Localised strain in fissured clays: the combined effect of fissure orientation and confining pressure

This paper reports the main results of an experimental study on the mechanics of intensely fissured natural clays, extending our previous studies on scaly clay from Santa Croce di Magliano. While previous work focused on the influence of the orientation of fissures with respect to the loading direction, the present investigation specifically explores an additional, important variable: the stress level. The combined effect of fissure orientation and confining pressure was studied by setting up a large campaign of plane strain compression experiments, in which different combinations of these two variables were tested. Conventional global stress–strain measurements were complemented by measuring displacement and strain fields through two-dimensional digital image correlation. Such rich information provided a clear and consistent picture of the interplay between fissure orientation and stress level and revealed complex deformation patterns, which cannot be ignored for a proper interpretation of the material response.

Water retention behaviour of compacted and reconstituted scaly clays

The paper presents the results of an experimental research devoted to investigate the response to suction variations of a scaly clay in compacted and reconstituted conditions. Different experimental techniques (axis translation, vapour equilibrium, dew point psychrometer suction measurements) were combined in order to explore the water retention properties in a wide suction range (0 ÷ 110 MPa). Experimental results allowed to define the water retention domains for a constant reference void ratio, highlighting the significant role of the microstructure on the response of the investigated clays. In particular, the collected results showed that in the low-medium suction range, the peculiar microstructural features give to the reconstituted clay a better retention capability than the compacted clay. However, the increasing suction induces a significative volumetric shrinkage on the saturated reconstituted clay, especially when the latter is initially normally consolidated. On the other hand, quite similar retention properties were recognized in the high suction range.

Influence of fissure inclination and confining pressure on the local behaviour of natural clays

In this experimental study the influence of fissuring orientation and confinement pressure on the mechanical behaviour of natural clays is investigated. The tested material, the scaly clay from Santa Croce di Magliano (south of Italy), is characterised by an intense network of pre-existing fissures of single orientation. Several plane strain compression tests have been conducted, under different confinement pressures (i.e., from 50 to 600 kPa), on specimens having fissures with vertical, medium and horizontal inclination. Digital Image Correlation has been used to follow the deformation processes of the specimens throughout the tests by measuring incremental shear and volumetric strain maps. The results showed a strong coupling between the total confinement and the fissure inclination, that is controlling both the onset and the development of the patterns of the localisation processes. The new results have been compared with previous ones carried out on the same material without confinement. The comparison shed light on the role of total confinement that becomes particularly relevant from certain levels of pressures and fissuring inclination.

Microstructures and deformation mechanisms in Opalinus Clay: insights from scaly clay from the Main Fault in the Mont Terri Rock Laboratory (CH)

The Main Fault in the shaly facies of Opalinus Clay is a small reverse fault formed in slightly overconsolidated claystone at around 1 km depth. The fault zone is up to 6 m wide, with micron-thick shear zones, calcite and celestite veins, scaly clay and clay gouge. Scaly clay occurs in up to 1.5 m wide lenses, providing hand specimens for this study. We mapped the scaly clay fabric at 1 m–10 nm scale, examining scaly clay for the first time using broad-ion beam polishing combined with scanning electron microscopy (BIB-SEM). Results show a network of thin shear zones and microveins, separating angular to lensoid microlithons between 10 cm and 10 µm in diameter, with slickensided surfaces. Our results show that microlithons are only weakly deformed and that strain is accumulated by fragmentation of microlithons by newly formed shear zones, by shearing in the micron-thick zones and by rearrangement of the microlithons.The scaly clay aggregates can be easily disintegrated into individual microlithons because of the very low tensile strength of the thin shear zones. Analyses of the microlithon size by sieving indicate a power-law distribution model with exponents just above 2. From this, we estimate that only 1 vol % of the scaly clay aggregate is in the shear zones.After a literature review of the hypotheses for scaly clay generation, we present a new model to explain the progressive formation of a self-similar network of anastomosing thin shear zones in a fault relay. The relay provides the necessary boundary conditions for macroscopically continuous deformation. Localization of strain in thin shear zones which are locally dilatant, and precipitation of calcite veins in dilatant shear fractures, evolve into complex microscale re-partitioning of shear, forming new shear zones while the microlithons remain much less deformed internally and the volume proportion of the µm-thick shear zones slowly increases. Grain-scale deformation mechanisms are microfracturing, boudinage and rotation of mica grains, pressure solution of carbonate fossils and pore collapse during ductile flow of the clay matrix. This study provides a microphysical basis to relate microstructures to macroscopic observations of strength and permeability of the Main Fault, and extrapolating fault properties in long-term deformation.

Microstructures and deformation mechanisms in Opalinus Clay: insights from scaly clay from the Main Fault in the Mont Terri Rock Laboratory (CH)

The Main Fault in the shaly facies of Opalinus Clay is a small reverse fault formed in slightly overconsolidated claystone at around one km depth. The fault zone is up to 6 m wide, with micron-thick shear zones, calcite and celestite veins, scaly clay and clay gouge. Scaly clay occurs in up to 1.5 m wide lenses, providing hand specimens for this study. After mapping of the macroscopic fabric at 10 cm–0.1 mm scale, Broad Ion Beam polishing combined with scanning electron microscopy (BIB-SEM) was used to study microscale deformation mechanisms. Results show a network of thin shear zones and microveins, separating angular to lensoid microlithons between 3 cm and 10 µm in diameter, with slickensided surfaces. Samples can be easily disintegrated into individual microlithons because of the very low tensile strength of the thin shear zones. Analyses of the size of microlithons show a power law distribution. We present a model to explain the progressive formation of a self-similar network of anastomosing thin shear zones during macroscopically continuous deformation in a fault relay. Localisation of strain in thin shear zones which are locally dilatant, and precipitation of calcite veins in dilatant shear fractures evolves into complex re-partitioning of shear, forming new shear zones at asperities while the microlithons remain much less deformed internally and the volume proportion of the µm-thick shear zones slowly increases. Grain scale deformation mechanisms are: microfracturing, boudinage and rotation of mica grains, pressure solution of carbonate fossils and pore collapse during ductile flow of the clay matrix. This provides a microphysical basis to relate the microstructures to macroscopic observations of strength and permeability of the Main Fault, and extrapolating fault properties in long term deformation.