A series of pile load tests were carried out in a geotechnical test pit and in a geotechnical centrifuge to determine the shaft resistance in compression and in tension during repeated loading. In the geotechnical test pit open ended steel pipe piles were jacked into compacted moist sand and after eliminating the end bearing by creating a void below the base, they were tested repeatedly in tension and in compression. For both cases the capacities decreased with increasing number of load cycles. The capacity in compression was sometimes larger and sometimes less than in tension, depending on the soil type. In the geotechnical centrifuge tests were performed on closed end pipe piles in medium dense dry silica sand. Both, the end bearing and the total pile resistance, were simultaneously measured to enable direct calculation of the shaft capacity. A continuous record of shaft and end bearing resistances, with respect to axial displacement and time, was obtained, suggesting an interaction between end bearing and shaft resistance during compressive loading.
It appeared that as soon as the end bearing was activated the mobilization of a pressure bulb can dominate the frictional behaviour of relatively short closed end piles. It is believed that the mobilization of end bearing generates a pressure bulb at the pile base that curls upward around the pile and increases the horizontal stresses acting on the pile shaft. The decrease of the shaft resistance during repeated loading is related to contraction and a decrease in grain size of the soil adjacent to the pile shaft. These findings will be compared with results from field load tests and observations reported in recent studies by other researchers.
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