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Edition:Final Report, 09/2017-08/2019
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Abstract:In Louisiana and many other states in the U.S., pre-boring procedures are routine practice for large displacement piles driven through hard/stiff cohesive soils. At present, the analysis and design of pre-bored piles rely primarily on local experiences, since the effects of pre-boring on the driving force as well as on the setup and long-term shaft resistances within the pre-bored zone remain unclear. Due to the high cost and time involved with field instrumentation and testing on pre-bored piles, this numerical study is funded by the Louisiana Department of Transportation and Development (DOTD) as a first step to explore the impacts of pre-bore size on the drivability and long-term shaft resistance reduction of piles. In this research, researchers developed simplified yet realistic numerical model, which considers a thin horizontal soil disc sufficiently far from the ground surface and the pile base using the ABAQUS package to investigate the effects of pre-boring on the pile behaviour over different installation and loading stages. Leveraging on the finite element model developed, the long-term shaft resistance reduction factor curves have been generated for typical Louisiana soil strata to provide guidelines for a better design and construction of pre-bored piles in Louisiana. On the basis of the numerical calculation results, a preliminary protocol, including the site selection, soil properties investigation, and the testing procedure and data collection, is subsequently proposed for future instrumented pile field tests of pre-bored piles in Louisiana. Finally, to facilitate the design of pre-bored piles, a set of practical formulas are developed to predict/evaluate conveniently the driving force, setup and long-term shaft resistances of pre-bored piles. The current research work not only reveals the shaft resistance reduction mechanism of pre-bored piles, but also provides a simple and reliable approach to evaluate the driving force and shaft resistance of pre-bored piles, which will be helpful and beneficial for geotechnical and construction engineers involved with the design and installation of the pre-bored pile foundations in Louisiana.
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