Unsaturated Soil Behavior Under Repeated Traffic Loading in Arid Climates: A Scenario-Based Numerical Study of Road Subgrade Performance Under Assumed Climate-Analogue Suction Pathways

Road subgrade response in arid regions is governed by the unsaturated state of the soil, yet pavement design frameworks represent moisture mainly through empirical, largely equilibrium environmental adjustments rather than through the explicit evolution of matric suction during wetting and drying. Climate influence is represented by a two-suction concept that separates the annual-average drying trend from the critical post-rainfall wetted state. The suction pathways are not derived from Coupled Model Intercomparison Project Phase 6 (CMIP6) output: their end-of-life values (140 and 55 kPa) are prescribed, and they are labelled Shared Socioeconomic Pathway (SSP) SSP2-4.5 and SSP5-8.5 analogues only because their direction and ordering are qualitatively consistent with the projections of the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) for arid regions. They are treated throughout as assumed scenario parameters. All soil, traffic and deformation parameters are representative literature-based values; no measured retention, resilient modulus or rutting data are used, so every result is a conditional model output and not a validated prediction. Under the assumed high-emission analogue the computed year-20 subgrade rut depth is 22.0 mm (parameter-perturbation range 12-31 mm), reaching the 20 mm rut-depth threshold adopted here between years 19 and 20, against 6.5 mm for the constant-climate baseline and 11.1 mm for the moderate analogue. A one-at-a-time sensitivity screening ranks the assumed pathway shift (+15.5 mm) first, followed by design suction (+4.6/-1.8 mm), the initial-strain (ε0) scaling and the exponent β, with traffic growth least influential.