Uncertainty Analysis of Micropile Pullout Based upon Load Test Results

Micropiles are applied in foundation rehabilitation projects to enhance the pullout capacity of the existing foundation system and minimize the vertical deflection of the structures. Consequently, the methods to calculate the pullout load–displacement behavior are important for the design of micropiles used for such rehabilitation projects. Production pullout load tests were performed on approximately 120 micropiles used for seismic retrofitting of the 580/980/24 freeway interchange in Oakland, Calif. These micropiles were required to satisfy the serviceability design capacity at a maximum deflection of 1.27 cm (0.5 in.), in addition to the ultimate design capacity. Because the existing structures varied in height and loading, micropiles of different pullout capacities were designed. In this paper, the measured data are using a closed-form “t–z” method to develop probability distribution functions for the model parameters. As the back-calculated model parameters are assumed to be random variables, the Monte Carlo simulation process is employed to develop a series of load–displacement curves. A method for determining the probability of micropile failure at the service limit state is developed using the results of the simulations. The method is utilized to obtain resistance factors that can be applied to LRFD based service limit- state design.