Upheaval buckling ("arch expansion") of cement-stabilized gravel (CSG) bases is a recurrent pavement distress in the saline arid regions of northwestern China; however, the failure pathway linking water-thermal-salt (WTS) ingress, microscopic deterioration, and field-scale buckling remains insufficiently understood. This study aims to clarify the mechanism of WTS-induced irreversible expansion and to establish a quantitative stability criterion that bridges laboratory expansion behavior with field buckling instability. To this end, a failure-oriented framework was developed by integrating field forensics from a highway in Southern Xinjiang, laboratory expansion tests under pure thermal cycling and WTS coupling, SEM/EDS microstructural characterization, and a plate-on-Winkler-foundation buckling model. Results show that pure thermal cycling produces only reversible deformation, whereas WTS coupling triggers a distinctive stepwise irreversible expansion. Increasing the internal sulfate content from 0% to 5% amplifies the free-expansion strain by approximately 230%, mainly due to the accelerated formation of ettringite, gypsum, and salt hydrates, which generate crystallization-induced internal stress. To bridge laboratory observations and field instability, an equivalent coupled expansion coefficient is introduced to convert WTS-induced free expansion into restrained membrane stress, and a unified stability index, eta, is proposed with temperature-gradient warping explicitly considered. The results indicate that eta increases from about 0.747 under thermal-only conditions to 5.478 under WTS coupling for the investigated case, which is consistent with the observed field upheaval. Parametric evaluation further indicates that, for the baseline mixture under the tested WTS envelope, the transition from stable to unstable occurs between 0% and 0.25% internal sulfate content, while GK gradation with low-to-medium cement dosage is more favorable for instability resistance. The novelty of this study lies in establishing a cross-scale mechanism-to-criterion framework that explicitly links WTS-driven microscopic expansive reactions to macroscopic structural buckling, thereby providing a practical quantitative tool for forensic diagnosis, mixture design, and risk screening of CSG bases in saline environments.