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This paper discusses some of the causes of residual installation forces in piles and the effects of the forces on the subsequent behavior of piles under structural or test loading. In the cases of jacked and driven preformed piles, residual forces arise as a result of the different rates of mobilization of bearing forces at the base and frictional forces on the shaft. The magnitudes of these residual forces can be large, and for this reason they may have a dominant effect on the mechanism of load transfer to the supporting soil at working loads. Residual forces are likely to be much smaller in the case of bored, cast in-situ concrete piles.
Some loading tests of jacked tubular-steel piles and bored cast-in-situ, concrete piles are described. All the piles were instrumented so that the distribution of load transfer could be examined, and all were in London clay. Results are presented to show that, as a result of the existence of residual forces, the proportions of shaft and base resistance mobilized at working loads are different for the two types of pile. Some evidence is presented to show that the settlements at working loads may also be influenced by residual forces.
bored pile, driven pile, jacked pile, clay, friction, negative friction, load test, settlement, instrumentation, stress distribution
Principal scientific officer, Building Research Station, Garston, Watford