Strain influence factors for footings on an elastic medium

Shallow foundations in granular soils are designed such that the settlements are within tolerable limits. Schmertmann's (1970) method is one of the most rational methods for computing settlements of footings in granular soils, and is commonly used world-wide. The method relies on a strain influence factor that varies with depth. Schmertmann et al. (1978) proposed separate strain influence factors for axi-symmetric and plane strain loading situations, representing circular and strip footings. The objective of this paper is to revisit Schmertmann's influence factors using FLAC and FLAC3D, explicit finite difference codes used widely in geotechnical modelling, and the theory of elasticity. Linear elastic and non-linear elastic constitutive models were used in the analysis. The influence factors derived are compared with those proposed by Schmertmann. For square and rectangular footings, the problem becomes three-dimensional and therefore FLAC3D was used in the analysis. The strain influence factors are developed for footings with breadth/length ratios of 0.25, 0.50, 0.75 and 1.0. The strain influence factors for the rectangular footings are presented along with those for the circular and strip footings. The effect of Poisson's ratio was also investigated. While verifying the original strain influence factors, the new factors proposed for the rectangular footings will be valuable in the design of shallow foundations on granular soils. The use of non-linear elastic constitutive model is more realistic than the traditional linear elastic model, and the differences are discussed