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The loading response of a single vertical pile was calibrated against strain gage load-history data acquired at a highway bridge abutment located on Steele County Highway 7 in Owatonna, MN. At this site, H-type piles were driven to a weathered bedrock layer and soil surcharging was used to reduce the anticipated settlement of a 15 m (50 ft) thick layer of clayey sand overburden. Evaluating the load in the bridge piling for this case study and subsequent sensitivity modelling provides a comprehensive case study for use as background to the Minnesota Department of Transportation's revised dragload design guidance. The simulation was performed with FLAC3D and the goal of the model was to investigate pile behavior subjected to negative skin friction (downdrag) from approximately 9m (30ft) of abutment backfilling being placed around the stickup length of the pile. The simulation approximated the construction sequence of backfilling by applying a layer-by-layer backfilling approach and approximated structural loading by applying a direct axial force to the top of the pile. A sensitivity analysis was conducted from the calibrated case by changing the stiffness of the strata along the frictional portion of the pile and in the endbearing strata. The variation in maximum force along the pile as well as the position of the neutral plane was observed by varying soil stiffness. The relative stiffness was defined as the ratio between the average Young's modulus along the shaft of the pile and the end-bearing stratum's modulus. The relative stiffness influences the amount of dragload, axial force distribution along the pile and the location of the neutral plane. From the simulations it was observed that for a relative stiffness below 0.1 (very stiff base layer), the neutral plane is at the bottom of the pile and maximum possible dragload forces are realized. At a relative stiffness above 10 (very soft base layer), the neutral plane is near the top of the pile and the drag load force is minimal. This research suggests that for many Minnesota state transportation projects the dragload is centered between the extreme cases. Findings indicate for piles driven to stiff rock, dragload must be evaluated to ensure pile structural capacity is sufficient; historically this check was often ignored.Keywords: Negative Skin Friction, Downdrag, Dragload, FLAC3D, Driven Piles Lucarelli, Blanksma, Dasenbrock, Peterson 3 INTRODUCTION Downdrag forces, dragload, on driven piles may be caused by a variety of site conditions; often dragload is a result of placing fill material on top of a consolidating soil layer near the pile which induces downdrag (a downward deformation of the piling). Design for piles or pile groups should consider the effect of downdrag in order to properly evaluate criteria for strength and service limit states in LRFD design. Current practice is still evolving and at present design guides may non-conservatively underestimate, or over-estimate, the maximum load on the pile if downdrag effects are not treated appropria...

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