Soil chemical washing has the disadvantages of long reaction time, slow reaction rate and unstable effect. Thus, there is an urgent need to find a cost-effective and widely applicable alternative power to facilitate the migration of washing solutions in the soil, so as to achieve efficient removal of heavy metals, reduce the risk of soil compaction, and mitigate the damage of soil structure. Therefore, the study used a combination of freeze-thaw cycle (FTC) and chemical washing to obtain three-dimensional images of soil pore structure using micro-X-ray microtomography, and applied image analysis techniques to study the effects of freeze-thaw washing on the characteristics of different pore structures of the soil, and then revealed the effects of pore structure on the removal of heavy metals. The results showed that the soil pore structure of the freeze-thaw washing treatment (FT) became more porous and complex, which increased the soil imaged porosity (TIP), pore number (TNP), porosity of macropores and irregular pores, permeability, and heavy metal removal rate. Macroporosity, fractal dimension, and TNP were the main factors contributing to the increase in TIP between treatments. The porous structure resulted in larger effective pore diameters, which contain a greater number of branching pathways and pore networks, allowing the chemical washing solutions to fully contact the soil, increasing the roughness of the soil particle surface, mitigating the risk of soil compaction, and decreasing the contamination of heavy metals. The results of this study contribute to provide new insights into the management of heavy metal pollution in agricultural soils.

Embankments and foundation geotechnical structures are frequently subjected to long-term cyclic loading due to traffic during their service life. Excessive cumulative deformation can lead to pavement cracking and uneven settlement of the subgrade. This study conducts a series of dynamic triaxial tests to analyze the effects of the number of cycles (N), effective confining pressure (sigma(c)), and dynamic stress amplitude (sigma(d)) on the axial cumulative strain (epsilon(d)) characteristics of solidified mud samples. Additionally, it investigates the evolution model of epsilon(d) of solidified mud and establishes a predictive model for this strain. In conjunction with the NMR tests, this research further investigates the effects of sigma(c) and sigma(d) on the pore distribution of solidified mud after loading. Ultimately, the correlation between microscopic pore structure indicators and epsilon(d) is elucidated. The results indicate that the epsilon(d) behavior of solidified mud under cyclic loading exhibits characteristics of plastic shakedown. Furthermore, the exponential hyperbolic function model more accurately characterizes the relationship between epsilon(d) of the samples and N. Before and after cyclic loading, the micropores of the samples accounted for over 95 % of the total pore volume, predominantly concentrated in the radius range of r < 0.3 mu m. A correlation exists between the average pore size of the sample and epsilon(d), which is primarily influenced by sigma(d) and sigma(c).

Xanthan gum biopolymers have gained increasing attention in geotechnical engineering due to the effectiveness and environmental-friendliness, and are proposed as a potential alternative to conventional materials for soil stabilization. Cyclic wetting and drying are the crucial factors that affect the behavior of surface soil, which are also a major challenge for biopolymer applications. This study aims to investigate the strength durability of xanthan gum-treated soil during wetting-drying cycles. The soil was treated with different contents of xanthan gum (0, 0.5, 1.5% by the mass of dry soil) and a total of 12 wetting-drying cycles were applied. Unconfined compression tests were performed to evaluate the changes in soil mechanical properties. The changes in microstructure were observed using nuclear magnetic resonance technology and scanning electrical microscopy. The results showed that soil mechanical properties decreased significantly in the first four cycles, and then tended to equilibrium. The compressive strength of soil treated with 1.5% xanthan gum could be approximately twice than that of non-treated soil after 12 cycles, and its strength reduction caused by wetting-drying cycling is about 20% less than that of the latter. When increasing the water content at drying stage, specimens subjected to wetting-drying cycles with less moisture change presented higher compressive strength, in which case the effectiveness of biopolymer treatment can be maximally retained. Xanthan gum treatment conferred great resistance to wetting-drying cycling due to its cementation and aggregation effects. The presence of xanthan gum leads to more inter-aggregate pores with a radius of about 0.1-1 mu m and limits the development of macropores. The strengthening effect of xanthan gum depends on direct clay particle-biopolymer interactions and inter-particle connection formed by xanthan gum matrix. From the results, xanthan gum biopolymers can significantly improve the mechanical properties of soil at shallow depth even after wetting-drying cycles.

The strength deterioration of soil-rock mixtures (SRM) subjected to freeze-thaw (F-T) cycles leads to instability and failure of upper engineering structures in cold regions. However, the mutual feedback response mechanism pertaining to the changes of pore and strength in SRM under F-T cycles are rarely addressed. Nuclear magnetic resonance and triaxial tests were carried out to study the pore structure characteristics and strength response patterns of samples. A correlation model of SRM porosity and strength deterioration was first proposed under F-T cycles, and the model rationality was verified by test data. The results demonstrated that the pore connectivity and porosity increased throughout the F-T process, with the T2 spectral distribution curves exhibiting three peaks. Among these peaks, the main peaks underwent slight changes, while the secondary and micro peaks presented significant changes. Before 3 F-T cycles, the pore distribution evolved to small pores uniformly, followed with the large pores increasing and the micropores disappearing. With increasing of F-T times, the strength and cohesion of SRM experienced a drastic decline, while the internal friction angle demonstrated a slight decrease accompanied by fluctuations. Based on the analysis of test results, a correlation model regarding the porosity and strength deterioration was proposed through the relationship between the micro-structure evolution and the macro-mechanical response during F-T cycles. Furthermore, intrinsic mechanism of SRM strength deterioration under F-T cycles was revealed by considering the pore structure characteristics. The results can provide theoretical insights for the analysis of F-T disaster mechanism and prevention of SRM in cold regions.