Subway subgrades typically consist of alternating deposits of soil layers with significantly different physical and mechanical properties. However, the overall dynamic characteristics and the evolution of micro-porous structures in stratified soils is often overlooked in current studies. In this study, cyclic triaxial tests were conducted on homogeneous sand, silt and stratified soils with different height ratios, and nuclear magnetic resonance (NMR) was used to investigate the changes in pore structure and moisture content. The dynamic behavior and macroscopic deformation mechanisms were systematically investigated in terms of stress amplitude, confining pressure, and layer height ratio (the ratio of sand to silt height). The results show that as the sand height ratio increases, the axial strain and pore water pressure first increase and then decrease, reaching the maximum when h(Sand): h(Silt) = 2:1. When the confining pressure is 100 kPa, the axial strain of h(Sand): h(Silt) = 2:1 is 181.08 % higher than that of silt. Under the dynamic loading, the stratified soils form a dense skeletal structure near the stratification plane, which hinders the flow and dissipation of pore water, so that the pore water agglomeration phenomenon occurs near the stratification plane, which aggravates the accumulation of residual pore pressure and reduces the deformation resistance. However, when h(Sand): h(Silt) = 4:1, the influence of the stratification planes is significantly reduced, and the deformation characteristics approach homogeneity. This study reveals the dynamic characteristics of stratified soils by comparing and analysing homogeneous samples.

In cold-region high-speed railway (HSR) subgrade engineering, coarse-grained soils are commonly used as frost heave prevention fillers. However, coupled water-heat migration during freeze-thaw cycles still induces frost heave. This study innovatively employs a nuclear magnetic resonance (NMR) system to elucidate the hydro-thermal transport mechanisms in coarse-grained soils during freezing. The results reveal that under identical temperature and freezing duration, high-water-content soils release substantial latent heat from pore water freezing, resulting in higher freezing zone temperatures than low-water-content soils. During freezing, unfrozen water content decreases as a power function with freezing time at different depths of soil samples, with the frozen zone experiencing the fastest water reduction, followed by the freezing front and then the unfrozen zone. Both free and bound water progressively decrease in frozen and unfrozen zones. After freeze-thaw, the change in soil pore structure leads to a decrease in bound water and an increase in free water in frozen zones, while both decrease in unfrozen zones. Furthermore, higher initial water content results in more pronounced reductions of bound water and increases of free water in frozen zones. These findings advance the understanding of hydro-thermal coupling mechanisms and provide theoretical foundations for frost damage mitigation in high-speed railway subgrades.

Understanding pore water distribution in soil is essential for elucidating water movement and mechanical properties, as it significantly influences soil strength and stability. Accurate assessment of this distribution provides a scientific foundation for civil engineering design, ensuring structural safety and durability. This study examines pore water distribution using plate load tests and Nuclear Magnetic Resonance (NMR). Results indicate that matric suction expels free water first, leaving bound water until a critical suction point is reached. As matric suction increases, the peak value of the T2 relaxation time curve decreases, shifting leftward, reflecting water drainage from larger to smaller pores. Then, water expulsion occurs in three stages, with Stage III primarily indicating bound water content, quantified at 19.23%, including 3.3% as strongly bound water. An equation is derived to calculate the surface relaxation rate of 0.0176 mu m/ms. Thus, the distribution of T2 relaxation time can be transformed into pore size distribution, summarizing the characteristics of pore water distribution during the drying process. Finally, comparative analysis confirms the effectiveness of NMR in measuring bound water. These findings enhance our understanding of soil water distribution and highlight the need for advanced models that incorporate pore connectivity and water retention dynamics.

The deformation of expansive soil in seasonally frozen regions caused by freeze-thaw cycles has severely affected the long-term performance of engineering applications. The alteration of expansive soil microstructure has resulted in many geotechnical engineering failures, such as soil cracking and settlement. Consequently, the micropore contraction and expansion mechanisms of expansive soil have drawn extensive attention. Nuclear Magnetic Resonance (NMR) is widely used as a rapid, non-destructive detection technique for moisture monitoring and microstructure evolution characterization in porous media. In addition, Magnetic Resonance Imaging (MRI) can visualize the migration pattern of pore water under different numbers of freeze-thaw cycles. SEM is the most effective and direct method to reveal the structure of particle and micropore arrangement. This paper investigates the pore size evolution and pore structure distribution characteristics of saturated expansive soil via 6 freeze-thaw cycle tests using NMR and SEM techniques. The evolution law of saturated expansive soil under freeze-thaw cycles is obtained. The results show that pore water migrates from the center to the periphery under freeze-thaw cycles. The pore size decreases as the number of freeze-thaw cycles increases and small particles increase significantly. During the freeze-thaw cycle, the arrangement pattern changed from surface-surface contact to stacking.

This study examines how acid rain affects the microstructure and mechanical properties of cement-amended loess, crucial for ensuring the safety of engineering projects. We aimed to investigate how acid rain influences the micro-mechanical behavior of cement-amended loess and its damage characteristics under combined acid rain and loading conditions. Cement-amended loess samples were exposed to artificial acid rain with varying pH levels, and changes in their strength and microstructure were analyzed using unconfined compression tests, SEM, NMR, and XRD techniques. Our findings reveal that acid rain erosion of cement-amended loess triggers hydration and erosion reactions. As acid rain concentration increases, the unconfined compressive strength of the amended soil gradually decreases, accompanied by an expansion of pore spaces from small to large-medium pores. Additionally, particle contacts shift from face-to-face and side-to-side to point-to-point and side-to-side configurations. Furthermore, prolonged erosion time exacerbates pore space expansion, indicating a time-dependent effect on soil integrity. To characterize these effects, we developed a constitutive equation within the framework of damage mechanics that incorporates both erosion and loading. This equation successfully aligns with experimental data, providing a comprehensive understanding of the coupled effects of acid rain erosion and mechanical loading on cement-amended loess. These insights are pivotal for designing resilient engineering solutions in environments prone to acid rain erosion.

Expansive soil is a special soil type that undergoes volume expansion during hygroscopicity and volume contraction during dehumidification. In this study, the effects of rainfall-evaporation cycles on the microscopic pores and cracks of expansive soils under different rainfall intensities were analyzed by simulating light rainfall, medium rainfall, and high-temperature drought environments using nuclear magnetic resonance (NMR) technology and image processing methods. The results showed that the micropores and small medium pores of the expanded soil gradually evolved into macropores during the cycling process, especially under stronger rainfall conditions. In addition, as the number of cycles increased, the expanded soil showed irrecoverable pore changes, which ultimately led to the scattering damage of the soil. By processing the surface crack images of expansive soils, the process of crack development was categorized into four stages, and it was found that the evaporation cycle of medium rainfall intensity caused the main cracks of expansive soils to develop more rapidly. A quantitative relationship model between the average crack width and the number of cycles as well as porosity was constructed, and the regression coefficient of determination R2 reached 0.98, 0.96, and 0.84, respectively. This study simulates the effects of real rainfall conditions on expansive soils and investigates the mechanism and evolution of cracks in expansive soils, which is of great theoretical and practical significance.

Cemented soils in coastal harbors are susceptible to adverse factors such as seawater corrosion and cyclic dynamic loading, which may consequently reduce their stability and durability. In recent years, Nano-SiO2(NS) has been widely used to enhance the mechanical properties of cemented soil. However, this enhancement may potentially lead to a reduction in ductility. Conversely, polypropylene fibers (PP) have attracted widespread attention for their potential to enhance the ductility of cemented soils, but their ability to improve the strength of cemented soils is limited. To address these issues, this study focused on utilizing five different nano-dosages combined with four different fiber dosages to enhance cemented soils. These enhanced soils were then subjected to curing periods of 7, 28, and 60 days in seawater environments. The study employed various tests including unconfined compressive strength tests (UCS), uniaxial cyclic loading tests, scanning electron microscopy tests (SEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) to investigate the potential impacts of these additives on durability, strength, corrosion resistance, and microstructure evolution. The results of the study indicate that seawater corrosion and cyclic loading contribute to a reduction in the stability of cemented soils. However, the addition of NS and PP effectively enhances the compressive strength and durability of these soils. The optimal combination ratio is achieved when the dosages of NS and PP are 3.6 % and 0.8 %, respectively. In this case, the growth rate of unconfined compressive strength of cemented soils surpasses the sum of each individual dosage, increasing by 137.7 %, 245.6 %, and 235.3 % after 7, 28, and 60 days of curing, respectively. Furthermore, the growth rate of PP on the compressive strength of cemented soils remains largely unaffected by seawater corrosion. The optimal composite dosage of cemented soils effectively mitigates the increase in porosity caused by seawater corrosion. C-S-H enhances the mechanical interlocking between hydration products and PP by encapsulating PP, reducing energy transfer losses in cemented soils, and increasing their dynamic modulus. The volcanic ash reaction and nucleation effect of NS further enhance this effect, and their combined use significantly improves the seawater corrosion resistance of cemented soils.

The decontamination of polluted soils is a major socioeconomic issue in many industrialized countries. In situ remediation approaches are nowadays preferred to ex situ techniques, but they require among others the use of bioindicators, which are sensitive to the progressive depollution on health effects. Animal species have been mainly used so far to monitor aquatic and air pollution. Current research focuses on the development of living indicators of soil pollution. In this study, the garden snail Helix aspersa maxima was acutely exposed to cadmium, one major soil contaminant causing severe health effects, including nephrotoxicity. Kidney and hemolymph were sampled and analyzed by a H-1-NMR-based metabonomic approach. Shortly after Cd exposure, numerous metabolic changes occurred in the hemolymph and kidney extracts. Altogether, they were indicative of a switch in energy sources from the Krebs cycle towards b-oxidation and the utilization of stored galactogen polysaccharides. Then, the activation of antioxidant defenses in the renal cells was suggested by the alteration in some precursors of glutathione synthesis, such as glutamate, and by the release of the antioxidant anserin. Cell membrane damage was evidenced by the increased levels of some osmolytes, betaine and putrescine, as well as by a membrane repair mechanism involving choline. Finally, the development of metabolic acidosis was suggested by the elevation in 3-HMG in the hemolymph, and the more pronounced lysine levels were consistent with acute excretion troubles. Cd-induced renal damage was objectified by the increased level of riboflavin, a recognized biomarker of nephrotoxicity.

This study focuses on the use of alkali-activated materials and geopolymer grouts in deep soilmixing. Three types of grouts, incorporating metakaolin and/or slag and activated with sodium silicate solution, were characterized at different scales to understand the development of their local structure and macroscopic properties. The performance of the soilmix was assessed by using combinations of the grouts and model soils with different clay contents. Feret's approach was used to understand the development of compressive strength at different water-to-solid ratios ranging from 0.65 to 1. The results suggested that incorporating calcium reduced the water sensitivity of the materials, which is crucial in soilmixing. Adding soils to grouts resulted in improved mechanical properties, due to the influence of the granular skeleton. Based on strength results, binary soilmix mixtures containing 75% of metakaolin and 25% of slag, with H2O/Na2O ratios ranging from 28 to 42 demonstrated potential use for soilmixing due to the synergistic reactivity of metakaolin and slag. The optimization of compositions is necessary for achieving the desired properties of soil mixtures with higher H2O/Na2O ratios.

Generally, artificial ground freezing (AGF) technology is utilized to guarantee tunnel safety during construction. However, the soil structure changes significantly after freeze-thaw, resulting in uneven deformation of the tunnel under traffic loading from subway vibration. To solve this problem effectively, it is necessary to consider the combined impact of freeze-thaw, salt, and traffic loading damage that marine soft soil must withstand simultaneously. For this reason, cyclic triaxial test and NMR test were performed on the silty clay saturated with NaCl solution in this study. The influence of three main factors on dynamic properties has been thoroughly investigated, namely freeze-thaw, salt content, and confining pressure. According to cyclic triaxial test, the shape of the hysteresis loop of the specimens after freeze-thaw changed more significantly with increasing loading cycles. The dynamic elastic modulus was weakened by freeze-thaw, while improved by the addition of NaCl. Damping ratio was consistent with the dynamic elastic modulus law. It was worth noting that the different freezing temperatures (-10 degrees C, -20 degrees C and - 30 degrees C) had only a slight impact on dynamic elastic modulus, as well as damping ratio. Mathematical models were proposed to forecast the dynamic elastic modulus and damping ratio regarding marine soft clay. NMR test indicated that the addition of salt made the internal pore environment of the specimens tend to be consistent and enhanced the water-solid interaction. The increase in porosity resulted in the decrease in dynamic elastic modulus. The results have provided valuable insights into the mechanical characteristics of marine soft clay when AGF technology is applied.