Experimental Study of the Shear Strength of Petroleum-Contaminated Soil under Freeze–Thaw Conditions

The development and exploitation of petroleum resources in cold regions have exacerbated the risk of petroleum spills, which can adversely affect the safety of infrastructure, particularly under freeze–thaw cycles. This study investigates the impact of freeze–thaw cycles on the shear strength of petroleum-contaminated soil. Direct shear tests are conducted on soil samples with varying water content, petroleum contamination levels, and freeze–thaw cycles. The results reveal that an increase in water content leads to a reduction in the shear strength of petroleum-contaminated soil. Furthermore, higher petroleum content fills the pore spaces of the soil, weakening its shear strength by diminishing both cohesion and internal friction. The freeze–thaw cycle further decreases soil shear strength, with a more pronounced effect at higher petroleum contents. Specifically, the shear strength undergoes a sharp decline after the first freeze–thaw cycle and stabilizes with subsequent cycles. Scanning electron microscopy observations postshear testing show that petroleum reduces interparticle cohesion, while freeze–thaw cycling induces particle size homogenization, both of which contribute to the decrease in shear strength. A Mohr–Coulomb-based shear strength model, incorporating water content, petroleum contamination, and freeze–thaw cycles, is developed and validated. The model demonstrates strong predictive accuracy, with a high correlation coefficient (R² = 0.968) between predicted and measured shear strength values, providing valuable insights for assessing the safety of structures in petroleum-contaminated sites in cold regions.