| The western Jilin area is located in the south of Songnen Plain with a warm temperate continental monsoon climate.It not only belongs to arid and semi-arid,fragile environment that is highly vulnerable to climate change,but also the seazonal frozen area.For half a century,the area and degree of land salinization increased has increased with numbers of lakes and marshes drying up,and many types of grassland have been degraded.Undoubtedly,soil salinization has limited the economy sustainability development,and posed a great threat to the local ecological environment.The special environmental climate and different protolith types in Zhenlai,Da'an,Nilin,Nong'an and so on,soils of different locations had its own unique composition and structure characteristic based on long-term field investigation and indoor experiments.In the investigation,it was found that there distributed areas of patchy saline-alkali land in Zhenlai area,which was one of the most serious salinization areas in Western Jilin.High soluble salt content,especially sodium salt in the soil,leads to strong soil dispersibility and poor engineering geological properties.Under the influence of seasonal frozen environment and high soluble salt content,there were a lot of cracks developed in the saline soil,forming heterogeneous and fractured soil structure.Therefore,it is necessary to study the formation mechanism and evolution law of saline soil structure in Zhen Lai.The research object was Zhenlai saline soil.Based on the long-term field investigation,indoor experiment,and analyzing great mass of data,by means of physical and chemical test,freezing test,scanning electron microscope,mercury injection test and nuclear magnetic resonance(NMR)test,the basic properties,hydrothermal properties,macro and micro structure of saline soil in Zhenlai were studied.The soil structure evolution under freeze-thaw cycles was discussed from the macro and micro perspectives.Also,its cause mechanism was analyzed.The dynamic changes characteristics of water and salt in saline soil were studied based on the water-salt transport monitoring experiment and water-heat-salt coupling model solved by COMSOL software.The rule of the thickness and depth of the frozen layer was obtained,which could explain the forming mechanism of saline soil structure during the freeze-thaw period.These researches could provide a wealth of theoretical basis on prevention and treatment of soil frost damage and secondary salinization process of saline soil.In this study,saline soil samples of different depths were carried out,and the composition and basic properties were obtained through laboratory tests.The results showed that the water and salt content changed with soils depth.The soil samples with 10~80cm depth had high salt content,the total soluble salt content was about 0.3~0.5%.The cation is mainly Na+,and anions mainly SO42-and HCO3-.However,during spring thawing period,the soil moisture content of 30~100cm depth was higher than that of other soil layers,and the evaporation depth was about 35 cm.The soil had a high content of colloidal particles,and the surface soil and the depth of more than 120 cm soil samples were silty heavy sub-clay,and other layers were all sandy light clay.Zhenlai saline soil is dispersive clay soil.The soil strength is large in dry state,if it encounters water,it will become muddy.The content of organic matter in each soil layer was lower,the cations content and clay mineral content was higher,the clay mineral crystallization was poor and it was transitional mineral.According to the compression curve,the structure strength of the undisturbed soil sample was smaller than that of the remolded soil.Repeated freeze-thaw reduced the soil strength.By studying the macro-micro structure of the undisturbed soil and remolded soil samples,it can be seen that the structural characteristics of the remolded soila after the freeze-thaw cycle were similar to undisturbed soil,all of which had developed a large number of cracks.It indicated the freeze-thaw cycle was one of the reasons for the development of soil fractures in Zhenlai soil.Remolded soil samples with different water-salt content had different structure failure modle after freeze-thaw cycles,and the effect of freeze-thaw cycle on the soil particles with different size is distinct,which has a destructive effect on the large particles,and has agglomeration ability on the colloidal particles.The results showed that the particle size of sulphate saline soil was mainly distributed in 0.075 ~ 0.005mm;the other remolded soil particle size was mainly distributed in 0.075 ~ 0.005 mm and <0.002 mm.There was large salt expansion in the sodium sulfate saline soil,salt heaving is soil failure modle.The more sodium sulfate content,the colloid particles decreased more after freeze-thaw cycles.And when the sodium sulfate content was more than 1.5%,the soil dispersion reduced greatly.Under the condition of freeze-hawing cycles,sodium sulfate enhanced the particles agglomeration,and the small particles were agglomerated into larger particles.While other remolded soil samples is mainly frost heave-salt expansion failure mode and salt expansion is small.The effect of ice and carbonate crystal on the large particles is obvious,and the aggregation effect is small.The comprehensive effect was the >0.075 mm particles content decreased a lot,and the colloidal particles content induced slightly.The microstructure and pore characteristics of the soil were studied based on scanning electron microscope(SEM)and mercury intrusion porosimetry(MIP).The pore distribution of undisturbed and remolded soil samples were studied by mercury injection test.The test showed that fissured soil contained 2 pore systems: fissure network and soil pore.While the pores distribution was bimodal distribution,which indicated the soil samples had double pore structure(intergranular pore and intragranular pore).The positions of the two peaks were in the range of 4 ~ 100?m and <0.4?m.It can be seen from the SEM images that the remolded soil structure was dense and smooth,and the microstructure is agglomerate structure.After the freeze-thaw cycle,soil micro-fractures were developed,and the microstructures of the soils were agglomerated-clotted.The microcrack density and width turned bigger with increase of water content,and decrease with increase of carbonate content.While the more carbonate content,the more carbonate crystals observed on the soil surface.The sulphate soil showed the salt-soil miscible entrainment granules morphological characteristics.After freeze-thaw cycles,a large number of overhead voids are developed in the soil and the structure unit body diameter turned larger.The content of fine particles reduced,which is in accordance with the results of particle analysis test.The results show that Zhenlai soil material composition(grain size composition,mineral composition,soluble salt species and content)is the inter cause of structure,and the freeze-thaw action is external cause.Freezing test was conducted through "ultra-cold environment rock freeze-thaw comprehensive experimental box".The experiment configured remolded soil samples with different moisture content,different salt type and content.The freezing process curves were measured,from which we could obtain the freezing temperature and stability period,and the influence factors were also studied.Compared with Nong'an saline soil,Zhenlai saline soil had a higher colloidal particle content,dispersibility and poorer crystallinity of clay minerals,those determined its freezing characteristics was different with Nong'an saline soil.The freezing process curve did not appear supercooling and jumping stage.Both sulphate saline soil and carbonate soil,the freezing temperature decreased with the increase of salt content.The freezing temperature had increase tendency with the freeze-thaw cycles.The stable period could reflect the freezing content of soil,and the change rule was not obvious with freeze-thaw cycle.The length of the stabilization phase can reflect the volumetric content of the diffusion layer in the soil,and fluctuate with the change of the freeze-thaw cycle.The unfrozen water content in Zhenlai saline soil was measured by nuclear magnetic resonance(NMR),and the effect of salt and other influencing factors was discussed.It was found that the non-frozen water content-temperature curves during cooling and heating were not the same,and the latter had hysteresis within a certain temperature range.The unfrozen water content-temperature curve was fitted using the exponential function,and the correlation is good.The fitting function is applied to numerical simulation of hydrothermal salt coupling in this study.The water-heat-salt of the Zhenli saline soil had a complex coupling relationship in seasonal frozen environment.Therefore,in order to obtain the coupling rules of water-salt in the freeze-thaw stages of Zhenlai saline soil,the outdoor monitoring experiment under natural freeze-thaw condition was carried out.The evolution law of water,temperature and salt in different layers could provide reference for the study of water-salt migration and salinization mechanism in western Jilin.However,the temperature change process is more complicated in the natural condition,the forward freeze and thaw occurs interactively.The precipitation of the salt is more sensitive to temperature change,and the trend of salt change is more volatile.Therefore,it is necessary to rationalize the boundary conditions when performing numerical simulations.On the basis of the physical and chemical properties of the soil samples and the determination of the hydrothermal parameters,the numerical simulation was carried out by establishing a water-heat-salt coupling model under the condition of freeze – thawing,and it is helped to analyze the outdoor monitoring test.The results showed that there were double frozen soil layers in the soil at a specific positive and negative temperature,indicating that establishing correct boundary condition was the key to the credible simulation results.Under the influence of snowmelt infiltration,only the shallow soil salt could leaching to the lower soil,and under the action of repeated freez-thaw,salt migrated with water to the frozen soil layer,so the content of soluble salt was higher in the 30~80cm in the soil,and the simulation results reflected the salt-water coupled transport in Zhenlai saline soil.The soil depth influenced by environment temperature was limited,under repeated freeze-thaw cycles,the soluble salt in the soil formed internal circulation within the range of 0 ~ 2m,and the simulation results can provide reference for the prevention and control of frozen soil disaster. |
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