Due to the similarity between the geometry and full displacement installation method of a cone penetrometer and displacement pile, the axial capacity of displacement piles is often assessed using data from a cone penetration test (CPT). As there are many more factors influencing pile axial capacity than affecting CPT cone resistance, there are a wide range of CPT-based empirical design methods in use. These methods have various levels of predictive success, which usually depends upon the soil conditions, pile geometry, pile installation method, and time between installations and loading. An improved understanding of the basis and reliability of respective design methods is essential to improve the quality of predictions in the absence of site specific load test data. This thesis explores the influence of soil state and drainage conditions on piezocone penetration test (CPTU) tip resistance (qc) and penetration pore pressures (u2). For cone penetration testing identified as 'drained', factors influencing the correlation between cone tip resistance and displacement pile shaft friction in sand are investigated through (i) a review of previous research and the performance of existing design methods; (ii) centrifuge studies of piles of differing widths with measurements of local lateral stress; (iii) field tension tests at different times between installation and loading for uninstrumented driven piles with different diameters and end conditions; and (iv) field tension tests at different times between installation and loading on closed ended strain gauged jacked segmented model piles with different installation sequences. CPTU qc and u2 are primarily controlled by soil state and drainage conditions, with effective stress strength parameters and soil stiffness also influencing the measurements. The primary mechanisms identified to control the correlation between cone tip resistance and shaft friction on displacement piles are identified as; (i) the initial increase in radial stress due to soil displaced during installation of a pile; (ii) different levels of soil displacement induced by open, closed, and partially plugged piles; (iii) reduction in radial stress behind the pile tip; (iv) additional reduction in radial stress with continued pile penetration (friction fatigue); (v) changes in radial stress during loading; (vi) constant volume interface friction angle between soil and steel; and (vii) changes in the effects of the above mentioned mechanisms with time between installation and loading. The relative effect of each of these factors is investigated in this thesis.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2007|