Experimental Study on Water and Salt Transport Law of Soil Under Complex Freeze-Thaw Cycle Conditions

Abstract:

To study the water-salt migration patterns in soil, using sandy loess as the research object, we conducted freeze-thaw, freeze-thaw-evaporation, and freeze-thaw-evaporation-rainfall three types of cycle tests under the conditions of unpressurized supplementation of 0.8% and 1.5% concentration sodium sulfate solution. We explored the water-salt migration patterns of the soil during freeze-thaw, evapo-ration, and rainfall processes, and analyzed the changes in temperature, moisture, and conductivity of the test soil column with respect to the working conditions. The results show that there is a lag phenomenon in the temperature changes of the soil during freeze-thaw cycles, and the amplitude of temperature changes decreases with the increase of soil depth. In all three working conditions, the moisture content and conductivity of the soil show periodic changes. After the freeze-thaw cycle, the water and salt contents of the soil increase. Under evaporation conditions, the salt is transported upward with water and causes oversaturation precipitation at the top of the soil column. Under rainfall conditions, the salt in the upper part of the soil column moves downward with water. Under the same working conditions, a higher concentration of the replenishing solution is more conducive to the accumulation of salt in the upper soil layer. When the unpressurized replenishing solution is 0.8% concentration, the increase in conductivity of the upper part of the soil column in the three working conditions is 0.12, 0.31, and 0.45 dS/m respectively. When the unpressurized replenishing solution is 1.5% concentration, the increase in conductivity of the upper part of the soil column in the three working conditions increases successively to 0.27, 0.36, and 0.52 dS/m. The changes in soil salt content are influenced by the combined effects of temperature, concentration gradient, and action time. Under the freeze-thaw-evaporation-rainfall cycle, the increase in conductivity of the upper soil layer is the greatest.