Shaft capacities of closed end piles under load reversals in sand

Hanke, Richard (2001) Shaft capacities of closed end piles under load reversals in sand. Masters thesis, Memorial University of Newfoundland.

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Abstract

Since a wide range of opinions exists on axially loaded friction piles in sand a further evaluation and determination of the mechanisms involved with pile shaft capacities has been undertaken. A series of model pile load tests have been carried out in a geotechnical centrifuge to determine the effects of continuous load reversals. Axial compression and tension tests were performed on closed end pipe piles in a medium to dense dry silica sand. Both the end bearing and the total pile capacity were simultaneously measured to enable direct calculation of the shaft capacity. Pile loads and displacements were measured, as was the pile's inclination off vertical. With this information and the soil properties obtained within the laboratory, back calculations were performed using various methods to theoretically determine the pile shaft capacities. The theoretical values were then compared with the pile model data at prototype scale. This comparison has revealed the lack of reliability of design procedures of full displacement-type friction piles in sand and has yielded recommendations for the further experimental testing of pile models, which may ultimately lead to recommendations to existing design guidelines. -- The study demonstrated that pile shaft capacities in non-cohesive soils can be both underestimated and dangerously overestimated by following conventional analytical design procedures. Pile inclination was determined to have a great affect on shaft capacity. Furthermore, the mobilization of pile end bearing was observed to have a distinct influence on compressive shaft resistances. Test results revealed a significant reduction in tensile shaft resistance after the first few load cycles and a continued reduction with an increasing number of load reversals. Conversely, an increase in the total pile compressive resistance after several cycles was observed and thought to be due to granular crushing and material densification at the zone of end bearing. Similar to the end bearing response, the compressive shaft resistance initially reduced with load reversals but a trend reversal did become apparent. As the end bearing began to increase with continuous load cycling, so did the compressive shaft resistance. Focussing on an individual load cycle, the observed tensile shaft resistances were approximately equal to the compressive resistances before the end bearing was mobilized. Beyond this point the compressive shaft resistances increased to values of 2 to 3 times that of the corresponding tensile capacities. The increase in compressive shaft resistance was observed to be proportional to the simultaneous increase in end bearing capacity. -- It is believed that a pressure bulb is generated at the pile base that in turn increases the lateral confining stresses along the lower portion of the pile shaft The study has shown that as soon as the end bearing is activated, the formation of a pressure bulb can dominate the frictional behaviour of relatively short piles.

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