Pile Run Initiation in Transitional Soils

Abstract

The phenomenon of unexpected pile runs during pile driving, also known as ’dropfalls’, in silty transitional soils remains a subject of limited understanding. These occurrences have the potential to result in damage to the pile, hammer, and the occurrence of costly installation delays. In response to the growing demand for renewable energy and the spatial constraints of nearshore sites, offshore wind farms have emerged as a promising solution. In the last fifteen years, there has been a marked increase in the dimensions of offshore wind turbines, with monopile foundations now reaching diameters of up to 10 m. However, the installation of these large foundation piles can be hindered by dropfalls, which occur when the bearing resistance of the soil is decreased. This study investigates the theoretical mechanisms by which excess pore-water pressure is generated in contractive silts during impact hammering. This temporarily reduces shaft friction and induces dropfalls. The primary objective of this study is to improve pile-run predictions, thereby enable the implementation mitigation measures. Firstly, the interaction between soil and pile under impact loading is reviewed, with particular emphasis on the hydro-mechanical response of silty soils. It is demonstrated that silts may exhibit behaviour similar to undrained soil during driving. Such behaviour has been shown to result in the accumulation of pore pressure within transitional layers, leading to a reduction in effective stress along the pile shaft. The application of insights to a case study at the Changhua offshore wind farm (Taiwan) is then undertaken, where drop-falls occurred at depths of 16–25 m. The conclusion drawn from this study is that soil permeability and contractivity have a significant influence on the bearing capacity of the soil. In light of the aforementioned conclusions, the thesis proposes the incorporation of these variables into Soil Resistance to Driving (SRD) methods for the purpose of forecasting dropfalls. The contractivity is in this thesis assessed with help of the state parameter, which describes the difference in void ratios. Lab tests on borehole samples and plotting of CPT data on an SBTn chart were used to identify semi-permeable soil layers. The findings contribute to the advancement of mechanistic understanding of pile runs in silty soils, thereby providing a predictive framework for engineers to ensure safer, more efficient large-diameter foundation pile installations in transitional soil environments.