Time and scale effects on the shaft friction of displacement piles in sand

JitKheng Lim

    Research output: ThesisDoctoral Thesis

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    The phenomenon of pile set-up (i.e. increase in pile shaft friction with time) in sand has been reported for decades but the observations are highly scattered and the mechanisms giving rise to set-up are poorly understood. The capacity gain with time, if successfully justified with greater understanding of its governing mechanisms, has major cost benefits for foundations and for the re-use of existing foundations. This thesis describes a series of laboratory and field experiments designed to track the changes of shaft resistance up to a maximum of 72 days after installation. The laboratory-scale pile tests were conducted in a pressure chamber, taking into account the interfering sample ageing effect, to study the relative influence of several important factors on pile set-up. The field tests employing three sizes of reduced-scale model piles (D = 65, 100 and 135 mm), each equipped with a surface stress transducer (SST), were installed and subsequently load-tested statically in tension at three sand sites of different mineralogy and groundwater conditions. The results, combined with other well-documented case histories, are examined to gain further insights on this longstanding contentious issue. Evidence shows that it is more appropriate to describe the phenomenon of pile set-up as a recovery process (rather than a gain) following the disturbance induced by pile installation. Set-up in the medium and longer term varies with the logarithm of time but involves a delay during the initial period and a limit after stress equilibration is reached, which can be better represented using a newly proposed expression. The measurements of SSTs reveal that the increases in stationary radial effective stress (ó’rs) over the ageing period are relatively small but considerable changes in the increases in radial effective stress during shearing (Δó’rd) are evident. The observation suggests that the major underlying mechanism of pile set-up in sand is that of constrained dilation due to an increase in the shear stiffness of the surrounding soil following installation disturbance. To assess how model pile test results can be extrapolated to full-scale conditions, the thesis examines scale effects using a database of tension load tests comprising pile diameters which vary by more than two orders of magnitude. The UWA-05 design method, which is adopted as a base calculation approach in the assessment, indicates that the prediction of full-scale offshore piles is reasonable but the capacity of small-scale model piles required a more complex representation of the shear stiffness of the sand mass surrounding the pile shaft.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2013


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