Research on Construction Countermeasures for Freeze–Thaw Deformation of Permafrost Subgrade in Forest Regions of Northeast China

The permafrost in forest regions of Northeast China is very sensitive to the disturbance of subgrade construction, which will aggravate the degradation of the permafrost upper limit, leading to freeze–thaw deformation of the permafrost subgrade. Based on the road construction project of Highway 332 in forest regions in Northeast China, through theoretical analysis, indoor experiments, on-site monitoring, and numerical simulation methods, a thermo-hydro-mechanical coupling numerical model of the permafrost subgrade was established. A “two-step” construction countermeasure for freeze–thaw deformation of permafrost subgrade based on rubble stone subgrade structure was proposed. The study indicates that the addition of rubble stones to the subgrade structure has a significant cooling effect. The optimal thickness for filling rubble stones is 1 m. The optimal construction timing for subgrade is a two-step construction across the year. The stamping construction of the rubble stones is in November. The filling construction of rubble stones and gravel is in April and May of the next year. Based on the proposed construction countermeasure for permafrost subgrade, the settlement at the center of the subgrade surface is 12.7 mm in the 5th year, 17.6 mm in the 10th year, 21.1 mm in the 15th year, and 23.5 mm in the 20th year. The settlement deformation of the subgrade tends to stabilize, which can ensure the long-term stability and safety of road operations.