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Behavior of micropiles in bridge bent applications.
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    NTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and Structures ; NTL-HIGHWAY/ROAD TRANSPORTATION-Construction and Maintenance ;
  • Abstract:
    This project concerned the behavior of micropiles under lateral loads. The North Carolina Department of Transportation was specifically interested in the use of micropiles to support bridge bents. 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 on lateral resistance of micropiles, determine the effect of casing joints on the lateral resistance of micropiles, determine the behavior of jointed micropile sections, evaluate the durability of micropile casings and jointed sections, and disseminate the experimental findings to NCDOT. 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. 14 of the 16 piles were two or three sections or more. A cap was cast around four of the piles 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 first predict the behavior of the load tests. The results of the field and lab tests were subsequently 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. 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 was observed in the load tests. b)Two feet of embedment for micropiles in this study was sufficient to carry lateral loads upward of 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) The strength of the micropiles with respect to the joints from field and laboratory tests was around 140 k*ft. d)Micropiles of this size can carry 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 in FB-Multipier.
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