Microstructure and mechanical behavior of smectite Holocene clays from the Queretaro Valley, central Mexico

This study investigates the mechanical behavior and microstructural evolution of undisturbed Jurica smectite-rich clays from the Querétaro Valley, central Mexico, through consolidated–undrained triaxial testing and scanning electron microscopy (SEM). The clays are classified as high-plasticity (CH) soils, with natural water content of about 30%, porosity of ∼66%, and a mineral assemblage dominated by Ca–and Na–Fe–smectites, with accessory volcanic-derived phases. Triaxial tests were conducted under confining pressures of 147–540 kPa on specimens with natural moisture (Ns) and re-saturated (Rs) conditions, and in both vertical (V) and horizontal (H) orientations. The results suggest that mechanical behavior is anisotropic and depends on confinement and saturation conditions. Vertically oriented specimens tend to exhibit higher strength and stiffness under low to intermediate confinement, although this relationship becomes less systematic at higher confining pressures. Re-saturated specimens show reduced stiffness and increased pore-pressure sensitivity relative to natural moisture conditions. Strength and pre-yield modulus increase with confining pressure, while friction angles tend to decrease at higher pressures, suggesting a progressive transition toward more ductile deformation, associated with pore collapse and particle rearrangement. A certain degree of variability in the results may be attributed to the inherent heterogeneity of the undisturbed smectite-rich clay fabric. SEM observations document stress-induced microstructural reorganization, including pore alignment, folding of flocculated (clay-silt) domains, localized deformation bands, and development of secondary porosity. Based on these observations, deformation is interpreted in terms of two conceptual modes: (i) distributed laminar sliding within clay domains, and (ii) localized shear associated with rigid inclusions and pore collapse. The results suggest a coupling between smectite microfabric and mechanical response, with potential implications for subsidence-prone lacustrine clays of urbanized volcanic-lacustrine basins.