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.

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.

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.

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.

Due to Paleo-clay's unique properties and widespread distribution throughout China, it is essential in geotechnical engineering. Rainfall frequently causes the deformation of Paleo-clay slopes, making slope instability prediction crucial for disaster prevention. This study explored Paleo-clay's strength degradation and slope stability under soaking and wet-dry cycles. Using Mohr-Coulomb failure envelopes from experiments, curve fitting was used to find the patterns of Paleo-clay strength degradation. Finite element simulations and the strength discounting method were used to analyze the stability and deformation of Paleo-clay slopes. The results indicate that wet-dry cycles impact them more than soaking. Paleo-clay's cohesion decreases exponentially as the number of wet-dry cycles and soaking times rise, but the internal friction angle changes very little. After 10 wet-dry cycles and 24 days of soaking, iron-bearing clay's cohesion decreased to 17% and 44% and reticular clay's to 32% and 48%. Based on the study area characteristics, three slope types were constructed. Their stability exhibited exponential decay. Under soaking, stability remained above 1.4; under wet-dry cycles, type I and II stability fell below 1.0, leading to deformation and failure. All types showed traction landslides with sliding zones transitioning from deep to shallow. Practical engineering should focus on the shallow failures of Paleo-clay slopes.

Expansive soils pose various problems to the existing transportation infrastructures by causing damages to pavements, railways, and embankments due to differential settlement, and volume changes in soils. Therefore, expansive soil if used in pavements must be stabilized by using some suitable means. The present study investigates the strength and durability of expansive soil stabilized with alkali-activated GGBFS (ground granulated blast furnace slag). In order to accelerate the hydration process, an alkali activator of low molarity (i.e., 5 M NaOH) is used to stabilize the subgrade expansive soil. GGBFS and alkali-activated GGBFS were added in the proportions of 5, 10, 15, 20, 25, and 30% to check the improvement in the strength properties of expansive soils after different periods of curing. The strength properties of stabilized soil were assessed by conducting various laboratory tests like unconfined compressive strength (UCS) and California bearing ratio (CBR). Durability study was also done by subjecting the soil specimens to 12 wet-dry cycles. The utilization potential of alkali activated GGBFS has been assessed from the mechanical, mineralogical, and morphological properties of stabilized soil. It was found that alkali-activated GGBFS can be effectively utilized for highway subgrade and sub-base applications.

Flooded rice cultivation, accounting for 75% of global rice production, significantly influences soil redox potential, element speciation, pH, and nutrient availability, presenting challenges such as extensive water usage and altered soil properties. This study investigates bacterial community dynamics in rice soils subjected to repeated draining and flooding in Rio Grande do Sul, Brazil. We demonstrate that bacterial communities exhibit remarkable resilience (the capacity to recover after being altered by a disturbance) but cannot remain stable after long-term exposure to environmental changes. The beta diversity analysis revealed four distinct community states after 11 draining/flooding cycles, indicating resilience over successive environment changes. However, the consistent environmental disturbance reduced microbial resilience, causing the bacterial community structure to shift over time. Those differences were driven by substitutions of taxa and functions and not by the loss of diversity. Notable shifts included a decline in Acidobacteria and an increase in Proteobacteria and Chloroflexi. Increased Verrucomicrobia abundance corresponded with lower pH levels. Functional predictions suggested dynamic metabolic responses, with increased nitrification during drained cycles and a surge in fermenters after the sixth cycle. Despite cyclic disturbances, bacterial communities exhibit resilience, contributing to stable ecosystem functioning in flooded rice soils. These findings enhance our understanding of microbial adaptation, providing insights into sustainable rice cultivation and soil management practices.

Cracking of compacted clays during cyclic wetting-drying poses significant challenges to the stability of channel slopes. This study performed a series of unidirectional wet-dry tests to evaluate the cracking behavior of expansive soils collected from a channel slope in northern Xinjiang, China. Using computed tomography scanning and three-dimensional (3D) reconstruction, the internal crack characteristics of expansive soils were quantitatively described. The results indicate that the penetration depth of the cracks was stabilized after five cycles, reaching 31.4% of the initial specimen height. The morphologies of internal cracks revealed a transition in the cracking mode, form shallow and scattered cracks in the initial stage to deeper and more clustered cracks in the final stage. Centralized cracks were prominent in the first three wet-dry cycles, followed by a shift to crack deflection from the vertical plane in subsequent cycles. Four indices (i.e., slice crack ratio, crack length, branching number, and dead-end point) provided a satisfactory quantitative depiction of the evolution of the spatial distribution and connectivity of the cracks over the number of cycles. Additionally, the crack volume fraction and fractal dimension effectively evaluated the 3D cracking behavior of soil crack networks.

The resilient modulus (MR) of different pavement materials is one of the most important input parameters for the mechanistic-empirical pavement design approach. The dynamic triaxial test is the most often used method for evaluating the MR, although it is expensive, time-consuming, and requires specialized lab facilities. The purpose of this study is to establish a new model based on Long Short-Term Memory (LSTM) networks for predicting the MR of stabilized base materials with various additives during wet-dry cycles (WDC). A laboratory dataset of 704 records has been used using input parameters, including WDC, ratio of calcium oxide to silica, alumina, and ferric oxide compound, Maximum dry density to the optimal moisture content ratio (DMR), deviator stress (sigma d), and confining stress (sigma 3). The results demonstrate that the LSTM technique is very accurate, with coefficients of determination of 0.995 and 0.980 for the training and testing datasets, respectively. The LSTM model outperforms other developed models, such as support vector regression and least squares approaches, in the literature. A sensitivity analysis study has determined that the DMR parameter is the most significant factor, while the sigma d parameter is the least significant factor in predicting the MR of the stabilized base material under WDC. Furthermore, the SHapley Additive exPlanations approach is employed to elucidate the optimal model and examine the impact of its features on the final result.