Probabilistic assessment of total and differential settlements due to spatial soil variability

Abstract

Design of shallow foundations is often guided by serviceability requirements. In particular, the amount of differential settlements becomes usually decisive in selecting the type of foundation – shallow or deep foundation. In practice, differential settlements are estimated by multiplying the average total settlement by an empirical factor accounting for inherent spatial variability of soils. For shallow foundations on clays and sands, ratios of differential to total settlements have been suggested varying roughly from 40 to 100%, respectively. Important factors of influence such as the spacing of footings, variability of the foundation soil, as well as variance reduction because of averaging are not taken into account. A general analytical approach to assess the total and differential settlements of strip footings is developed. The compressibility of the soil is modeled as a spatial random field. The probabilistic approach explicitly considers the inherent soil variability, the zone of influence of settlement to account for variance reduction, and the spacing of adjacent footings because of horizontal spatial correlation. The method presented is compared to the approach advocated in Eurocode EC7. The findings indicate the widely used design criteria used to judge differential settlements to be overly conservative, in particular for sandy subsoil. Examples are presented to illustrate the method.