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.

For the soils in sloping ground, the effect of static shear stress must be considered to evaluate the cyclic behaviors of soils when subjected to seismic loading. This study aims to reveal the effect of both static shear stress magnitude and direction on the cyclic behaviors of the medium-dense sand based on a series of multi-directional cyclic simple shear tests. It is found that the effect of static shear stress on the liquefaction resistance of the medium-dense sand is detrimental in both parallel and perpendicular loading modes. The detrimental effect is more pronounced in parallel loading mode. Under the perpendicular loading mode, the full liquefaction of the specimens cannot be reached. The deformation pattern of the specimens is cyclic mobility along the cyclic loading direction, and plastic strain accumulation along the static stress direction. A modified pore pressure prediction model with two fitting parameters is further proposed to incorporate the effect of static shear stress.

The residual shear strength (RSS) of unsaturated soils is a crucial parameter for the reliable analysis and design of geostructures constructed with or within unsaturated soils undergoing large shear deformation. For investigating the RSS of unsaturated soils, two sets of data are specifically generated on the poorly graded sand with silt and Indian Head till using suction-controlled ring shear tests and three more sets (i.e., silty sand (SM), silty clayey sand, and fat clay) are gathered from the literature. A model is proposed extending two approaches for predicting the RSS for both coarse- and fine-grained unsaturated soils. In this model, the suction contribution was calculated considering the loss of degree of saturation due to shearing, which was described as a nonlinear function of degree of saturation. The capability of the proposed model is validated with the five sets of data using two different approaches. The best-fitting approach that is based on three fitting parameters provides good predictions. The approximate approach performs well for four studied soils, except for SM soil; this approach is simple for use in engineering practice because no fitting parameters are required. The proposed model is valid for the suction range where degree of saturation is higher than the residual degree of saturation.

Energy dissipation can macroscopically synthesize the evolutions in the microstructure of the marine clay during cyclic loading. Hence an energy-based method was employed to investigate the failure criterion and cyclic resistance of marine clay. A series of constant-volume cyclic direct simple shear tests was conducted on undisturbed saturated marine clay from the Yangtze Estuary considering the effects of the plasticity index (IP) and cyclic stress ratio (CSR). The results indicated that a threshold CSR (CSRth) exhibiting a power function relationship with IP exists in marine clay, which divides the cyclic response into non-failure and failure states. For failed specimens, the development of energy dissipation per cycle (Wi) with the number of cycles (N) exhibited an inflection point owing to the onset of serious damage to the soil structure. In this regard, the energy-based failure criterion was proposed by considering the inflection point as the failure point. Consequently, a model was proposed to quantify the relationships between failure energy dissipation per cycle (Wf) [or failure accumulative energy dissipation (Waf)], initial vertical effective stress, IP, and the number of cycles to failure (Nf,E). An evaluation model capturing the correlation among CSR, IP, and Nf,E was then established to predict the cyclic resistance, and its applicability was verified. Compared with the strain-based cyclic failure criterion, the energybased failure criterion provides a more robust and rational approach. Finally, a failure double-amplitude shear strain (gamma DA,f) evaluation method applicable to marine clay in different seas was presented for use in practical geotechnical engineering.

Long-term traffic loadings will induce strong vibrations in the saturated ground, and it probably produces excessive settlements of saturated ground and even various distresses (such as cracks and leakage) of the tunnel structure. To better understand the long-term cyclic deformation behaviors of saturated clay subjected to cyclic traffic loading, a series of cyclic undrained hollow cylinder apparatus tests were performed on Shanghai saturated clay. The secondary cyclic compression stage of permanent axial strain, energy dissipation, and damping ratio are employed to identify the distinct shakedown ranges of saturated clay. Moreover, attempts are made to establish a link between the permanent deformation behavior invoked by different levels of dynamic stress and a kinematic yielding framework. The cyclic test results of Shanghai clay can be classified as plastic shakedown, plastic creep, and incremental collapse, and Y-2 and Y-3 yield limits are interpreted as threshold cyclic dynamic stress to divide the shakedown ranges. Additionally, the effective cyclic dynamic stress ratio can better identify the shakedown ranges of saturated clay. Eventually, a criterion is recommended to identify distinct shakedown ranges of saturated clay. The findings will contribute to the safe design of the transport infrastructure in saturated ground.

The amount of energy dissipated in the soil during cyclic loading controls the amount of pore pressure generated under that loading. Because of this, the normalized dissipated energy per unit volume is the basis for both pore pressure generation models and energy-based liquefaction analyses. The pattern of energy dissipation in the soil in load-controlled cyclic triaxial and load-controlled cyclic direct simple shear tests and displacement-controlled cyclic triaxial and displacement-controlled cyclic direct simple shear tests is quite different. As a result, the pattern of pore pressure generation associated with load-controlled tests is markedly different from that in displacement-controlled tests. Pore pressure generation patterns for each of the four test types were proposed based upon the manner in which the load was applied during the test and the soil's response to that loading. The results of four tests, two load controlled and two displacement controlled, were then used to verify these patterns. Pore pressure generation rates in load-controlled and displacement-controlled tests are different when plotted against their cycle ratios. Conversely, the tests produce nearly identical patterns when plotted against energy dissipation ratio. This occurs because of the relationship between energy dissipation ratio and pore pressure generation is independent of the loading pattern.

In ocean engineering, polymer layer is often adopted as waterproof materials, and the mechanical behaviour of marine sand-polymer layer interfaces has significant influence on the engineering safety. In the research, based on the bespoke large temperature-controlled interface shear equipment, direct shear experiments were performed on the interfaces between polymer layer and marine sand with the particle size ranging from 1 mm to 2 mm (S1 marine sand) and from 2 mm to 4 mm (S2 marine sand) in the temperature range of-5 degrees C-80 degrees C. The test outcomes manifest that, both the change rules of interface peak shear strength and its sensitivity to normal stress variation are temperature dependent; The variation rules of the interface peak shear strength in elevated temperature are different in diverse normal stress. By adopting the experimental outcomes, machine learning models were established to predict the interface shear stress under the effects of temperature and soil particle, with higher estimating precision and efficiency. The research findings are beneficial for the correct design of marine engineering facilities related to marine sand-polymer layer interfaces.

In order to understand the mechanical characteristics of tree roots and their mechanical effects on slopes, the landslide in Wuping high vegetation coverage area of Fujian province was selected as the research site, and the root tensile mechanical properties of typical tree roots in the study area were tested after classification by diameter class. Furthermore, in-situ direct shear tests of root-soil composites under different root cross-sectional area ratios (RAR) and moisture content were conducted at the landslide site, and investigations were made into the distribution characteristics of roots in the profile to explore the mechanical effects of roots on shallow landslides. The results showed as follows: (1) The tensile force of Pinus massoniana and Cunninghamia lanceolata ranged from 12.45-673.09 N in 1-7 diameter class, and the tensile force was positively correlated with the root diameter by power function; The tensile strength ranges from 7.16 MPa to 60.95 MPa, and the tensile strength is negatively correlated with the root diameter as a power function. The average tensile force and tensile strength of Cunninghamia lanceolata root were higher than those of Pinus massoniana. (2) Tree roots significantly improved the shear strength of soil, and the additional cohesion provided by roots to soil was significantly positively correlated with the shear plane RAR. The root structure of Cunninghamia lanceolata is closer to R type, and that of Pinus massoniana is VH type. Under similar RAR, Cunninghamia lanceolata roots has a better reinforcing effect on the soil than Pinus massoniana. (3) With the increase in moisture content, the shear strength of the root-soil composites of Pinus massoniana and Cunninghamia lanceolata significantly decreases, as water infiltration diminishes the additional cohesion provided by the root systems to the soil. (4) Based on the Wu model, considering the influence of moisture content on soil cohesion and additional root cohesion, an estimation model for the shear strength value of root-soil composites considering moisture content was established. Upon verification, the accuracy of this model proved to be higher than that of the Wu model, and the results were reasonable. (5) Although the root system has a reinforcement effect on shallow landslides, its contribution to the stability of shallow landslides under heavy rainfall is limited due to the influence of root distribution depth, density and water infiltration.

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.

This paper presents a specifically designed experimental study aimed at exploring the role of fines in altering the behaviour of sand under the constant shear drained (CSD) stress path. The novelty of this study includes that the interplay of several key factors (fines shape, fines content, void ratio) was investigated systematically and the true constant shear stress condition was fulfilled by means of an advanced servo system which allowed the entire loading response to be captured. One of the marked findings is that the presence of fines not only alters the onset of instability of loose sand but also affects the deformation development thereafter. Drained instability can be triggered more easily in loose sand mixed with silica fines compared with the sand on its own. At the same quantity of fines, instability can be triggered more easily for sand mixed with rounded fines. However, the effect of fines appears to be marginal for sand at dense state. For all tested specimens, values of the axial strain rate at instability fall in a narrow range (0.008%/min-0.016%/min), meaning that the axial strain rate can be potentially a useful guide for quantitatively determining the inception of instability under the CSD conditions. The stress ratio q/p ' at onset of instability under the CSD conditions is also state dependent as under the undrained conditions.