Understanding the mechanical properties of solidified oil-contaminated soil and the evolution of contaminant migration and diffusion under environmental effects is a key prerequisite for promoting the reuse of contaminated soil projects. Lime and fly ash with a low-carbon concept were selected as solidification materials. Combined with a leaching test and COMSOL Multiphysics software, the control of the solidification effect on oil migration under leaching environment was evaluated macroscopically by the oil content after leaching, and the mechanical stability under leaching was evaluated by soil strength and deformation. The results indicate that the solidification of lime-fly ash has the potential to address the issue of contaminant migration and diffusion, particularly in cases where a significant number of contaminants are migrating under the leaching effect of contaminated soil, and to achieve effective control of the migration of oil contaminants in soil. The oil content at each interface of solidified contaminated soil under the action of leaching is always close to the initial oil content setting state, and the migration rate is only 1.35%-2.76%. The variation range of the mechanical parameters of the solidified contaminated soil under leaching is positively correlated with confining pressure and contaminant concentration, but only fluctuates within 10 s of the initial stress and then the strength value is stable at 5.77×10⁴-6.07×10⁴ N/m², and the maximum fluctuation value of displacement is 1.73×10^-3-6.46×10^-2 mm. The mechanical stability of solidified contaminated soil is good, and the safety factor F_s is more than 10. Lime-fly ash solidified oil-contaminated soil can take into account both environmental and engineering requirements, and exhibits the potential for engineering reuse.

The permeability of treated contaminated soil is an important factor to consider when reusing polluted soil in engineering projects. In this study, lime and fly ash were chosen as solidification materials due to their ability to both adsorb and solidify contaminants. The permeability coefficients of petroleum-contaminated soil before and after solidification, as well as the residual petroleum content within the soil, was investigated under varying parameters such as confining pressure, osmotic pressure and contamination intensity. X-ray diffraction and scanning electron microscopy were used to analyze the evolution of permeability and the migration and diffusion patterns of pollutants, providing insights into the engineering reutilization potential of solidified petroleum-contaminated soil. The results showed that the adsorption effect of the solidified product on petroleum molecules weakened the hydrophobicity of the petroleum, increasing the effective permeation pathways in the soil. The permeability coefficient of solidified petroleum-contaminated soil was two orders of magnitude higher compared to unsolidified soil. Both solidified and unsolidified petroleum-contaminated soil exhibited decreased permeability due to the enhanced adsorption and interception capacity of the soil matrix for petroleum, as well as the elevated confining pressure, osmotic pressure, and contamination level, which intensified the interception among soil particles. The solidification process effectively controlled the migration and diffusion of petroleum contaminants under permeation conditions. The residual petroleum content in various locations closely approximated the initial content, reducing the risk of pollution through permeation. Considering the mechanical properties (compressive strength of 1280.1 kPa, shear strength of 388.88 kPa), permeability (ranging from 4.28×10⁻⁶ cm/s to 7.39×10⁻⁶ cm/s), and migration control characteristics (fluctuation rate from 0.3% to 4.9%) of lime and fly ash, it can be concluded that lime and fly ash solidified petroleum-contaminated soil can be reused in the construction of subgrade materials that require impermeability.

The repeated freeze-thaw cycles with seasonal alternations have an obvious effect on soil structure. To reduce the temperature sensitivity of saline soil and then use it in engineering, a combined treatment method is proposed, where lime, fly ash and modified polyvinyl alcohol (MPA) are used as solidified materials. Unconfined compressive strength (UCS) tests and microstructure characterization are firstly used to evaluate the solidified effect and obtain the parameters range of solidified materials. Then, the shear strength tests for determining cohesion and internal friction angle are conducted. Experiments are conducted by considering separate and combined treatments of materials mentioned above. The results indicate that the combined treatment with lime, fly ash and MPA can improve the strength of saline soil. After combined treatment, the UCS is 1130.25 kPa, which is 5.18 times than that of saline soil (218 kPa). The strength of combined solidified saline soil meets the requirements of engineering specification (JTG 3430-2020). The stable value of UCS of combined solidified saline soil is 700 kPa under freeze-thaw cycles. The fluctuation is about 5% after three freeze-thaw cycles. The cohesion and the internal friction angle of combined solidified saline under the optimal proportion can be 208.2 kPa and 38.56°, respectively after three freeze-thaw cycles. The sensitivity of the factors is in an order of decreasing importance: curing time, lime content, MPA content, dry density, salt content, and freeze-thaw cycles. With an increase of lime, fly ash and MPA content, the strength of combined solidified saline soil increases and then tends to become stable. The optimization of solidification parameters can effectively weaken the influence of freeze-thaw on coastal saline soil. Based on tests results of compressive strength and shear strength, it can be concluded that the optimal combination of solidified parameters is 14% of lime, 30% of fly ash, 1% of MPA, 28 days of curing time, and a dry density of 1.65 g/cm³.