【作者】 张先伟；

【导师】 王常明；

【作者基本信息】 吉林大学 ， 地质灾害防治工程， 2010， 博士

【摘要】 软土在我国分布广泛,具有较差的工程特性,如灵敏度高、强度低、承载力低以及固结时间长、蠕变特性显著的特点。随着沿海地区大型工程的兴起,对软土工程的研究提出了更新更高的要求,软土的结构性、蠕变特性及力学模型的研究是其中突出而紧迫的问题。软土的蠕变特性是决定软土地基及其上部结构工后沉降和稳定性的重要因素,而软土特殊的结构性又使蠕变特性“复杂化”。因此,结构性软土蠕变特性以及本构模型的研究对正确描述结构性软土的工程性状具有重要的理论意义,对正确分析、评价软土地基变形和稳定性具有重要的现实意义。论文以黄石、漳州、青岛地区软土为研究对象,通过土力学和土质学有机结合的研究途径,对软土的结构性和蠕变特性的力学行为和物化组成、微观结构特征的测试,及相关性分析,从宏观力学表现与微观结构两个方面对结构性软土蠕变机理进行了探讨。基于扰动状态概念,建立反映软土蠕变特性的结构性模型,通过室内蠕变试验数据验证模型的可靠性,达到可靠预测软土地基的变形的目的。主要获得以下结论:1)系统分析了结构性对软土的固结特性、固结系数、次固结系数、应力—应变关系、孔隙水压力特性、强度包线、蠕变曲线、粘滞系数、非线性蠕变特性的影响。相对于非结构性土和结构性较弱的土,结构性软土表现出特殊的工程特性。如随着压力的增大次固结系数Ca出现峰值的现象正是结构性所引起的,通过Ca/Cc来估算Ca时必须考虑结构性的影响;结构性软土的粘滞系数随剪应力的增加达到峰值,且固结压力越大,峰值对应的剪应力也越大。2)根据重塑土还原后的压缩曲线和原状土的压缩曲线之间存在的一个结构强度区域,提出结构破损系数S的概念,该值获取简单,表示土体在压缩过程中结构破损的情况,可用来定量判别不同地区结构性土的结构强弱,由此分析得到结构性最强的为青岛软土、黄石软土次之、漳州软土最弱。3)在试验数据处理和结果表达方面获得了一些有意义的结论。对于非线性特性显著的软土蠕变试验数据应采用陈氏法处理;复杂度应作为衡量软土在蠕变过程中孔隙或颗粒形状规则程度的重要参数,它能较好反映微结构的轮廓形状,还可以衡量平均粒径与实际情况差距的大小;用雷达图表示定向频率分布,能同时对比很多组数据,还可设定数据标准圈便于观察分布强度。4)通过定性与定量的对比软土原状样与蠕变破坏后土样的电子显微镜照片,得到了蠕变条件下结构的变化规律,结果表明:软土主要以絮凝结构、骨架结构、蜂窝结构、团聚结构为主。在蠕变条件下,颗粒间以边对边、边对面为主的接触形式向以面对面为主的接触形式过渡,状态逐渐向重塑土的性状逼近;孔隙变化遵循孔隙匀化原理,即体积收缩过程中大孔隙减少多,小孔隙变化小,孔隙分布逐步均匀化;长条形孔隙与颗粒数量减少,等轴形孔隙与颗粒几乎不存在,整体上向扁圆形发展;颗粒与孔隙的趋于“圆滑”,且孔隙比颗粒复杂度减少幅度要大;颗粒的定向性趋向有序性发展。由于蠕变过程中软土微结构参数变化受蠕变试验方法影响较大,对不同蠕变试验方法得到的微观数据要综合分析,这样才能较真实的接近实际情况。5)根据蠕变机理的分析表明:结构性软土的蠕变过程可概括为内部结构不断改变自我调整再造以适应外力变化的过程。在这个过程中,软土主要经历了结构基本完整、结构大量破损、与重塑土土性相似的三个阶段。结构性软土具有蠕变特性的根本原因是软土在复杂的沉积环境下使颗粒与颗粒间形成边-边、边-面的接触,形成了絮凝、蜂窝、团聚、骨架等结构,颗粒表面的粘土矿物带有电荷,吸附大量的游离氧化物和阳离子,将颗粒通过胶结联结粘结起来,使软土具有一定的结构强度。软土普遍具有高含水率、大孔隙比、高粘粒含量的特点,同时含有大量的伊利石、蒙脱石、高岭石等粘土矿物,一些海相软土还含有大量有机质,软土的易溶盐含量也较高,这些因素使软土具有较大的吸水性,使孔隙中充满大量的结合水,使软土在外力作用下变形表现显著的时效性。6)基于扰动状态概念,将结构破损系数加以处理代替扰动函数,建立了软土的结构性蠕变模型。根据扰动函数在压缩过程中的直观表现,验证了该模型可以描述蠕变过程中结构性逐渐衰减的过程。通过对比Burgers体模型对试验数据的拟合结果,表明模型能较好地表现软土减速蠕变与稳定蠕变阶段,验证了模型的可靠性。更多还原

【Abstract】 Soft clay distributes widely in China, has poor engineering properties such as high sensitivity, low strength and bearing capacity, consolidation for a long time and creep characteristics is remarkable. The rapid development of coastal areas huge-scale constructions presents us newer and higher tasks on engineering researches of soft clay, and the structural characteristics, creep characteristics and mechanical models of soft clay are the prominent and urgent ones up till now. The creep characteristics is an important factor that decides the settlement and stability of soft clay foundation and superstructure after construction, and the special structural characteristics of soft clay causes the creep characteristics―the complication‖. Therefore research on the creep characteristics of the structural soft soil and constitutive model has important theoretical significance for the correct description of the engineering properties of the soft soil, and have important practical significance for analyzing and evaluating soft clay ground deformation and stability.This paper was based on the soft clay in Huangshi and Zhangzhou and Qingdao areas, according the approaches that combined the soil mechanics and soil material, researched on soft clay structural characteristics, creep mechanics behavior, physical chemical composition, microscopic structure characteristics and their relationships, investigated the creep mechanism of the structural soft clay from two aspects which were macroscopic mechanics and microscopic structure. Through introducing the disturbed state concept to establish soft clay creep model, and verified the reliability of model through the creep test data fitting, for the purpose of forecasting reliably the foundation deformation of the soft clay, for the purpose of forecasting reliably the foundation deformation of the soft soil.The primary results and conclusions ware gained as following:1) Analyzed systematically the influence of the structural characteristic on solidifying characteristics, consolidation coefficient, secondary consolidation coefficient, stress-strain relations, pore-water pressure characteristics, strength envelope,viscosity coefficient,nonlinear creep characteristics and creep curves. Results showed that structural soft clay displayed special engineering characteristics relatived to non- structural soil and structural weak soil. Such as the phenomenon that the secondary consolidation coefficient Ca achieved peak value with the pressure increasing was caused by the structural characteristics, so estimating Ca through Ca /Cc must consider the influence of the soft clay structure; structural soft soil’s viscosity coefficient achieved peak value along with shear stress increasing, and the bigger peak value of consolidation pressure the bigger the shear stress.2)According the structural strength region between the compression curve of reconstituted soil and the compression curve of primary soil, proposed the concept of structural damage coefficient S, this parameter was gained simply which showed soil structural damage condition in the process of compression, could be used to distinguish quantitatively the structural strength of the different structural soils, by using this method obtained that the structure characteristics of QingDao soft clay was the strongest and the HuangShi soft clay was second and the ZhangZhou soft clay was weakest.3)Some meaningful conclusions about test data processing and result expression were obtained. Such as soft clay creep test datas that exhibited nonlinear characteristics should use Chen’s method; the complexity should be taken as the important parameters measuring soft soil pore or particle shapes during the creep, which not only could reflect microstructure outline shape, but also measure the size between the average size of the gap and the actual size; radar chart was used for showing directionality frequency distribution, that simultaneously contrast many group of datas, and it was advantageous of the observation on distributed intensity by setting the data standard ring.4)Researched on soft clay microstructure change rule in the pro and post creeping through qualitative and quantitative comparison of electron microscope photographs of the primary soil and the soil sample after the creep test. Results showed that soft clay mainly by flocculated structure, skeleton structure, honeycomb structure, aggregate structure. In the Condition of creep the particles contact forms transited the primarily form by side-to-side, side-to-face to the form by face-to-face, and the state was gradually approaching to the characteristics of reconstituted soil; micro-poor size changed fellow theory of homogenize which maropore reduced many and the fine pore changed was small,the pore distribution turned to be homogenization gradually in the volume compression process; the number of elongated pores and particles reduced, the equiaxed shape pore and particle were almost non-existent, on the whole the pore turned to be oblateness; the pore and particle outline was trended to be smooth, and the pore complexity reduced more bigger than the particle complexity reduction scope; particle’s directionality trended orderliness. Because in the process of soft clay creep the soft clay microstructure parameters influenced by the test method more, should analyze comprehensively the microscopic test datas by different methods, for approaching to the actual situation more realistically.5)By the creep mechanism the analysis showed that: the structural soft clay’s creep process could be summarized to the process that internal structure changed to be self-adjustment restoration in order to adapt the external force change. In this process, the soft soil experienced three stages which were structure basic complete, structure massive breakages, and reconstituted soil similar.The root cause of the soft clay creep characteristics was the soft clay formed the flocculated structure, skeleton structure, honeycomb structure, aggregate structure, which particles contacted form primarily by side-to-side,side-to-face. The clay mineral in the particle surface had electric charges, adsorbing massive free oxide and cationic, then bonding particles through the cemented joint, so that the soft clay had certain structural strength. Generally soft clay was characteristics that high moisture content large void ratio, high clay content, simultaneously soft soil contained massive clay minerals such as illite, montmorillonite and kaolinite, some marine clay also contained organic matter, and the soluble salt content is higher. These factors enabled soft soil to have the big water absorption, and holes were filled with massive bound waters, causing the soft soil showed remarkable timeliness by the external force.6)On base of the conception of disturbed state, replacing the disturbance function by the structural damage coefficient processed, established the creep model of the structural soft clay.According to the visual performance of disturbance function during the compression, confirmed that this model was possible to describe creep process that the structural characteristics weakened gradually. According to creep test data fitting by contrasting Burgers models the results showed that model can describe deceleration creep stage and steady creep stage well, verifying the reliability of the model.更多还原