A microstructure approach to the sensitivity and compressibility of some Eastern Canada sensitive clays

Investigation of micro-macro relationships in soils most often concerns granular soils in which both the elemental unit (the grain) and the physical laws governing inter-grain interactions appear to be better known. The situation is different for clays because the elementary unit and the inter-unit interactions at the micro-scale are more difficult to characterise. In fine-grained soils, it has been shown that an intermediate level, corresponding to the way clay and fine-grained particles are arranged together, had to be considered so as to link microscopic features to macroscopic behaviour. An investigation of the change in microstructure during compression carried out some time ago on a sensitive clay from Canada demonstrated that studying the changes in pore size distribution provides a satisfactory description of microstructure changes during compression. The analysis is applied here to six other sensitive clays. First, a careful examination of the intact and remoulded microstructure is conducted in order to understand better the relationship between microstructure and sensitivity. Second, an interesting correlation between the compressibility coefficient and the slope of the cumulative pore size distribution curve is observed in Champlain clays. This confirms the analysis conducted that compression in loose, low-plasticity soils can be attributed to the progressive and ordered collapse of pores, starting from the largest existing ones and progressively affecting smaller and smaller pores. A conclusion drawn from this work is that a micro-macro analysis in terms of changes of a rigid fragile porous matrix appears to be more relevant than a standard analysis based on the behaviour of individual grains, where the grains are taken to be the relevant elemental microstructure unit (a unit difficult to identify in natural clay soils). Also, further insight is provided for the interpretation of microstructure effects through comparing the compression curves of intact soil samples to those of remoulded samples.