Effects of Freeze–Thaw Cycles and Water Content on the Mechanical Properties of Artificially Prepared Structured Loess

Loess is a type of structured soil widely distributed in seasonal frozen regions. Its mechanical properties are significantly influenced by the number of freeze–thaw cycles (N) and water content (ω), often leading to engineering hazards such as slope instability and foundation settlements. To overcome the challenges associated with undisturbed loess with unstable structures, this study proposes a method for artificially preparing structured loess with a mass ratio of loess:kaolin:urea:gypsum powder of 70:20:7:3. A series of laboratory tests, including freeze–thaw cycling, consolidation, unconsolidated–undrained triaxial shear, and scanning electron microscopy, are systematically conducted. The results indicate that water content is the primary factor controlling the strength of loess. The increase of water content significantly reduces the peak strength and cohesion of the structured soil, shifting the stress–axial strain curve from a strain-softening to a strain-hardening pattern. Microstructural analysis reveals that freeze–thaw action disrupts soil aggregates, leading to a transformation from a dual-porosity structure to a single-porosity structure. Based on the experimental data, a modified Duncan–Chang model incorporating confining pressure, water content, and the number of freeze–thaw cycles is established. Validation confirms that the model accurately reflects the stress–axial strain behavior of the structured loess.