One-dimensional finite element limit analysis for hydraulic pile extraction

Abstract

This paper presents a numerical method to efficiently calculate the required overpressure for hydraulic pile extraction. The method uses a one-dimensional (1D) model involving two steps: (1) the steady state hydraulic pressure distribution is calculated for a unit pressure using a finite element model and (2) the required overpressure is calculated using finite element limit analysis. This combined model leads to an efficient calculation tool for estimating the required hydraulic overpressure to extract the pile. The model can also capture other failure mechanisms, such as failure of the pile plug, and the 1D approach allows for alternative soil resistance formulations to be incorporated easily. This allows the method to be used for preliminary analysis as well as detailed extraction assessments for an entire wind farm site. The proposed method is applied to study a pile in sand, clay and layered soil profiles. The analyses in sand show a significant increase in the plug shaft resistance due to the hydraulic gradient, resulting in a super-linear increase of break-out pressure with embedment depth. In clay soils, the break-out pressure increases linearly, but plug failure may occur at large embedment depths. Furthermore, soil layering can have a significant effect on the break-out pressure.