Analysis of shaft resistance of jacked and drilled-displacement piles
Significant developments in piling technology over the last few decades have yielded a variety of new pile types. Jacked and Drilled-Displacement (DD) piles, for example, are two emerging pile types that are becoming increasingly popular in geotechnical engineering practice. Jacking of steel or precast concrete piles into the ground is an effective and environmentally friendly pile installation technique (produces minimal noise, vibration and air pollution). DD piles, which are installed using a drilling tool consisting of a partial-flight auger and a displacement body, are a new generation of rotary displacement piles. Different types of DD piles are available in practice; each type is classified according to the design of the drilling tool and associated installation method.^ Installation of a pile often involves complex loading modes that cause substantial changes in the state of the soil surrounding the pile. As a result of these changes, the response of piles to structural loads varies greatly depending on the installation method used. For reliable estimation of pile capacity, design methods should account for the effect of the installation technique. Empirical or semi-empirical equations, based on results of load tests performed on driven piles (i.e., piles installed using hammers or vibrators), are often used to calculate the capacity of jacked piles. However, the mechanics involved in pile driving (a dynamic process) differ from those prevalent in pile jacking (a quasi-static process). Similarly, the currently available design methods for DD piles rely exclusively on empirical relations developed based on the results of load tests performed on particular types of DD piles installed at specific sites.^ No theoretical research has been performed to assess the effect of the installation method on the shaft resistance of jacked and DD piles. In order to develop design equations for the calculation of the shaft resistance of these piles, we performed FE analyses using advanced constitutive models for sand and clay. The plasticity-based constitutive models capture key features of soil behavior during pile installation and loading. The proposed design equations involve installation parameters (e.g., number of jacking strokes for jacked piles, and rate of drilling for DD piles), in situ soil state and properties (e.g., relative density and confinement for sand and undrained shear strength for clay), and some other key soil characteristics (e.g., initial fabric anisotropy for sand, and residual shear strength for clay). Additionally, setup factors are proposed that can be used to estimate the gain in shaft resistance as a function of time after installation of jacked piles in clay. Good agreement was obtained between the shaft resistance values calculated with the proposed equations and the data available in the literature. ^
Monica Prezzi, Purdue University.
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