Effect of soil-foundation interaction on the modulus of subgrade reaction

Abstract

For design of raft foundations, the soil is commonly modelled by using elastic springs, which is based on the Winkler’s theory. In the static analyses, these springs are considered as a constant value k called the modulus of subgrade reaction. However, no foundation is infinitely flexible due to the interaction between the soil and foundation, which is investigated in the present work. For this purpose, the effect of foundation dimensions and foundation material on the subgrade reaction modulus is examined for square-shaped foundations with uniformly distributed load, and the results obtained from these analyses are compared with those in infinitely flexible foundations. It is shown that the effect of Young’s modulus of foundation material on the k-value can be ignored while the foundation dimensions have a great effect on the k-value. The ratio of the subgrade reaction modulus obtained from the analyses taking into account the soil-foundation interaction to the subgrade reaction modulus in infinitely flexible cases varies between 0.36 and 0.45 in the middle area of foundations as well as 0.45 and 0.76 on the edges of foundations with a width between 5 m and 20 m as well as a thickness between 0.25 m and 1.0 m.