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.

Soil aggregate stability and pore structure are key indicators of soil degradation. Waves generated by the water-level fluctuations could severely deteriorate soil aggregates, which eventually induce soil erosion and several other environmental issues such as sedimentation and flooding. However, due to limited availability of the hydrological alteration data, there is a limited understanding of soil aggregates, intra-aggregate pore dynamics, and their relationships under periodically flooded soils. The present study has relied on long-term hydrological alteration data (2006-2020) to explore the impacts of inundation and exposure on soil aggregates and pore structure variations. Soil samples from increasing elevations (155, 160, 163, 166, 169, and 172 m) in the water-level fluctuation zone of the Three Gorges Reservoir were exposed to wet-shaking stress and determined soil structural parameters. The overall inundation and exposure ratio (OvI/E) gradually decreased from 1.87 in the lowest to 0.27 in the highest elevation, respectively. Predominant distribution of macropores was recorded in lower elevations, while micropores were widely distributed in the upper elevations. The mean weight diameter (MWD) was significantly lower in the lower (2.4-3.7 mm) compared to upper (5.3-6.0 mm) elevations. The increase in MWD has increased the proportion of micropores (PoN < 50 mu m), with R-2 = 0.59. This could suggest that the decrease in flooding intensity can create favorable conditions for plant roots growth. The strong flooding stress in lower elevations (i.e., higher values of the OvI/E) accelerated the disintegration of soil aggregates and considerably increased the formation of macropores due to slaking and cracking. The findings of the present study emphasize the need to restore degraded soils in periodically submerged environments by implementing vegetation restoration measures. This could enhance and sustain aggregate stability, which was also proved to increase functional pores under hydrological alterations.

This study investigates the corrosion behaviour of grounding down leads in transmission towers subjected to wet-dry cycle in saline soils of Northwest China through accelerated corrosion experiments. Using saline soil from the Hexi Corridor, rich in chloride and sulphate ions, corrosion rates were assessed via weight loss, polarisation curves, scanning electron microscopy and X-ray diffraction analyses. Results demonstrate that wet-dry cycle significantly accelerates corrosion due to enhanced chloride ion diffusion and corrosion kinetics, with the highest average weight loss rate (3.08%) and corrosion current density (0.3526 mA/cm(2)). Scanning electron microscopy analysis revealed extensive cracking in corrosion product layers under cyclic wet-dry conditions, weakening their protective capability and further intensifying corrosion. The primary corrosion products identified were FeO and Fe2O3, consistent with field samples, indicating that the corrosion mechanism remains unchanged under accelerated conditions. This study provides novel insights into how cyclic moisture conditions affect grounding materials in saline environments, guiding material selection, maintenance strategies and site selection to improve transmission line reliability and safety.

The development pattern of shrinkage cracks in sandy clay under dry wet cycling conditions is relatively complex. This study employed indoor experiments and image analysis methods to explore the inhibition mechanism of jute fiber on drying shrinkage cracks in sandy clay under dry wet cycling conditions. The results demonstrated that the jute fiber effectively inhibits crack propagation through friction, overlap, and anchoring mechanisms. Notably, increasing the fiber content can considerably reduce soil crack rate and crack width and promote the micro crack formation. The water absorption capability of jute fiber helps to evenly distribute water in the soil, thereby slowing down the evaporation rate and limiting crack formation. For instance, the addition of 0.6 % jute fiber led to a decrease in its crack rate and average crack width by 15.4 % and 53.3 %, respectively, compared to pure clay. Furthermore, after 5 cycles of wet-dry cycles, the crack rate and average crack width of sandy clay with different dosages decreased by 65-80 % and 69-75 %, respectively. This study provides a theoretical basis and technical support for incorporating jute fiber in clay improvement, which is immensely significant for enhancing the durability and stability of clay in engineering applications.

Sandy red clay, abundant in clay minerals, exhibits a marked sensitivity to variations in water content. Several of its properties are highly prone to deterioration due to wet-dry cycling, potentially leading to slope instability. To investigate the multi-scale deterioration patterns and the underlying chain mechanism of sandy red clay subjected to wet-dry cycles, this study conducted systematic tests on remolded sandy red clay specimens through 0 to 5 wet-dry cycles, with the number of cycles (N) as the variable. The study's results indicated the following, under wet-dry cycling: (1) Regarding the expansion and shrinking properties, the absolute expansion rate (delta a) progressively increased, whereas the absolute shrinkage rate (eta a) gradually decreased. Concurrently, the relative expansion rate (delta r) and relative shrinkage rate (eta r) gradually declined. (2) At the microscale, wet-dry cycles induced significant changes in the microstructure, characterized by increased particle rounding, disrupted stacked aggregates, altered inter-particle contacts, enlarged and interconnected pores, increased number of pores, and a reduction in clay mineral content. (3) At the mesoscale, cracks initiated and propagated. The evolution of cracks undergoes stages of initiation stage, propagation stage, and stable stage, and with the crack rate increasing to 2.0% after five cycles. (4) At the macroscale, the shear strength exhibited a continuous decline. After five cycles, cohesion decreased by as much as 49.6%, whereas the internal friction angle only decreased by 4.3%. This indicates that the loss of cohesion was the primary factor contributing to the strength deterioration. (5) A 19.4% decrease in the slope factor of safety (Fv) occurred after five cycles. This reduction was primarily attributed to the decrease in material cohesion and the upward shift in the potential sliding surface. Under the influence of wet-dry cycles, slope failures typically transitioned from overall or deep sliding to localized or shallow sliding.

Expansive soils, characterized by significant volume changes in response to moisture fluctuations, present substantial engineering challenges globally. This study explores the efficacy of lignosulfonate (LS), an industrial by-product, as a sustainable stabilizer for expansive soils. Three soil samples with varying degrees of expansiveness (weak, mid, and strong) were treated with LS, and their geotechnical properties were evaluated. For weak, mid, and strong expansive soil, the optimum lignosulphonate content (OLS) determined based on the free swelling rate and plasticity index was 0.75%, 2%, and 6%, respectively. The addition of LS resulted in a reduction of the liquid limit, plasticity index, and free swell index across all soil types. Furthermore, LS-treated soils exhibited enhanced resistance to volume changes and improved shear strength under cyclic wet-dry conditions. Moreover, crack development is inhibited in LS-modified soil. LS decreases the soil's affinity for water by creating a hydrophobic barrier around soil particles. Furthermore, the interaction between LS and the layered clay minerals results in stronger binding, which contributes to the stabilization process. The findings indicate that LS not only reduces the swelling nature of expansive soils and improves their shear strength and stability under wet and dry cycling conditions, but also provides an environmentally friendly solution for soil stabilization and sustainable construction practices.

The substantial development of desiccation cracks profoundly impacts the mechanical and hydraulic properties of clayey soils, potentially leading to various engineering challenges such as slope failures. Therefore, identifying the soil's cracking potential is crucial for guiding engineering design and construction processes. The aim of this study was to propose a method for cracking potential classification for clayey soils. To this end, standard cyclic wet-dry tests, capable of maximizing the soil's cracking potential, were proposed based on an analysis of the cracking behavior of lateritic soils under different wet-dry conditions. Subsequently, the cracking characteristics of several typical clayey soils (i.e., lateritic soil, kaolinite, bentonite, and attapulgite) were examined by standard cyclic wet-dry tests. Finally, a novel method for cracking potential classification of clayey soils was proposed incorporating the entropy weighting method. The results show that the most significant degree of cracking in lateritic soil is observed under vacuum saturation and 60 degrees C oven-drying, which is identified as the standard wet-dry condition. When the crack development stabilizes after multiple standard wet-dry cycles, the cracking potential of the soil is characterized by parameters such as the total crack length, maximum crack width, surface crack rate and the fractal dimension of the cracks. On this basis, a classification method is proposed to categorize the cracking potential of clayey soils into five levels: extremely weak, weak, medium, strong, and extremely strong. The cracking potential of different clayey soils was evaluated using this method, revealing that bentonite exhibited the highest cracking potential, classified as extremely strong.

This study investigates the role of polypropylene fibers (PFs) in mitigating the combined effects of wet-dry (W-D) cycles and vibration event (VE), such as earthquake or machine vibrations, on the desiccation cracking and mechanical behavior of clay through model tests. A comprehensive experimental program was conducted using compacted clayey soil specimens, treated with various PF percentages (i.e., 0.2 %, 0.4 %, 0.6 %, and 0.8 %) and untreated (i.e., 0 % PF). These specimens were subjected to multiple W-D cycles, with their behavior documented through cinematography. Desiccation cracking and mechanical responses were evaluated after each W-D cycle and subsequent VE. Results indicated that surface cracking, quantified by morphology and crack parameters i.e., crack surface ratio (Rsc), total crack length (Ltc), and crack line density (Dcl), increased with progressive W-D cycles. Higher PF content in soil significantly reduced desiccation cracking across all W-D phases, attributable to the enhanced tensile strength and stress mitigation provided by the fibers. Following VE, surface crack and fragmentation visibility decreased due to the shaking effects, as indicated by reductions in Rsc and Dcl. However, Ltc increased slightly, suggesting either crack persistence or lengthening. Higher PF content resulted in a more substantial reduction in Rsc and Dcl and a reduced increase in Ltc after VE. W-D cycles led to increased cone index (CI) values, reflecting enhanced compactness due to shrinkage which enhances with PF content showing improved soil resistance to loading. Meanwhile, VE reduced CI values following W-D cycles, particularly in nearsurface layers, PF content mitigates this reduction, demonstrating that PF contributes to a more stable soil matrix. Also, PF content decreased the soil deformation under W-D cycles and subsequent VE.

BackgroundBiochar is widely recognized for its capacity to capture and store carbon in soil attributed to its stable structure. However, in most field studies examining the effects of biochar application on soil respiration, the impact of rainfall events on the experimental outcomes has not been taken into account. To address the existing gap in this research field, we conducted a one-year study on soil respiration in an urban camphor forest and collected the data of soil respiration, soil temperature, soil moisture, and the rainfall events closest to the soil respiration monitoring time. We specifically examined how different stages of rainfall events influenced soil respiration in relation to biochar application.ResultsThis study found that the annual average soil respiration rate increased with the doses of biochar application, and the soil respiration rate under the biochar application at the dose of 45 t/ha showed a significant rise. The stages of rainfall events, rainfall amount, and the interaction effect of the two, and biochar doses significantly affected soil respiration. The parameters in the regression model for soil respiration, soil temperature and moisture varied with the different stages of rainfall events and the doses of biochar application. The biochar application eliminated the significant effect of soil moisture on soil respiration during one day after rainfall events. The significant correlation between soil moisture and the temperature sensitivity of soil respiration (Q10) was eliminated by biochar application, both during one day after rainfall events and more than eight days after rainfall events.ConclusionsOur findings indicated that the rice straw biochar application has a short-term positive effect on soil respiration in urban camphor forests. The rainfall events affect the field soil respiration monitored in the biochar applications, possibly by affecting the soil respiration response to soil temperature and moisture under different doses of biochar application. The impact of rainfall events on soil respiration in biochar application experiments should be considered in future forest monitoring management and practice.

Cement soil stabilization is widely used in civil engineering to improve the performance of soils subjected to freeze-thaw (F- T), wet-dry (W-D), and sulfate attack (SA). Due to the negative impacts associated with manufacturing cement, the development of eco-friendly and sustainable additives is highly desirable. Coal-derived char is a cost-effective byproduct of the coal pyrolysis process. In this study, the influence of coal char on mineralogical, microstructural, physical, and mechanical properties of cement stabilized soils (with cement contents of 0%-20% and char contents of 0%-30%) subjected to F-T cycles, W-D cycles, and SA is investigated. Compared to cement stabilized soils, char-cement stabilized soils exhibit up to 60.8% fewer volume changes during F-T cycles and 31.6% fewer during W-D cycles. The compressive strength of char-cement stabilized soils with cement contents of 5%, 10%, and 20% are on average 7.9%, 17.6%, and 11.0%, respectively, higher than that of cement stabilized soil subjected to F-T cycles, W-D cycles, or SA. The inclusion of char promotes cement hydration and results in the formation of more amorphous hydration products that fill voids or cover soil minerals. The findings indicate the promising potential of coal char in enhancing soil performance under a range of challenging environmental conditions.