Strength softening in clay rocks: key factors and micro-scale clay mineral correlations

In this study, the mechanisms of strength weakening in clay-rich rocks following water exposure were investigated by examining the relationship between mineral composition, microscopic pore structure and gaseous water adsorption. X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and molecular dynamics (MD) simulations were used to characterise mineralogy, pore features and clay–water interactions. Results showed that water absorption is positively correlated with total clay content, particularly montmorillonite (MMT). A novel contribution of this work is the quantitative determination of weight coefficients for the main influencing factors, which clarifies the relative roles of hydrophilic mineral composition and pore structure in post-absorption strength reduction – an aspect not addressed in prior studies. The weight coefficients ranked as follows: pore fractal dimension > effective porosity > clay mineral (MMT) content. MMT content exhibited a negative correlation with post-absorption strength, with MMT identified as the primary mineral driving strength softening. MD simulations further demonstrated the micro-scale weakening effect of hydrated MMT. These findings provide new insight into the water sensitivity of clay-rich rocks and support improved assessment and mitigation of water-induced engineering failures.