Determining the Capacity of Reused Bridge Foundations from Limited Information

Abstract

Reuse of bridge foundations often requires determining the capacity of existing foundations, including driven piles. These piles have a proven history of load-carrying capacity but may lack test data from which the capacity can be obtained. In general, pile capacity can be estimated from the pile geometry and soil properties using empirical calculations. However, these produce highly uncertain results due to the variable nature of soil and pile-driving methods. As a result, LRFD practice uses low resistance factors for pile design based on these calculations, which can produce inefficient designs, sometimes with lower capacity than the loading observed during the initial service life. This research proposes a novel reliability-based methodology to update the capacity of individual piles based on previously observed loading to the pile group. A likelihood function is developed that accounts for the probability that a single pile has low capacity given that the total group capacity was greater than the observed loading during its initial service life. The results are tabulated to show the increases to the LRFD resistance factors that are possible while maintaining the reliability index (and corresponding probability of failure) for standard LRFD designs.