Pre-boring is a technique widely used to facilitate large displacement pile driving in hard/dense soils, but usually at the expense of reducing the long-term shaft resistance of the pile. In the US, it is a routine practice for large displacement pile to be driven through dense cohesive soils with the ultimate capacity of pre-bored piles usually being determined based on empiricism and local experience. Understanding and quantifying the impacts of pre-boring installation on pile capacity can greatly help geotechnical design engineers to understand the interactions among the factors of pre-boring, pile size, soil conditions, and so forth and to improve the design and construction qualities of pile foundations in hard/dense soils. Due to the high cost and time involved with field instrumentation and testing on pre-bored piles, a numerical analysis study was funded by the Louisiana Department of Transportation and Development (DOTD) as a first step to exploring the impacts of pre-bore size on the long-term shaft resistance reduction of piles. This study was conducted using a practical finite element model that integrates the entire process from pile installation through subsequent consolidation to pile loading. The long-term shaft resistance was then examined after full consolidation by applying a vertical shear displacement on the soil element adjacent to the pile until the ultimate state was reached. Based on the numerical simulations, a set of reduction factor curves were then developed for a typical Louisiana soil stratum, which may provide guidelines for better design and construction of pre-bored piles in Louisiana.
