Performance of Precast Driven Piles in Marine Clay
The research reported is concerned with the behavior of precast reinforced concrete piles driven through soft marine clay and founded in residual soil and weathered rock of sedimentary origin; to study the load-transfer behavior of such piles, strain gages installed along the pile shafts were monitored frequently during the static loading tests and at regular intervals during and after the construction of the superstructure. Piezometers were installed in proximity to one of the piles. The dissipation of excess pore water pressures indicated that setup in the marine clay was highly significant. Hence, pile capacities calculated using wave-equation models based on stress-wave measurements during initial pile driving gave consistently lower values than those of static loading tests one week later. Long-term instrument readings indicated development of negative skin friction when the piles were in service. The downdrag increased with time, and this additional loading should be accounted for in the pile design as part of the service load on the pile.
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International Journal of Geomechanics 03/2006; 6(2). DOI:10.1061/(ASCE)1532-3641(2006)6:2(89) · 1.20 Impact Factor
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ABSTRACT: Several full-scale, long-term tests on instrumented piles performed since the 1960s and through the 1990s are presented. The results of the tests show that a large drag load will develop in piles installed in soft and loose soils. The test cases are from Norway, Sweden, Japan, Canada, Australia, United States, and Singapore and involve driven steel piles and precast concrete piles. The test results show that the transfer of load from the soil to the pile through negative skin friction, and from the pile back to the soil through positive shaft resistance, is governed by effective stress and that already a very small movement will result in mobilization of ultimate values of shaft shear. The pile toe resistance, on the other hand, is determined by downdrag of the pile and the resulting pile toe penetration. Reconsolidation after the pile installation with associated dissipation of pore pressures will result in significant drag load. An equilibrium of force in the pile will develop, where the sustained loads on the pile head and the drag load are equal to the positive shaft resistance plus the pile toe resistance. The location of the force equilibrium, the neutral plane, is also where the pile and the soil move equally. The drag load is of importance mostly for very long piles (longer than 100 pile diameters) for which the pile structural strength could be exceeded. Downdrag, i.e., settlement of the piled foundation, is a very important issue, however, particularly for low-capacity short piles. Soil settlement at the neutral plane will result in a downdrag of the pile. The magnitude of the downdrag will determine the magnitude of the pile toe penetration into the soil, which will determine the pile toe resistance and affect the location of the neutral plane. Nature's iteration of force and soil settlement will decide the final location of the neutral plane.
Canadian Geotechnical Journal 04/2006; 43(4):409-430. DOI:10.1139/t06-009 · 1.33 Impact Factor
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