Pile Diameter Effects on Initial Modulus of Subgrade Reaction for Laterally Loaded Piles-Paper

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The study examines how pile diameter affects the initial modulus of subgrade reaction used to model lateral pile response. This stiffness is particularly important for large bridge piles, whose service-level movements are small and whose seismic behavior may therefore depend strongly on the initial slope of the p-y curves.<br /><br />The authors developed a three-dimensional finite-element model of a soil-pile system and validated it against field measurements from a 24-inch pile in sand. They then analyzed piles 0.5–10 ft in diameter, keeping pile flexural stiffness constant, and matched the results to beam-on-nonlinear-Winkler-foundation analyses to estimate equivalent initial spring stiffness. Sensitivity checks considered changes in soil and pile stiffness and different pile-head fixities.<br /><br />The results indicate that API-based values, calibrated to a 2-ft reference pile, can underestimate initial soil stiffness for larger piles and overestimate it for smaller piles. The proposed diameter adjustment is:<br /><br />- For piles larger than 2 ft: multiply the reference modulus by \((D/2)^{0.4}\).<br />- For piles smaller than 2 ft: multiply it by \((D/2)^{0.2}\).<br /><br />Thus, a 10-ft pile may have an initial modulus about 1.9 times the API reference value, while a 0.5-ft pile’s value may be about 0.76 times the reference value. Field-test data for 1.3–4-ft piles supported the proposed adjustment; accounting for diameter improved predictions of deflections and bending moments.<br /><br />The study addresses only the initial, small-strain portion of the p-y curves. Its conclusions may not apply once soil or pile behavior becomes nonlinear, and further work is needed to assess diameter effects on the nonlinear portions of those curves.

Keywords

pile diameter

subgrade reaction

lateral pile response

p-y curves

initial modulus

finite-element model

bridge piles

soil-pile interaction

diameter adjustment

API reference value