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There is increasing evidence from researchers on clay mineralogy that the density of water attached to clay minerals may be greater than 1 . 0 Mg/m 3 : This fact becomes especially evident in compacted highly expansive clays close to water saturation. This kind of material is being considered in the design of engineered barriers for radioactive waste disposal, because of its sealing and retention properties. From a geotechnical point of view, most of the analyses required to check the performance of the barrier are sensitive to the value considered for the water density. This is the case for the unsaturated flow calculations, which depend on the water-retention properties of the soil. This paper presents, first, a review of measurements performed at microscopic level. A description of the hydration process of expansive clay considering the micro and macrostructure is included. Then a method to obtain an average water density as a function of suction for sodium and calcium bentonites is proposed. The method is used to correct the waterretention curves expressed in terms of the degree of saturation.

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