Evaluation of the lateral response of micropiles via full scale load testing

Micropiles are a relatively new deep foundation technology in the United States. As an alternative to driven piles or drilled shafts, micropiles can provide substantial support while minimizing cost, environmental impact, and harmful construction vibrations. In order to implement micropiles for new construction on bridges with unsupported lengths, a better understanding of the performance of micropile constituent materials and the structural performance of single micropiles and micropile groups is required. This research addressed the behavior of micropiles under lateral loads. In this configuration, micropiles would be subjected to lateral loads. Thus, there was a need to evaluate the behavior of micropiles as bridge bent foundations with respect to joints between micropile sections and embedment or plunge in rock. The objectives of this study were to demonstrate the lateral performance of micropiles in single and group configurations, determine the effect of casing plunge into rock on lateral resistance of micropiles, determine the effect of casing joints on the lateral resistance of micropile, determine the behavior of jointed micropile sections, and evaluate the durability of micropile casings and jointed sections. These objectives were investigated using a three pronged approach of numerical modeling, full scale field lateral load tests, and laboratory testing. Sixteen sacrificial micropiles were installed in order to perform six lateral load tests. Rock plunge depths of 1, 2, 5 and 10 feet were investigated. Fourteen of the 16 piles comprised two or three sections. A cap was cast around four of the micropiles to create a bent that was load tested against a group of reaction piles. In addition, nine jointed micropile specimens were fabricated and tested in the laboratory under four- point flexure. Numerical models were developed to predict the behavior of the load tests. Subsequently, the results of the field and lab tests were used to calibrate the model for DOT use. A long term study of the impact of corrosion on micropile sections is submitted for future implementation. The main findings of this study include: a) The casing joint has a large impact on the lateral capacity of micropiles. In cases where the micropiles were sufficiently embedded in rock, rather than yielding there was an abrupt failure at the casing joint. This occurrence was observed in the load tests. b) In this study, two feet of embedment for micropiles was sufficient to carry lateral loads greater than 30 kips. Embedment at 5 and 10 feet produced similar results to 2 feet. One foot of embedment does not appear to be sufficient based upon results of the field tests and numerical models. c) Based on field and laboratory tests, the strength of the micropiles with respect to the joints in bending moment was approximately 115 kips*ft. d) Micropiles of 10.75 in. diameter, 0.50 in. wall thickness carried significant lateral load with little deflection. However, the failure mode is brittle, as the piles tested failed abruptly with little lateral displacement. e) Reduction of the section area at threaded joint by 60% to 70% results in a reasonably accurate model for the behavior of the casing joint using FB- MultiPier computer program