Determination method of permafrost table in seasonal frozen soil areas under "Water-Heat-Salt" coupling

The permafrost table is an important index for the design and construction of roads in cold regions, so it is necessary to find a convenient, accurate and fast judgment method to determine the permafrost table. In this study, a three-field coupled model was established based on the hydrothermal salt coupling within the permafrost and the similarity theory, and the changes of the permafrost table under different temperature, moisture and salt conditions were numerically simulated by considering the transient temperature change and the influence of the permafrost layer on the seasonally thawed layer. In addition, an accelerated permafrost table test method was designed based on the time-domain variation and hydrothermal salt coupling by the similarity theory, which rapidly simulated the permafrost table change under different temperatures, moisture, and salts in the natural environment. Comparing the simulation and test results with the measured values in the field, the errors are less than 3%, which verified the feasibility of the method for determining the permafrost table, and the simulated results are better than the test results. Results show that the results of determining the permafrost table with a single index have different degrees of deviation, and the permafrost table obtained by the temperature index is the most accurate in general, and it is more accurate to use the average value of the three indexes as the permafrost table compared with a single index.