Post-grouting pile technology has gained extensive application in collapsible loess regions through the injection of slurry to compress and consolidate the soil at the pile base, thereby forming an enlarged base that enhances the foundation's bearing capacity and reduces settlement. Despite the prevalent unsaturated state of loess in most scenarios, the conventional design methodologies for piles in collapsible loess predominantly rely on saturated soil mechanics principles. The infiltration of water can significantly deteriorate the mechanical properties of loess due to the reduction in matric suction and the occurrence of collapsible deformation, leading to a substantial degradation in the bearing behavior of piles. To explore the variations in load transfer mechanisms of post-grouting piles in collapsible loess under conditions of intense precipitation, a comprehensive large-scale model test was conducted. The findings revealed that the post-grouting technique effectively mitigates the adverse effects of negative pile shaft friction in saturated zones on the pile's bearing behavior. Furthermore, the failure criteria for piles may shift from the shear failure of the base soil to excessive pile settlement. By incorporating principles of unsaturated soil mechanics, modified load transfer curves were developed to describe the mobilization of both pile shaft friction and base resistance. These curves facilitate the extension of the traditional load transfer method to post-grouting piles in collapsible soils under extreme weather conditions. The proposed revised load transfer method is characterized by its simplicity, requiring only a few soil indices and mechanical properties, making it highly applicable in engineering practice.

The span of pile foundations beneath metro depots typically ranges from 10 to 20 m, exhibiting a notably large span. This structural characteristic results in the pile foundations bearing a more concentrated upper load, while the interstitial soil between the piles bears minimal force. Concurrently, global climate change and enhanced urban greening initiatives have led to a significant increase in rainfall in northwest China, a region traditionally characterized by arid and semi-arid conditions. This climatic shift has precipitated a continuous rise in groundwater levels. Furthermore, the extensive distribution of collapsible loess in this region exacerbates the situation, as the rising groundwater levels induce loess collapse, thereby adversely affecting the mechanical behavior of the pile foundations. In light of these factors, this study utilized the pile foundations of a metro depot in Xi'an as a prototype to conduct static load model tests under conditions of rising groundwater levels. The experimental results reveal that the load-settlement curve of the pile foundations in the absence of groundwater exhibited a steep decline with distinct three-stage characteristics, and the ultimate bearing capacity was determined to be 5 kN. When the groundwater level is situated below the loess stratum, the settlement of both the pile foundations and the foundation soil, as well as the axial force, skin friction, and pile tip force, remains relatively stable. However, when the groundwater level rises to the loess stratum, there is a significant increase in the settlement of the pile foundations and foundation soil. Negative skin friction emerges along the pile shaft, and the bearing type of the pile foundation transitions gradually from a friction pile to an end-bearing pile. The influence range of the pile foundation on the settlement of the foundation soil is approximately three times the pile diameter.

Collapsible loess has special sensitivity to water, and its engineering mechanical properties deteriorate significantly after immersion in water, causing the foundation to sink, which seriously threatens the safety and stability of the high-speed railway subgrade under train vibration loading. Studying this effect is essential to prevent and control the disasters of high-speed railway subgrades. In this study, a model with the function of simulating foundation settlement is established to conduct disaster testing of high railway subgrade under train vibration loading. The results indicate that when different foundation shapes are settled, the surface of the subgrade under static load is gradually settled in a short time, and the settlement value of the track surface is lower than that of the corresponding subgrade surface. Under train vibration load, the maximum dynamic settlement occurs at the middle of the subgrade slope, which is smaller than the corresponding settlement under static load. The number of stabilization times required from different monitoring positions on the subgrade surface is different under different excitation forces, and the number of stabilization times required is more in the middle of the subgrade slope and the slope shoulder. The influence of train speed on subgrade has a critical respond speed that increases with increasing vibration times. There are horizontal, vertical and 45 degrees angle cracks in the middle of subgrade slope. It is qualitatively assessed that the slope of the high-speed railway subgrade in the collapsible loess area is unstable under the effect of train load. The data and rules provided in this document provide some reference values for the construction of a high-speed railway in the collapsible loess area.

Collapsible loess is characterized by its unique soil-forming environment, mineral composition, and microstructure, resulting in poor engineering properties such as high water sensitivity, high collapsibility, high compressibility, and low strength. To improve the poor engineering properties of collapsible loess, we selected a suitable eco-friendly material-guar gum (GG)-for its improvement and reinforcement, and investigated the improvement effect of different GG dosages (0.5 similar to 1.5%) and curing ages (0 similar to 28 days) on collapsible loess. The mechanical properties of soil samples were determined by direct shear tests, unconfined compressive strength tests, and splitting tests. The water stability of soil samples was evaluated by both cube and sphere crumb tests. SEM and EDS analyses were also conducted to determine the microstructural and mineral changes in soil. The results indicate that the incorporation of GG is beneficial to inhibit the collapsibility of the soil and improves the water stability and strength of the soil. The collapsibility coefficient of loess is reduced to below 0.015 when 0.75% and above of GG is admixed, which is considered a complete loss of its collapsibility. When the GG dosage increases from 0% to 1.25%, the compressive strength and tensile strength of the soil samples increase by 43.5% and 34.9%, respectively. However, by further increasing the GG dosage to 1.5%, the compressive strength and tensile strength decrease by 3.8% and 6% compared to those with 1.25% GG. This indicates that the strength of the specimens shows an increasing trend and then a decreasing trend with the increase in GG dosage, and 1.25% GG was found to be the best modified dosage. Microstructural and mineral analyses indicate that the addition of GG does not change the mineral composition of loess, but, rather, it significantly promotes the agglomeration and bonding of soil particles through cross-linking with Ca2+ ions in the soil to form a biopolymer network, thus achieving a reliable reinforcement effect. Compared with the existing traditional stabilizers, GG is a sustainable and eco-friendly modified material with a higher low-carbon value. Therefore, it is very necessary to mix GG into collapsible loess to eliminate some of the poor engineering properties of loess to meet engineering needs. This study can provide test support for the application and promotion of GG-modified loess in water agriculture and road engineering.

A water conveyance open channel project in the northern Xinjiang region crosses a large area of collapsible loess. The mechanical properties of the collapsible loess have undergone severe degradation after years of exposure to rainfall, evaporation, and seasonal temperature fluctuations, making it highly susceptible to engineering phenomena such as channel foundation collapse and slope failure. To delve into the deterioration mechanism, direct shear, compression, and microscopic scanning tests were conducted on the collapsible loess under dry-wet & freeze-thaw cycles. The deterioration patterns of shear strength and compression properties, as well as their damage mechanisms, were analyzed at both macro and microscopic scales. The results of the study indicate (1) Straight shear test: with increasing the number of dry-wet-freeze-thaw cycles, the peak shear strength exhibits a three-stage trend: rapid decrease, decelerated rate of decrease, and eventual stabilization. The cohesion decreased exponentially, with the largest reduction occurring during the first cycle, and stabilizing after 5 cycles, reaching a degradation degree of 44.55%. The change in internal friction angle, which varied within 2.1 degrees, was less affected by the wet-dry-freeze-thaw cycles, with a maximum degradation of 7.04%. (2) Compression test: the compression curve can be divided into two stages of elastic deformation and elastic-plastic deformation according to the consolidation yield stress sigma(k), and sigma(k) shifts forward as the cycle times increase. The compression coefficient and compression index decreased exponentially or in a power function form with increasing cycle times, indicating reduced overall compressibility of the soil body. (3) Microstructure: through scanning electron microscope (SEM) analysis, under cycling, the number of large pores decreased while the number of medium and small pores increased, with the arrangement tending towards disorder. Large particles gradually transformed into medium and small particles, and their morphology tended to become rounded. Correlation analysis indicates that pore size and its angle are the main factors influencing shear strength. Pearson's correlation coefficient reveals that particle morphology and pore size have the greatest influence on compression indices.

Metro transit construction has begun to develop rapidly in northwest China because of the acceleration of urbanization. Accordingly, metro depots are also regarded as an essential auxiliary facility for stopping, operation, and maintenance of trains. Meanwhile, many commercial buildings are constructed over metro depots to improve the utilization rate of land due to the increasingly scarce urban land resources, known as transit-oriented development (TOD). These buildings have a large covered area and transfer concentrated loads to the bases. Therefore, pile bases under metro depots have the bearing characteristics of undertaking large concentrated loads, while lesser loads are placed on the soil between the adjacent pile bases. Additionally, the main ground in northwest China is collapsible loess, so the collapsibility should also be considered. Based on the above background, this research performed static loading tests with and without immersion in a reduced scale of adjacent pile bases under a metro depot in Xi'an. The remolding process of natural loess could destroy its structure and the anisotropy of natural loess could also affect the test results. Therefore, four kinds of artificial collapsible loess with different mass ratios of barite powder, kaolin, river sand, cement, industrial salt, and calcium oxide were made by the free-drop method. This method could make the artificial loess simulate the structure of natural loess reasonably. Then, the artificial loess with the most similar properties to intact loess was selected by comparison. Finally, static loading tests with this artificial loess were implemented. The results showed that the ultimate bearing capacity was 4.5 kN. At the same time, the axial force decreased along depth, since the pile shaft friction was positive, and the load sharing ratio of pile tip force increased to 0.58 when the load exceeded 4.5 kN in the situation without immersion; the settlement of pile bases increased significantly after immersion, while the negative shaft friction occurred at the depth of -8 cm similar to-35 cm, and the load sharing ratio of pile tip force reached 0.92.

The Yuncheng Basin is part of the Fenwei Graben System, which has developed ground fissure hazards that have caused serious damage to farmland, houses, and roads and have brought about huge economic losses. Located in Wanrong County on the Emei Plateau in the northwestern part of the Yuncheng Basin in China, the Wangjiacun ground fissure is a typical and special ground fissure developed in loess areas, and its formation is closely related to tectonic joints and the collapsibility of loess. In order to reveal the formation and genesis of the Wangjiacun ground fissure, the geological background, developmental characteristics, and genesis pattern of the Wangjiacun ground fissures were studied in detail. A total of three ground fissures have developed in this area: a linear fissure (f1) is distributed in an NNE-SSW direction, with a total length of 334 m; a circular fissure (f2) is located near the pool, with a total length of 720 m; f2-1, a linear fissure near f2, has a fissure length of 110 m and an NE orientation. This study shows that tectonic joints in loess areas are the main controlling factors of the linear fissure (f1); differential subsidence in the pool caused by collapsible loess is the main source of motivation for the formation of the circular fissures (f2, f2-1), and tensile stresses produced by the edges of subsidence funnels lead to the cracking of shallow rock and soil bodies to form ground fissures (f2, f2-1). This study enriches the theory of ground fissure genesis and is of great significance for disaster prevention and the mitigation of ground fissures in loess areas.

Steel slag has the potential to replace cement and lime as a cementitious material to eliminate the collapsibility of loess. In this content, the physical and mechanical properties (compaction, compressive strength and shear strength) and engineering properties (California bearing ratio, permeability and collapsibility) of steel slag improved collapsible loess (SSICL) were analyzed by adjusting the steel slag-loess ratio (SS/L). The results show that the maximum dry density (MDD) of the mixture increases and the optimum moisture content (OMC) decreases with increasing steel slag content. Improved compressive strength can be achieved by increasing the SS/L ratio and curing period, and the 7-day strength of SSICL with SS/L ratio of 15 % meets the specification requirement of 0.5 MPa. The internal friction angle increases with increasing SS/L ratio, while the cohesion has the opposite trend. The permeability coefficient of SSICL with different SS/L ratios ranges from 1.14 x 10-6 cm/s to 7.59 x 10-6 cm/s, only when SS/L ratio of 15 % was used did the permeability continue to decrease with increasing curing period. The CBR and immersion expansion rate obtained in the study range are much higher than the specification requirement of 8 % and lower than the specification limit of 2 %, respectively. The results also show that the collapsibility of SSICL was completely eliminated within the study range. Finally, the optimal steel slag content of 15 % was proposed and the field test was carried out, which proves the feasibility of the proposed steel slag for improving collapsible loess subgrade.

The Guanzhong Plain city group is located in the centre of inland China, which is an important fulcrum of the Asia-Europe Continental Bridge, and also the second largest urban agglomeration in the western region of China. However, the large thickness collapsible loess has always been a pain point in the process of urban development. In this paper, for the problem of collapsible loess in Guanzhong area, five typical geomorphological units in Xi'an, Xianyang, Tongchuan and Sanmenxia are targeted for sampling. Through a large number of indoor and outdoor experiments, the relevant data of physical properties, hydrophysical properties, mechanical properties, collapsible index of collapsible loess and the distribution pattern of each index in the study area are analysed.By analyzing the correlation among the indexes, four indexes which are closely related to the collapsibility of loess are obtained: natural density, void ratio, dry density and saturation, and the regression analysis of these four indexes and the collapsibility coefficient is carried out respectively.This finding provides a reference for a more in-depth study of the characteristics of collapsible loess in Guanzhong area, and can optimise the engineering investigation process in urban construction, which is very important for the urban construction in this area.