The application of novel materials that enhance soil engineering properties while maintaining vegetation growth represents an innovative strategy for ecological protection engineering of expansive soil slopes. Laboratory tests, including wetting and drying cycle tests, direct shear tests, unconfined swelling ratio tests, and vegetation growth tests, were conducted to analyze the effects of xanthan gum on both engineering and vegetation-related properties of expansive soil. The feasibility of xanthan gum for soil improvement was systematically evaluated. The interaction mechanism between xanthan gum and expansive soil was elucidated through scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. Results demonstrated that xanthan gum effectively inhibited crack development and strength loss. With increasing xanthan gum content, the crack area ratio decreased logarithmically by up to 58.62%, while cohesion increased by 82.96%. The unconfined swelling ratio exhibited a linear reduction, with a maximum decrease of 43.58%. Notably, xanthan gum accelerated seed germination rate but did not significantly affect long-term vegetation growth. Mechanistically, xanthan gum improved soil properties via two pathways: (1) forming bridging structures between soil particles to enhance cohesion and tensile strength; (2) filling soil voids and generating a polymer film to inhibit water-clay mineral interactions, thereby reducing hydration membrane thickness. These findings offer both theoretical insights and practical guidelines for applying xanthan gum in ecological protection engineering of expansive soil slopes.

This study addresses the utility of polyelectrolytes, i.e., cationic poly(diallyldimethylammonium chloride) (PDADMAC) and anionic polystyrene sulfonate (PSS), as additives to improve properties of the polymer-stabilized soil. This paper specifically focuses on the resistance of polymer-stabilized soils to degradation and/or damage during and following multiple wetting-drying cycles (zero, one, two, three, five, and seven cycles). Each cycle consisted of 24 h of moisture conditioning using capillary rise followed by 24 h of drying. Then, these specimens were evaluated for their unconfined compressive strength (UCS). The microstructure and composition of the soils were investigated using scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and X-ray fluorescence analysis (XRF). Based on the results, the soils used in this study for polymer treatment were primarily composed of carbonates and silicates with a small amount of clay minerals. The polyelectrolyte stabilizers (PDADMAC and PSS) and polyelectrolyte complexes (PECs) were added to the soils at dosages ranging from 0.2% to 1.6% by weight of dry polymer to dry soil. Treated soils demonstrated increased UCS compared with untreated counterparts. The untreated soils exhibited rapid degradation of UCS and mechanical collapse within three to four wetting-drying cycles. On the other hand, the polymer-treated soils exhibited a strength reduction of between 10% and 50% following the first cycle and then maintained the UCS of about 3-6 MPa after completion of all wetting-drying cycles. Furthermore, the stabilized soil demonstrated significant improvement in toughness compared with their untreated and cement-treated counterparts. The ability of the polymer-stabilized soils to stand up to wetting-drying cycles is a key finding and contribution of this study.

The variation in soil moisture can lead to unfavourable deformation of highway embankments, threatening their long-term stability under seasonal groundwater level fluctuations and frequent changes in evaporation and precipitation. This paper conducted unsaturated soil triaxial tests to examine soil water retention and volumetric deformation behavior during wetting-drying cycles. The results show that soil water retention decreases with increasing wetting-drying cycles, particularly in the low suction range from 0 to 100 kPa, where gravimetric moisture content (GMC) declines sharply. With more wetting-drying cycles, the soil's capacity for volumetric deformation diminishes. The soil has a loose soil structure and is more prone to plastic deformation. Furthermore, three soil water retention models, the Gallipoli, Tarantino, and Hu models were employed to analyse soil's hydromechanical behaviours and evaluate the effect of wetting-drying cycles. It was found that Tarantino's model used only three fitting parameters, which were more concise and maintained a good fitting effect. This study clarifies soil-water retention and volumetric deformation behavior during wetting-drying cycles, which is essential for effective water control in subgrade construction and operation.

Electroosmotic drainage has been proposed as a method for reducing moisture content and simultaneously increasing shear strength, thereby enhancing the stability of soft clays. Understanding electroosmotic consolidated soil behavior under wet-dry cycles is vital for assessing long-term stability and performance in a changing environment. In this investigation, electroosmosis-treated soft clay specimens were prepared and subjected to different wetting-drying cycles. The experimental results emphasized that in the case of soft clay which has been treated under identical electroosmosis conditions and subsequently subjected to varying numbers of wetting-drying cycles, it was determined that with an increment in the number of wetting-drying cycles, the crack ratio exhibits an increasing trend. However, the extent of the crack ratio exerts a minimal and almost negligible effect on the average moisture content of the soil mass. Specifically, five wetting-drying cycles can induce a pronounced reduction in the coefficient of variation (COV) of the soil moisture content distribution. Moreover, it was observed that a relatively smaller crack ratio is associated with a relatively greater average shear strength. Simultaneously, the corresponding COV will be larger. Conversely, a larger crack ratio gives rise to a smaller average shear strength, and the corresponding COV will be smaller.

Red mudstone is highly sensitive to water content variations. Lime treatment is recommended when using red mudstone as subgrade fill material. The mechanical properties of lime-treated red mudstone fill material (LRMF) degrade due to wetting-drying (WD) cycles caused by seasonal environmental effects. A series of WD cycle tests, unconfined compression tests, and bender element tests were conducted to investigate the degradation of strength and small strain stiffness of LRMF. Combining with the successive water-dripping scanning electron microscope (SEM) tests, the microstructure disturbance of LRMF after WD cycles was examined. Swelling of specimens on both the wet and dry sides was observed during low-amplitude WD cycles. For high-amplitude WD cycles, swelling on the wet side was also observed. On the dry side, initial volume shrinkage was recorded, followed by swelling in successive cycles, even though the water content was significantly lower than the initial state. Swelling results in the degradation of strength and small strain stiffness. Volumetric shrinkage increased strength, but small strain stiffness was still reduced due to crack propagation. A unified model is proposed to identify the degradation of strength and volumetric strain, while the small strain stiffness for dry specimens under large-amplitude WD cycles is significantly below the degradation line. The degradation rate of small strain stiffness is significantly higher than that of strength. After water exposure, the LRMF generally retains its initial microstructure. However, loosened aggregates, slaking, and crack propagation are clearly seen in water-exposed specimens. Degradation of the mechanical properties of LRMF can be attributed to damage to the soil fabric.

In this paper, the EC-5 water sensor and the MPS-6 water potential sensor were used to measure water content and suction, respectively, to investigate the evolution of soil-water retention properties of compacted loess samples prepared at different dry densities and subjected to different numbers of wetting-drying cycles. The water retention data were integrated with a detailed microstructural investigation, including morphological analysis (by scanning electron microscopy) and pore size distribution determination (by nuclear magnetic resonance). The microstructural information obtained shed light on the double porosity nature of compacted loess, allowing the identification of the effects of compaction dry density and wetting-drying cycles at both intra- and inter-aggregate levels. The information obtained at the microstructural scale was used to provide a solid physical basis for the development of a simplified version of the water retention model presented in Della Vecchia et al. (Int J Numer Anal Meth Geomech 39: 702-723, 2015). The model, adapted for engineering application to compacted loess, requires only five parameters to capture the water retention properties of samples characterized by different compaction dry densities and subjected to different numbers of wetting-drying cycles. The comparison between numerical simulations and experimental results, both original and from the literature, shows that only one set of parameters is needed to reproduce the effects of dry density variation, while the variation of only one parameter allows the reproduction of the effects of wetting and drying cycles. With respect to the approaches presented in the literature, where ad hoc calibrations are often used to fit density and wetting-drying cycle effects, the model presented here shows a good compromise between simplicity and predictive capabilities, making it suitable for practical engineering applications.

Expansive clays present serious issues in a variety of engineering applications, including roadways, light buildings, and infrastructure, because of their notable volume changes with varying moisture content. Tough weather conditions can lead to drying and shrinking, which alters expansive clays' hydro-mechanical properties and results in cracking. The hydro-mechanical behavior of Al-Ghatt expansive clay and the impact of wetting and drying cycles on the formation of surface cracks are addressed in this investigation. For four cycles of wetting and drying and three vertical stress levels, i.e., 50 kPa, 100 kPa, and 200 kPa, were investigated. The sizes and patterns of cracks were observed and classified. A simplified classification based on main track and secondary branch tracks is introduced. The vertical strain measure at each cycle, which showed swell and shrinkage, was plotted. The hydromechanical behavior of the clay, which corresponds to three levels of overburden stress as indicated by its swell potential and hydraulic conductivity was observed. It was found that at low overburden stresses of 50 kPa, the shrinkage is high and drops with increasing the number of cycles. Al-Ghatt clay's tendency to crack is significantly reduced or eliminated by the 200 kPa overburden pressure. The results of this work can be used to calculate the depth of a foundation and the amount of partial soil replacement that is needed.

Expansive soils are extremely susceptible to environmental changes. Under conditions of drought or heavy rainfall, the soil tends to develop cracks, thereby affecting its mechanical properties. This study aims to investigate the formation of cracks and the mechanisms of strength degradation in compacted expansive soils under different wetting-drying circumstances. Seven wettingdrying (W-D) cycles were conducted at various temperatures, relative humidity (RH) levels, and water content ranges. Scanning electron microscopy (SEM), image processing techniques, and direct shear testing were integrated to analyze the soil structure deterioration at micro, meso, and macro scales, respectively. Experimental results indicate that the crack parameters exhibit an initial growth followed by stabilization with increasing W-D cycles, while the shear strength gradually decreases. The conditions of higher temperature, lower RH, and larger W-D ranges correspond to the greater initial growth rate of the crack parameters, along with a higher cohesion decay rate. The internal friction angle tends to fluctuate within a narrow range, so the reduction in cohesion is the primary cause of shear strength degradation during W-D cycles. Under distinct overburden stress conditions, the shear strength exhibits non-linear characteristics. The measured values of cohesion under low-stress conditions are lower than theoretical values, while the internal friction angle demonstrates the opposite trend. Correlation analysis shows that the crack rate is more closely associated with the cohesion degradation rate, presenting a certain linear relationship. The slope and intercept of this relationship are related to environmental temperature and relative humidity. Overall, the macroscopic strength degradation is due to the coupled effects of microscopic structural damage and mesoscopic crack development. In a cyclic W-D environment, the imbalance of the tensile stress field leads to the accumulation of microscopic fatigue damage. It promotes the formation of mesoscopic cracks, creates weak areas, and damages soil integrity, ultimately manifesting as deterioration in macroscopic mechanical properties. This study provides valuable insights into the cognitive understanding of the mechanical structure damage evolution of expansive soil under extreme environmental factors.

The cyclic swell-shrink behavior of expansive soils poses formidable challenges to both rigid and flexible structures within pavement engineering, necessitating effective mitigation strategies. This research explores the utilization of waste expanded polystyrene (EPS) beads, a byproduct of hand-crushed EPS blocks, to construct recycled geofoam granules columns (GGC) in expansive soil. The objective is to assess the potential of GGC in mitigating swell-shrink phenomena through rigorous cyclic wetting-drying tests. A series of cyclic swelling-shrinkage experiments were conducted in a purpose-built swell-shrink apparatus, maintaining precise laboratory conditions. Remolded soil samples, incorporating GGC with two distinct diameters (40 mm and 75 mm) and a GGC density of 15 kg/m3, underwent cyclic wetting-drying cycles. The experimental data reveals a consistent reduction in the swell-shrink pattern with an increasing number of applied wet-dry cycles. Notably, the largest diameter GGC exhibited a pronounced decrease in the swell-shrink pattern compared to plain soil. Quantitatively, the findings demonstrate a remarkable 28% and 46% reduction in full swelling for 40D and 75D GGC, respectively, showcasing the efficacy of GGC in countering expansive soil tendencies. Equilibrium conditions were rapidly achieved by the 4th and 5th cycles, leading to a substantial 42% and 53% reduction in time requirements for 40D and 75D GGC. These quantitative assessments underscore the promising application of GGC in pavement engineering, offering a sustainable and technically sound solution to the cyclic swell-shrink challenges. The discussion delves into the mechanisms underlying GGC's influence on controlling swell-shrink behavior, emphasizing the pivotal role of soil-geofoam interaction.

Temporal variability in the macro-mechanics and microstructure induced by periodic water fluctuations during reservoir operation is widespread but adverse for slip zone soils. Herein, taking the slip zone soils of Huangtupo No. 1 landslide in the Three Gorges Reservoir area as a research case, the consolidation undrained (CU) triaxial tests coupled with wetting-drying cycles are organized to address macroscopic temporal variability of shear strength parameters. Then, quantitative microscopic characterizations are performed based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with mercury compression test (MIT). Eventually, the macro and micro connections are identified via gray rational analysis (GRA) and dynamic time warping (DTW) to be thus mathematized. Moreover, the weakened constitutive model is constructed. The test results show that the temporal variability of macroscopic shear strength parameters can be quantified as negative exponential decay. The wetting-drying cycles prominently contribute to the generation of intra-agglomerate pores (0.9-35 mu m). Besides, the inter-granular pores (0.007-0.9 mu m) and porosity are the connections to bridge microstructural parameters and macroscopic shear strength parameters. Furthermore, empirical equations for macro and micro connections are tentatively derived; the temporal variability of slip zone soils is invited to appropriately model the weakening laws of stress-strain. This study is expected to provide ingenious perspectives and promising references in stability evaluation and even disaster prevention of reservoir landslides.