Diameter Effects on p-y Curves-PDF

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The paper reviews whether conventional API p-y curves adequately account for pile-diameter effects. API’s sand curves derive from Reese et al., while its soft-clay curves follow Matlock. The author clarifies that p-y stiffness is not wholly independent of pile diameter: only the initial tangent stiffness in Reese’s sand method is diameter-independent. Ultimate soil resistance generally increases with diameter, making secant stiffness diameter-dependent as deflection grows.<br /><br />The author argues that both methods have a sound theoretical basis in Terzaghi’s modulus-of-subgrade-reaction framework, which accounts for foundation size when extrapolating from smaller tests. The proposed modifications by Stevens and Audibert and by O’Neill and Gazioglu are reviewed, but their alternative clay-curve formulations have technical limitations and are not shown to justify replacing Matlock’s method.<br /><br />Apparent increases in resistance observed in tests of larger piles may instead reflect additional resistance caused by pile rotation, especially in free-head tests. Such rotational effects are not represented by the translational soil resistance modeled by standard p-y curves. They can be significant for short shafts or piles extending above ground, but are generally inappropriate to assume for offshore jacket piles whose heads are restrained against rotation.<br /><br />For large or unusual foundations, the paper recommends modeling additional rotational and base resistances explicitly, where practical, rather than altering standard p-y curves. Otherwise, designers may use sensitivity analyses with p- and y-multipliers, while accounting consistently for uncertainty and loading conditions. The author concludes that API’s conventional curves remain suitable for their intended applications, particularly rotationally restrained offshore piles.

Keywords

API p-y curves

pile-diameter effects

Reese sand method

Matlock clay method

soil resistance

secant stiffness

Terzaghi subgrade reaction

pile rotation

offshore jacket piles

foundation design