Experimental Study on Microstructural and Mechanical Characteristics of Expansive Soil under Freeze–Thaw Actions

Expansive soil, widely recognized as problematic, have caused numerous engineering disasters in cold regions during freeze–thaw cycles (FTCs) due to their swelling and shrinkage properties and susceptibility. Various modifiers have been used to mitigate these adverse features to enhance the stability of expansive soil. In this study, octadecylamine (ODA) was used as a modifier to investigate its effects on the mechanical properties and microstructural evolution of expansive soil given FTCs. Extensive tests were conducted to evaluate the expansion rate, mechanical strength, and microstructural changes of both unmodified and ODA-modified expansive soil. The results showed that ODA effectively reduces both the free expansion rate and expansion rate without loading. At ODA contents of 0.2% and 0.5%, the shear strength, cohesion, and friction angle of expansive soil all exhibit significant increases, persisting even during FTCs. The micropores of ODA modified expansive soil gradually decrease, whereas the macropores gradually increase given FTCs. The shape factor reaches a maximum value of 0.638 at a content of 0.5% after seven FTCs. Considering the evolution of shear strength and microstructure given FTCs and the reduction in the expansion rate for the ODA-modified expansive soil, the optimum ODA content is 0.5%. This study provides valuable scientific evidence and guidance for the engineering design of expansive soil and disaster prevention in cold regions.