Based on long term investigations of the microstructure of various geomaterials as 14 different as sensitive low plastic clays from Eastern Canada, unsaturated compacted silt from 15 the Paris area, highly plastic compressible deep marine clay from the Gulf of Guinea, 16 compacted MX80 bentonite and the Callovo-Oxfordian claystone, considered as possible 17 barriers in deep geological radioactive waste disposal, two fundamentals and distinct nano 18 and micro mechanisms governing their macroscopic volume changes have been identified. In 19 low plastic structured clays and dry compacted soils, in which an aggregate microstructure has 20 been evidenced, the decrease in volume under mechanical compression result from the 21 collapse of inter-aggregates pores in an ordered manner, from the larger to the smaller, with 22 no effect on the intra-aggregate porosity. The soil skeleton can hence be modelled as an 23 elastic fragile porous matrix, affected by the ordered collapse of its pores. Things are different 24 in plastic soils, due to the significant reactivity of the montmorillonite minerals to changes in 25 water content that results in significant changes of the initial porous matrix. The ordered 26 adsorption of layers of water molecules in clay platelets with respect to the suction or stress 27 applied, evidenced long time ago in Soil science through X-ray diffraction techniques, appear 28 to be able to help better understanding various phenomena like the compression of plastic 29 compressible soils and the hydration of compacted bentonites. Interestingly, it also applies to 30 understand the effects of smectite minerals in the volume changes behaviour of the Callovo-31 Oxfordian claystone. These two mechanisms at nano and microscopic scales hence provide a 32 better understanding of the macroscopic volume changes of a large variety of natural and 33 compacted clayey soils and rocks. 34 Keywords: clayey soils and rocks; microstructure; montmorillonite; pore size distribution; 35 nano scale; water adsorption. 36 37 2