Strength degradation law and nonlinear model of remolded loess under dry-wet freeze-thaw cycles condition

A large area of collapsible loess area is part of a water conveyance open channel project in Xinjiang. Channel slope sliding failure often occurs due to intermittent water supply and seasonal temperature changes. To further study the deformation and failure mechanisms of canal slopes, triaxial tests and scanning electron microscope tests were conducted on loess under dry-wet freeze-thaw cycles to examine its deformation law and microscopic mechanism. Finally, the Duncan-Chang model was improved based on the deformation law of loess. Triaxial test results show that: With the increase of the number of dry-wet freeze-thaw cycles, the stress-strain curve of loess decreases, while the volume change curve increases, with diminishing change ranges. After more than 5 times, the stress-strain and volume change curves stabilize, and the cohesion and internal friction angle show an exponential attenuation trend. The cohesion is significantly affected by the dry-wet freeze-thaw cycles. Scanning electron microscopy test results show that: The microscopic analysis was conducted using scanning electron microscopy. Under dry-wet freeze-thaw cycles, large particles and aggregates split, the cementing material between particles decreased, the surface contact increased, the proportion of large pore areas decreased, while the proportion of small and medium pore areas, three-dimensional porosity and pore roundness increased, and the fractal dimension decreased. Three-dimensional porosity and fractal dimension are significantly correlated with cohesion, while the proportion of large pore area is significantly correlated with internal friction angle. The original Duncan-Chang model fails to reasonably reflect the softening phenomenon of soil. To address this, a quadratic function is used to modify the Duncan-Chang model, and Poisson's ratio is fitted using a formula, resulting in an improved Duncan-Chang model. The improved model effectively fits the triaxial test results of loess under different cycles. As the number of cycles increases, the stress-strain model parameters k and q show an exponential decrease, while the stress-strain model parameter o shows an exponential increase. Similarly, the volume change model parameters j and l show an exponential increase, whereas the volume change model parameter t shows an exponential decrease.