Soybean urease-induced calcium carbonate precipitation (SICP) is an innovative and eco-friendly approach with demonstrated potential for mitigating soil liquefaction. However, the specific impacts of the concentrations of soybean urease and salt solutions require further elucidation. The research examines how the two compositions influence calcium carbonate formation. Dynamic characteristics of one-cycle SICP-treated clean and silty sand were analyzed based on cyclic triaxial tests. It was revealed that SICP-treated specimens of both liquefied sand and silty sand exhibit reduced accumulation of excess pore pressure and diminished strain growth under cyclic loading, thereby delaying liquefaction failure. Although higher concentrations of both soybean urease and salt solution can enhance liquefaction resistance, salt solution concentration has a more pronounced effect on improving liquefaction resistance due to the more production of calcium carbonate. Scanning electron microscopy observations confirmed the presence of calcium carbonate crystals at the interfaces between sand particles and between sand and fine particles. These crystals effectively bond the loose sand and fine particles into a cohesive matrix, reinforcing soil structure. A direct linear correlation was established between the liquefaction resistance improvement and precipitated calcium carbonate content. Notably, the one-cycle SICP treatment method adopted in this study demonstrates a better biocementation effect compared to cement mortar or multi-cycle MICP-treated sand under the same content of cementitious materials. These findings provide valuable insights for optimizing SICP treatments, aiming to reduce the risk of soil liquefaction in potential field applications.

Red mud is a kind of solid waste, which can be used as engineering roadbed filler after proper treatment. Due to the special physical and chemical properties of red mud, such as high liquid limit and high plasticity index, it may affect the stability of soil. Therefore, red mud can be improved by adding traditional inorganic binders such as lime and fly ash to improve its road performance as roadbed filler. Red mud-based modified silty sand subgrade filler will be affected by dry-wet alternation caused by various factors in practical application, thus affecting the durability of the material. In order to study the strength degradation characteristics and microstructure changes of red mud, lime and fly ash modified silty sand subgrade filler after dry-wet cycle, the samples of different curing ages were subjected to 0 similar to 10 dry-wet cycles, and their compressive strength, microstructure and environmental control indexes were tested and analyzed. The results show that the sample cured for 90 days has the strongest toughness and the best ability to resist dry and wet deformation. With the increase of the number of dry-wet cycles, the mass loss rate of the sample is in the range of 6 similar to 7 %, and the unconfined compressive properties and tensile properties decrease first and then increase. There are continuous hydration reactions and pozzolanic reactions in the soil, but the degree of physical damage in the early stage of the dry-wet cycle is large, and the later cementitious products have a certain offsetting effect on the structural damage. The internal cracks of the sample without dry-wet cycle are less and the structure is dense. After the dry-wet cycle, the microstructure of the sample changed greatly, and the cracks increased and showed different forms. Through SEM image analysis, it was found that the pore structure of the sample changed during the dry-wet cycle, which corresponded to the change law of mechanical properties. After wetting-drying cycles, the leaching concentration of heavy metals in the modified soil increased slightly, but the overall concentration value was low, which was not a toxic substance and could be used as a roadbed material. The study reveals the influence of dry-wet cycle on the strength characteristics and microstructure of red mud, lime and fly ash synergistically improved silty sand, which provides a technical reference for the engineering application of red mud-based materials.

This study explores the mechanical properties and synergistic mechanisms of silty sand modified with guar gum (GG) and polypropylene fiber (PP fiber) through a series of unconfined compressive strength (UCS) tests, direct shear tests, and direct tensile tests. The test results reveal that the unconfined compressive strength (UCS) of silty sand can be dramatically improved by incorporating GG, boosting its strength by up to 23 times compared to the natural soil. Adding PP fiber further enhances the UCS and effectively mitigates brittle failure. GG dominates the increase in shear strength by enhancing cohesion, while the PP fiber optimises the shear stability by increasing the internal friction angle. The shear strength of the GG-PP fiber-enhanced soil can be boosted by 235% compared to natural soil. The synergistic effect of GG and PP fibers enables the tensile strength of the improved silty sand to reach 122.75 kPa, representing a 34.15% increase compared to soil with only GG incorporated. However, if the fiber content is too high (> 0.5%), the tensile strength will decrease due to increased porosity. The study found that GG enhances the cohesion between soil particles through hydrated gel, and the PP fiber inhibits crack propagation and improves ductility through the bridging effect. The two form a bonding-bridging synergistic system, which significantly optimises the mechanical properties of the soil. This combined improvement scheme has both high strength and high ductility and can replace traditional inorganic cementitious materials, providing new ideas and methods for the application of silty sand in roadbed engineering, slope reinforcement, and other fields.

The soil construction materials cured with biopolymers are gradually being recognized and widely used in engineering areas, such as roadbeds or foundation fills. The strength of biopolymer-solidified soils (BSS) is easily influenced by the change of internal residual moisture content (RMC), however, the quantitative relationship between them remains unclear. Xanthan gum, as a representative of biopolymer, was used in this study to enhance the mechanical properties of silty sand dredged from the Yellow River under different initial water contents and curing temperatures. The unconfined compressive strength (UCS), curing time, water stability and microscopic properties of BSS were investigated via a series of indoor experiments. Results show that the proposed method for quantitatively evaluating the BSS strength using different RMC values was found to be workable compared to that of the traditional cement-treated method under different curing ages. The curing time required for BSS to reach a certain target strength, i.e. 2900 kPa, is reduced to 9.3 h at a higher curing temperature of 90 degrees C. Moreover, BSS exhibits the self-healing properties of strength recovery after re-temperature drying, with a strength recovery ratio above 45%. The control raw soil samples completely disintegrate in water within 10 s, and even lower xanthan gum biopolymer dosages, such as 0.5%, improved stability in water by reducing permeability by sealing the internal voids of the soil. SEM results indicate that the initial water content and curing temperature mainly affect the distribution of effective xanthan gum linkages, and thus significantly improve the strength and water stability of BSS. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

This study investigated the consolidation behavior of silty sand improved with stone columns using a laboratory model based on the unit cell concept. The research focused on analyzing settlement, total stresses, and pore water pressures. Quantitative results showed that increasing the area replacement ratio and relative density reduced settlement, stress distribution, and pore pressure dissipation. The settlement reduction ratio increased with higher area replacement ratios, showing values of 1.12, 1.38, and 1.73 for ratios of 0.03, 0.06, and 0.11 at 40% density and values of 1.23, 1.48, and 2.18 at 90% density. Stress on the stone column increased by 3.7 to 5.0 times for 40% density and 3.0 to 4.2 times for 90% density compared to unimproved soil. Stone columns improved load-bearing capacity and accelerated consolidation by reducing pore water dissipation paths. Column efficacy increased with higher area replacement ratios and relative density, indicating effective stress transfer. Analytical comparisons showed that consolidation in model tests occurred faster than predicted by the Terzaghi and Barron methods, with results of the Carillo method aligning closely with laboratory tests. The study confirmed that stone columns significantly enhanced the consolidation behavior and stability of silty sand.

The understanding of rainfall-induced landslides on gentle, loose-fill slopes is limited in comparison to steep slopes. Hence, two physical model tests were conducted on silty sand slopes under continuous rainfall: one on a bare slope and the other on a slope planted with ryegrass. The slope angle of 25 degrees is much lower than the internal friction angle of slope material (34.3 degrees), which makes the model test fall well into the category of gentle slope. For the initially unsaturated bare slope, a rainfall event with return period of 18 years could trigger a rapid and retrogressive global sliding, which differs from previous findings that gentle slopes would only experience shallow failure. A sudden increase in pore-water pressure was simultaneously observed, which might be generated by the wetting-induced collapse of unsaturated loose soil. On the other hand, the stability of the slope with grass plantation was significantly enhanced, and it was able to withstand rainfall event more severe than those with a return period of 100 years, with only minimal deformation. The results suggest that the gain in shear strength due to ryegrass roots surpasses the additional sliding force caused by the increased water retention capability. Additionally, it is found that the abrupt change in pore pressure was no longer indicative of slope failure in the case of the grass-reinforced slope.

Silty sandy soils usually have low shear strength due to their non-cohesive structure, weak internal bonds, and high porosity. Environmental challenges, such as freeze-thaw (F-T) cycles, also reduce the mechanical characteristics and instability of infrastructures and structures built on these soils. Biopolymers and fibers offer a sustainable solution to improve soil strength and F-T strength. However, while much research focuses on stabilizing silty sand, fewer studies examine the combined effects of biopolymers and fibers on soil properties under F-T cycles. Additionally, the correlation between ultrasonic pulse velocity (UPV) and unconfined compressive strength (UCS) in biopolymer-stabilized and fiber-reinforced soils still needs to be explored. This study examines the stabilization of silty sand using Persian gum (PG) (0.5-3%) and kenaf fibers (KF) (0-1.5%) with lengths of 6, 12, and 18 mm at the curing times of 7, 28, and 90 days. The samples were subjected to F-T cycles (0, 1, 2, 3, 6, and 12). The results showed that the highest UCS was achieved with 2.5% PG and 1% KF (12 mm) after 28 days. After 12 F-T cycles, the UCS reductions were 41% for sample with 2.5% PG and 34% for sample 2.5% PG and 1%KF. The swelling after freezing for the 2.5% PG and 1% KF sample and the 2.5% PG sample was 4.8% and 3.45%, respectively. A correlation between UPV and UCS after various F-T cycles was suggested. The scanning electron microscopy (SEM) analysis revealed increased voids, weakened polymer bonds, and cracks after 12 F-T cycles.

Due to continuous water level fluctuations and changes in climatic boundary conditions, river embankments experience frequent variations in the degree of saturation and suction distributions, which strongly influence earthworks performance, both in terms of infiltration regime and stability conditions. For these reasons, an experimental campaign aimed at investigating the hydro-mechanical response of a compacted silty sand mixture, representative for the embankment earthfills of the river Po tributaries (Italy), has been carried out and the main results are reported and discussed in this paper. To promote homogeneity and initial conditions consistency, the specimens were reconstituted at target dry unit weight and moisture content, by using the standard Proctor compaction energy. Suction-controlled triaxial and oedometer tests were designed and carried out to highlight the effect of drying and wetting stress paths, as well as confining stresses, on strength and compressibility characteristics of the tested material. The well-established axis translation technique has been used for controlling matric suction inside the samples. Typical suction ranges for the embankment, assessed through field monitoring of seasonal and daily hydraulic changes, have been imposed during the tests. Unsaturated specimens consistently exhibited a higher shear strength and stiffness with the increase of suction, compared to the saturated samples. The outcome of the present mechanical characterization may provide some meaningful geotechnical insights for the assessment of river embankment actual safety margins under transient seepage groundwater flow.

As metro lines continue to expand rapidly in urban areas, the excavation of twin tunnels in shallow depths using shield tunnelling methods has become widespread. By analysing field data obtained from an actual shield tunnelling project, it has been observed that the post-ground settlement occurring over the preceding tunnel during the excavation of the following tunnel in silty sand is approximately 42% of the green field settlement, which cannot be disregarded. Accurate approximation of the post-ground settlement is useful for preventing any damage due to excessive deformation and to determine the total ground settlement profile during twin tunnel construction stage. And yet, only a few number of studies have focused on investigating and predicting the postground settlement that occurs during twin tunnel construction in soft soils. Therefore, this study develops a transparent model using the multi-gene genetic programming (MGGP) method, enabling the prediction of postground settlement during twin tunnelling. Comparative analysis demonstrates that the proposed model is userfriendly and capable of generalising to unseen data. The reliability of the MGGP-based model has been validated through sensitivity and parametric analyses. Additionally, when estimating post-settlement during twin tunnelling, it is essential to consider the spacing between twin tunnels, soil cohesion, and crucial operational parameters of the shield, such as torque and face pressure.

Freeze-thaw (F-T) cycling poses a significant challenge in seasonally frozen zones, notably affecting the mechanical properties of soil, which is a critical consideration in subgrade engineering. Consequently, a series of unconfined compressive strength tests were conducted to evaluate the influence of various factors, including fiber content, fiber length, curing time, and F-T cycles on the unconfined compression strength (UCS) of fiber-reinforced cemented silty sand. In parallel, acoustic emission (AE) testing was conducted to assess the AE characteristic parameters (e.g., cumulative ring count, cumulative energy, energy, amplitude, RA, and AF) of the same material under F-T cycles, elucidating the progression of F-T-induced damage. The findings indicated that UCS initially increased and then declined as fiber content increased, with the optimal fiber content identified at 0.2%. UCS increased with prolonged curing time, while increases in fiber length and F-T cycles led to a reduction in UCS, which then stabilized after 6 to 10 cycles. Stable F-T cycles resulted in a strength loss of approximately 30% in fiber-reinforced cemented silty sand. Furthermore, AE characteristic parameters strongly correlated with the stages of damage. F-T damage was segmented into three stages using cumulative ring count and cumulative energy. An increase in cumulative ring count to 0.02 x 104 times and cumulative energy to 0.03 x 104 mvmu s marked the emergence of critical failure points. A sudden shift in AE amplitude indicated a transition in the damage stage, with an amplitude of 67 dB after 6 F-T cycles serving as an early warning of impending failure.