Displacement pile installation effects in sand

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Abstract

Installation effects govern the post-installation behaviour of displacement piles in sand. These effects are currently not completely understood. Suitable experimental techniques to model these installation effects include field, laboratory and experimental models. In the current thesis a small-scale laboratory model is used to investigate the installation effect of displacement piles in sand, to complement a numerical study of the same subject. The current knowledge of installation effects is initially discussed. The distribution of shaft friction, resulting from cyclic loading, and the effect of initial relative density on the installation effects are of particular interest. Aims and scope for the current thesis are subsequently discussed, and consist of investigating the effect of installation mode (continuous or incremental installation), and the effect of the initial relative density of the soil before installation. The experiments are carried out in the geotechnical centrifuge, and considerable effort is taken to ensure that the similarity between the scale model and the prototype is assured. This concerns the model itself, the size of the soil grains, as well as the boundaries of the soil container. This process is guided both by theoretical and empirical consideration. The experimental model is then elaborated. This includes the geotechnical centrifuge, as well as the electrical and communications system installed to drive electric control motors and transmit measurement and control data. The model control system is described, including the computer programs that control the model tests. Deformation measurements are carried out by an in-flight camera. The deformation measurements are subsequently analysed with a series of computer programs to adjust for lens distortion and to retain displacement increments with a Particle Image Velocimetry (PIV) program. The soil sample preparation procedure is described, including preparation at different initial relative densities.The small scale model pile is described. This model pile included horizontal contact stress and axial stress measurement sensors that were included in the small membranes inside the model pile. The model pile was analysed with a FE-program to estimate the effect of the loading conditions. Calibration was carried out in custom-made calibration equipment for the horizontal stress sensors and the axial stress sensor.The experimental measurement results consisted of stress measurements and deformation measurements. The stress measurements consisted of the horizontal contact stress and the axial stress, as well as the ratios between these. The deformation measurements were presented as displacement paths in which the soil displacements were analysed, as well as incremental strains. The interpretation of the measurements focused on the effect of initial relative density, and the effect of load cycles. The initial relative density was shown to have a large influence on the horizontal contact stress during installation, and during extraction of the pile. The deformation measurements showed a similar influence in which the denser soil samples exhibited more horizontal displacement. The effect of incremental installation was analysed in the stress and deformation measurements, and indicates that the compaction of the soil during cyclic loading results in lower horizontal contact stress. The measurements were compared to numerical models that display a similar result, giving more confidence in the current theory of installation effects that is included in empirical design methods.

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