Mechanical Properties and Degradation Mechanisms of Shallow Ili Loess Under Freezing and Thawing Conditions

Existing freeze–thaw cycle tests are based on freeze–thaw cycles under fully constrained conditions, but under natural conditions, the vertical deformation of soil during freeze–thaw cycles is much greater than the lateral deformation. Therefore, to better simulate the freeze–thaw process of shallow soil under natural conditions, a method of preparing specimens is proposed. Specimens with five freeze–thaw cycle gradients, four water content gradients and three dry density gradients were prepared. This method realises the top-unconstrained role of the specimens during freeze–thaw cycles and provides space for vertical deformation. At the same time, it enables the observation of height and surface degradation. Triaxial tests of shallow Ili loess were carried out after indoor freeze–thaw cycles under fixed confining pressure (100 kPa). The results show that surface degradation and vertical deformation of Ili loess without top restraint increase gradually with the number of freeze–thaw cycles; strength decreases gradually and damage morphology changes from shear to bulging. Threshold conditions for the transition from softening to hardening in the strength curve of Ili shallow loess are proposed, as well as damage parameters related to freeze–thaw cycles, water content, and dry density. A coupled damage constitutive model applicable to shallow Ili loess has been established that takes these three factors into account.