Macroscopic effects of nano and microscopic phenomena in clayey soils and clay rocks

Based on long term investigations of the microstructure of various geomaterials as different as sensitive low plastic clays from Eastern Canada, unsaturated compacted silt from the Paris area, highly plastic compressible deep marine clay from the gulf of Guinea, compacted MX80 bentonite and the Callovo-Oxfordian claystone, considered as possible barriers in deep geological radioactive waste disposal, two fundamentals and distinct nano and micro mechanisms governing their macroscopic volume changes have been identified. In low plastic structured clays and dry compacted soils, in which an aggregate microstructure has been evidenced, the decrease in volume under mechanical compression result from the collapse of inter-aggregates pores in an ordered manner, from the larger to the smaller, with no effect on the intra-aggregate porosity. The soil skeleton can hence be modelled as an elastic fragile porous matrix, affected by the ordered collapse of its pores. Things are different in plastic soils, due to the significant reactivity of the montmorillonite minerals to changes in water content that results in significant changes of the initial porous matrix. The ordered adsorption of layers of water molecules in clay platelets with respect to the suction or stress applied, evidenced long time ago in Soil science through X-ray diffraction techniques, appear to be able to help better understanding various phenomena like the compression of plastic compressible soils and the hydration of compacted bentonites. Interestingly, it also applies to understand the effects of smectite minerals in the volume changes behaviour of the Callovo-Oxfordian claystone. These two mechanisms at nano and microscopic scales hence provide a better understanding of the macroscopic volume changes of a large variety of natural and compacted clayey soils and rocks. (C) 2020 Elsevier Ltd. All rights reserved.