Evaluation of Pile Diameter Effect on Initial Modulus of Subgrade Reaction

This paper presents the results of a study on the effect of pile diameter on the initial modulus of subgrade reaction. A series of ambient and impact vibration tests were performed on four different diameters of cast-in-drilled-hole piles to determine the natural frequencies and damping of the soil-pile systems. The measured natural frequencies were then compared with those estimated from a numerical model. The soil springs in the numerical model were established by implementing two different concepts on initial modulus of subgrade reaction. One is based on Terzaghiʹs concept in which the modulus of subgrade reaction is independent of pile diameter. The other was based on recent research suggesting that the initial modulus of subgrade reaction may be linearly proportional to pile diameter. It was found that the measured natural frequencies were in good agreement with the computed ones when the diameter-independent modulus of subgrade reaction was employed. In addition, the test results show that the damping ratio of the system varied with pile diameter from 3% for 0.4-m pile to 25% for 1.2-m pile.