Diameter Effects on Pile Head Lateral Stiffness and Site Investigation Requirements for Pile Foundation Design*

Abstract

The effect of pile shaft diameter on unrestrained pile head lateral and rotational stiffnesses of long piles is considered in relation to required site investigation data. Analysis of field test data for lateral load tests on piles of different diameters published in the literature led to the suggestion that the modulus of subgrade reaction appears to increase with increasing pile shaft diameter. This is contrary to the usual understanding that the modulus of subgrade reaction is independent of pile shaft diameter. This puzzle is resolved in the realization that as the pile diameter increases the depth of soil which contributes to the lateral stiffness also increases. If the soil stiffness increases with depth, then the pile head lateral stiffness will be greater than would be predicted on the basis of lateral load testing of smaller diameter piles. Included in the article is discussion of the effect of the variation with pile shaft diameter of the unrestrained pile head lateral and rotational stiffnesses for three distributions of soil modulus with depth, as well as the effect of the pile head moment to shear ratio. A corollary of the article is that accurate estimates of pile head stiffness during foundation design require better than routine site investigation data.