Settlement Uncertainty Analysis for Continuous Spread Footing on Elastic Soil

In conventional design, continuous spread footings are always designed on the basis of their transverse behavior, where the natural variability of soil properties and any uncertainties in the geometrical and mechanical properties of the structure are usually not considered. In order to reveal the effects of these uncertainties and the soil variability on the behavior of spread footings, the longitudinal direction needed to be studied. Numerous mechanical models, which describe the soil-structure interaction as a beam on elastic foundation, were developed. The common parameter between these models is the subgrade reaction coefficient which depends on numerous parameters. In this paper, the FOSM method was used, first on four semi-empirical models which give the subgrade reaction coefficient, and secondly on the analytical solution of a beam from Winkler’s hypothesis including different boundary conditions. Uncertainties in the subgrade reaction coefficient, differential settlement and bending moment were obtained for the following case: continuous wall footing in residential construction, where a low stiffness zone or shrinkage of clayey soil is possible. Results from a global uncertainty analysis demonstrated the major effects of uncertainties in soil modulus; Poisson’s ratio and width of the spread footing on the uncertainty of the subgrade reaction coefficient, and thus on uncertainties of the differential settlement and bending moment. Finally, it was shown that when the soil modulus and geometrical dimensions of the foundations are uncertain and where a zone of weak soil or shrinkage of clayey soils are present, the longitudinal behavior of continuous spread footings for residential construction should also be considered in their design.