Adding biochar to soil can improve the soil’s physical–chemical properties, microscopic pore structure, and bacterial habitat. This affects the soil’s strength characteristics and the oxidization of methane. Using a Humboldt pneumatic direct shear instrument, this study investigated the effect of the amount of biochar in the soil, the soil’s methane-oxidizing bacteria, aeration time, and carbon content on the strength characteristics of a biochar-amended clay. The results show that when the biochar content is low, the soil’s stress–strain curve shows a strain hardening state as the strain increases. When the biochar content is greater than 10%, the methane-oxidizing bacteria increase as the shear strain increases. The stress–strain curves of the biochar–clay mixture all showed a softened state. Under the same biochar content, the soil’s stress–strain curves show strain softening as the methane filling time increases. However, with an increase in the amount of biochar, cohesion gradually increased and the internal friction angle did not change significantly. A scanning electron microscope (SEM) image of the biochar–clay mixture with methane oxidizing bacteria revealed the influence of the evolution law of the samples’ micropore structure on the soil’s stress–strain curve and strength properties.

Water seepage in saline soils can compromise the stability of landfill cover layers and cause ecological pollution. Biochar, recognized as a novel sustainable material for soil improvement, has garnered significant attention due to its large specific surface area and complex pore structure. Its addition can alter the physico-chemical properties of soil, including pore structure characteristics, compaction behavior, and liquid seepage paths. This study therefore focuses on saline soil, investigating the effects of biochar type and addition rate on the soil’s hydraulic conductivity through variable-head permeability tests. The correlation between changes in microscopic structural characteristics and hydraulic conductivity was analyzed using SEM and MIP techniques. The results indicate the following: (1) Both corn straw and wood biochar affect the hydraulic conductivity of the modified soil, with the degree of influence varying based on the presence and dosage of biochar, generally leading to a reduction. (2) The corn straw biochar-modified soil exhibited a more significant decrease in hydraulic conductivity—by approximately one order of magnitude—compared to wood biochar, reaching a minimum value of 1.04 × 10⁻⁶ cm/s at a 10% addition rate. (3) Analysis from SEM and MIP tests reveals that incorporating biochar alters the original pore structure of the soil, causing macropores to transition towards mesopores and micropores, and resulting in the narrowing or clogging of pore channels, which ultimately reduces the hydraulic conductivity. The finding that biochar amendment decreases hydraulic conductivity provides valuable data to support the design of landfill cover layer schemes.

The durability deterioration of concrete under the coupled action of freeze–thaw cycles and corrosive media is a critical technical challenge for engineering structures in cold regions and salt corrosive environments. This paper systematically reviews the research progress on the mechanical properties of concrete subjected to coupled freeze–thaw and corrosion effects. Starting from the mechanisms of freeze–thaw damage and corrosion damage, it analyzes the material degradation laws under individual factors and the synergistic failure mechanism of the coupled freeze–thaw–corrosion condition. The coupling effect is revealed: freeze–thaw-induced microcracks accelerate the penetration of corrosive media, while the expansion of corrosion products in turn aggravates freeze–thaw damage. Building on this, from the perspective of factors influencing concrete failure, this paper systematically summarizes the key factors governing concrete damage under single-factor and coupled-factor conditions as well as their nonlinear response characteristics. The review indicates that the damage degree under the coupled action is far greater than the simple superposition of damage caused by individual factors and presents complex patterns, including the concentration threshold effect, the time-sequence effect, and sensitivity to a low water–cement ratio. Existing reviews predominantly focus on qualitative descriptions of single-factor deterioration mechanisms, while systematic comparative analyses of threshold behaviors under multi-factor coupling and quantitative consolidation of mechanical degradation metrics remain limited. Furthermore, targeted durability design guidance tailored to cold saline environments is rarely summarized in the prior literature, which motivates the present comprehensive review. Although existing studies are relatively well-established for single damage mechanisms, further efforts are still needed to deepen the understanding of multi-factor interaction thresholds and dynamic evolution processes. The findings of this review can provide theoretical support and engineering reference for the durability design and service life prediction of concrete structures in cold regions and salt corrosive environments. The summarized threshold laws and quantitative mechanical degradation data can provide targeted parameter guidance for the durability design of hydraulic structures, bridge substructures, and port engineering in northwest saline soil, northern severe cold, and eastern coastal salt fog areas.