United States Department of Transportation
i
Liquefaction-Induced Downdrag on Continuous Flight Auger (CFA) Piles From Full-Scale Tests Using Blast Liquefaction: [techbrief]
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2017-08-01
[PDF-418.69 KB]
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Abstract:Deep foundations typically support dead and live loads through a combination of positive skin friction acting along the sides of the pile and end-bearing at the toe of the pile; however, during seismic events, negative skin friction (or downdrag) can potentially develop. Downdrag occurs because of increases in effective stress caused by pore pressure dissipation and settlement by the liquefiable soil layer(s) relative to the pile (figure 1). This phenomenon creates a dragload that the pile must support in addition to its permanent service pile head load (Qτ). As illustrated in figure 1, the depth of the dragload extends to the neutral plane. The neutral plane is the depth where the settlement of the pile equals the liquefaction settlement of the soil and where the axial load in the pile is the greatest. The elevation of the neutral plane is found by trial and error such that the service load plus negative friction should equal the positive friction plus end-bearing resistance.
Fellenius reviewed many case histories involving downdrag on piles and concluded that the dragload does not change the ultimate bearing capacity of a pile foundation, but it should be added to the dead load in design.(1) In addition, the designer must also still determine that both the structural capacity of the pile and the tolerable settlement is not exceeded. For seismic events, it has been found that assuming skin friction equals zero in the layer(s) of surrounding soil in which liquefaction occurs will result in little error.(2,3) The American Association of State Highway and Transportation Officials recommends that the residual strength of the soil in that zone be assumed when estimating negative skin friction in a liquefied layer(s).(4)
To evaluate the skin friction of CFA piles, a blast-induced liquefaction test program was conducted at a site in Christchurch, New Zealand. During the tests, load versus depth was measured in three instrumented, full-scale, 0.6-m diameter CFA piles of various lengths. The test program included two sets of blast-induced liquefaction, the first without load on the piles and the second with load on the piles. The blasts liquefied a 10-m layer of sand along the length of the pile. The axial load distribution along the length of the pile because of negative skin friction was measured after liquefaction and reconsolidation. This report focuses on the test results of the loaded piles as compared to conventional design practice.
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