Mechanisms of setup of displacement piles in sand: laboratory creep tests

A series of triaxial creep tests is described, with the aim of shedding light on the mechanisms behind displacement pile setup in granular soils. The stress path leading to the creep stages has been designed to mimic, as far as possible, the installation of a pile adjacent to an element of soil. A complex strain response is found with creep in dense sands at high stress ratio, with rotation of the creep strain vector over time. A new hypothesis, involving kinematically restrained dilation of the soil close to the pile shaft and soil ageing, is proposed to explain pile setup. Influences on the creep behaviour, and hence degree of setup, are found to be particle shape and strength, relative density, and rate of loading. The application of small cyclic perturbations during creep is found to accelerate the onset of volumetric dilation and hence is proposed to accelerate setup.