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INVESTIGATION OF MONOTONIC AND CYCLIC LOADING OF PILES IN SAND USING A DIC CALIBRATION CHAMBER
Understanding the response of piles subjected to cyclic loads is critical for piles subjected to extreme loading conditions, particularly in the offshore environment in which platforms and wind farms operate. Not only there is no specific understanding of quantitative aspects of the impact of cyclic loading on pile resistance, but also the mechanisms governing the cyclic and post-cyclic response of piles in silica sands are not well understood. The mechanisms governing pile resistance mobilization under monotonic (tensile and compressive) or cyclic axial loading were investigated by performing instrumented model piles tests in a novel half-cylindrical calibration chamber with three viewing windows that allow the capturing of digital images of the sand domain and the instrumented model pile during installation and testing.
A set of tensile-compressive and compressive-tensile load tests were performed to study the effect of loading direction on the shaft resistance of model displacement and non-displacement piles. Measurements of displacements and deformations in the sand domain were obtained through the digital image correlation (DIC) technique. The tensile-to-compressive shaft resistance ratios were found to be a function of the loading history (installation method), loading sequence, and pile surface roughness. The results show that the tensile-to-compressive shaft resistance ratios are always less than one for jacked piles and nearly one for fresh preinstalled piles. The results from DIC analyses revealed that, when the loading direction is reversed, the soil elements near the pile shaft contract, and the direction of the principal strains rotate by about 90 degrees.
A series of model pile experiments that included installation, monotonic and cyclic load tests were performed to study the effect of cyclic loading on the limit unit shaft resistance and limit and ultimate unit base resistance of displacement piles. The impact of (1) cyclic displacement half amplitude, (2) number of displacement cycles, (3) relative density, and (4) initial stress state on the pile resistance was assessed based on the pile load measurements obtained before and after cycling and on the displacement and strain fields from the DIC analysis. A minimal effect on the limit unit shaft resistance was observed after cycling in tests performed with small cyclic displacement half amplitudes (= 0.25 mm), regardless of the number of cycles (up to 2,000 cycles). For 100 cycles or more applied with cyclic displacement half amplitudes greater 0.7 mm, the limit unit shaft resistance after cyclic loading was found to be always smaller than the limit unit shaft resistance before cyclic loading. It was observed that the degradation of the limit unit shaft resistance after cycling increases with increasing initial vertical stress and with decreasing relative density. From the DIC analysis, it was found that the decrease in the limit unit shaft resistance after cyclic loading is linked to the radial contraction and the development of cyclic and permanent shear strains in soil elements near the pile shaft during cyclic loading. Finally, the results show that the ultimate unit base resistance can drop significantly after cycling. The magnitude of the drop in the ultimate unit base resistance depends on both the magnitude of the cyclic displacement and the number of cycles. This experimental program's results provide a framework to improve the prediction of the capacity of piles subjected to cyclic loading.