Evaluation of the Long-Term Thermal–Mechanical Stability of an Embankment Replaced with Crushed Rocks in Cold Regions

Conventional measures of protecting permafrost cannot improve embankment stability in warm permafrost regions. Therefore, based on the principle of allowing permafrost to thaw, a method of replacing the 4.5 m underlying permafrost layer with 200–400 mm diameter crushed rocks was proposed to reduce embankment settlement. To evaluate the long-term stability of the embankment in question, a hydro-thermomechanical coupling model considering condensation is established for unsaturated frozen soil; the water, heat, and deformation conditions of the embankment in 20 service years are calculated; and its working mechanism is analyzed. In addition, the optimal replacement depth of the crushed rocks is discussed from thermodynamic and economic perspectives. The results show that: (1) an increase in replacement depth can increase the permafrost table under the embankment centerline, thus improving the thermal stability of the embankment; (2) the increase in replacement depth can reduce the unfrozen water content from the deep foundation to the embankment filling layer, thus reducing the cumulative settlement; (3) if only the embankment stability is considered, the embankment stability is better with the greater replacement depth. If both stability and economy are considered, a replacement depth of 4.0 m is the optimal solution. The maximum settlement, maximum horizontal deformation, maximum uneven settlement, and maximum horizontal deformation difference of this embankment are −0.693, −0.241, 0.306, and −0.358 cm. This study provides a reference for the settlement control of embankments and the optimal design of crushed-rock embankments in warm permafrost regions.