Multiphysics Assessment of Mexico City Soils: Structure and Thixotropy

The unique properties of Mexico City soils arise from their complex lacustrine–alluvial–volcanic origin. This study employed multiphysics techniques to characterize both intact and remolded specimens: stiffness and compressibility measurements were used to evaluate the granular skeleton, and complex permittivity and low-field nuclear magnetic resonance (NMR) were used to evaluate the pore fluid and fluid–mineral interactions. Below the yield effective stress σv′<σy′, the undisturbed specimen maintained interparticle bonding, a high void ratio, low compressibility and nearly constant shear stiffness. Beyond the yield stress σv′>σy′, the specimen gradually lost interparticle bonding, and its behavior began to resemble that of the remolded specimen, exhibiting stress-controlled properties such as shear stiffness, stress ratio K0, and compressibility. Differences in shear stiffness and stress ratio during unloading from the maximum vertical effective stress σmax′≈30σy′ suggest a residual memory of the initial fabric in the undisturbed specimen. Low-perturbation monitoring after remolding showed that thixotropic recovery of shear stiffness is closely linked to changes in ion distribution and mobility, as well as to alterations in pore size.