Effect of microstructure on hydric strain in clay rock: A quantitative comparison

Abstract This paper presents a new method able to compare quantitatively strains and microstructure of a clay material. This method was applied to a Tournemire clay rock sample subjected to cyclic hydric loading in laboratory conditions. Local strains of a centimeter-size Tournemire clay rock sample were quantified by digital image correlation (DIC) on a field of view of 5.5 × 4.1 mm2 at the resolution of 2 μm.pixel−1, at a temperature of 22 °C and between 98 and 33% of relative humidity. At the end of the hydric loading, at dry state, the microstructure of the same field of view was mapped by an extended mosaic of scanning electron microscopy images at the resolution of 0.625 μm.pixel−1 and superimposed to the reference images used for DIC. Two microstructural parameters: the mean area and the area fraction of rigid clasts and clay matrix, were quantified by image analysis to be directly superimposed to local strain values. Speckle artefacts partly due to polished surfaces of clasts were filtered in the comparison between strains and microstructure. Strains concentrated in parts of the clay matrix, and at the interface between matrix and several quartz clasts which showed a slight detachment from the matrix. Constant clay and clast area fractions and sizes corresponded to very variable strain values at the microstructure scale. The relationship between strain values, mineral size and area fractions was not linear. Micro-mechanisms of deformation were therefore very complex at the scale of the experiment. On a very large field of view, very different microstructure behaviors were averaged which avoided any simple relationship between microstructure and strains. Heterogeneous water distribution and non-local microstructural control factors were suspected.