Seasonally frozen soil where uneven freeze-thaw damage is a major cause of highway deterioration has attracted increased attention in China with the rapid development of infrastructure projects. Based on Darcy's law of unsaturated soil seepage and heat conduction, the thermal-hydraulic-mechanical (THM) coupling model is established considering a variety of effects (i.e., ice-water phase transition, convective heat transfer, and ice blocking effect), and then the numerical solution of thermal-hydraulic fields of subgrade can be obtained. Then, a new concept, namely degree of freeze-thaw damage, is proposed by using the standard deviation of the ice content of subgrade during the annual freeze-thaw cycle. To analyze the freeze-thaw characteristics of highway subgrade, the model is applied in the monitored section of the Golmud to Nagqu portion of China National Highway G109. The results show that: (1) The hydrothermal field of subgrade has an obvious sunny-shady slopes effect, and its transverse distribution is not symmetrical; (2) the freeze-thaw damage area of subgrade obviously decreased under the insulation board measure; (3) under the combined anti-frost measures, the maximum frost heave amount of subgrade is significantly reduced. This study will provide references for the design of highway subgrades in seasonally frozen soil areas.
Keywords: COMSOL multi-physics; freeze–thaw damage degree; frost heave; highway subgrade; seasonally frozen soil; thermal–hydraulic–mechanical coupling.