Molecular dynamics simulation of small-strain anisotropy of unsaturated montmorillonite
QIAN Jiangu1, 2*, ZHOU Gaoyun1, SHI Zhenhao1
(1. Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China;
2. State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China)
Abstract：Anisotropy is a fundamental characteristic of the small-strain behavior of unsaturated clay minerals, with its microscopic mechanisms governed by the coupled effects of interparticle hydration and stress levels. To elucidate the multiscale mechanisms through which confining pressure and suction influence the small-strain stiffness of clay minerals, this study employs a mesoscale coarse-grained molecular dynamics (CGMD) approach, calibrated against full-atomistic potential of mean force (PMF) calculations. An interparticle potential function that explicitly accounts for macroscopic relative humidity (RH) is introduced by incorporating an exponentially decaying oscillatory term, enabling the quantitative characterization of the periodic oscillatory decay of interparticle forces arising from the ordering of water molecules. Montmorillonite platelets are idealized as oblate ellipsoidal particles, and the evolution of small-strain elastic anisotropy is systematically investigated under various RH and confining pressure conditions. Mesoscale simulations are performed using the LAMMPS platform to capture the small-strain elastic response of clay minerals, and the simulation results exhibit a consistent trend with experimental data at both macro- and microscopic scales. The evolution of stiffness anisotropy is further analyzed in relation to fabric evolution, elucidating the microscopic mechanisms underlying the effects of pressure and RH. The results indicate that under constant RH, increasing confining pressure enhances small-strain stiffness and anisotropy, with the fabric progressively evolving toward an ordered, transversely isotropic structure characterized by face-to-face particle stacking. Under low confining pressures, an increase in RH leads to a reduction in stiffness, a slight decrease in anisotropy, and a tendency toward fabric loosening, accompanied by reduced stacking density and order. Furthermore, a well-defined linear correlation is observed between the macroscopic anisotropy parameter and the mesoscopic order parameter.
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2026, Vol. 45 
