Design Considerations for Rocking Foundations on Unattached Piles

Abstract

By appropriately reducing the size of the footing, rocking behavior caused by seismic loading can occur about the footing base. Recent research has shown how rocking shallow foundations can result in improved seismic performance of bridges. One concern of rocking systems is the potential for significant settlement accumulation in poor soil conditions. This research explores the effectiveness and practicality of using unattached piles to mitigate settlement while still allowing rocking. Two series of centrifuge tests investigated the performance of individual footing-column-mass structural models in these soil conditions on unattached piles. The lateral load connection between the rocking foundation and piles is an important consideration; a variety of possible shear key configurations capable of transferring lateral and dead loads to the piles while allowing separation associated with rocking are discussed. Unique design issues associated with rocking on unattached piles and methods to address these issues are described. Concepts are presented to support the evaluation of the effect of excess pore pressure on the pile capacity. Methods of evaluating effects of pile capacity, number of piles, and arrangement of piles on the residual settlement and base shear coefficient to initiate rocking on piles are also described.