Expansive soil, characterized by significant swelling-shrinkage behavior, is prone to cracking under wet-dry cycles, severely compromising engineering stability. This study combines experimental and molecular dynamics (MD) simulation approaches to systematically investigate the improvement effects and micromechanisms of polyvinyl alcohol (PVA) on expansive soil. First, direct shear tests were conducted to analyze the effects of PVA content (0 %-4 %) and moisture content (30 %-50 %) on the shear strength, cohesive force, and internal friction angle of modified soil. Results show that PVA significantly enhances soil cohesive force, with optimal improvement achieved at 3 % PVA content. Second, wet-dry cycle experiments revealed that PVA effectively suppresses crack propagation by improving tensile strength and water retention. Finally, molecular dynamics simulations uncovered the distribution of PVA between montmorillonite (MMT) layers and its influence on interfacial friction behavior. The simulations demonstrated that PVA forms hydrogen bonding networks, enhancing interlayer interactions and frictional resistance. The improved mechanical performance of PVAmodified soil is attributed to both nanoscale bonding effects and macroscale structural reinforcement. This study provides theoretical insights and technical support for expansive soil stabilization.

Commercial software packages for FEM analysis have been used to numerically simulate the behaviour of the complex systems of bentonite-bonded sand mould under pressure and subjected to stress distributions and to predict their performance. The Drucker-Prager model and the Mohr-Coulomb model are two well-known mathematical models used to describe the plastic non-linear behaviour of the soil. Conducting direct shear tests on varying densities of sand can provide the individual parameters necessary for the simulation of the moulding process. A new approach is based on making relationships between micro-mechanical parameters and changes in sand density during the compaction process. COMSOL Multiphysics is a popular software tool used to implement FEM simulations. The steps involved drawing geometry, inserting material properties, mesh generation and time-dependent density, and solving the model. The boundary conditions depend on the particular problem being analysed, which defines the external forces and constraints acting on the structure. The use of a coarse mesh and stationary study may be a computationally efficient approach for the evaluation of the compaction process of green sand. The study found that the maximum displacement value is 6.1*10-3 mm, the maximum volumetric strain value is 8.88*10-5, and the von Mises stress is 4.14*103 N/m2. On a utilise des progiciels commerciaux disponibles pour l'analyse FEM pour simuler numeriquement le comportement des systemes complexes de moules en sable lie a la bentonite sous pression et soumis a des distributions de contraintes, et pour predire leurs performances. Le modele Drucker-Prager et le modele Mohr-Coulomb sont deux modeles mathematiques bien connus utilises pour decrire le comportement plastique non lineaire du sol. Mener des essais de cisaillement directs sur des densites variables de sable peut fournir les parametres individuels necessaires a la simulation du procede de moulage. Une nouvelle approche est basee sur l'etablissement de relations entre les parametres micromecaniques et les changements de densite du sable au cours du procede de compactage. COMSOL Multiphysics est un outil logiciel populaire utilise pour mettre en oe uvre des simulations FEM. Les etapes impliquaient le dessin de la geometrie, l'insertion des proprietes des materiaux, la generation du maillage et de la densite en fonction du temps, ainsi que la resolution du modele. Les conditions aux limites dependent du probleme particulier analyse, qui definit les forces et les contraintes externes agissant sur la structure. L'utilisation d'un maillage grossier et d'une etude stationnaire peut constituer une approche informatique efficace pour evaluer le procede de compactage du sable vert. L'etude a trouve que la valeur de deplacement maximale etait de 6.1*10-3 mm, la valeur de deformation volumetrique maximale etait de 8.88*10-5 et la valeur de contrainte de von Mises etait de 4.14*103 N/m2.

To investigate the interaction mechanism between the sand-structure interface under cyclic loading, a series of cyclicdirect shear tests were conducted. These tests were designed with various surface roughness values represented by the jointroughness coefficient (JRC) of 0.4, 5.8, 9.5, 12.8, and 16.7, and normal stresses of 50, 100, 150, and 200 kPa. A 3D printerwas employed to accurately control the surface roughness and obtain concrete samples with varyingJRCvalues. The testresults were used to establish discrete element method models, which facilitated the analysis of the mesoscopic shearbehavior at the sand-structure interface during the cyclic direct shear process. The results revealed that the sand-concreteinterface demonstrated softening behavior. There is a critical value for the surface roughness corresponding to themaximum interface shear strength. The thickness of shear band, where the changes in porosity were concentrated within,increases with higher surface roughness and cycle number. The coordination number stabilizes after 80 cycles. Thedistributions of the contact normal direction and tangential contact force exhibited nearly isotropic characteristics aftercyclic loading. It was observed that surface roughness amplifies the deflection angle of the main axis in the normal contactforce distribution, while reducing that in the shear contact force distribution.