Numerical Analysis Of Coupled Water Heat Salt And Stress Of Saline Soil In Western Jilin Province

The western Jilin province is not only a saline soil region, but also a typical seasonal frozen soil region, which freezing time of surface soil is up to 5 months in winter. Some engineering diseases and environmental problems such as frost heave, thaw collapse, salt expansion and salinization are related to the special geological environment and climate characteristics. As the typical multi-phase porous medium, the effect coupled solid and liquid-phase, thermal, salt and stress of soil mass in western Jilin province is essence of these engineering diseases and environmental problems mentioned above. The macroscopic expression of multi-physical field interaction is the water and salt migration in soil mass. The internal structure of soil can be changed which caused by water migration so that frost heave is occurred and triggers a series of engineering problems, salt migration causes the salt to accumulate in the surface and then aggravates the degree of secondary salinization. In summary, the multi-physical field interaction which contained hydraulic, thermal, salt and stress in the internal soil mass of western Jilin province is the basis to study various engineering diseases and disaster evolution mechanism of geological environment system of this area. In this paper, the following conclusions are obtained:1. Through on-site investigation and sampling on the dry land which located in the northeastern Aobaotupao of Nong’an, western Jilin province, and a series of basic tests are carried out. Results show that soil samples have obvious stratification phenomenon in the longitudinal direction from the material composition of view in the study area. The shallow surface layers above 30 cm are mainly silty or sandy loam, and the following depths are mainly silty clay. The clay content of all sampling depths are above 30%, and clay content increased gradually with the increase of depth. Mineral components which tested by X-ray analysis shows that primary mineral including quartz and feldspar accounted for 84% of the total mineral content, and secondary mineral are mainly kaolinite, illite and illite-Smectite layer-mixed minerals. The content of kaolinite and illite are generally higher than illite-Smectite layer-mixed minerals which accounted for 13% the total minerals. Results of soluble salt test shows that the total amount of soluble salt of all sampling points are more than 1% in the study area, and the amount is up to 1.69% on the surface. According to the classification of salt content, each sampling depth is strong saline soil, and according to the classification of salt type it belongs to the weak carbonate saline soil. Cation is mainly Na+ among the component of soluble salt, and anion are mainly SO42-, HCO3-and Cl-. The content of Na+ is very high, which reached 0.38%, and it has a certain impact on the soil-water interaction, dispersion, disintegration of the soil in the study area.2. The microstructure and pore characteristics of the soil were studied based on scanning electron microscope(SEM) and mercury intrusion porosimetry(MIP) in the study area. Results show that the crystallinity of clay minerals is poor in this area. The microstructure types are mainly aggregate structure, clay minerals are mostly arranged in the way of face to face and edge to edge. The main pore size interval is 6~30μm, and macro pore(4~40μm) ranges from 40% to 60% of different sampling depths. Freezing temperature test is conducted of each sample and the freezing temperature of soil samples with 20%, 25% and 30% initial water content are obtained. At last, the relationship between unfrozen water content and negative temperature of soil samples in the study area was predicted by two point’s method.3. According to different density, three kinds of compaction degree(85%, 90% and 95%) test soil columns were made in the study area, and the water and salt migration tests were carried out by the direct method and the freezing method. Results of direct water migration test show that: the height of three kinds of soil samples with different compaction degrees under natural conditions(the soil is not frozen) caused by capillary water rise are very small. The capillary water rise height of soil samples with 85% compaction degree is the highest, which is 3.2cm. Through the water and salt migration test of saline soil which were frozen for 5 days and 90 days under a certain temperature gradient, the results show that the amount of water and salt migration of three kinds of compaction degree are very small. After freezing for 90 days, the water and salt migration under 40 cm in the bottom is obvious, the amount of water migration decreases with the increase of compaction degree, and the temperature gradient has an obvious effect on the migration of water and salt.4. The western Jilin province is not only a seasonal frozen soil region, but also distributes a large area of saline soil. Under the influence of external environment, the internal moisture field, temperature field, chemical field and stress field of saline soil interact with each other, forming a complex hydrodynamic coupling system. The coupled model of hydraulic, thermal, salt and mechanics(HTSM) of saline soil in western Jilin province is established based on the test of basic physical and chemical properties as well as the determination of the hydraulic, thermal and salt parameters of soil samples in the study area, combining with water, heat and salt transport equation, mass continuity equation, energy conservation equation, equilibrium differential equation and so on.5. The coupled model of hydraulic, thermal, salt and mechanics(HTSM) of saline soil in western Jilin province is solved by the physical fields which are established by the PDE equations in mathematical model of COMSOL Multiphyscis. The effectiveness of this model was verified by comparing with the indoor water and salt migration test. At the same time, a numerical example including water and salt migration and associated phenomena of saturated frozen soil under the condition of an open system is designed with simplified boundary conditions. And the mechanism of water and salt migration in saline soil is analyzed.