A microstructural insight into the compression behaviour of scaly clays

Scaly clays are intensely fissured clays with lens-shaped elements of millimetre size and they show a complex compression behaviour that poses challenges to the design and construction of geostructures (excavations, retaining diaphragms and tunnels). Scaly clays show a normal compression line (NCL) where plastic deformation accumulates, as typically observed in non-scaly clays. Yet the response observed upon unloading and subsequent reloading is very peculiar, (a) the unloading–reloading cycle is typically a closed loop with relatively large hysteresis; (b) the compressibility recorded at high overconsolidation ratio of the unloading or reloading branches is close to the NCL compressibility. This paper presents a microstructural study on an Italian scaly clay where scanning electron microscope observations are integrated with mercury intrusion porosimetry analyses and X-ray computed tomography images. The mechanism associated with the closing of inter-scale porosity and the generation of new intra-scale porosity was identified as the process responsible for the plastic deformation. Experimental observation of reconstituted clay showed a ‘quasi-reversible’ behaviour upon loading and unloading and a pore size distribution characterised only by interparticle porosity. The observation that unloading and reloading curves are parallel in natural and reconstituted clays, led to the postulation that the interparticle porosity is controlling the elastic response.