Setup and Relaxation in Glacial Sand

Abstract

Setup and relaxation were encountered during testing and subsequent installation of piled foundations in a glacial deposit of clean sand. Displacement piles driven into this medium dense deposit show time‐dependent increases in load capacity of from 40–80% as determined by loading tests and the results of wave equation analyses made using input obtained from dynamic measurements. Evidence is cited to show that setup results from a loss of strength due to disruption of the structure of the sand during pile driving, followed by a recovery in strength as the soil structure heals at constant effective stress. Relaxation occurred in large groups of piles where the cumulative effects of driving piles into the ground brought the sand to a dense state, creating conditions where subsequent pile driving caused sands in the failure zone to dilate during shearing displacements. Relaxation is attributed to dissipation of the resulting negative pore pressures and relief of high lateral ground stresses within the dilated sand.