Design Methodology for Axially Loaded Auger Cast-in-Place and Drilled Displacement Piles

With the increasing use of augered cast-in-place and drilled displacement piles in new construction, it is important that proper design parameters be incorporated when evaluating pile performance using reliability-based design methods. Although augered piles can be distinguished from bored piles, including drilled shafts, and driven piles by the magnitude of the effective stress changes they produce in the vicinity of the pile during construction, the current design methods for augered piles generally use the same design methods as those used for bored or driven piles. To enhance the efficiency of the augered piles, a unique design method must be developed. This paper focuses on developing a design methodology for axially loaded augered piles installed in predominately sandy soils using the t-z method. To develop the design parameters for augered piles, back-calculation of the t-z parameters was conducted using static load–test data. The data from 17 static pile load tests conducted on augered piles from construction sites were obtained. Load-settlement and load-transfer curve fittings were performed using the t-z model to back-calculate a set of soil-pile interfacial and tip parameters, along with their variability. Correlations of the back-calculated t-z model parameters with the standard field investigation data were conducted, and the most-promising correlations were incorporated into the t-z model–based design methodology. Subsequently, the t-z method for augered pile design was evaluated by comparing the predicted and measured load-settlement and load-transfer curves. In addition, the t-z method was used to perform probabilistic load-settlement analyses and obtain resistance factors applicable to the load and resistance factor design approach.