Search Results (9)

Extreme climatic conditions characterized by drastic temperature fluctuations exacerbate soil erosion through intensified thermo-mechanical weathering processes. Loess-covered regions are particularly vulnerable to such conditions because of the inherent thermo-sensitivity of loess. A comprehensive investigation of mechanisms of thermo-mechanical weathering in loess under extreme temperature regimes holds critical importance for elucidating soil degradation patterns. It is also essential for formulating mitigation strategies in climate-sensitive loess terrains, especially given the increasing frequency of extreme weather events under global warming scenarios. This study employed integrated physical monitoring experiments and numerical modeling. The evolutionary patterns of temperature fields and corresponding thermal stress distributions in loess subjected to both heat shock (rapid heating) and cold shock (rapid cooling) conditions were systematically examined. The key findings are as follows: (1) Soil temperature variations demonstrate phase-lagged responses to ambient thermal variations during both shock scenarios, exhibiting distinct thermal inertia effects. (2) The spatial distribution pattern of thermal stress is predominantly governed by the temperature gradient within the soil matrix. (3) While the magnitude ranges of thermal stress remain comparable between shock types, their directional characteristics fundamentally differ; heat shocks induce surface compressive stresses and internal tensile stresses, whereas cold shocks generate inverse stress patterns. (4) Compared to heat shock, cold shocks trigger obvious surface degradation through tensile stress-induced failure of particle bonds. These mechanically weakened zones establish favorable conditions for subsequent erosion processes in loess landscapes. Full article

The Henan area of the middle and lower reaches of the Yellow River is situated within the third sedimentary loess area, positioned as the southeasternmost segment within the transitional belt connecting the Loess Plateau with the North China Plain. Addressing concerns related to loess collapse, landslides, and subgrade settlement across various regions attributable to the collapsible nature of Malan loess in western Henan, this study undertook collapsibility testing of undisturbed Malan loess in the province. The different mechanisms of loess collapsibility in different regions were explained from the microstructure by using the indoor immersion-compression test double-line method, scanning electron microscope (SEM), and particles and cracks analysis system (PCAS). The relationship between quantitative factors of microstructure and collapsibility of loess was analyzed by linear regression analysis. The findings indicate that under identical overburden pressure and immersion conditions, the collapsibility of Malan loess in western Henan diminishes progressively from west to east. Microstructural tests were conducted on various loess specimens using scanning electron microscopy, revealing that the distribution of loess particles is notably concentrated in the Xingyang and Gongyi areas, leading to a reduction in pore area compared to the Shanzhou and Mianchi areas. While the Mianchi and Shanzhou areas exhibit a loose arrangement of loess particles, those in Xingyang and Gongyi are comparatively denser. Analysis of microstructural images through the particles and cracks analysis system elucidated that the pore arrangement in the Gongyi and Xingyang areas is more stable than in the Mianchi and Shanzhou areas. Additionally, there is a gradual concentration in particle distribution, accompanied by an increase in agglomeration degree. According to the analysis and comparison of microstructure and quantitative parameters of four groups of loess samples before and after collapsibility, it is revealed that the change mechanism underlying loess collapsibility in various regions of western Henan primarily stems from the external factors influencing the microstructural alterations within the loess. The microstructural determinants contributing to collapsibility changes in different regions encompass three principal aspects: Firstly, modifications in the grain morphology of the Malan loess skeleton in western Henan are notable. Secondly, variations in the internal pore characteristics of loess microstructure are observed. Thirdly, disparities exist in the interconnections between soil particles. The findings of this research hold significant worth for improving construction safety and geological hazard prevention within the Loess region of western Henan. Full article

Due to its unique geotechnical properties, loess presents itself as a cost-effective and energy-efficient material for engineering construction, aiding in cost reduction and environmental sustainability. However, to meet engineering specifications, loess often requires enhancement. Evaluating its permeability properties holds significant importance for employing improved loess for construction materials in landfills and artificial water bodies. This study investigates the influence of dry densities, grain size characteristics, grain size distribution, and admixture contents and types on the permeability of improved loess, focusing on the Malan and Lishi loess. The falling head permeability test was conducted to analyze how each factor affects the permeability of the improved loess. The findings indicate that the permeability coefficient decreases with increased dry density and admixture content. Conversely, it demonstrates a linear increase with the average grain size (d₅₀), restricted grain size (d₆₀), and the product of the coefficient of uniformity and coefficient of curvature (C_u × C_c). The primary influencing factor is the type of admixture, followed by C_c and d₆₀. Furthermore, this study developed a predictive model for permeability using a support vector machine (SVM), surpassing the predictive accuracy of linear regression and neural network models. The model provides a robust prediction for the permeability of superior loess material. Full article

The micro pores in loess show regional variation in structure on the Loess Plateau and greatly influence the physical properties and macro behaviors of loess. In this study, the 3D microstructures of Malan loess from Lanzhou (LZ), Qingyang (QY), Hengshan (HS) and Jingyang (JY) were established based on μ-CT scanning, and the corresponding microstructural parameters were compared and analyzed quantitatively. The results indicate that the LZ and HS loess both show overall homogeneous structures with dominant inter-particle pores, while the QY and JY loess have more intra-aggregate and constricted pores. Overall, the LZ loess has the largest pore size, followed by the JY loess, QY loess and HS loess, which is consistent with the throat sizes of the four loess samples. The average coordination numbers (CNs) of the LZ and HS loess are lower than those of the QY and JY loess, while the throat lengths of the former two loess are larger than those of the latter two loess. Analysis of the correlation between the micropore parameters and macro behaviors of the loess suggests that the void ratio is the precondition for loess collapse, but it shows weak relevance to collapsibility; meanwhile, the size of pores contributing to the major pore space presents a strong positive correlation. The throat length representing the pore structure is more closely related to loess permeability compared with the void ratio, average CN and throat size. Full article

Malan loess is a wind-formed sediment in arid and semi-arid regions and is an important constituent of the Earth’s critical zone. Therefore, the study of the relationship between microstructure and heat transfer in Malan loess is of great significance for the in-depth understanding of the heat transfer mechanism and the accurate prediction of the heat transfer properties of intact loess. In order to quantitatively characterize the heat transfer processes in the two-phase medium of solid particles and gas pores in the intact loess, this study used modern computed tomography to CT scan the Malan loess in Huan County, Gansu Province, the western part of the Loess Plateau, China and used the specific yield of the intact Malan loess as the parameter basis for extracting the threshold segmentation of the large pores in the scanned images for the three-dimensional reconstruction of the connected large pores. An experimental space for heat conduction of intact Malan loess was constructed, and the surface temperature of Malan loess was measured on the surface of the space with a thermal imager. The simulation of the heat conduction process was carried out using the solution program in AVIZO (2019) software using the volume averaging method combined with CT scanning to reconstruct the 3D pores. The experiments of heat conduction in the intact Malan loess showed that for a given external temperature pressure, the temperature decreases along the heat flow direction as a whole. The temperature of the pores in the normal plane along the heat flow direction is higher than the temperature of the solid skeleton. Abnormal temperature points were formed at the junction of the surface and internal pores of Maran loess, and the temperature of the jointed macropores was about 1 °C higher at the surface of the sample than that of the surrounding solid skeleton. Simulation of heat conduction in Malan loess showed that the heat transfer process in Malan loess was preferentially conducted along the large pores and then the heat was transferred to the surrounding Malan loess particle skeleton. The simulation results of heat conduction in Malan loess were in high agreement with the experimental results of heat conduction in Malan loess, which verifies the reliability of the calculated model. Full article

Malan loess is an eolian sediment in arid and semi-arid areas. It is of great significance to study the pore structure of Malan loess for its evolution, strength, and mechanical properties. In order to quantitatively characterize the absolute permeability tensor of Malan loess and to simulate the seepage process of Malan loess, this study calculated the specific yield of intact Malan loess with a homemade seepage experimental device and recorded the water flow process on the surface of Malan loess during the seepage process. Modern computed tomography was used to scan the intact Malan loess samples from Jiuzhoutai, Lanzhou (western part of the Loess Plateau, China); the specific yield of the intact loess was used as the parameter value for the threshold segmentation of the scanned image for the 3D reconstruction of the connected pore space, the solver program in AVIZO software was used to solve the absolute permeability tensor of Malan loess using the volume averaging method combined with the CT scan to reconstruct the 3D pore space, and the simulation of the seepage process was carried out. The simulation results showed that Malan loess is a highly anisotropic loess; the absolute permeability in the vertical direction is 9.02 times and 3.86 times higher than the permeability in the horizontal direction. The pore spaces are well connected in the vertical direction (forming a near-vertical arrangement of pipes) and weakly connected in the horizontal direction. In the seepage simulation, it was found that the water flows first along the vertically oriented channels and then fills the horizontally oriented pores; the absolute permeability coefficient was calculated to be 0.3482 μm². The indoor seepage experiment was consistent with the simulation experiment, which verifies the reliability of the calculated model. Full article

The uniformity of remolded loess samples prepared in the laboratory directly determines the reliability of various test results. While many tests have been designed to reflect the properties of undisturbed loess, there are obvious structural differences between remolded and undisturbed loess. Therefore, it is of great theoretical and practical significance to explore a non-destructive evaluation index of the uniformity and structure of loess samples for indoor geotechnical tests. In this study, the loess from the Malan loess section of the Chanhe River in Xi’an, China, was used as the experimental material to analyze the uniformity of remolded loess samples in terms of the stress distribution, stratification range, and sample variance. Based on the test results, the feasibility of using the air permeability coefficient as an index by which to evaluate the uniformity and structure of loess samples is investigated for the first time. The results demonstrate that when the overall height of the sample is fixed, the higher the designed dry density of the sample and the lower the air permeability of the sample. Moreover, when the designed dry density of the sample is constant, with the increase in the overall height of the sample, the range of the stratified dry density of the sample gradually decreases, as does the sample variance. The more uniform the sample, the greater the air permeability, and under the same conditions, the air permeability of remolded soil is lower than that of undisturbed loess. SEM images reveal that there is a considerable difference between the structures of undisturbed and remolded loess samples. Remolded loess is characterized by the greatly reduced effectiveness of cementation between skeleton particles, which causes its structure to be weaker than that of undisturbed loess. In this paper, a quantitative index of loess structure based on air permeability is proposed, and the test results show that the index can adequately reflect the loess structure. Due to its obvious advantages of non-destructiveness, high efficiency, economy, convenience, and high speed, it has a unique advantage for the evaluation of the uniformity and structure of remolded soil samples. Full article

The shortage of land resources on the Loess Plateau has long been a thorny problem. Many high-fill projects are carried out, accompanied by a series of geological hazards, which threaten the ecological environment and personal safety. The creep characteristics of loess are an essential part of loess landslide research. The microstructural changes are closely related to creep behavior. By conducting triaxial creep experiments on Malan loess from the Yan’an area in China, scanning electron microscope (SEM) experiments on loess in different creep stages were carried out. Using qualitative and quantitative analyses of the microstructural characteristics of intact loess and remodeled loess during creep, the microstructural parameters were compared, and finally, the microscopic mechanisms during creep were analyzed. The qualitative analysis of remodeled loess during creep returned significantly higher results than it did for intact loess. During the creep process, among the microstructural parameters of loess change, the change in pore and particle size and shape were the most obvious, while the complexity of other microstructural parameters and orientation changed little. During the creep process of loess, the soil skeleton changed continuously, but the loess microstructure featured different changes at different levels of deviator stress. Full article

This paper presents a model for estimating the moisture of loess from an image grayscale value. A series of well-controlled air-dry tests were performed on saturated Malan loess, and the moisture content of the loess sample during the desiccation process was automatically recorded while the soil images were continually captured using a photogrammetric device equipped with a CMOS image sensor. By converting the red, green, and blue (RGB) image into a grayscale one, the relationship between the water content and grayscale value, referred to as the water content–gray value characteristic curve (WGCC), was obtained; the impacts of dry density, particle size distribution, and illuminance on WGCC were investigated. It is shown that the grayscale value increases as the water content decreases; based on the rate of increase of grayscale value, the WGCC can be segmented into three stages: slow-rise, rapid-rise, and asymptotically stable stages. The influences that dry density and particle size distribution have on WGCC are dependent on light reflection and transmission, and this dependence is closely related to soil water types and their relative proportion. Besides, the WGCC for a given soil sample is unique if normalized with illuminance. The mechanism behind the three stages of WGCC is discussed in terms of visible light reflection. A mathematical model was proposed to describe WGCC, and the physical meaning of the model parameters was interpreted. The proposed model is validated independently using another six different types of loess samples and is shown to match well the experimental data. The results of this study can provide a reference for the development of non-contact soil moisture monitoring methods as well as relevant sensors and instruments. Full article