Microstructure Of Loess Research Articles

Model interpretation and microscopic characteristics of collapsibility evolution of compacted loess under dry-wet cycles

The evolution of loess microstructure exerts a direct impact on its collapse evolution during dry and wet (DW) cycles. In this study, a hydro-mechanical coupling numerical model considering DW cycles and mechanical loading was established by extending the Barcelona Basic model, meanwhile combining with the test results to reveal the effect of DW cycling on the collapse deformation and strength response of loess. Additionally, the microscopic mechanism of loess collapse evolution was revealed through microscopic tests. Results indicated DW cycles caused the net compaction of loess, with the first DW cycle exerting the most significant effect on its deformation, consequently deteriorating the loess. Wetting under constant loading leads to a collapse of macrostructures formed by aggregates. Moreover, DW cycles transformed the structural units from line and surface contact to point. The basic structural units exhibited obvious grade properties, in which DW cycles trigger the collapse of compound aggregates, with the number of relatively stable mononuclear aggregates and intergranular pores increasing. DW cycles in an open environment induced the loss of cementing materials such as soluble salts and reduced the bonding strength among basic structural units. This subsequently tended to weaken the structural properties of loess and decreased the mechanical properties.

Advanced multi-scale characterization of loess microstructure: Integrating μXCT and FIB-SEM for detailed fabric analysis and geotechnical implications

Loess, a Quaternary wind-blown deposit, is a problem soil that gives rise to frequent geohazards such as landslides and water-induced subsidence. The behaviour of loess is controlled by its microstructure, consisting of silt-sized skeleton particles and complex bonding structures formed by clay-sized particles. Achieving a deep understanding and precise modelling of loess behaviour necessitates comprehensive knowledge of the realistic 3D microstructure. In this paper, a correlative investigation of the 3D loess microstructure is performed using X-ray micro-computed tomography (μXCT) and focused ion beam scanning electron microscope (FIB-SEM). Details of clay structures in loess, such as clay coatings, clay bridges and clay buttresses, are visualized and characterized in 3D based on FIB-SEM images with a voxel size of 10 × 10 × 10 nm3. The clay structures exhibit a diverse degree of complexity and their impact on the mechanical properties of loess is highlighted. Statistical analysis of the skeleton particles, including size, shape and orientation, are derived from μXCT images with a voxel size of 0.7 × 0.7 × 0.7 μm3. The findings provide insights into the collapse mechanism and particle-scale modelling of loess. The combination of μXCT and FIB-SEM proves to be a powerful approach for characterizing the intricate micro-structures of loess, as well as other geomaterials.

Wetting-induced collapse of loess: Tracing microstructural evolution

Loess is a silt-dominated, clastic yellow-to-yellowish brown aeolian sediment with high porosity and low density. The metastable microstructure of loess makes it highly susceptible to collapse, which plays a major role in landform evolution, geohazard development and engineering damages, particularly the widespread occurrences of settlement, deformation and cracking of civil engineering structures. Differing from the existing investigations based on pre- and post-collapse comparison, the present study is to report the microstructural changes during the collapse process and to determine how each microstructural constitutive element participates in this process. A series of laboratory tests was conducted on undisturbed loess specimens to simulate and capture snapshots of microstructure at five representative points during the entire collapse process. Results show that the loess microstructure becomes more homogeneous as the outstanding macropores are crushed into smaller (meso- and mini-pores) pores, the number of pores is dramatically increased, and the pore throats become narrower. The pore shapes have no obvious changes and remain elongated and rough-edged morphology from the point of view of statistic. Regarding solids, the pre-collapse unstable point-to-point contacts tend to transform into relatively stable edge (edge-edge or point-edge) type contacts, whilst coarse particles tend to reorient along the horizonal direction. XRD, SEM and EDS analysis indicates that these changes in the nature and distribution of pores and particles result from (1) the breakage and disintegration of moderately to highly weathered particles, (2) the disintegration of the matrix due to the swelling of clay minerals and the dissolution of calcium carbonates, (3) the consequential modifications of the microscopic forces which lead to slipping, rotation and therefore dislocation of particles, and (4) the collapse of mesoscale force chains along particles, reforming on smaller scales with larger numbers, shorter lengths and horizontal alignments.

State-of-the-Art Research on Loess Microstructure Based on X-ray Computer Tomography

Computer tomography (CT), combined with advanced image processing techniques, can be used to visualize the complex internal structures of living and non-living media in a non-destructive, intuitive, and precise manner in both two and three-dimensional spaces. Beyond its clinical uses, CT has been extensively employed within the field of geotechnical engineering to provide both qualitative and quantitative analyses of the microstructural properties of loess. This technology has been successfully applied in many fields. However, with the rapid development of CT technology and the expansion of its application scope, a reassessment is necessary. In recent years, only a few documents have attempted to organize and review the application cases of CT in the field of loess microstructure research. Therefore, the objectives of this work are as follows: (1) to briefly introduce the development process of CT equipment and the basic principles of CT and image processing; (2) to determine the current state and hotspots of CT technology research based on a bibliometric analysis of the literature from the past three decades in the Web of Science Core Collection and CNKI databases; and (3) to comprehensively review the application of CT to explore the microstructural characteristics (such as particle size, shape, arrangement, and the connectivity, orientation, and pore throats of pores, etc.) and the evolution of structural damage in loess within geotechnical science. In addition, the progress and deficiencies of CT applications in the field of loess microstructure are summarized, and future prospects are proposed.

Permeability and Disintegration Characteristics of Loess Solidified by Guar Gum and Basalt Fiber.

Loess has the characteristics of loose, large pore ratio, and strong water sensitivity. Once it encounters water, its structure is damaged easily and its strength is degraded, causing a degree of subgrade settlement. The water sensitivity of loess can be evaluated by permeability and disintegration tests. This study analyzes the effects of guar gum content, basalt fiber content, and basalt fiber length on the permeability and disintegration characteristics of solidified loess. The microstructure of loess was studied through scanning electron microscopy (SEM) testing, revealing the synergistic solidification mechanism of guar gum and basalt fibers. A permeability model was established through regression analysis with guar gum content, confining pressure, basalt fiber content, and length. The research results indicate that the addition of guar gum reduces the permeability of solidified loess, the addition of fiber improves the overall strength, and the addition of guar gum and basalt fiber improves the disintegration resistance. When the guar gum content is 1.00%, the permeability coefficient and disintegration rate of solidified soil are reduced by 50.50% and 94.10%, respectively. When the guar gum content is 1.00%, the basalt fiber length is 12 mm, and the fiber content is 1.00%, the permeability of the solidified soil decreases by 31.9%, and the disintegration rate is 4.80%. The permeability model has a good fitting effect and is suitable for predicting the permeability of loess reinforced with guar gum and basalt fiber composite. This research is of vital theoretical worth and great scientific significance for guidelines on practicing loess solidification engineering.

Study on the Shear Strength of Loess Solidified by Guar Gum and Basalt Fiber.

Loess is widely distributed in the northwest and other regions, and its unique structural forms such as large pores and strong water sensitivity lead to its collapsibility and collapse, which can easily induce slope instability. Guar gum and basalt fiber are natural green materials. For these reasons, this study investigated the solidification of loess by combining guar gum and basalt fiber and analyzed the impact of the guar gum content, fiber length, and fiber content on the soil shearing strength. Using scanning electron microscopy (SEM), the microstructure of loess was examined, revealing the synergistic solidification mechanism of guar gum and basalt fibers. On this basis, a shear strength model was established through regression analysis with fiber length, guar gum content, and fiber content. The results indicate that adding guar gum and basalt fiber increases soil cohesion, as do fiber length, guar gum content, and fiber content. When the fiber length was 12 mm, the fiber content was 1.00%, and the guar gum content was equal to 0.50%, 0.75%, or 1.00%, the peak strength of the solidified loess increased by 82.80%, 85.90%, and 90.40%, respectively. According to the shear strength model, the predicted and test data of the shear strength of solidified loess are evenly distributed on both sides of parallel lines, indicating a good fit. These findings are theoretically significant and provide practical guidance for loess solidification engineering.

Hazard assessment of seismic-collapsed loess landslides on the Loess Plateau as exemplified by the M6.2 earthquake in Jishishan County, China

The Loess Plateau is marked by intense neotectonic activity and frequent earthquakes. Its unique physico-mechanical properties, combined with the granular overhead pore structure of loess, render it prone to seismic landslides triggered by strong earthquakes. Different types of loess seismic landslides have distinct formation mechanisms, disaster-causing characteristics, and risk assessment programs. In this study, the risk of seismic-collapsed loess landslides as one of the types of loess seismic landslides was evaluated on the Loess Plateau. A risk zoning map for seismic-collapsed loess landslides on the Loess Plateau, considering various exceedance probabilities, was compiled by assessing eight factors. These factors include peak ground acceleration, microstructure of loess, and were evaluated using both the minimum disaster-causing seismic peak ground acceleration zoning method and the analytic hierarchy process. The following conclusions were obtained: (1) Earthquakes are the primary inducing factor for seismic-collapsed loess landslides, with other factors serving as influencers, among which the microstructure of loess carries the highest weight; (2) Across various exceedance probabilities, the likelihood of seismic-collapsed loess landslides occurring at 63% of the 50-year exceedance probability is low. Moreover, as the minimum hazard-causing seismic peak ground acceleration increases, the risk of occurrence of seismic-collapsed loess landslides rises, leading to a gradual expansion of the area share in moderate and high-risk zones; (3) Hazard evaluation results align well with existing data on seismic-collapsed loess landslides and findings from field investigations. The case of seismic-collapsed loess landslides induced by the M6.2 magnitude earthquake in Jishishan County, China, is presented as an illustration. The combined use of the minimum hazard-causing seismic peak ground acceleration zoning method and the analytic hierarchy process method offers a reference for geohazard hazard assessment, with earthquakes as the primary inducing factor and other factors as influencers.

Exploring the mechanical behavior and microstructure of compacted loess subjected to dry-wet cycles and chemical contamination

Due to climatic factors and rapid urbanization, the soil in the Loess Plateau， China, experiences the coupled effects of dry-wet cycles and chemical contamination. Understanding the mechanical behavior and corresponding microstructural evolution of contaminated loess subjected to dry-wet cycles is essential to elucidate the soil degradation mechanism. Therefore, direct shear and consolidation tests were performed to investigate the variations in mechanical properties of compacted loess contaminated with acetic acid, sodium hydroxide, and sodium sulfate during dry-wet cycles. The mechanical response mechanisms were investigated using zeta potential, mineral chemical composition, and scanning electron microscopy (SEM) tests. The results indicate that the mechanical deterioration of sodium hydroxide-contaminated loess during dry-wet cycles decreases with increasing contaminant concentration, which is mainly attributed to the thickening of the electrical double layer (EDL) by Na+ and the precipitation of calcite, as well as the formation of colloidal flocs induced by OH−, thus inhibiting the development of large pores during the dry-wet process. In contrast, the attenuation of mechanical properties of both acetic acid- and sodium sulfate-contaminated loess becomes more severe with increasing contaminant concentration, with the latter being more particularly significant. This is primarily due to the reduction of the EDL thickness and the erosion of cement in the acidic environment, which facilitates the connectivity of pores during dry-wet cycles. Furthermore, the salt expansion generated by the drying process of saline loess further intensifies the structural disturbance. Consequently, the mechanical performance of compacted loess is sensitive to both pollutant type and concentration, exhibiting different response patterns in the dry-wet cycling condition.

Using Cement and Calcium Lignosulfonate to Improve the Mechanical Properties and Microstructure of Loess in a Seasonal Freezing Zone

The cement composite calcium lignosulfonate is used to enhance the mechanical properties and the freeze–thaw resistance of loess. Based on an unconfined compressive test under different freeze–thaw cycles, the influence of cement dosage, curing age, and freeze–thaw cycles on compressive strength are discussed. The results indicate that the strength of loess can increase by up to 13 times, and the loss of strength is reduced from 72% to 28% under the reinforcement of cement dosage and curing age. The loss of strength is mainly concentrated in the initial 5 freeze–thaw cycles, and the structure gradually stabilizes after 10 freeze–thaw cycles. In addition, according to the X-ray diffraction test, it is found that the stabilized loess exhibits a comparatively more stable mineral composition. The scanning electron microscope results reveal that hydration products enveloped the soil particles, forming a mesh structure that strengthens the connection between the soil particles. The freeze–thaw damage makes the small and medium pores turn into large pores in loess, while the stabilized loess changes micro and small pores into small and medium pores, with no large pores found. It is feasible to improve loess with the cement composite calcium lignosulfonate, which can provide references for the reinforcement treatment of loess.

Quantitative characterization of microstructure and research on spatial variation characteristics of loess of different strata in Luochuan, Shaanxi, China.

The complete sequence of loess strata in Luochuan has become a typical section in loess strata, and is the main focus of research for many scholars studying loess. We were based on the theory of aeolian loess and established a set of quantitative index parameters for loess microstructure through our previous research, such as equivalent diameter, sphericity, morphology ratio, orientation angle Phi, orientation angle Theta, pore Eq-Radius, throat Eq-Radius and throat channelLength. Through the quantitative characterization of various index parameters of the Luochuan loess, we found that the probability density of each index parameter meets a specific distribution well, and in terms of spatial dimension, it shows that as the depth of the strata increases, the average particle size and the mode of pore Eq-Radius, throat Eq-Radius and throat channelLength generally increase, while the mode of particle morphology ratio generally decreases. In addition, loess particles in deeper strata are less prone to vertical sedimentation and tend to deposit gently or horizontally. Most particles in different strata are distributed in a northwest or southwest direction. During the formation period of strata, the main cause for spatial differences is the material carrying force. We conducted a statistical analysis on the correlation between the macroscopic physical properties of loess and its microstructure index parameters. Specifically, we found a positive correlation between loess density and the average particle size and the mode of particle equivalent diameter, Additionally, we found a negative correlation between loess liquid limit and plastic limit, and the mode of particle morphology ratio. Furthermore, there was a negative correlation between permeability coefficient and the mode of pore Eq-Radius, throat Eq-Radius, and throat channelLength.

Analysis of Soil–Water Characteristic Curve and Microstructure of Undisturbed Loess

Long-term irrigation promotes the infiltration of water in the thick, stratified loess layer, significantly raising the groundwater table and triggering a series of landslides in loess platform areas. The soil–water characteristic curve (SWCC) of loess buried at different depths affects the unsaturated infiltration process and is intricately connected to the soil’s microstructure. The SWCCs, scanning electron microscope (SEMs), and pore size distributions (PSDs) for five sets of undisturbed loess samples at depths ranging from 3.4 to 51.9 m are shown in this paper. The results indicate that the fitting parameter air entry value (AEV) of the SWCC rises from 13.67 kPa to 40.19 kPa as the depth increases from 3.4 to 51.9 m. And the saturated volumetric water content drops by 10.9%, with a notable SWCC shape difference between the transition and residual zones observed. Additionally, the total porosity of undisturbed loess falls by 12% when the depth increases from 3.4 to 51.9 m, while the macropores and mesopores reduce by 3.6% and 12.1%, respectively. These findings highlight the control of the pore structure on the SWCC and emphasize the correspondence between the SWCC and PSD. The conclusions also illustrate that the compaction effect changes the microstructure characteristics of loess, thereby affecting the soil’s water retention behavior.

Experimental Study on Strength and Microstructure of Loess Improved by CG-2 Curing Agent and Cement

In order to study the improvement effect of the CG-2 curing agent and cement on loess, a series of physical and mechanical property tests and microstructure tests were carried out on loess improved with different dosages of curing agent and cement to study the physical and mechanical properties, durability and microscopic pore characteristics of the CG-2 curing agent and cement-improved loess. The results show that the unconfined compressive strength of improved loess increases gradually with the increase in curing agent and cement dosage, and the higher the compaction degree and the longer the curing age, the higher the unconfined compressive strength. In the case of the same cement content, the higher the dosage of curing agent, the more the unconfined compressive strength of improved loess increases. Under the condition of reaching the same unconfined compressive strength, the addition of curing agent can significantly reduce the amount of cement. The more the content of cement and curing agent, the less the unconfined compressive strength decreases after a certain number of freeze–thaw cycles, and the higher the dry-wet cycles index after a certain number of dry-wet cycles, indicating that the addition of curing agent can significantly improve the ability of the sample to resist freeze–thaw cycles and dry–wet cycles. According to the microscopic test results, it is found that the addition of curing agent can reduce the porosity of soil particles, change the contact and arrangement mode between soil particles, and enhance the agglomeration and cementation characteristics between soil particles, and obviously improve the physical and mechanical properties of soil. The research results can provide new ideas and methods for the improvement technology of loess.

Deterioration mechanism of loess from Ili valley region of China under wet and dry cycles: evidences from shear tests

The properties of loess in the Ili region of China are significantly affected by repeated cycles of rainfall and evaporation. It is thus essential to investigate the deterioration mechanism of loess subjected to the wet and dry cycles. This paper employs various methods, including the direct shear and triaxial shear tests, as well as the scanning electron microscopy (SEM), to evaluate the variation patterns of shear strength parameters and microstructure of Ili loess. The direct shear test was conducted on loess samples experiencing a limited number of wet and dry cycles (0, 1, 3, 5, 7, and 9), while the triaxial shear test focused on a more extensive range of wet and dry cycles (0, 1, 3, 10, 20, and 30). In parallel, the alterations in the shear strength parameters of the loess material under different shear tests were also scrutinized. The findings obtained from this research revealed that the shear strength of Ili loess decreased to varying degrees based on the two test methods when they are affected by the wet and dry cycles. Comparing the results with the same number of wet and dry times (0, 1 and 3 times), both the shear strength and cohesion obtained from the triaxial shear test were greater than those from the direct shear test, while the results for the angle of internal friction were reversed. Moreover, the scanning electron microscope tests on Ili loess did indicate that the micro-particle size, pore space, morphology, soil structure, and particle contact mode exhibited the deterioration with different degrees. The micro-structural change is believed to be the main reason for the deterioration mechanism of the shear strength. The research outcomes will enrich the understanding about the loess properties in Central Asia, providing data reference and theoretical basis for engineering construction in these region.

Experimental investigations on the physico-mechanical and microstructural properties of loess reinforced with anionic polyacrylamide

The typical characteristics of loess, such as macropores, weak cementation, and strong collapsibility, can adversely affect the stability of building foundations, transportation infrastructure, and other geotechnical systems. To address this problem, a water-soluble anionic polyacrylamide (APAM) was used to improve the characteristics of loess. A series of multi-scale geotechnical engineering tests, such as sedimentation column, zeta potential, Atterberg limits, standard compaction, permeability, triaxial compression strength, scanning electron microscope (SEM), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD) tests were conducted on the loess samples. The results showed that the APAM polymer could flocculate fine loess particles into stable agglomerates via a series of physical actions, including charge neutralization, adsorption and bridging, and hydrophobic interactions. The contact form among the loess particles changed from loose point contact to strong mosaic contact with the addition of the APAM polymer, the surface porosity, fractal dimension, and interplanar spacing of the treatment material decreased significantly. The considerable improvement of loess microstructure affected its macroscopic physical and mechanical properties. The optimum moisture content and permeability coefficient of the treated loess decreased with the increases of the APAM content, whereas the plasticity index and maximum dry density increased. Additionally, the failure strength of the loess samples first increased then decreased as the APAM content increased, which showed the maximum value at an addition amount of 0.30% content, beyond which its strength value decreased. The test results demonstrated that the APAM polymer treatment could provide significant improvement in the macroscopic physico-mechanical and microstructural properties of loess. This study will serve as a crucial resource for loess foundation stabilization using the environmentally friendly and cost-effective polymer.

Assessing unsaturated permeability of loess under multiple rainfalls

Multiple rainfalls may give rise to changes in the microstructure of loess which in turn alter its unsaturated permeability. This study evaluates the unsaturated permeability of compacted loess using an infiltration device. In addition, Scanning Electron Microscopy (SEM) and Nuclear Magnetic Resonance (NMR) are used to document the microstructure before and after rainfall. The results demonstrated that the void ratio mostly controls permeability. In particular, the unsaturated permeability of loess samples featuring a low dry density (≤1.35 g/cm3) displays a decreasing trend with increasing rainfall times due to the occurrence of pore collapse. In contrast, rainfall events lead to an increase in pore size for loess samples with high dry density (≥1.45 g/cm3) as a result of the dissolution of soluble salts and cement inside the soil samples, leading to an increase in unsaturated permeability. Based on the experimental results, a novel model is developed to estimate the unsaturated permeability of compacted loess.

Experimental study on the disintegration characteristics of undisturbed loess under rainfall-induced leaching

Loess is prone to collapse as the soil approaches saturation, leading to severe geological hazards. However, how the undisturbed loess microstructure evolves due to rainfall-induced leaching is poorly understood and a detailed assessment of the corresponding disintegration characteristics is lacking. In this study, we conducted a series of leaching and disintegration tests on undisturbed Q3 loess from Yan’an, China. Loess disintegration on a macroscopic level were characterized using a self-developed disintegration apparatus, and the relationship with influencing factors such as moisture content, temperature, and salinity were explored. At the microscopic scale, scanning electron microscopy (SEM) was performed to qualitatively probe the underlying mechanisms of loess disintegration under varied leaching conditions. The microstructure properties of the loess samples revealed in the SEM images were examined using Image-Pro Plus (IPP) software. A quantitative relationship between the microstructural parameters of the pores and particles with disintegration was established based on the grey correlation theory. The results show that the disintegration process of undisturbed loess under the action of leaching can be divided into three stages: soaking, softening, and collapsing. Compared with samples without leaching, the study demonstrates that leaching can facilitate the loess disintegration. Under otherwise similar conditions, loess disintegration ratio decreases with increasing moisture content and salinity, whereas it increases with water temperature. In addition, microstructural observation shows that the undisturbed loess microstructure changed from interlocking to overhead with increased leaching, inducing favorable conditions for loess disintegration. Moreover, grey correlation analysis shows that the particle size distribution and macropore content of undisturbed Q3 loess yields a satisfactory correlation with the loess disintegration ratio. In general, rainfall-induced leaching effect is conducive to the disintegration of loess by degrading its internal structure.

Loess microstructure indication indexes for the study of palaeoclimatic conditions in northwest China

AbstractThe Loess Plateau of China, located to the west of the Liupan Mountains and north of the Qinling Mountains stretching across the Yellow River, is the main loess deposit area in Northwest China. The loess in the northwest of China has deposits with large thickness and extensive distribution. Scanning electron microscopy (SEM) and energy spectrum analysis of loess have been performed to study the relationship between the loess microstructures and the forming climates Era. The relevant indexes were evaluated including the sand‐dropping speed (Vn), certain sedimentary depths (hn sand particle volume (Vd) and element ratios of Ca/Fe, K/Al, Si/Al and Ca/Mg. The microstructure indexes of loess accumulation and evolution reflect paleoclimate conditions and time scales to a certain extent. The important discovery is the microscopic sand‐dropping speed (Vn) and the sedimentary depth hn) calculation method of different sedimentary ages. These indices are compared with the record of major aeolian‐forming climates from the Guliya ice core, and provide a reliable benchmark for studying climate change It also can be used as important indicators of monsoonal change and environmental evolution reconstruction. The index of sand sedimentation speed (Vn) got from loess microstructure could reflect sand‐dropping speed and loess deposition course. According the article can serve as new indicators of climatic changes of different forming loess layers. It can also be concluded that the climatic indexes obtained from loess microstructure can reflect climate conditions of loess forming. The loess forming climatic parameters are synchronous correspond to Tengger Desert and Guliya ice core for studying climate change, then microscopic parameters can also be used for preliminary analysis of loess climate formation and has be found corresponding evidence, and the loess climatic parameters correspond to the other two indexes. The analysis of loess microstructure indexes is very useful in researching climate change. Loess microstructure indexes can find new indicators and information about the monsoon climate evolution and paleoclimate changes.

Study on Shear Behavior and Mechanism Based on Shear Functional Unit of Loess Microstructure

The structural specificity and hydrological sensitivity of loess have a strong impact on its long-term stability and safety. This topic is being actively researched and focuses on the macromechanical behavior of the shear strength of loess disturbed and its micromechanisms from the perspective of the dry–wet cycle (especially involving soluble salt erosion). In this paper, the correlation between micro-structural shear functional units and macroscopic degradation behavior was established by combining the changes in physicochemical properties of mass loss, surface cracking, strength deterioration, and structural disturbance of the loess with scanning electron microscopy (SEM) microscopic images in different dry–wet cycles and different salt contents. Results revealed that with the increase in dry–wet cycles and salt content, the mass loss of soil deteriorated and the surface crack rate increased. The cohesion of soil showed an overall decreasing trend, which decreased more obviously in the early stage of the dry–wet cycle, followed by a slow decrease, and tended to be constant after nine dry–wet cycles. However, the internal friction angle increased and then decreased during the whole cycle, and its value generally changed little. According to the deterioration and decay of shear strength, it can be concluded that the structural disturbance of loess increased with the increase in dry–wet cycles and salt content. At the same time, further linear quantization fitting of the structural disturbance parameters showed that the structural parameters had a positive correlation with salt content and a power function with dry–wet cycles, where dry–wet cycles seemed to play a dominant role in the loess structural deterioration rather than salt content. The microscopic study demonstrates that the dry–wet cycles and salt content do not directly affect the cohesion and internal friction angle of soil but change the basic shear structural unit of aggregate and then cause an essential impact on c and φ, which in turn have an essential impact on soil strength attenuation. This paper not only helps to elucidate the essence of water–soil–salt structural interactions but also provides theoretical references for sustainable development research in environmental engineering, geological engineering, and other related fields.

Microstructure response to shear strength deterioration in loess after freeze-thaw cycles

Shear strength deterioration owning to freeze-thaw (F-T) cycles affects loess slope stability in seasonal frozen region. The change in loess shear strength is closely related to particle microstructure. In this study, multiple F-t-tests (i.e., 0, 1, 3, 5, 10, 15, 20, and 30 cycles) were performed on natural loess from Jing'yang, China. Unconsolidated-undrained triaxial tests and scanning electron microscopy (SEM) were performed on the specimens after the predefined number of F-T cycles. Particle microstructure was quantitatively analyzed using fractal dimension theory. The relationship between shear strength index and particle microstructure characteristics parameters was evaluated using the grey correlation method. Changes in the loess microstructure involved two main processes: transformation and stabilization, and the particle shape, arrangement, and interparticle contact were transformed into different types. The particle shape changed from complex to round with debris detaching from the particle surface. The orderliness of particle arrangement increased and point-face intercontact was formed. The internal friction angle and cohesion decreased with an increasing number of F-T cycles, and the change pattern was similar to that of the particle microstructure characteristics, both of which changed significantly before 5 F-T cycles and stabilized after 15 F-T cycles. An evaluation of the correlation between shear strength index and particle microstructure characteristic parameters demonstrated that, the change in particle shape contributed the most to deterioration of shear strength during F-T cycles, followed by particle arrangement and particle distribution. These results provide insights into the mechanism of degradation of loess shear strength in seasonally frozen areas at the microstructural scale.

Quantification of the spatial-temporal evolution of loess microstructure from the Dongzhi tableland during shearing

The physical and mechanical properties of loess are significantly influenced by its microstructural characteristics. The microstructure evolution is of great importance to better understand the loess deformation mechanism. To quantitatively and qualitatively characterize the microstructure evolution of intact loess during shearing, SEM and EDS were carried out at 5 typical strain states during triaxial tests. In addition, the microstructure at different positions inside the samples was also observed to study the spatial evolution. The results indicate that the intact Malan loess studied has a metastable structure with a high porosity of 0.45, widely distributed spaced pores, point contacts and weakly cemented chains. During deformation, the microstructure in the middle part of the sample is most sensitive to deformation and has the significant variations and local difference. The microstructure evolution can be described in four stages: weak compaction stage, initial stage of structural collapse, massive structural collapse stage and structural stability loss stage. These four stages correspond to the four special stages of deformation, and the deformation process is expounded from the perspective of microstructure. The peak stress point is the critical point at which the external stress exceeds the initial structural yield strength. The failure of weak cementation and unstable contact, which leads to the massive collapse of the spaced pores, is the intrinsic factor of structural system instability. The appearance of local microcracks indicates that the shear strength of loess sample begins to decrease, and the deformation velocity will increase. These results from the study of the spatial–temporal evolution of loess microstructure contribute to a better understanding of the catastrophic disaster–causing behavior and the physical mechanism of deformation in the study area.