Granular materials usually copossess inherent and stress-induced anisotropy that significantly influences their mechanical behaviors. This paper presents a series of true-triaxial tests on aeolian sands to consider the inherent and stress-induced anisotropy in terms of soil deposition angles and intermediate principal stress coefficients, respectively. These results show that the deposition angle primarily affected the elastic-plastic stage under axisymmetric conditions. Otherwise, the deposition angle affects all deformation processes after the elastic stage when the intermediate principal stress coefficient changes. Moreover, the critical state is not unique but depends on the combined effect of the deposition angle and the intermediate principal stress coefficient, which indicates that the strength, stress-strain response, and dilatancy behavior of sands are affected by both inherent and stress-induced anisotropy.

Aeolian sand along the Hojiakueri Railway in the Taklimakan Desert exhibits poor mechanical properties for direct use as a filler for railway subgrades. Although cemented soil reinforced with single fibers can improve mechanical properties, its limited effectiveness and high cement usage pose significant economic and environmental concerns. This study investigated the improvement of splitting tensile strength (STS) in cemented aeolian sand through hybrid fiber reinforcement. An orthogonal test was designed to evaluate four factors-fiber types (pairwise combinations of basalt, polypropylene, and glass fibers), fiber lengths (3, 6, and 9 mm), hybridization ratios (1:1, 1:3, and 3:1), and fiber contents (4 %o, 8 %o, and 12 %o) - along with their interactions. The performance of cemented aeolian sand reinforced with hybrid fiber (CASRHF) was evaluated through STS tests and scanning electron microscopy (SEM). The results identified the optimal combination as a 1:1 mix of 6 mm basalt and polypropylene fibers with a fiber content of 12 %o. The interaction between hybrid fiber type and fiber length was the most critical factor influencing STS, followed by hybrid fiber type, fiber length, and fiber content. SEM analysis further revealed a linear negative correlation between STS and porosity, providing new insights into the microscopic mechanisms. The findings underscore the importance of optimizing hybrid fiber combinations to meet the performance requirements of railway subgrade beds in aeolian sand regions.

Mining leads to soil degradation and land subsidence, resulting in decreased soil quality. However, there are limited studies on the detailed effects of mining activities on soil properties, particularly in western aeolian sand. This study, therefore, quantitatively assessed the aeolian sandy soil disturbance induced by mining activities in the contiguous regions of Shanxi, Shaanxi, and Inner Mongolia. The following soil physical quality indices were measured in the pre (May 2015), mid (October 2015), and postmining period (April 2016), such as the soil water content (SWC), particle size (PS), soil penetration (SP), and soil saturated hydraulic conductivity (SSHC). The results showed that mining activities brought irreversible effects on soil structures. In the pre-mining period, land subsidence broke up large soil particles, destroying soil structure, leading to decreased PS (218.33 vs. 194.36 mu m), SP (4615.56 vs. 2631.95 kPa), and subsequently decreased SSHC (1.12 vs. 0.99 cm/min). Rainfall during the midmining period exacerbated this fragmentation. Thereafter, low temperatures and humidity caused the soil to freeze, allowing the small soil particles to merge into larger ones. Meanwhile, the natural re-sedimentation, subsidence, and heavy mechanical crushing in the post-mining period increased PS and SP. The SSHC hence increased to 1.21 cm/min. Furthermore, the evaluation of soil indices from different stress zones showed that the external pulling stress zone always had a higher SSHC than the neutral zone in any mining period, possibly due to the presence of large cracks and high SWC. This study contributes to the understanding of the impact of mining activities on soil physical qualities, providing a theoretical basis and quantitative guidance for the surface damage caused by coal mining in the aeolian sandy area in Western China.

Microbial-induced calcite precipitation (MICP) is an eco-friendly soil stabilization technology widely applied to the solidification of aeolian sand. To further enhance the effectiveness of MICP in cementing aeolian sand, this study introduced wheat straw powder (WSP) as a reinforcing material and conducted experimental research on WSP-enhanced microbial cemented aeolian sand. By combining macroscopic physical and mechanical tests with discrete element method (DEM) simulations, this study systematically investigated the mechanisms by which WSP enhances microbial cementation and the mesoscopic failure characteristics of the material. The results indicated that adding WSP significantly increased the calcium carbonate content, resulting in uniform calcite deposition and encapsulation of sand particles. This enhancement increased the compressive strength and deformation resistance of the cemented sand columns, with a notable increase in strain at failure. DEM simulations further revealed that as the calcium carbonate content increased, macroscopic cracks within the sand columns evolved from single to multiple pathways, eventually penetrating the entire sand column along the loading direction. The internal bonding failure process could be divided into compaction, expansion, and rapid growth stages. Additionally, the uniformity of particle bonding in WSP-reinforced sand columns significantly impacted their macroscopic mechanical behavior, with uneven interparticle bonding likely inducing microcrack accumulation, leading to severe fracture patterns. These findings provide valuable insights for optimizing microbial cementation techniques for aeolian sand.

Aeolian sand widespread in the arid and semi-arid regions has been taken use as the cost-optimal subgrade filling materials. Under the drought climate, the performance of the compacted aeolian sand subgrade is largely dependent on its unsaturated strength. In this work, the shear strength with respect to the matric suction of unsaturated aeolian sand was investigated. A series of triaxial tests were conducted on the compacted specimens with different matric suctions. Results showed that the shear strength of specimen firstly increased with matric suction and then dropped off to a certain value. The maximum shear strength was reached at the matric suction of 40 kPa, which locates in the residual zone on the soil-water characteristic curve (SWCC) of the tested soil. Such phenomena were analysed from the perspective of capillary pore distribution, as well as the internal tri-phase (air-water-solid) structure that identified by microfocus X-ray computed tomography (mu CT) technique. According to the capillary pore size distribution of the specimen, pores with radius smaller than 21 mu m are theoretically saturated with water at suction of 10 kPa. The identified delimiting pore radius was found to be comparable to that of 25 mu m as identified by mu CT. On this basis, the role of water bridges in unsaturated aeolian sand and the pore size-level that govern the mechanical properties were discussed.

Against the backdrop of saline soil solidification and the resource utilization of solid waste and aeolian sand in cold and arid regions, this study employs locally accessible fly ash and aeolian sand to solidify saline soil. By combining unconfined compressive strength tests, X-ray diffraction analysis, scanning electron microscopy, orthogonal experiments, and single-factor analysis, the strength characteristics, mineral composition, and interfacial structure changes of saline soil solidified with different freeze-thaw cycles and varying amounts of fly ash, aeolian sand, and alkali activators were investigated. The effects of each factor were analyzed to determine the optimal mixture ratio and to explore the solidification mechanism.The results indicate that the unconfined compressive strength of saline soil is most significantly enhanced when solidified with a combination of fly ash, aeolian sand, and alkali activators. The optimal mixture ratio was found to be 24 % fly ash, 7 % aeolian sand, and 4.5 mol/L alkali activator. With the incorporation of these solidifying materials, the failure mode of saline soil transitions from plastic to brittle, and the stress-strain curve exhibited a strain-softening behavior. The combined solidification method demonstrated the most pronounced effect in mitigating freeze-thaw damage, with the unconfined compressive strength of the solidified soil reaching 7.01 MPa after seven freeze-thaw cycles, compared to 0.03 MPa for the untreated soil, an increase by a factor of 234.This significant enhancement is attributed to the formation of substantial gel substances, which mitigate the strength loss caused by freeze-thaw cycles. The gel locking mechanism between particles in the solidified soil far exceeds the detrimental effects of freeze-thaw cycles, effectively inhibiting freeze-thaw deterioration. Additionally, the reaction pathways involving AFt and AFm phases reduce the content of SO 4 2- and Cl-in- in the solidified soil, effectively suppressing salt expansion and significantly improving the soil's strength.

The dynamic resilience characteristics of aeolian sand subgrade are influenced by salt content and water content, exhibiting significant stress dependence and anisotropy. The resilient modulus(MR) M R ) of aeolian sand represents the stress-strain nonlinearity under cyclic loading, serving as an important parameter for the design of aeolian sand subgrade in desert areas. In order to investigate the variation of M R of aeolian sand subgrade with salt content and water content under traffic loading, as well as the M R characteristics under these conditions, three types of aeolian sand samples with varying water content and four sulphate contents were prepared. The variation of M R of aeolian sand under different confining pressures and deviator stress levels, as well as the influences of water content and salt content, was studied through indoor dynamic triaxial testing. Based on the pattern of the fitting parameters of the benchmark model, a prediction model suitable for the M R of aeolian sand was constructed. The results indicate a rise in aeolian sand's M R with increasing deviator stress and confining pressure, with confining pressure having a more significant impact than deviator stress. With the increase in water content, the M R of aeolian sand decreases nonlinearly, and with the increase in salt content, it exhibits a wave-shaped trend of increasing-decreasing-increasing, which is related to the dissolution state of sodium sulfate in the soil. Based on the experimental results, a prediction model of the M R of aeolian sand was established, derived from the benchmark model, which can reflect the influence of salt content and water content on the M R , introducing them as variables within the model.

The cementation of desert aeolian sand is a key method to control land desertification and dust storms, so an economical, green and durable process to reach the binding between sand grains needs to be searched. The method based on the microbially induced calcite precipitation (MICP) appeared in recent years as a promising process that proved its efficiency. The feasibility of the MICP technique to treat aeolian sand composed by low clay content, fine particles, low water content and characterized by weak permeability was demonstrated in the present paper. The effects of initial dry density, cementation number and curing time on the permeability and strength of MICP-treated aeolian sand were investigated using permeability tests and unconfined compressive strength (UCS) tests. The microstructure of aeolian sand was observed by scanning electron microscopy (SEM) tests and X-ray diffraction (XRD), aiming to reveal the solidification principle of MICP. The tests result indicated that when the initial dry density and the cementation number rose, the hydraulic conductivity of aeolian sand decreased while the mechanical strength given by UCS values improved. When the initial dry density was 1.65 g/cm3, the curing time was 3 h and the cementation number reached 20, the hydraulic conductivity and UCS reached 0.00151 cm/s and 1050.30 kPa, respectively. With increasing curing time, the hydraulic conductivity first decreased, followed by an increase, while the UCS exhibited an up and then a downtrend. Furthermore, the correlation between UCS values and the CaCO3 content reached a high R2 value equal to 0.912, which confirmed that the cementation occurred in sandy material and governed the soil strengthening. Indeed, the calcium carbonate crystals observed by SEM and XRD enhanced the friction between particles when they wrapped around the sand grains surface, while carbonates reduced the soil permeability when filling the pores and sticking the sand particles together. Finally, the theoretical and scientific knowledge brought by the present study should help in managing sand in desert areas.

To verify the novel method of achieving a true-triaxial stress path with the pseudo-triaxial apparatus, a series of drained and undrained tests were carried out for the identical scheme with pseudo-triaxial apparatus and true-triaxial apparatus respectively. The differences between the two types of tests were quantified. The results show that the novel method effectively achieved the true-triaxial stress path by controlling the loading ratio of the pseudo-triaxial apparatus. The relationships of q - epsilon 1 and eta - epsilon s measured by the two apparatuses had a higher similarity which decreases slightly with the b increase. When 0 <= b < 0.5, the slope of the critical state line measured by both apparatuses was almost identical. When 0.5 <= b <= 1, the slope of the critical state line measured by the novel method was slightly lower, but the biggest change was within 10% compared with the two Mohr-Coulomb criteria, the peak strength measured by the two apparatuses was distributed near the criteria, indicating the feasibility and rationality of the novel method. The tests show that the novel method greatly enriches the test range of pseudo-triaxial apparatus, which not only simplifies the process of soil 3D testing but also reduces the test cost.

Aeolian sand (AS) is locally available in desert area that can be used as road construction material. However, AS is a loose granular material with low bearing capacity which needs to be stabilized. This paper presents a novel study of using geopolymer (GP) and fines to stabilize AS. A series of cyclic triaxial tests was conducted to study the effect of GP and fines contents on cyclic response of stabilized AS. The experimental results show that adding fines into AS can effectively increase the cyclic loading capacity but increase the accumulated axial strain of the mixture; inclusion of GP into the AS-fines mixture greatly enhances the cyclic loading capacity and reduces the accumulated axial strain of the mixture. The shakedown response of the untreated AS changes from plastic shakedown to incremental collapse with increase in cyclic stress ratio (CSR); however, the GP-fines-AS mixture with higher fines and GP contents mainly experiences plastic shakedown. The modulus index of untreated AS or fines-containing AS shows an increase-stable trend with loading cycles, indicating strengthening in the soil matrix, but that of the GP-fines-AS mixture shows increase-stable, stable or decrease-stable trend with loading cycles, depending on the CSR and fines and GP contents. Microcharacterization using scanning electron microscope (SEM) shows that the added fines greatly alter the microstructure of AS by filling the voids and acting as lubricant, which facilitates the movement of AS particles and thus induces larger axial strain. The added GP increases the cyclic loading capacity of the treated soil by inducing a chemical fabric in the treated soils. Increase in fines and GP content results in larger contact area and stronger fabric leading to enhanced stabilizing effect.