Experimental microscopic investigation of the cyclic swelling and shrinkage of a natural hard clay

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In this paper, a microscopic experimental investigation of the swelling–shrinkage behaviour of a hard clay subjected to wetting and drying (W–D) cycles is presented. Three W–D cycles with a range of relative humidity from 20 to 98% were performed on the hard clay sample through varying the temperature and the vapour pressure in an environmental scanning electron microscope (ESEM). The obtained ESEM images of the material at different hydric stages were analysed using a digital image correlation method, and full-field strains at the micrometre scale were quantified. The results show that the deformation of the material when subjected to W–D cycles is significantly heterogeneous and it is closely related to its microstructure. The different contributions of clay particles and of inclusions to swelling and shrinkage of the hard clay are revealed: the clay minerals swell, which is, however, constrained by the no-swelling inclusions; the interaction between the different phases is significant. A residual swelling is found at the end of the first W–D cycle. The residual swelling continues to accumulate during the second cycle but becomes stable for the third cycle. Further microstructural analysis shows that the irreversible swelling can be mainly attributed to the appearance and propagation of microcracks accompanied by a limited unrecoverable deformation of the clay minerals.