Deformation Mechanisms of Sulfate–Saline Soil Subgrades in Cold Arid Regions: Thermal Differential Responses in Pavement Structural Layers

To investigate the deformation characteristics of coarse-grained sulfate–saline soil subgrades to the thermal responses of pavement structures in cold arid environments with significant diurnal temperature variations, this study focuses on a highway located in the Tarim Basin in Northwest China. Three test sections (K-I, K-II, and K-III) were constructed, each featuring distinct configurations of pavement base layers. The designs of these sections depend on the characteristics of the pore structure and cement content. Multifactor monitoring of moisture, heat, salt, and mechanics was conducted to evaluate the deformation behavior of the subgrade under varying thermal conditions. Section K-I with the insulated low-cement-dosage skeleton-dense gradation (insulated LC-SDG) base exhibits the smallest daily temperature amplitude of 3.1°C and low thermal sensitivity. Section K-III with the pavement base of high-cement-dosage suspension-dense gradation (HC-SusDG) displays the highest thermal sensitivity, recording an annual temperature amplitude that exceeds that of Section K-II (LC-SDG) and Section K-I by 2.5%–4.0%. This intensifies the phase transition in high-sulfate subgrade soils of Section K-III, where the latent heat released during crystallization slows the temperature decline at the subgrade surface during cooling periods. In Section K-III, pronounced dissolution-crystallization cycles of moisture and salt, coupled with steep thermal gradients, result in excessive moisture-salt accumulation beneath the upper impermeable layer, which is 6.9–15.4 times higher than that in Sections K-II and K-I. Consequently, the subgrade surface experiences a peak soil pressure of 229.6 kPa, cyclic pressure amplitudes of 3.7 kPa, and salt-induced pressure increments of 3.4 kPa in Section K-III. These stresses promote the accumulation of tensile strain, and during the optimal salt heaving period (December–February), the peak reaches only 212.8 × 10−6 at the subbase of Section K-I. The thermal regulation and cement content-optimization of pavement structures are critical for mitigating the deformation of sulfate–saline soil subgrades in cold arid regions. These findings provide theoretical guidance for enhancing regional pavement design and construction techniques.