Abstract
Secondary salinization is a common problem in saline soil projects. In order to grasp the mechanism of water and salt migration of high-chlorine saline soil during the cooling process, the saline soils along the Qarhan-Golmud Highway in the Qinghai-Tibet Plateau were selected as test samples. Firstly, the basic physical parameter test and the soluble salt chemical experiment were carried out and obtained liquid and plastic limits, dry density, etc. Secondly, freezing temperature experiments and water-salt migration experiments under one-way cooling conditions were conducted according to the actual environmental conditions, and after the temperature gradient line of the soil sample was stable, water content and labile salt chemistry experiments were conducted to obtain the distribution of water and salt contents of soil samples. Finally, the effect of crystallization-water phase transition on water and salt migration and the effect of chloride salt on the temperature of crystallization-water phase transition were considered, and a mathematical model applicable to the water and salt migration of highly chlorinated saline soils under the effect of unidirectional cooling was established and solved with COMSOL Multiphysics software, and the correctness of the model was verified by comparing the simulation results with the experimental results. The study found that (1) during the one-way cooling process, both water and salt showed a tendency to migrate to the cold end. The MC (saline soil with medium chlorine content) with an initial water content of 16.9% and Cl− content of 3.373% was measured to reach a 17.6% water content and 3.76% Cl− content at the cold (top) end after the experiment. The HC (saline soil with high chlorine content) with an initial water content of 6.6% and Cl− content of 17.928% was measured to reach a 6.83% water content and 18.8% Cl− content after the experiment and (2) after the one-way cooling experiment of the MC, the water content at a distance of 1–2 cm from the cold end has abrupt changes, which may be caused by a small amount of crystallization—water phase transition at this location. At the same time, according to the temperature change graph during the cooling process, the phase change temperature was set to −9°C in the numerical simulation process to match the experimental results.
Highlights
Qinghai province is located in the northwest inland area of China, which is the gateway of the Qinghai-Tibet Plateau
It is verified according to the fitting equation of the freezing temperature of saline soil given by Chen et al [4]
After the test, when the water content of MC was measured, it was found that the water content at 1–2 cm from the top changed significantly and the water content here reached 17.6% from 16.9%, which implies that a small amount of crystallization-water phase transition may have occurred here, producing a phase change zone that led to a large amount of water accumulation at the top part; at the same time, during the simulation, the phase change temperature was set to −9°C; according to Figure 10, the numerical simulation curve and the actual measurement curve are in good agreement
Summary
Qinghai province is located in the northwest inland area of China, which is the gateway of the Qinghai-Tibet Plateau. Klas et al [18] proposed a new method for calculating the phase transition in the fully implicit numerical model of coupled heat transfer and variable saturation water flow at temperatures above and below zero This method is based on a mixed equation of water flow and heat transfer. Most of the existing studies are based on the low salt content and freezing conditions and the study on the freezing temperature and water-salt migration mechanism of high-chlorine salts is not sufficient. In order to better understand the law of water and salt migration in high chloride saline soils and to better solve practical engineering geological problems, the following studies were conducted. Water-heat-salt coupling equation of high-chlorine saline soil under the action of temperature is established This model introduces the relationship between the ice content, water content, and negative soil temperature. The second development module of the COMSOL Multiphysics finite element software is used to solve the model and compare the experimental results
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