Abstract:
Drilled Displacement Piles (DDP) provide an ideal deep foundation solution that combines the benefits of
ground improvement with traditional advantages of piling systems. This paper consolidates data from 20
construction projects in which more than 30 DDPs were installed and tested axially. High quality site
exploration data (e.g., Cone Penetration Test and Standard Penetration Test) were evaluated to derive
geotechnical analysis parameters. The test sites consisted of mostly mixed soil types with strongly stratified
layers of sand, silt, and clay. Pile diameters ranged between 40 and 61 cm. Test results were compared with
predictive methods developed specifically for DDPs as well as methods used in general geotechnical
practice. The accuracy of the empirical methods is presented in terms of total pile capacity, pile
length/diameter, and soil type. The large majority of all examined methods underpredict the in-situ pile
capacities. It was also found that the difference between the tested and predicted capacity is strongly related
to the level of improvement in the surrounding soil due to the piles’ installation. A general comparison
between predictive axial accuracy and the observed level of ground improvement upon installation is also
discussed.