镇赉盐渍土冻融条件下水热盐运移试验及结构演变研究

发布时间：2018-06-08 06:27

  本文选题：盐渍化冻土 + 异构-裂隙土　； 参考：《吉林大学》2017年博士论文

【摘要】：吉林西部属于干旱—半干旱温带大陆性季风气候,并且为季冻土区。近半个世纪以来,吉林西部土地盐碱化面积和程度都有所增加,多处湖沼干涸、草地退化,而土体盐渍化也无疑限制了当地经济的可持续发展,并且给生态环境造成巨大威胁。由于吉林西部特殊的环境气候,加之原岩类型的不同,使吉林西部不同地点土体具有独特的土体结构。在长期对该区多点多地的野外调查和室内试验的基础上,了解吉林省西部的镇赉、大安、乾安、泥林、农安等地盐渍土的物质组成和结构各不相同。在调查中发现镇赉地区分布有大面积斑状盐碱土地,该区为吉林西部盐渍化最为严重的地区之一,土中易溶盐含量较高,并且以钠盐为主,导致土体分散性较强,工程地质性质较差。在季冻区环境和高易溶盐含量的影响下,镇赉盐渍土发育有大量裂隙,形成了特殊的“蒜瓣”结构,土体类型为异构破碎型。因此,为了搞清该区土体的水热盐运移规律,有必要对其特殊结构的成因机制和演变规律进行研究。本次研究以镇赉盐渍土为研究对象,以长期的野外调查以及室内试验为基础,通过大量收集分析已有成果资料,利用理化试验、冻结试验、扫描电镜、压汞试验、核磁共振试验等多种手段,对镇赉盐渍土基本物质组成、水热特征、宏微观结构进行了研究。从宏微观角度探讨了冻融条件下土体的结构演变规律,从机制学角度分析了其结构成因;在室外水盐运移监测试验的基础上,结合COMSOL进行盐渍土水热盐耦合数值模拟,掌握正向冻融期盐渍土中水热盐动态变化特征,分析盐渍土冻结层厚和深度的变化规律。有助于了解正向冻融期土体结构的成因机制,为深入探究该区土体的次生盐碱化进程和防治土体冻害提供丰富的理论依据。本文首先对研究点不同埋深的盐渍土进行取样测试,通过室内试验了解镇赉盐渍土的成分特征和基本性质。镇赉土样水盐含量随埋深变化明显,埋深10~80cm土样含盐量较高,总含盐量为0.3~0.5%,阳离子主要为Na+,阴离子以硫酸根和碳酸氢根为主;春融期埋深30~100cm的土样含水率较高,该区蒸发深度约为35cm。镇赉表层以下各层土样胶粒含量均较高,表层土和深度大于120cm土样为粉质重亚粘土,其他土样均为粉质轻粘土。镇赉盐渍土具有分散性,土体干时硬度大,遇水即刻变浑浊,各层土样有机质含量均较低,阳离子含量较高,土样中粘土矿物含量高且结晶较差,为过渡型矿物。根据压缩曲线求得原状土样结构屈服压力要小于重塑土样,说明土体中的裂隙削弱了土体的结构强度。通过研究镇赉原状和重塑土样的宏微观结构可知,重塑土样冻融循环后与原状土样结构相似,均发育有大量的裂隙,这说明冻融作用是镇赉土体裂隙发育的原因之一。不同水盐含量的重塑土样经历冻融循环后结构改变方式不同,冻融循环作用对不同粒径的土颗粒作用效果不同,对大颗粒有破坏作用,但对胶粒有团聚作用,综合作用结果是硫酸盐渍土中粒径主要分布在0.075~0.005mm,碳酸盐渍土和不同含水率土样粒径主要分布在0.075~0.005mm和0.002mm。硫酸钠盐渍土累积盐胀量较大,土体以盐胀破坏为主,随着硫酸钠含量的增加,冻融循环后土样胶粒含量愈小;并且当硫酸钠含量大于1.5%时,土样由分散性土转变为非分散性土;在冻融循环条件下,硫酸钠增强了土颗粒的团聚性,使胶粒团聚成较大的颗粒。而原盐含量和碳酸盐渍土样主要为冻胀-盐胀破坏模式,土体盐胀量较小,冰晶和碳酸盐晶体对土颗粒中大颗粒的破坏作用较显著,对胶粒的团聚作用较小,综合作用结果是0.075mm的颗粒含量减小,胶粒含量略有减少。利用室内压汞试验对原状土样和重塑土样的孔隙分布进行定量研究,发现镇赉裂隙土包含有2个孔隙系统:裂隙网络和土孔隙,并且孔隙分布呈双峰特征,即土样呈双孔隙结构(粒间孔隙和粒内孔隙两种孔隙结构),双峰位置分别位于孔径为4~100μm和0.4μm范围。通过扫描电镜试验得知,未经冻融循环的重塑土样结构密实、平整,无明显裂隙发育,微结构类型为团聚状结构,土结构较完整。而经历冻融循环后各土样均发育有宏微观裂隙,各土样微结构类型为团聚状-凝块状结构,微裂隙密度和宽度随含水率的增大而增大,随碳酸盐含量增大而逐渐减小,在土样表面观察到的碳酸盐结晶也随含盐量的增大而增多。硫酸盐渍土微结构呈现盐-土粒混溶态包裹土团粒的形态特征,冻融循环后土中发育大量架空孔隙,结构单元体直径较大,细颗粒含量含量减少,与颗粒分析试验结果相符。结果表明:镇赉土体的物质组成(粒度组成、矿物组成、易溶盐种类和含量)是裂隙结构形成的内因,冻融循环作用是外因。本文利用课题组自主研发的“超冷环境岩土冻融综合实验箱”进行室内冻结试验研究,配置不同易溶盐含量、类型和不同含水率的重塑土样,测得各土样的冻结过程曲线,获得土样冻结温度以及稳定阶段时长,并研究土体冻结温度的影响因素。与农安盐渍土相比较,镇赉盐渍土由于其胶粒含量高、分散性强、粘土矿物结晶差,这种特殊土质决定了土样的冻结过程曲线与农安盐渍土差异很大,其冻结过程曲线均未出现过冷和跳跃阶段;不论是硫酸盐渍土还是苏打盐渍土,冻结温度随含盐量增大而降低;冻结温度有随冻融循环次数略微升高的趋势,稳定阶段时长可反映出土中扩散层体积含量,随冻融循环次数变化剧烈波动。利用核磁共振仪测定镇赉盐渍土不同水盐含量时未冻水含量随温度的变化曲线,并讨论了影响未冻水含量的因素,发现降温和升温过程中的未冻水含量-温度关系曲线并不相同,在一定温度范围内后者存在滞后现象,对曲线进行拟合,并将拟合函数用于水热盐耦合数值模拟。季冻区盐渍土中水热盐耦合关系复杂,为了准确了解在冬季冻结和春融阶段镇赉盐渍土的水热盐耦合规律,本次利用室外正冻和正融的条件下进行镇赉盐渍土水热盐现场监测试验,获得盐渍土土体不同层位水分、温度和盐分的演变规律,为今后研究吉林西部盐渍土的水盐迁移以及盐渍化机理提供参考。但自然条件下气温的变化过程较复杂,整个冬季来说正向冻结和正向融化时交互发生的,并且盐分的结晶析出对温度变化较敏感,盐分的变化趋势波动性较大,数值模拟时有必要进行合理简化。在对研究区土样进行基本物理化学性质以及水热盐参数测定的基础上,通过建立正冻-正融条件下盐渍土水热盐耦合模型,对室外监测试验进行数值模拟。结果表明在冻融作用下,在特定的正负温差时,土体内部可存在双冻土层,设立正确的边界条件是模拟结果可信的关键。在融雪入渗的影响下仅浅层土体盐分可淋溶至下层土体,在反复冻融作用下盐分随水迁移聚集至冻土层,因此原位土体中30~80cm范围内易溶盐含量较高,模拟结果较好的反映了镇赉原状土体内部的水盐运移规律。受温度环境影响的土体深度范围有限,在反复冻融循环作用下,土体中易溶盐在0~2m范围内形成内部循环,模拟结果可为季冻区冻土灾害防治提供参考。
[Abstract]:The western Jilin belongs to the arid and semi-arid temperate continental monsoon climate, and it is the Quaternary permafrost region. Since nearly half a century, the area and degree of salinization of the land in Western Jilin has been increased, and many marshes have dried up and the grassland is degraded. Soil salinization also undoubtedly restricts the sustainable development of the local economy and causes great ecological environment. As a result of the special environmental climate in Western Jilin and the different types of original rock, the soil of different sites in Western Jilin has a unique soil structure. On the basis of a long field investigation and laboratory test of many sites in this area, the material composition of the saline soil in Zhenlai, Da'an, Qian an, mud forest and Nong'an in Western Jilin province is understood. It is found that there is a large area of saline alkali soil in the Zhenlai area, which is one of the most salinized areas in Western Jilin, with high soluble salt content in the soil and sodium salt, which leads to strong dispersion of soil and poor engineering geological properties. It is affected by the environment of the frozen area and the content of high soluble salt. There are a lot of cracks in the saline soil in Zhenlai, which forms a special "garlic valve" structure, and the type of soil is heterogeneous and broken. Therefore, it is necessary to study the formation mechanism and evolution law of the special structure of the soil in this area. This study takes Zhenlai saline soil as the research object, and takes a long field in the long field. Based on the investigation and laboratory test, through a large amount of data collected and analyzed, the basic material composition, hydrothermal characteristics and macro and micro structure of Zhenlai saline soil were studied by means of physical and chemical tests, freezing test, scanning electron microscope, mercury injection test and nuclear magnetic resonance test. The soil mass under freezing and thawing conditions was discussed from the macro and micro angle. The structure evolution law is analyzed from the mechanism point of view. On the basis of the outdoor water salt migration monitoring test, the coupling numerical simulation of Saline Soil Hydrothermal salt is carried out on the basis of the outdoor water salt migration monitoring test. The characteristics of the dynamic change of the hydrothermal salt in the salted soil during the positive freezing and thawing period are mastered, and the variation of the thickness and depth of the frozen soil layer is analyzed. It is helpful to understand the positive of the salt soil. The formation mechanism of the soil structure during the freezing and thawing period provides a rich theoretical basis for deep exploration of the secondary salinization process and the prevention and treatment of soil frost damage in this area. Firstly, the paper sampled the saline soil with different buried depth at the research point, and studied the composition and basic properties of the saline soil in Zhenlai by indoor test. The water and salt of Zhenlai soil sample contained water and salt. As the depth of buried depth changes obviously, the salt content of buried depth 10~80cm soil is higher, the total salt content is 0.3~0.5%, the cation is mainly Na+, the anion is mainly sulphuric acid root and bicarbonate, the soil sample water content of the buried depth of 30~100cm in the spring thawing period is higher. The depth of evaporation in this area is about 35cm. Zhenlai surface layer with higher soil sample, surface soil and depth. The soil samples of 120cm are silty heavy subclay and other soil samples are powder light clay. Zhenlai saline soil is dispersed. Soil dry hardness is large, and water is cloudy with instant water. The content of organic matter in each layer is low, the content of cations is high. The clay mineral content is high and crystallization is poor, which is the transition mineral. According to the compression curve, the original form is obtained. The yield pressure of the soil sample is less than the remolded soil sample, indicating that the crack in the soil weakens the structural strength of the soil. Through the study of the macro and micro structure of the original and remolded soil samples of Zhenlai, it is known that the frozen thawing cycle is similar to the original soil sample structure after the frozen thawing cycle, which indicates that the freezing thawing is the development of the fracture of the soil in Zhenlai. One of the reasons is that the remolded soil samples with different water and salt content have different structural changes after freezing and thawing cycles. The effect of freezing and thawing cycles on different particle size soil particles is different, and it has a destructive effect on large particles, but the aggregate effect on the colloidal particles is the result that the particle size distribution in the sulphate saline soil is mainly distributed in 0.075~0.005mm and carbonate saline soil. The grain size of soil and different water content is mainly distributed in 0.075~0.005mm and 0.002mm. sodium sulfate saline soil. The soil mass is mainly salt expansion, and with the increase of sodium sulfate content, the smaller the content of the clay particles after the freezing and thawing cycle; and when the content of sodium sulfate is greater than 1.5%, the soil sample is transformed from dispersed soil to non dispersive soil. Under the conditions of freeze-thaw cycle, sodium sulfate enhances the agglomeration of soil particles and aggregates the particles into larger particles. The original salt content and the carbonated soil sample are mainly frost heave salt expansion failure mode, the soil salt expansion is smaller, the ice crystal and carbonate crystals have a significant damage to the large particles in the soil particles, and the aggregate effect on the colloidal particles is small and comprehensive. The result is that the particle content of 0.075mm is reduced and the content of the colloid is slightly reduced. The pore distribution of the original and remolded soil samples is quantitatively studied by the indoor mercury pressure test. It is found that the Zhenlai fissure soil contains 2 pore systems: the fracture network and the soil pore, and the pore distribution is Shuangfeng characteristic, that is, the soil sample is double pore structure (intergranular). Two pore structures in pore and intragranular pore, the Shuangfeng position is located at the range of 4~100 mu m and 0.4 mu m, respectively. Through scanning electron microscopy, it is found that the structure of remolded soil samples without freezing thawing cycle is dense, flat, without obvious fissure development, and the microstructure type is agglomerate structure, and the soil structure is more complete. The micro structure type of each soil sample is agglomerate like structure. The density and width of microcracks increase with the increase of water content, and gradually decrease with the increase of carbonate content. The carbonate crystallization on the surface of the soil increases with the increase of salt content. The microstructure of the salinic saline soil presents a mixture of salt and soil particles. A large number of aerial pores were developed in the soil after the freezing and thawing cycle. The diameter of the structure unit was larger and the content of fine particles decreased. The results showed that the material composition of the soil in Zhenlai (size composition, mineral composition, soluble salt type and content) was the internal cause of the formation of fracture structure, and the freezing and thawing cycle was used as a result. In this paper, the freezing and thawing experimental box of "super cold environment, rock and soil freezing and thawing", which is developed by the project group, is studied in the indoor freezing test, and the remolded soil samples with different soluble salt content, type and different water content are arranged, the freezing process curves of various soil samples are measured, the freezing temperature of the soil samples and the length of the stable stage are obtained, and the soil freezing is studied. Compared with the saline soil, Zhenlai saline soil has a high concentration, strong dispersion and poor crystallization of clay minerals. This special soil determines that the freezing process curve of the soil sample is very different from that of the saline soil. The freezing temperature of soda saline soil decreases with the increase of salt content; the freezing temperature has a tendency to slightly increase with the number of freezing and thawing cycles. The length of the freezing and thawing period can reflect the volume content of the diffusion layer in the unearthed soil, and the change of the number of the freezing and thawing cycles violently. The determination of the unfrozen water content with the temperature of different water and salt content in Zhenlai saline soil with the NMR instrument The factors affecting the content of unfrozen water are discussed. It is found that the temperature relation curve of the unfrozen water content in the process of cooling and heating is not the same. The latter exists the lagging phenomenon in a certain temperature range, fitting the curve and applying the fitting function to the numerical simulation of the coupling of water and heat salt. The coupling of water and heat in the saline soil of the season freezing zone The relationship is complicated. In order to understand the coupling law of water and salt in the saline soil of Zhenlai in the freezing and spring melting stage of winter, the field monitoring test of water and heat salt of Zhenlai saline soil is carried out under the conditions of outdoor positive freezing and normal melting, and the evolution law of water, temperature and salt in different layers of saline soil is obtained, which will be used to study the salt in the west of Jilin in the future. The water and salt migration and the mechanism of salinization provide a reference. However, the change process of the temperature is more complex under natural conditions. The interaction between the freezing and the positive melting occurs during the whole winter, and the crystallization of the salt is sensitive to the temperature change, and the variation trend of the salt is large, and it is necessary to simplify the numerical simulation. On the basis of the basic physical and chemical properties of the soil samples in the study area and the determination of the parameters of the hydrothermal salt, the water and hot salt coupling model of the saline soil under the positive freezing and thawing condition is established. The numerical simulation of the outdoor monitoring test is carried out. The results show that under the specific positive and negative temperature difference, there is a double frozen soil layer under the specific positive and negative temperature difference. The exact boundary condition is the key to the reliability of the simulation results. Under the influence of the infiltration of snow melting, the salt of only shallow soil can be dissolved into the lower soil, and the salt content of the soil is aggregated with the frozen soil under repeated freezing and thawing, so the content of the salt is high in the 30~80cm range of the soil in situ. The simulation results reflect well the internal soil of the original soil of Zhenlai. The depth of soil affected by temperature and environment is limited. Under the action of repeated freezing and thawing cycle, the soluble salt in the soil forms internal circulation within the range of 0~2m. The simulation results can provide reference for the prevention and control of frozen soil disaster in the season.
【学位授予单位】：吉林大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P642.1
，

本文编号：1994932