Insights into the thermal insulation capability of a new polyurethane polymer subgrade material: An in-situ field test on the Qinghai–Tibet highway

The frost heave and thaw settlement of subgrades in permafrost regions severely threaten highway safety during repeated freeze–thaw cycles. However, these freeze–thaw cycles are difficult to overcome because of the lack of thermal insulation materials (or structures). To fill this gap, we developed a polyurethane (PU) polymer material with low thermal conductivity and designed a double–layer PU–grouted structure. The water–vapor–thermal coupling governing equations of the grouted subgrade were established. A numerical simulation was conducted using the finite element software COMSOL Multiphysics. Ten-month in-situ tests were conducted on the Qinghai–Tibet Highway from November 2021 to August 2022 to investigate the thermal insulation properties of the grouted subgrade. The results indicated that the PU exhibited excellent thermal insulation performance and considerably reduced the disturbance depth of the air temperature. Freeze and thaw depths were reduced by 0.65 and 1.65 m, respectively. During the freeze period, the surface and deep layers of the subgrade showed a temperature difference of 6 °C due to the upper PU layer, preventing 32.4 % of the liquid water from being frozen. The upper PU layer caused a temperature difference of up to 7 °C between the surface and the deep layers of the subgrade during the thawing period. Water evaporation was prevented by the upper PU layer, and water gathered between the grouted layers, which was defined as the "pot-cover effect." The pot cover effect significantly reduced the soil temperature and maintained a stable temperature in the deeper subgrade during thawing.