A microstructural insight into the compression behaviour of scaly clays

Scaly clays are intensely fissured clays with lens shaped elements of millimetre size and 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, i) the unloading-reloading cycle is typically a close-loop with relatively large hysteresis; ii) the compressibility recorded at high OCR 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 SEM observations are integrated with Mercury Intrusion Porosimetry (MIP) analyses and X-ray Computed Tomography (XCT) 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 characterized only by interparticle porosity. The observation that unloading and reloading curves are parallel in natural and reconstituted clays, led to postulate that the interparticle porosity is controlling the elastic response.