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.

Friction characteristics are critical mechanical properties of clay, playing a pivotal role in the structural stability of cohesive soils. In this study, molecular dynamics simulations were employed to investigate the shear behavior of undrained montmorillonite (MMT) nanopores with varying surface charges and interlayer cations (Na+, K+, Ca2+), subjected to different normal loads and sliding velocities. Consistent with previous findings, our results confirm that shear stress increases with normal load. However, the normal load-shear stress curves reveal two distinct linear regions, indicating segmented friction behavior. Remarkably, the friction coefficient declines sharply beyond a critical pressure point, ranging from 5 to 7.5 GPa, while cohesion follows an inverse trend. The elevated friction coefficient at lower pressures is attributed to the enhanced formation of hydrogen bonds and concomitant changes in density distribution. Furthermore, shear strength was observed to increase with sliding velocities, normal loads, and surface charges, with Na-MMT exhibiting superior shear strength compared to KMMT and Ca-MMT. Interestingly, the friction coefficient shows a slight decrease with increasing surface charge, while ion type exerts a minimal effect. In contrast, cohesion is predominantly influenced by surface charge and remains largely unaffected by ion type, except under extreme pressures and velocities.

In view of the pollution of unpaved road dust in the current mines, this study demonstrated the excellent dust suppression performance of the dust suppressant by testing the dynamic viscosity, penetration depth and mechanical properties of the dust suppressant, and apply molecular dynamics simulations to reveal the interactions between substances. The results showed that the maximum dust suppression rate was 97.75 % with a dust suppressant formulation of 0.1 wt% SPI + 0.03 wt% Paas + NaOH. The addition of NaOH disrupts the hydrogen bonds between SPI molecules, which allows the SPN to better penetrate the soil particles and form effective bonding networks. The SPI molecules rapidly absorb onto the surface of soil particles through electrostatic interactions and hydrogen bonds. The crosslinking between SPI molecules connects multiple soil particles, forming larger agglomerates. The polar side chain groups in the SPN interact with soil particles through dipole-dipole interactions, further stabilizing the agglomerates and resulting in an enhanced dust suppression effect. Soil samples treated with SPN exhibited higher compressive strength values. This is primarily attributed to the stable network structure formed by the SPN dust suppressant within the soil. Additionally, the SPI molecules and sodium polyacrylate (Paas) molecules in SPN contain multiple active groups, which interact under the influence of NaOH, restricting the rotation and movement of molecular chains. From a microscopic perspective, the SPN dust suppressant further strengthens the interactions between soil particles through mechanisms such as liquid bridge forces, which contribute to the superior dust suppression effect at the macroscopic level.

To overcome the limitations of microscale experimental techniques and molecular dynamics (MD) simulations, a coarse-grained molecular dynamics (CGMD) method was used to simulate the wetting processes of clay aggregates. Based on the evolution of swelling stress, final dry density, water distribution, and clay arrangements under different target water contents and dry densities, a relationship between the swelling behaviors and microstructures was established. The simulated results showed that when the clay-water well depth was 300 kcal/mol, the basal spacing from CGMD was consistent with the X-ray diffraction (XRD) data. The effect of initial dry density on swelling stress was more pronounced than that of water content. The anisotropic swelling characteristics of the aggregates are related to the proportion of horizontally oriented clay mineral layers. The swelling stress was found to depend on the distribution of tactoids at the microscopic level. At lower initial dry density, the distribution of tactoids was mainly controlled by water distribution. With increase in the bound water content, the basal spacing expanded, and the swelling stresses increased. Free water dominated at higher water contents, and the particles were easily rotated, leading to a decrease in the number of large tactoids. At higher dry densities, the distances between the clay mineral layers decreased, and the movement was limited. When bound water enters the interlayers, there is a significant increase in interparticle repulsive forces, resulting in a greater number of small-sized tactoids. Eventually, a well-defined logarithmic relationship was observed between the swelling stress and the total number of tactoids. These findings contribute to a better understanding of coupled macro-micro swelling behaviors of montmorillonite-based materials, filling a study gap in clay-water interactions on a micro scale. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The mechanical behavior of expansive soil in geotechnical engineering is significantly sensitive to loading rates, hydration, confining pressure, etc., where most engineering problems are attributed to the existence of montmorillonite in expansive soil. Here, the hydration, confining pressure, and loading rate effect on the mechanical behavior of montmorillonite were investigated through the triaxial tests and molecular dynamics (MD) simulation method, revealing their fundamental mechanism between the microscale and macroscale. The average basal spacing of hydrated montmorillonite system, the diffusion coefficient and density distribution of interlayer water molecules were calculated for the verification of MD model. The experimental results indicated that the stress-strain relationship of montmorillonite was the strain-hardening type. The failure stress did not increase monotonously with the increase in loading rate, and there were two obvious critical points. The failure stress of the soil sample increased with the increase of the confining pressure, and the decrease of the water content, where their fundamental mechanism between microscale and macroscale were adequately discussed. Furthermore, the stress-strain response, total energy evolution, deformation evolution of atomistic structure, and broken bonds evolution were analyzed to deeply understand the fundamental deformation mechanism at the microscale. The multi-scale studies could effectively examine the macroscopic mechanical behavior of expansive soil and elucidate its microscopic mechanisms.

The effective stress principle is the fundamental theory of soil mechanics. The effective stress transmitted between particles dominates the mechanical properties of soil, such as strength, deformation, and drainage. However, there remains a paucity of research on the effective stress in the compression of nano-scale clay minerals. This study explored the application of the effective stress principle in the consolidation behavior of kaolinite through the Molecular Dynamics method. The calibration and correction for micro effective stress and pore water pressure were first proposed. Micro-effective stress is the stress on the mineral itself in the contact part of two particles, while micro-pore water pressure always represents that on the weakly bound and free water in the same part. The strongly bound water film between particles can indirectly transmit the micro-effective stress through the electrical double-layer repulsion. The calculation of micro stress has been corrected according to the derivation of macro theory, and the results obtained corresponded well with that in the macro experiment. Moreover, the evolution of effective stress was analyzed by observing the interparticle water film. The increase in effective stress during consolidation was mainly due to the compression of the strongly bound water and the drainage of weakly bound and free water.

The physics of granular materials, including rheology and jamming, is strongly influenced by cohesive forces between the constituent grains. Despite significant progress in understanding the mechanical properties of granular materials, it remains unresolved how the range and strength of cohesive interactions influence mechanical failure or avalanches. In this study, we use molecular dynamics simulations to investigate simple shear flows of soft cohesive particles. The particles are coated with thin sticky layers, and both the range and strength of cohesive interactions are determined by the layer thickness. We examine shear strength, force chains, particle displacements, and avalanches, and find that these quantities change drastically even when the thickness of the sticky layers is only 1% of the particle diameter. We also analyze avalanche statistics and find that the avalanche size, maximum stress drop rate, and dimensionless avalanche duration are related by scaling laws. Remarkably, the scaling exponents of the scaling laws are independent of the layer thickness but differ from the predictions of mean-field theory. Furthermore, the power-law exponents for the avalanche size distribution and the distribution of the dimensionless avalanche duration are universal but do not agree with mean-field predictions. We confirm that the exponents estimated from numerical data are mutually consistent. In addition, we show that particle displacements at mechanical failure tend to be localized when the cohesive forces are sufficiently strong.

The creep behavior of net-like red soils mainly depends on the micromechanical behavior of clay mineral atoms at the nanoscale. The 1M-tv configuration of illite determined by the experiments of XRD and SEM-EDS, was utilized to address the mechanical properties along various loading directions using the conventional molecular dynamics (MD) simulation method. Furthermore, a novel MD simulation method based on transition state theory was proposed to discuss temperature effects. Simulated results indicate that the ultimate stress value under tensile perpendicular to the illite layer is minimal relative to the transverse direction, the in-plane shear has more resistance to overcome than the transverse shear. Amounts of the tensile, compressive, and shear strengths of illite decrease with increasing temperature, while the strain of steady-state creep at the same loading applied time increases with the temperature. An energy barrier to enter the accelerated creep destruction phase is about 18 kcal/mol. Moreover, the improved MD simulation method can extend the time scale from 200 ps to 186 days. These results may conclude that the proposed MD simulation method may provide a powerful tool to investigate the creep behaviors of clay minerals at experimentally relevant timescales at the nanoscale.

Melt-processed starch-based film formulations with market-competitive qualities and scalability for commercialization are developed in this study, unlike solvent casting, which has major technical and operational restrictions. First, a computational approach was utilized to understand the plasticization effect at molecular level and the same was validated with the experimental approach. The experimental process involved varying of glycerine content in the formulations (15-25 wt%) along with melt processing temperature (80 degrees C-140 degrees C), to deliver superior properties without the use of secondary fillers and additives. In comparison to other compositions, the starch-based system with 15 % glycerine and a processing temperature of 140 degrees C demonstrated the best properties in terms of mechanical (tensile strength: 20.5 MPa) and wettability (contact angle similar to 93.8 degrees). The thermal stability of films declined as the glycerine level was increased. It is noteworthy that the films underwent similar to 94 % weight loss within 30 days in soil compost admixture under ambient conditions. This study would facilitate future development of starch-based low-cost, high-value packaging products.

The development of biodegradable slow-release fertilizers derived from lignocellulosic materials is essential for mitigating environmental pollution and ecological damage associated with petroleum-based components in conventional fertilizers, as well as for enhancing agricultural productivity. In this study, a Camellia oleifera Abel. shell based slow-release fertilizer (COSU) was prepared by molten urea impregnation method. FTIR NMR, SEM, EDX, BET and molecular dynamic simulation were used to reveal the urea storage and slow-release mechanisms of COSU at the cell wall and molecular level. These results indicated the role of the cellular tissue structure with its pore structure in the storage and slow release of urea and demonstrate the molecular behavior of urea adsorption and release on lignocellulosic chemical component. The maximum nitrogen loading rate of COSU was 36.58 % and the cumulative release rate over 28 days was 75.08 %, which met the GBT23348-2009 standard. The multiple coupling regulatory mechanism of the cell wall - lignocellulosic molecules of urea store and releasing were discussed and proposed. Pot experiments confirmed that the prepared slow-release fertilizer not only stimulated the growth of corn seedlings but also contributed to an increase in soil humus. The findings of this research provide a new insight and a solid theoretical foundation for the development of lignocellulose-based slow-release fertilizers, offering a sustainable alternative to traditional fertilizers and contributing to a greener agricultural future.