Swelling and collapse behavior of expansive marl and its implications in foundation engineering

Abstract

Expansive soils increase their volume upon wetting, but under high vertical stresses may exhibit collapse, with the latter aspect being rather overlooked in foundation engineering. Oedometer swelling/collapse tests on expansive Pliocene marls of various calcium carbonate content and plasticity index sampled from Nicosia, Cyprus, indicate that, unlike the free swelling strain, the swelling pressure is rather insensitive to the initial degree of saturation. Consequently, significant collapse strains upon wetting are not unlikely to occur in practical situations. The paper discusses the implications of this behavior in the case of shallow foundations and proposes methods of analysis. Under certain conditions, foundation design based on settlement calculations that use as input the fully saturated soil properties may be unconservative and supported structures may be exposed to severe risk of damage. For the design of mat foundations, stepwise distributions of the equivalent modulus of subgrade reaction across the mat as a function of the average bearing pressure are proposed based on the results of coupled finite element simulations. These distributions allow prediction of the peak bending moments produced at the end of the wet and dry seasons due to ground swelling and shrinkage, respectively, for the climatic conditions of Cyprus.