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 47 construction projects in which more than 120 DDPs were installed and tested axially. High quality site exploration data (e.g., Cone Penetration Test and Standard Penetration Test) were analyzed to derive geotechnical design parameters. The test sites consisted of mostly mixed soil types with strongly stratified layers of sand, silt, and clay. Pile diameters ranged between 35 and 61 cm and pile lengths ranged between 6 to 24 m. The slenderness ratio (L/D) ranged between 14 and 70.
Approximately 50% of all pile tests did not reach failure. Therefore, tests with known failure loads were first utilized to examine a variety of failure interpretation methods in literature and their ability to accurately predict the experimental failure. The Hansen 80% method was found to be the most reliable method. Hereafter, field test results were compared with pile design formulations commonly found in literature. General axial capacity equations as well as equations specifically developed for DDPs were utilized. The accuracy of the empirical methods is presented in terms of total pile capacity, pile length/diameter, and soil type. A 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.