Strain Path Analysis of Setup Time Around Piles and Caissons

Abstract

Installation of driven piles and suction caissons in clayey soils generates excess pore pressures that temporarily reduce load capacity due to side resistance. Time dependent dissipation of these excess pore pressures leads to recovery of side resistance, a process known as ‘setup’. Since many facilities cannot be put into operation until sufficient pile load capacity has been mobilized, realistic predictions of setup time can be important. This study consists on the analysis of setup time following open ended pile and caisson installation. Initial excess pore pressures due to installation disturbance are predicted based on a strain path analysis based on a ring source moving at constant velocity in an incompressible medium. It is assumed that the setup occurs primarily due to dissipation of excess pore pressures generated during the installation process; thixotropic effects are neglected. The analysis employs an elastic perfectly plastic model of soil behavior and an uncoupled analysis of consolidation to simulate conditions on the pile shaft outside of the influence of tip effects. A parametric study shows that wall thickness and soil rigidity index can exert order of magnitude differences on setup time. Strain path solutions show reasonable agreement to laboratory and field measurements of pore pressure dissipation around thin-walled piles typical of suction caissons. Strain path solutions tend to underestimate setup time for driven piles, likely due to partial plugging during pile driving.