Experimental investigation of the mechanical properties of hydrophobic polymer-modified soil subjected to freeze–thaw cycles

Freeze–thaw action, as a strong weathering process, could severely affect the engineering properties of soil in cold regions. Repeated freeze–thaw cycles could deteriorate the structure and mechanical properties of soil, thus changing its engineering properties when the water content changes slightly. To overcome these problems, an eco-friendly hydrophobic polymer was utilized to improve a naturally occurring saline soil in a seasonally frozen region. Both treated samples and untreated samples were prepared at five initial water contents and subjected to seven freeze–thaw cycles (0, 1, 5, 10, 30, 60, and 120) with an original salt content. The results showed that the unconfined compressive strength and deformation modulus of the treated soil increased significantly, with enhanced interparticle linkages and a denser structure. Unlike traditional soil additives, the modification mechanism consisted of physical binding, i.e., electrostatic interactions between the soil and polymer charges. However, successive freeze–thaw cycles degraded the strength and toughness of the treated soil, especially after 10 cycles. From a microscopic view, freeze–thaw action resulted in particle breakage, expanded soil porosity, and cracks that formed between the agglomerated particles, which reduced the soil structural integrity and lowered the strength. This might attribute to both the expansion and migration of pore water. Nevertheless, the hydrophobic property of the polymer-bond agglomerates mitigated the effects of unfrozen water migration and water redistribution, improving the frost resistance of treated soil after successive freeze–thaw cycles. Hence, it has certain potential to use hydrophobic polymers as a soil additive for application in seasonally frozen regions.