InterPore2026

From Crystalline Swelling to Shear Rheology: Multiscale Mechanics of Hydrated Smectite Faults

21 May 2026, 10:05

1h 30m

Poster Presentation (MS12) Coupled Flow-Deformation Processes in Porous Media Poster

Speaker

hassan breiteh

Description

Smectite-rich fault zones play a central role in controlling the mechanical behavior of shallow plate boundaries, where the transition between seismic and aseismic slip remains poorly understood. The frictional and rheological properties of smectite are strongly governed by the hydration state of its interlayer space. Classical thermodynamic and geochemical models generally assume equality between confining pressure and pore fluid pressure, leading to the conclusion that smectite remains fully hydrated (3W) at depth. However, faults are porous, stressed systems in which these pressures are decoupled, potentially allowing hydration transitions with major mechanical consequences.

In this work, we develop a multiscale framework linking nanoscale hydration thermodynamics to the mechanical response of smectite under shear. First, molecular dynamics simulations of Na-montmorillonite are performed under controlled water activity, allowing spontaneous access to all stable and metastable hydration states (0W–3W). Pressure–basal spacing isotherms are constructed and integrated to derive the swelling grand potential, enabling a rigorous stability analysis. This approach reveals hydration phase transitions and metastable states that emerge when confining and pore pressures are independently controlled, consistent with recent XRD observations on compacted clays.

Based on these results, an analytical swelling model is developed and calibrated on molecular simulation data. The model reproduces the full hydration phase diagram and provides an efficient tool to predict hydration transitions along coupled mechanical and chemical loading paths.

We then investigate the shear response of hydrated smectite using molecular dynamics simulations initialized from fully equilibrated swelling states (1W, 2W, and 3W). Simple shear deformation is applied in the XZ plane under realistic temperatures, pore water pressures, and confining pressures representative of shallow fault zones. Shear stresses are analyzed using block-averaging techniques to account for thermal fluctuations. The results reveal systematic shear-thinning behavior across all hydration states, with a clear strength hierarchy such that 1W systems exhibit the highest resistance to shear, followed by 2W and 3W. Increasing interlayer water content leads to reduced shear stress and apparent viscosity, indicating enhanced lubrication and facilitated sliding. Temperature increase further promotes mechanical weakening through thermal softening. Within the explored stress range, the shear response shows weak sensitivity to confining and water pressures. No resolvable yield stress is detected within the investigated shear-rate window, suggesting a dominantly viscous to viscoplastic response.

Together, these results provide a consistent multiscale picture in which hydration state governs both swelling thermodynamics and shear rheology, offering new insights into how nanoscale hydration mechanisms may control fault weakening, creep, and the seismic versus aseismic behavior of smectite-rich faults.