Effects of Dry–Wet Cycles on Permeability and Shear Strength of Yuanmou Red Clay

The shear strength of soil is influenced by drying and wetting cycles in practical engineering applications. This study focuses on the Q2 loess from the Xiaokongtong Valley in Qingyang City, specifically targeting the loess residual tableland and gully region. By using the direct shear test, we studied the shear strength parameters and the attenuation characteristics of shear strength in Longdong Q2 loess under different moisture contents and drying-wetting cycles. Using Longdong Q2 loess as an example, the results indicate that shear strength is inversely related to drying and wetting cycles and moisture content. In the context of more than three drying-wetting cycles, loess exhibited a reduced decrement in shear strength. The influence of these cycles was more pronounced on cohesion compared to the internal friction angle, with moisture content exerting an even greater effect on both parameters. Consequently, a functional relationship was established between the number of drying-wetting cycles, internal friction angle, and cohesion. This relationship facilitated the development of a shear strength formula, wherein the number of drying-wetting cycles was employed as the independent variable.

Evolution of Tensile Properties of Compacted Red Clay under Wet and Dry Cycles

The phyllite-weathered soil is a regional speciality. It is essential to study the changes in shear strength of phyllite-weathered soil under dry-wet cycles to understand the changes in mechanical properties of phyllite-weathered soil in the process of dry-wet climate and to manage the slope of phyllite-weathered soil. This paper simulated 12 dry-wet cycles on the specimens of remodelled phyllite-weathered soil. Direct shear and SEM tests were conducted on the specimens in the 0th, 3rd, 6th, 9th, and 12th drying paths. The effects of moisture content and the number of dry-wet cycles on the shear strength of phyllite-weathered soil were analysed macroscopically and microscopically. The following conclusions were obtained: (1) The cohesion of the weathered soil of phyllite will be reduced by increasing the number of cycles, and the more the number of dry-wet cycles, the more pronounced the reduction; the internal friction angle of the weathered soil of phyllite will be reduced by increasing the number of cycles, but the pattern of the decrease in the internal friction angle is not obvious. (2) The increase in the number of dry-wet cycles will increase the stiffness and brittleness of the phyllite-weathered soil specimen, and it will change from the weak hardening type of plastic damage to the solid softening type of brittle damage after a certain number of cycles. (3) The SEM test found that phyllite-weathered soil particles in Longsheng, Guilin are large, and most of the particles are in face-to-face and angle-to-face contact, which is easy to form a hollow structure, and the dry density value of the soil in the natural state is small. At the same time, the soil is reddish-brown in colour because of the leaching of Fe2 O3. The shear strength index of the cemented phyllite-weathered soil with Fe2 O3 is more significant than that of phyllite-weathered soil in other areas. The soil has a good shear strength index and a small dry density.

Loess landslides caused by the dry-wet cycling processes are the most common geological disasters in the Yili region of China and have caused significant economic loss and casualties. Therefore, there is an urgency to study the mechanism of landslide disasters. However, research on loess landslide disasters under dry-wet cycling conditions in the Yili River Valley is still underdeveloped, and the research foundations are relatively weak. Based on the characteristics of high and stable mica content in Yili loess, this research probed the changes in shear strength and microstructure of loess with different mica contents (0%, 5%, 10%, and 15%) after different dry-wet cycles (0, 1, 3, 5, 7, 10, 15, and 20) using direct shear testing and a scanning electron microscopy. The results showed that the mica content had a negative relationship with the shear strength of loess. For the same number of dry-wet cycles, the higher the mica content was, the lower the loess’ shear strength, especially in the first five dry-wet cycles. The influence of mica content on shear strength parameters was not similar. The impact was more significant for cohesion. With increased mica content, cohesion gradually decreased. The effect was minor with the internal friction angle. With the rise in mica content, the angle slightly increased with slight variations. Under certain dry-wet cycling conditions, micro-particle content in the loess decreased continuously, the average reduction can reach 11.25%, the content of small and medium particles tends to increase, the average increments were 6.21% and 3.1%, and volatility changes in large particle content. However, the overall increasing trend remains. Accordingly, the number of micropores and small pores decreased, the average reduction was 7.63% and 5.48%, the number of medium pores and large pores increased, and the average increments were 6.13% and 6.99%, respectively. All these changes were more evident in the first three dry-wet cycles and when the mica content increased from 0% to 5%. This study will be beneficial as a reference for the occurrence mechanisms of loess landslide under dry-wet cycling conditions in the Yili area.

Granite residual soil is widely distributed in Southeast Fujian. Large-scale engineering construction leads to the exposure of residual soil slopes to the natural environment. Affected by seasonal climate factors, the soil of slopes experiences a dry–wet cycle for a long time. The repeated changes in water content seriously affect the shear strength of soil, and then affect the stability of the slope. In order to explore the influence of the dry–wet cycle on the shear strength of granite residual soil in Fujian, an indoor dry–wet cycle simulation test was carried out for shallow granite residual soil on a slope in Fuzhou, and the relationship between water content, dry–wet cycle times, and the shear strength index, including the cohesion and internal friction angle of the granite residual soil, was discussed. The results show that when the number of dry–wet cycles is constant, the cohesion and internal friction angle of the granite residual soil decrease with an increase in water content. The relationship between the cohesion, internal friction angle, and water content can be described using a power function. Meanwhile, the fitting parameters of the power function are also a function of the number of wet and dry cycles. The prediction formulas of the cohesion and internal friction angle considering the number of dry–wet cycles and water content are established, and then the prediction formula of shear strength is obtained. The ratio of the predicted value of shear strength to the test value shall be within ±15%. An error transfer analysis based on the point estimation method shows that the overall uncertainty of the predicted value of shear strength caused by the combined uncertainty of the predicted value of cohesion and the internal friction angle and the single-variable uncertainty of the predicted value of shear strength caused only by the uncertainty of the predicted value of either the cohesion or internal friction angle increases first and then decreases with an increase in the number of dry–wet cycles. All increase with an increasing water content. The maximum standard deviation of the proposed shear strength prediction model of granite residual soil is less than 9%.

In order to research the effect of cracks in red clay on shear strength through dry-wet cycle test, the experimenters used imaging software and a mathematical model to determine fractal dimension and crack ratio of surface cracks in red clay in Guilin, China. After each dry-wet cycle, direct shear tests were carried out on the sample, and such variables as matrix suction on the crack propagation process of red clay were analyzed. The mechanics model was established and obtained the critical condition of soil cracks. The results show that with the increase in the number of dry-wet cycles the shear strength of the samples would decrease. But the rule of shear strength of sample 3 is slightly different from samples 1 and 2. The shear strength of red clay has a good correlation with fractal dimension and crack ratio, which could be an identification index of the strength of red clay.

In the Yili region, China, complex environmental conditions induce repeated wet–dry (WD) and freeze–thaw (FT) cycles, deteriorating soil shear strength and producing frequent loess landslides. In this study, we collected soil samples from the Alemale landslide, Yili Prefecture and performed their triaxial shear tests with different numbers of WD, FT, and WD-FT cycles. In addition, we summarized the change mechanisms of loess mechanical properties and its deterioration, in the Yili region, under different cyclic effects. Subsequently, the test results under the three cycling modes were compared and analyzed, the differences in the deterioration effects of different cyclic conditions on loess were discussed in depth, and finally, a multiple linear regression model was established and the weights of single factors under the action of coupled cycles were analyzed. The results show the following: (1) Regardless of the confining pressure values, the principal stress evolution trends in soil samples under different cycling modes were generally consistent, i.e., after an initial increase, peak values were reached, followed by a final decline. (2) Under unconsolidated undrained (UU) conditions, shear strength values of all soil samples tested under the three cycling modes dropped after the first twenty cycles, exhibiting different evolution patterns. (3) Coupled WD-FT cycling most significantly promoted soil shear strength degradation, with less WD cycling effect, and FT cycling had the least significant effect; in all three modes, the first cycle had the highest contribution to this effect. From the perspectives of cohesion, angle of internal friction, and decay of shear strength attenuation, the coupled WD-FT cycling effect on soil shear strength could not be reduced to a simple single-factor addition–subtraction relationship. (4) Weight analysis of soil samples after WD, FT, and WD-FT cycling revealed that WD cycles in the coupled WD-FT cycling mode had the most significant impact on the shear strength attenuation of soil samples (contributing 57%), FT cycles had a medium impact (contributing about 33%), while the effect of the total number of cycles was negligible (about 10%). The research results provide experimental and theoretical bases for subsequent control of loess landslides.

Study the effects of dry-wet cycles and cadmium pollution on the mechanical properties and microstructure of red clay

Geological hazards such as gully erosion, collapse and slope failure occur frequently in loess areas, which are closely related to the soil disintegration characteristics. Understanding the impact of freeze–thaw and wet–dry action on soil disintegration in the context of climate change is essential to establish effective soil and water conservation strategies and prevent engineering geological hazards in loess areas. In this study, sodic–saline loessial soils with different clay content were subjected to freeze–thaw and wet–dry cycles, followed by aggregate durability tests, direct shear tests and disintegration tests to investigate the effects of the two natural processes on soil disintegration characteristics. The results showed that the samples subjected to freeze–thaw cycles primarily exhibited rapid and stable disintegration, followed by slow disintegration, whereas the samples subjected to wet–dry cycles revealed weight gain, continuous slow disintegration and eventual sudden disintegration. Freeze–thaw action continuously deteriorated the disintegration resistance of soil, while wet–dry action improved the disintegration resistance of soil after the first cycle, and gradually weakened it in subsequent cycles. Statistical analysis showed that, for samples undergoing freeze–thaw cycles, the number of cycles and clay content were positively correlated with the disintegration rate, while the aggregate durability was negatively correlated with the disintegration rate. For samples undergoing wet–dry cycles, the number of cycles had a positive effect on the disintegration rate, while the clay content, shear strength and cohesion had a negative correlation with the disintegration rate. At a certain clay content, there was a positive correlation observed between the surface crack ratio, crack length and width with the disintegration rate of the wet–dry samples, while shear strength and cohesion had a negative correlation with the disintegration rate of both freeze–thaw and wet–dry samples. Furthermore, the study outlined the disintegration mechanism of loessial soils based on internal factors, driving factors, resistance factors and evolutionary factors. This study contributes to the in‐depth understanding of the catastrophic mechanism of geological hazards in cold and arid areas and provides experimental evidence for its control and management.

The Inner Mongolia section of the Yellow River is a seasonal frozen soil area, where the freeze–thaw effect can alter soil strength and compressibility, affecting bank stability. This study takes the banks sandy silt of the Inner Mongolia section of the Yellow River as the research object. It systematically investigates the relationship between shear strength parameters and compression index of sandy silt and the initial dry density, water content, and freeze–thaw cycles of the soil. It analyzes the order and significance of influencing factors, establishes prediction models of shear strength and compression index, and evaluates the effects of freeze–thaw cycles on soil cohesion and shear strength. The results show that the shear strength index of sandy silt is proportional to changes in initial dry density and inversely proportional to changes in water content. After 10 freeze–thaw cycles, the cohesion of the soil decreases by 22.53 to 58.85%, and the shear strength decreases by 22.67 to 58.91%. The internal friction angle is less affected by freeze–thaw and tends to be stable overall. The smaller the initial dry density and the greater the water content, the greater the compression index and compressibility of the soil, but freeze–thaw has little effect on compression index. The factors affecting sandy silt shear strength and compression index are ranked as dry density > moisture content > freeze–thaw cycles. The stepwise regression model of soil shear strength and compression index based on initial dry density, water content, and freeze–thaw cycles is effective, providing technical guidance for engineering practice.

Soil reinforcement is a reliable and effective technique for enhancing soil resistance. The present study aims to investigate the impact of water content on the mechanical behavior of a fiber-reinforced clay. To this end, clay specimens were prepared with three water contents (15%, 17.5%, and 20% by clay weight) and three contents of fiber (0.1%, 0.2%, and 0.3% referred to clay weight). Next direct shear and unconfined compressive strength tests were performed to study the effect of water content on the modulus of elasticity, compressive strength, strain energy, shear strength, shear strength ratio, stress–strain behavior, internal friction angle and cohesion coefficient of the prepared clay. The results indicate when the water content is lower than the optimum moisture content (OMC), fiber decreases modulus of elasticity and increases ductile behavior, compressive and shear strength, strain energy, shear strength ratio and cohesion coefficient but when the water content is more than the OMC, fiber increases brittle behavior, compressive and shear strength, strain energy, shear strength ratio and friction angle.

The degradation of shear mechanical properties of rock fracture surfaces was determined after applying multiple dry-wet cycles. Artificially fractured feldspathic sandstone specimens were soaked in chemical solutions with pH values of 2, 7, and 12 for 3, 6, 9, 12, and 15 dry-wet cycles, followed by direct shear tests under normal stresses of 3, 6, 9, 12, and 15 MPa. The results showed that the pre-peak shear stiffness and peak shear strength of the fracture surfaces decreased, and the peak shear displacement increased progressively after cumulative dry-wet cycling treatments compared to the behavior of oven-dry rock fractures. Additionally, the pre-peak shear stiffness, peak shear strength, peak shear displacement, and residual shear strength decreased cumulatively as the number of dry-wet cycles increased. However, the chemistry of the wetting solution had little effect on mechanical behavior. Based on the Barton formula for describing the peak shear strength for rock fractures, an empirical formula for peak shear strength for irregular rock fractures under dry-wet cycling conditions is proposed by introducing a proportionality factor to describe the degree of deterioration of the rock fracture surface shear strength. The modified formula has a good fitting accuracy for the test shear strength data of sandstone fractures under dry-wet cycling conditions, which may assist in the practical estimation of the peak shear strength of rock fractures under dry-wet cycling conditions in engineering practice.

Instability of tailings dams may result in loss of life and property and serious environmental pollution. The position of the tailings dam’s phreatic line varies due to continuously changing factors such as rainfall infiltration and discharge of tailings recycling water. Consequently, tailings dams undergo dry-wet (DW) cycles, accompanied by the appearance of a hydro-fluctuation belt. With dynamic development of the physical and chemical properties of tailings sand in the hydro-fluctuation belt, the stability of tailings dams is uncertain. In this study, direct shear tests were performed on the tailings sand collected from a tailings dam in Luonan, through which the shear strength parameters of tailings sand with DW cycles were obtained. Then, a method that efficiently calculates the phreatic line of the tailings dam under DW cycles was proposed. In addition, based on laboratory tests and the proposed phreatic line calculation method, we used a finite element program to evaluate the stability of the tailings dam that experienced different DW cycles. The calculated results showed that: (i) the damage effects of DW cycles gradually weakens as the number of DW cycles increases. (ii) With the increasing of DW cycles, the maximum displacement of the tailings dam increases from 0.5 mm to 22 mm, and the area of maximum displacement expanded mainly at the toe of the tailings dam and at the front edge of the hydro-fluctuation belt. (iii) The tailings dam safety factor decreases continuously with increasing DW cycles. This study may provide a novel method for analyzing the stability of tailings dams under different DW cycles as well as an important reference for improving tailings dam stability.

Weak interlayers in reservoir slopes can significantly impact the construction and operation of hydropower projects. Considering the existence of multiple weak interlayers facing outward on the slope within the left bank slope of Caiziba pumped storage plant located in Fengjie, Chongqing, there is an urgent need to identify the effects of high-frequency, rapid, and cyclic water level changes on its stability. The study involved creating a geological model and using 3D laser scanning and 3D printing technology to prepare several groups of weak interlayers with varying roughness. Shear tests were conducted under natural and fully wet conditions to investigate the strength deterioration of weak interlayers with different roughness after multiple wetting-drying cycles. The results were verified through numerical simulation. The results indicate that the shear strength increases with the roughness of the weak interlayer under the same number of cycles. Additionally, the residual curve stepwise decreases with an increase in the number of cycles. When the roughness remains constant but the number of cycles is different, an exponential decrease in the shear strength of the interlayer occurs with an increase in the number of cycles. The impact of the number of wetting-drying cycles on the strength gradually weakens. After six cycles, the shear strength decreases by 90% and stabilizes at a value that is positively correlated with the roughness. Multiple wetting-drying cycles significantly impact the strength of the weak interlayer and the slope stability in the reservoir area. This study summarizes the deterioration law, which can serve as an important reference for analyzing the slope stability in similar projects.

Due to world-wide distributions and extensively used as construction materials, geotechnical engineers are interested in understanding the mechanical behavior of residual soils which are sometimes referred in the literature as problematic soils. The climatic zones where residual soils occur are often experienced by many drying-wetting cycles due to seasonal variations. This seasonal variation in the water content termed as drying-wetting cycle is regarded as the most destructive environmental factor that may cause many foundation diseases. Considering these, the purpose of this study is to understand the effects of drying-wetting cycles on saturated shear strength characteristics of undisturbed residual soil. A series of consolidated drained (CD) triaxial tests are conducted on multiple drying-wetting soil specimens to analyze the saturated shear strength. The test results indicate that the stress-strain relationships appear to be strain-hardening. The deviatory stress and initial stiffness of saturated soils increase and the volume of soils becomes contractive as the net normal stress increases but decrease with increasing drying-wetting cycle numbers. The cohesion (c_d) and internal friction angle (φ_d) decrease with increasing cycle number (N) but the attenuation rate of φ_d is less than c_d. The variations of c_d and φ_d with respect to drying-wetting cycles can be expressed by exponential function. The saturated shear strength and it’s attenuation rate due to drying-wetting cycles are analyzed. The effect is more significant for the first cycle and decreases with subsequent cycles and finally reaches to a constant state after 4 cycles. Furthermore, a mathematical function is proposed in this paper which can describe the saturated shear strength attenuation rate of drying-wetting cycle samples. Such studies are useful to understand the possible changes in shear strength behavior of residual soils below the engineering structures that are subject to periodic drying and wetting from climatic variations.