a pile using a series of twodimensional elastoplastic Winkler models along the pile length.