【作者】 张旭东；

【导师】 王清；

【作者基本信息】 吉林大学 ， 地质工程， 2018， 博士

【摘要】 位于吉林西部季冻区的盐渍土,兼具季冻土、盐渍土、分散土的特性。由于该区原岩类型、特殊气候条件的原因,吉林西部土体具有典型的工程地质特征,其中冬季气温长达5个月均为零下,促使土体内部流-固-热-应力-应变等不同物理场之间相互耦合,进而导致各种类型的工程病害和农业病害。由于冻结过程中,土体内部温度场、水分场、溶质场、应变场、应力场的相互作用,一方面导致土体内部水分迁移,土体内部孔隙压力改变,应力场、应变场分布改变,引发土体发生冻胀、导致路基变形、构筑物破坏等工程病害;另一方面水分的迁移带动了土体内部溶质的运移,随着水分的迁移和蒸发,盐分不断在近地表聚集,盐分在地表的聚集不仅会引起土体盐渍化的环境病害、还可能引起道路翻浆、侵蚀构筑物等工程病害。因此,对吉林西部土体内部流-固-热-应力-应变多物理场耦合的研究是研究区环境工程地质灾变机理的研究基础。本文通过选取吉林西部典型的盐渍土分布地区——农安地区进行取样研究,并通过研究得到以下几点结论:通过对2015年春季野外调查结合样品的室内测试,获得取样点盐渍土的成分特征、基本物理化学性质以及热力学参数。测试结果表明,采样点原生矿物比例较高,次生矿物以高岭石为主;土体为弱碳酸盐渍土,其阳离子主要为钠离子,阴离子主要为碳酸氢根和硫酸根,其中钠离子和碳酸氢根对土体酸碱度起主导作用;土体粉粒为主,而粘粒含量也相对较高,以30cm深度为界限,性质有所不同,其中深度小于30cm的土体主要为重亚粘土,属于低液限粘土,而深度大于30cm的土体主要为粉质轻粘土,且属于高液限粘土;且小于30cm土体含水率低、含盐量高,通过参考2014年入冬前研究区土体剖面的含水率、盐分的含量特征及变化,得到了土体受蒸发作用影响的深度,并初步探究了研究区土体次生盐渍化的原因。不同的含水率、不同压实度的土样,经过冻融循环实验后,观察盐渍土样内部结构的变化特征,并测得经过不同冻融循环次数后土结构单元体的变化情况。借助于扫描电子显微镜,发现土样经历不同冻融循环作用后,粒径较小的单元体或颗粒的百分比先增加后减小,而粒径较大的单元体或颗粒的百分比先减小后增加;分析认为,冻融循环作用下,一方面,土体结构单元体内部或孔隙内水在冻结成冰的过程中、体积会增大,而冰融化成水的过程、体积又会减小,可导致结构单元体形态的变化或破坏;另一方面,在冻结过程中,水分的迁移作用导致孔隙压力变化,并在新的位置排开土颗粒、分凝成冰、使结构进一步变化、甚至破坏结构连结。利用核磁共振技术测试不同初始含水率、不同含盐量土体的未冻水含量,分析了初始含水率、含盐量对土体未冻含水率曲线的影响。通过利用课题组自主研发的“超冷环境岩土冻融模拟平台”对最优含水率配置好的土样进行室内冻结试验研究,得到土样的冻结曲线和冻结温度,通过利用测温法对土体的未冻含水率进行研究预测;最终利用核磁共振的测试结果对比验证预测结果的有效性。考虑土体在不经过冻结作用的自然条件下的毛细水上升和冻结条件下土体内部的水分迁移,分别进行不同压实度土体的毛细水上升实验,以及不同实验条件(有/无水源补给、单/双向冻结、不同温度梯度等)下的冻结实验,选择冻结时间为5天。通过对比实验结果,确定引起土体内部水分迁移的驱动力,进而分析发现实验土样中温度压力势是土体内部水分迁移的主要因素,重力势对水分迁移的作用较弱,而基质势对土体内部水分迁移作用非常弱。在合理假设条件的基础之上,系统性地从土体的三维单元体出发、建立水-热-盐-应力-应变多场耦合模型,并针对研究区特定环境特征对方程进行分析简化,得到一维条件下的水-热-盐-应力-应变耦合模型。模型的建立结合了物质守恒定律、能量守恒定律、有效应力原理、弹性本构方程、几何方程、达西定律、克拉伯龙方程等,通过结合未冻水函数、热力学参数、渗透系数、压缩模量、结晶函数等建立完整的五场耦合方程组,并通过引入“透镜孔隙比”的概念对冰透镜的形成进行判定,通过溶解度确定盐结晶的位置。借助于“超冷环境岩土冻融模拟平台”,考虑土体冻结速率,设置适当初始条件和边界条件,在室内有水源补给的条件下进行土柱的单向冻结实验,实验过程中对温度和冻胀量实时监测、观察实验现象,并对实验结束后含水率、含盐量的分布特征进行测试。利用COMSOL Multiphyscis数学模块中的PDE方程接口建立物理场对吉林西部盐渍土水-热-盐-应力-应变(HTSPE)耦合数学模型进行求解,并与室内明确边界条件的水盐迁移和冻胀试验结果进行对比,验证模型的有效性,结果表明模型可以很好地预测冻结过程中土体内的多场耦合作用。同时,基于室内实验模拟的有效性,对野外取样现场进行模拟对比并分析模拟计算效果,通过合理选取野外边界条件,对现场水、盐含量进行计算,结果表明计算结果与测试结果基本一致,但地表30cm范围内的土样含水率较小而含盐量较大,分析认为这是由于野外土体30cm以上近地表土体受蒸发作用,水分向空气中蒸发、而盐分在地表残留聚集。更多还原

【Abstract】 The soil in Western Jilin,has the properties of seasonal frozen soil,saline soil and dispersive soil.Since the special protolith and the weather conditions,the soil has special engineering geological characteristics.The winter lasts 5 months,which prompts coupling process of the different fields of flow-solid-thermal-stress-strain.As a result,different disease in engineering and agriculture is induced.In freezing,the distribution of temperature field,water field,salt field,strain field and stress field change,which leading to frost heave.Besides,water migration affects salt transport,and the salt accumulate at the soil surface,which leading to salinization and environmental disaster.Thus,study of the multi-field mathematical coupled model is the research foundation for the study of environmental engineering geology disastrous mechanism in the study area.This article selected Nong’an county,which is the typical saline soil area in Western Jilin,as the study area for study.And some conclusion are drawn as following,Through the indoor test for the soil samples collected in the Spring of 2015,we obtained the characteristics of the content,basic physical properties and the thermal parameters.The results indicated that the proportion of the primary mineral for the soil is high.The secondary mineral is mainly dominated by kaolinite.The soil is carbonated saline soil.The positive ion is mainly Na~+,and the negative ions are mainly bicarbonate radical and sulfate radical,which affect the pH value.In the soil,the silt has a large content.While 30cm is the boundary for the particle distribution,the soil that less than 30cm is mainly low liquid limit clay,and the soil at the depth over 30cm is mainly high liquid limit clay.Besides,the soil at the depth less than30cm has low water content and high salt content.Comparing with the water content and salt content of the soil in 2014,the influence depth of evaporation for the soil is obtained,and the reason for the soil salinization is primarily investigated.The soil samples with different water contents and different compaction degrees,which is subjected to freezing and thawing,are studied for the structures observation.With the assistance of scanning electron microscope,it was found that after freezing and thawing,the proportion of the small particles increases firstly and then decreases,and the bigger particles decreases firstly and then increases.Through the analysis,it is considered to be influenced in two parts.On the one hand,the volume will increase if the pore water transfer into ice,and the volume will decrease if the ice melt into water,which leading to the particles transformation or broken.On the other hand,water migration induces the changes of the pore water pressure,and freezes into ice in a new position,or even breaks the structural connection.The unfrozen water content for the soil samples with different water contents or different salt contents is tested by using nuclear magnetic resonance.Then we analyzed the influence of the initial water content and salt content on the unfrozen water content curve.By using“Rock&soil Freezing-Thaw Trial and Error imitating comprehensive experimental platform with ultracold condition”that independently designed and invented by our research group,the freezing tested is developed on the soil samples with the optimum water content.Then we obtained the freezing curve and freezing temperature.And based on the thermometry,the unfrozen water content is forecasted.As a result,the forecasted results is proved to be valid by comparing with the testing result by the nuclear magnetic resonance.Upon considering the capillary water raising under unfrozen condition and water migration under freezing condition,we carried out the capillary water raising experiment and water migration tests under different conditions(whether there is water recharging,unidirectional or bidirection freezing),which are subjected to freezing.And the freezing time is 5 days.By comparing the experimental results,the driving force for the water migration is determined,and the temperature potential and the pressure potential is the main reason for water migration,gravitational potential affects water a little,and the matric potential has little influence on water migration.On the basis of the reasonable assumption,we systematically established the mathematical coupled model coupled water-heat-salt-stress-strain by analyzing three-dimensional representative elementary volume.And then simplified the model into one-dimensional coupled model.The model is based on the conservation law of energy and mass,principle of effective stress,constitutive equations,Darcy law and clapeyron equation.And the mathematical equation set contains some parameters including unfrozen water content,thermodynamic parameter,osmotic coefficient.The concept of total void ratio is proposed to determine the formation of ice lenses,and the positions of the salt crystallization is determined by solubility.Unidirectional freezing test with the water recharge is carried out by means of“Rock&soil Freezing-Thaw Trial and Error imitating comprehensive experimental platform with ultracold condition”,considering the freezing speed,initial condition and boundary condition.And the temperature,frost heaving amount are monitored.Also,the water content and salt content is tested after the experiment.Through using the PDE module of COMSOL Multiphyscis,the experiment is simulated based on the mathematical coupled model of hydraulic-thermal-salt-pressure-elastic(HTSPE).The simulated results have an agreement with the experimental results,which verified the validation of the model.Thus,the special engineering boundary condition is considered for simulation.The calculated results of the water content and salt content indicated that the calculated results are agree with the monitored data.While the water content of the monitored water content for the soils at the depth less than 30cm is small and the salt content is high,which is considered to be influenced by evaporation,which leads the moisture go into the air and the salt accumulated at the soil surface.更多还原