Results from long-term measurement in piles of drag load and downdrag

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.Key words: piles, negative skin friction, drag load, downdrag, neutral plane, pile settlement.

Group effect of dragload in pile groups embedded in consolidating soil under embankment load

“…Field tests and model tests focused on dragload (additional compressive force caused by NSF) of piles subjected to surcharge load were analyzed (Okabe, 1977;Shibata et al, 1982;Combarieu, 1985;Fellenius, 2006;Yang et al, 2009;Wang et al, 2011;Kim and Chung, 2012). These research results show that dragload is influenced by pile-soil interface coefficients, and soil consolidation conditions.…”

Settlement calculation method for single pile in negative friction case considering consolidation of surrounding soils

“…NSF reduces pile bearing capacity and increases axial force and settlement, leading to serviceability problems (Li 2014). Based on filed experiments, Fellenius (2006) observed that NSF can produce additional pile axial force for more than 3000kN. Walker and Darvall (2013) reported that surcharge loading could lead to 35mm ground settlement and NSF developed to the depth of 18m in single pile foundation.…”

“…Based on full-scale pile test within 1960s~1990s, Fellenius (2006) reported that pile-soil skin friction increased significantly, besides, the neutral point came up with the dissipation of pore water pressure. The NSF development characters for friction pile are studied by centrifuge tests (Lam et al 2009;Ng et al 2008).…”

Pile Drag Load and Downdrag in a Liquefaction Event

“…The AASHTO specifications recommend adding the factored drag load from the soil layers above the liquefying layer to the factored loads from the structure and requires the factored shaft resistance in the soil layers below the liquefying layer plus the factored toe resistance to be equal or larger than the combination of the factored loads mentioned. However, the AASHTO specifications do not recognize that a drag load is typically present in the pile prior to the earthquake (Fellenius 2006) Geotechnical and Geoenvironmental Engineering, Vol. 134, No.…”