Design and Applications of Drilled Displacement (Screw) Piles

Drilled displacement (DD) piles are being increasingly used as foundation elements for structures, particularly in projects requiring accelerated construction or involving the rehabilitation of foundations of existing, overstressed structures. Different types of DD piles are available in practice; each type is classified according to the design of the drilling tool and associated installation method. Installation of DD piles causes substantial changes in the state of the soil surrounding the pile. These changes result from the complex loading imposed on the soil by expansion of a cylindrical cavity to make room for the specially designed drilling tool, by torsional and vertical shearing as the drilling tool gradually moves down into the ground, and by the reversed vertical shearing caused by extraction of the drilling tool from the ground. This report consolidates the information available on DD piling technology, reviews and compares the empirical design methods typically used for these piles, and presents a numerical approach to model the shaft resistance of DD piles in sand. The installation of DD piles produces greater radial displacement of soil than that produced by nondisplacement piles (e.g., drilled shafts), particularly in the case of sandy soils which gain additional strength through densification. This radial displacement of soil around the pile shaft contributes to the high capacity obtained for DD piles. Accordingly, our focus has been on analyzing the shaft resistance of DD piles in sand and proposing a design procedure based on the results of the analyses.