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. Analysis of field test data for lateral load tests on piles of different diameters at particular sites had lead to the suggestion that 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 realisation that only if the soil modulus is constant with depth is a constant modulus of subgrade reaction appropriate. A corollary of the work is that accurate estimates of pile head stiffness require better than routine site investigation data. Included in the paper 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.