Database Calibration of Design Methods and LRFD Resistance Factors for Auger Cast-in-Place and Drilled Displacement Piles

Auger cast-in-place (ACIP) and drilled displacement (DD) piles are routinely considered as an alternative to drilled shafts or driven piles. However, their acceptance to support bridges has been lagging. The load and resistance factor design (LRFD) procedures in the bridge design specifications of the American Association of State Highway and Transportation Officials (AASHTO) do not address ACIP and DD piles. To fill this gap, a load test database DUT/CFA/836 is developed in this paper, including 615 ACIP piles and 221 DD piles. The load tests in DUT/CFA/836 are analyzed from three aspects: (1) determine the bearing capacity of piles (Qfi) based on the measured load-displacement (Qt-w) curves by three methods in the International Building Code for comparison purposes; (2) calculate the bearing capacity of piles (Ruc) by 23 methods based on soil parameters (indirect) or in situ test data (direct); and (3) predict the Qt-w curves by seven sets of nonlinear t-z and q-w models to account for the load transfers in shaft shearing and end bearing, respectively. For each quantity of interest—bearing capacity, permissible load, and displacement at a service load—the bias and scatter of the predictions are characterized by the mean and coefficient of variation (COV) of the ratio of measured (or interpreted) to calculated values (called model factor). Following the AASHTO LRFD methodology, the values of the mean and COV of the capacity model factor are used to calibrate the LRFD resistance factors for ACIP and DD piles at the Strength I limit state.