Numerical study on thermal regimes beneath canal in permafrost zones with different geological conditions

In permafrost regions, construction and operation of hydraulic projects will exert considerable thermal impact on underlying permafrost, and its degree will be different at locations with different permafrost geological conditions. Taking a canal in a high altitude permafrost zone as an example, a water-thermal coupled mathematical model for freeze-thaw soils was established in this study. The water migration, ice-water phase change and nonlinear relationship between unfrozen water content and temperature in freezing soils were considered in the model. Using this model, the long-term evolution of the thermal conditions of permafrost subgrade under the canal in the context of climate warming was numerically investigated, and the influences of the mean annual ground temperature (MAGT) and ice content (i_v) of the permafrost subgrade were considered. The results show that when the permafrost is ice-poor (i_v < 10%), both the vertical and lateral thermal erosions of the canal are significant. After 50 years of the excavation, there is no permafrost under the mid-bottom and bank slope of the canal. When the MAGT is −0.5 ℃, the permafrost under the canal has degraded within about 10 m from the bank slope outward. When the MAGTs are −1.0 or −1.5 ℃, there is still permafrost under the bank slope of the canal. With the increase in i_v, the thermal inertia of permafrost increases significantly. When the i_v increases from ice-poor to ice-rich (20% < i_v ≤ 30%), there is still permafrost existing under the canal in 50^th year after the excavation even when the MAGT is −0.5 ℃. However, a thawed layer shaped as a pot-bottom develops beneath the canal. Under the impacts of climate warming and water thermal erosion, the permafrost beneath the canal experiences significant downward degradation, or quick descend of the permafrost table. Only under the impacts of climate warming, the permafrost beneath the slope of canal and the natural ground surface experiences a slow descent of the permafrost table and slight warming of the top permafrost layer.