Strain influence diagram for estimation of settlement of isolated and combined ring footings

Abstract

The Schmertmann’s concept of strain influence diagram provides a convenient framework for estimating settlement of footings by incorporating the influence of shear deformations in soils. This method is being widely used in practice to estimate settlement of circular, rectangular and strip footings by using the strain influence diagrams available in literature. This study extends Schmertmann’s framework to propose new set of strain influence diagrams for isolated and combined ring footings. The proposal is based on threedimensional elastic finite element analysis for various configurations pertaining to geometry of rigid ring footings, loadings and soil conditions. In case of isolated footing, the model has been verified by comparing the simulation results of circular footing with predictions from the Schmertmann solution and a closed form solution in elastic domain. The numerical results indicate that with increase in ratio of inner-to-outer radius of ring footing ‘n’, the settlement decreases and so does the depth of maximum strain. This observation has been accounted for by varying the depth of the assumed simplified strain influence diagram. Based on the new strain influence diagrams, empirical equations correlating the influence depth with ‘n’ values are proposed for estimating settlement of ring footings under various degrees of heterogeneity in sand. Likewise, in combined footings, 3D numerical modeling and analysis similar to the isolated footings are performed at various spacings and configurations of ring footing and a detailed procedure is outlined to obtain settlement for combined ring footings based on a proposed empirical equation derived along lines similar to the case of isolated footings. The proposed method is finally validated by comparing with experimental results available in literature for combined footings.