Scanning electron microscopy and mercury intrusion porosimetry are used in parallel to identify the structure of a medium sensitivity Champlain clay. The clay structure is observed firstly on intact, remolded, and oven-dried soils and secondly on soils consolidated at various levels in one-dimensional compression. Both methods of investigation reveal for the intact soil the existence of an aggregated structure characterized by an interaggregate and an intra-aggregate porosity. Remolding affects interaggregate links but does not destroy aggregates.The observation of clay structure at various levels of one-dimensional compression shows that the collapse of the structure is progressive, the largest interaggregate pores being the first affected. As the consolidation proceeds, smaller and smaller pores are affected. For a given pressure increment, only the largest existing pores are affected. A structure anisotropy has been seen to develop with increasing compression.The scanning electron microscope and the mercury intrusion porosimeter used in conjunction with each other appear as a powerful approach for clay structure observation. Keywords: natural clay, microstructure, freeze-drying, porosimetry, microscopy, consolidation, aggregate.

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This paper describes the use of quantitative mineralogy and specific surface area in interpreting the index properties of clayey soils from nine sites in Eastern Canada. Samples representative of the Tyrrell, Laflamme, Champlain, and Goldthwait marine seas and Lac Barlow–Ojibway have been studied.Quantitative X-ray diffraction analyses may be satisfactorily obtained using potassium metaperiodate (Foscal-Mella 1976) as an internal standard in these soils. Determination of contained amorphous matter requires extraction procedures less brutal than those employed in the Ségalen method.Correlations between Atterberg limits and specific surface area are believed to be more useful than those between specific surface area and the grain size or clay fraction. Increases in plasticity and specific surface area are related to increases in the amount of contained phyllosilicates and amorphous matter.The mineralogical composition of the soils studied is dominated, even in the clay fraction, by felsic minerals (plagioclase, quartz, microcline, and hornblende). Samples close to the Canadian Shield contain relatively more felsic minerals than those away from it. Key words: mineralogy, index properties, sensitive clays, physicochemistry.

A landslide occurred on 10 May 2010, along the Salvail River, in the municipality of Saint-Jude, Quebec. Debris of the landslide was formed of clay having horst and graben shapes, typical of spreads in sensitive clays. A detailed investigation was carried out by the Ministère des Transports, de la Mobilité durable et de l'électrification des transports du Québec, in collaboration with Université Laval, with the objective of characterizing this landslide, determining the causes, and learning about its failure mechanism. The soil involved was a firm, grey, sensitive, lightly overconsolidated clay with some silt. Data from piezometers installed near the landslide indicated artesian conditions underneath the Salvail River. Cone penetration tests allowed the location of two failure surface levels: the first one starting 2.5 m below the initial river bed, extending horizontally up to 125 m, and a second one 10 m higher, reaching the backscarp. Investigation of the debris with onsite measurements, light detector and ranging surveys, cone penetration tests, and boreholes allowed a detailed geotechnical and morphological analysis of the debris and reconstitution of the dislocation mechanism of this complex spread.

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