Performance of Micropiles Used to Underpin Highway Bridges

Micropiles, which are small-diameter, drilled piles, are being used more frequently to support structures where it is not feasible to use driven piles or larger-diameter, drilled shafts because of conditions such as limited overhead or restraints on vibrations. The construction procedures used for micropiles typically include the performance of verification load tests (i.e., compression load tests to 2.5 times the design capacity on test piles) and the performance of proof tests (i.e., load tests on a limited number of production piles, typically about 5%, to 1.67 times the design capacity). Because of concerns over postconstruction settlement and the difficulty of proof loading individual production piles to 1.67 times the design capacity, all production micropiles used to underpin four bridges were preloaded, and the deflection of each pile was measured to verify acceptable postconstruction performance. The collection of deflection data for such a large number of production micropiles is unusual and serves as a valuable case study showing the successful performance of drilled micropiles.