Evaluation of Negative Skin Friction on Bridge Abutment Piles-Article

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The study evaluated negative skin friction (downdrag) on bridge abutment piles using four years of field monitoring at a three-span bridge in Ontario, Canada. Researchers instrumented three H-piles and nearby soil; the paper focuses on one pile. Strain gauges measured pile forces, while extensometers and piezometers tracked pile and soil settlement and pore-water pressure during construction of the approach embankment and bridge.<br /><br />Embankment loading caused the soft clay around the pile to settle, generating downward drag forces. Soil settlement reached approximately 64 mm on the more heavily loaded side of the abutment. The measured maximum drag force was 306 kN—about 77% of the pile’s design dead load—and developed after excess pore-water pressure had dissipated. The neutral plane, where soil and pile settlements are equal, shifted as construction and settlement progressed.<br /><br />The researchers compared measurements with the unified design method using the total-stress (α) approach. Assuming constant undrained shear strength gave a reasonable drag-force estimate but poorly predicted the neutral-plane location. Accounting for how undrained shear strength varies with effective stress improved predictions of both. Estimates based on direct simple shear test results were closest to measurements: predicted drag force was 284 kN, and the predicted neutral-plane elevation was close to the observed value.<br /><br />The findings show that drag-force assessments should consider stress changes from embankment loading and soil disturbance during pile driving. The study also highlights the value of long-term field instrumentation for understanding downdrag development.

Keywords

negative skin friction

downdrag

bridge abutment piles

field monitoring

H-pile

soft clay settlement

pore-water pressure

neutral plane

embankment loading

undrained shear strength