Effects of longitudinal variability of soil on a continuous spread footing

A simple model, describing the soil–foundation interaction for continuous wall footing, and which takes longitudinal variations in soil's properties into account, is presented in this paper. This model has been developed using a finite element representation in which a 1D structure (continuous spread footing) rests on a spatially variable soil, described by Winkler-type springs, characterized by the subgrade reaction coefficient.Geotechnical (pressuremeter tests) and geophysical (VLF-R: Very Low Frequency-Resistivity mode) surveys, performed over a selected study area resting on an alluvial terrace, were used to provide information on the soil's variability, and to reveal the presence of clayey soil. Three kinds of natural variability are considered: natural soil variations, rapid variations in the soil's properties, and shrinkage of clayey soil.A relationship between the coefficient of subgrade reaction and the resistivity was observed. The spatial variability of the coefficient of subgrade reaction was analyzed using geostatistics, and then used in the model to derive differential settlements and bending moment values of a continuous spread footing, which cannot be predicted if the soil is assumed to be homogeneous, or if shrinkage under the continuous wall footing is not taken into account. Results show that, as is often the case in practice, when the longitudinal soil variability is not taken into account, the design may be unsafe.

► Simple FEM model taking longitudinal soil variation into account is developed for shallow foundation.
► Natural soil variation, rapid lateral variation and shrinkage of clayey soil are considered.
► VLF method is used to obtain spatial variability of soil properties.
► A relationship between soil resistivity and deformation modulus is highlighted.
► Shrinkage of clayey soil gives the major effect on continuous spread footing.