【作者】 梁健伟；

【导师】 房营光；

【作者基本信息】 华南理工大学 ， 结构工程， 2010， 博士

【摘要】 软土是工程建设中常见的天然材料,广泛分布于沿海、河流中下游及湖泊的三角洲地区,其物理力学性质非常复杂,具有含水量高、孔隙比大、压缩性高、强度低、渗透性差、结构性和流变性显著等特点,其中具有代表性的淤泥和淤泥质土,主要由极细的粘土颗粒、有机物、氧化物等固相物质和水组成。软土的颗粒微小,比表面积大,颗粒表面富集电荷,在颗粒—水—电解质系统的相互作用下,颗粒表面形成的吸附结合水膜是显著影响软土工程特性的重要微观物质因素。结合水的厚度随矿物成分、孔隙液性质及浓度、温度等因素的变化而改变,以致改变软土的塑性、流变性、渗流与强度等宏观工程特性。长期以来,人们主要从宏观层次对软土的工程特性进行研究,无法从根本上解释其产生不良工程特性的机制和本质规律。本文通过实验,从微细观的角度分析细颗粒软土变形和渗流等工程特性及其变化的微观机制。通过软土的矿物成分及比例、比表面积、阳离子交换量、孔隙特征及分布等微细观参数的测试,对不同微细观参数组合的软土试样进行变形与渗流特性试验,基于颗粒—水—电解质系统相互作用的扩散双电层理论,由定量计算给出变形和渗流特性参数与各微细观参数的定量关系,分析特性参数与微细观参量的内在关系,解释影响软土特性的微观机制。最后,在试验分析的基础上,建立了软土的圆孔和平行平板微观渗流模型,通过实测结果证实了模型的合理性和有效性。本文开展的研究工作和取得的成果有如下几个方面:(1)通过微细观试验测试软土的矿物成分及比例、颗粒尺度、比表面积、阳离子交换量(CEC)、微孔隙的尺度及分布,由颗粒表面电荷密度计算颗粒表面电位。测试结果表明,不同矿物成分的颗粒比表面积、CEC差别较大,以致其表面电荷密度差别较大,矿物晶层结构是导致这些差别的最重要因素;颗粒表面电位随孔隙液离子浓度的提高而降低;软土中的孔隙尺度以微米级孔隙为主,孔径分布特征与其矿物成分、颗粒尺度和内部结构有关。(2)研究不同矿物成分、孔隙液离子浓度和含水量的软土的强度与塑性变形特性。根据试验分析,认为矿物颗粒之间的摩擦与胶结粘聚作用是软土强度特性的决定因素,结合水膜的变化将改变矿物颗粒之间的摩擦与胶结粘聚作用机制,从而改变强度性质;对于盐渍土,可溶盐离子通过颗粒表面微电场作用改变结合水膜性质是其具有低液塑限和较特殊强度特性的主要微观机制。(3)软土直剪蠕变试验与微细观参数测试分析相结合,研究粘土矿物、有机物和氧化物、含水量对软土流变特性的影响。试验结果表明,软土中的各种成分通过表面吸附结合水膜影响土样的流变性质;不同物质成分对软土流变性产生不同程度的影响,从而呈现出软土流变性的成分因素,而影响软土流变性的微观机制都是通过改变吸附结合水膜的厚度和性质产生的;强结合水是影响软土流变性质的主要因素,弱结合水则是相对次要因素。(4)采用不同电解质离子浓度的孔隙液对人工土和天然土进行渗透试验,研究极细颗粒软土微孔隙渗流的“微电场效应”和“微尺度效应”,并通过压汞法研究了固结过程中渗流固结特性的变化。试验分析认为,颗粒表面电荷的微电场对粘土微孔隙渗流会产生显著影响,不同的矿物成分和孔隙液离子浓度使颗粒表面具有不同电场,使孔隙液粘度和“有效渗孔”改变,从而影响渗流特性;对于微米级以下的微孔隙渗流,出现渗透系数随水力梯度降低而增大的“异常”现象,采用微孔隙渗流的“边界滑移”作出一种与试验相符的解释;孔隙尺度及分布特征是影响土体渗流固结特性的重要因素。(5)建立了软土的圆孔和平行平板微观渗流模型,通过实测结果证实了模型的合理性和有效性。在微观渗流模型中,考虑颗粒表面微电场对孔隙液粘度的影响以及对运动离子的电场力作用,导出了“圆孔”形和“平行平板”形渗孔的流体运动方程,由模型求得的渗透系数与系统微细观参数的关系得到了试验结果的验证,并证明了Darcy定律为本微观渗流模型的一个特例。更多还原

【Abstract】 Soft soil is a kind of common natural materials for civil engineering, which is widely distributed in coastal areas, middle-lower reaches of rivers and lacustrine delta areas. Slit and mucky soil are the most typical ones that characterized with complex physical-mechanical properties, such as high water content, large void ratio, high compressibility, low strength, weak permeability, remarkable structural and rheological properties and so on. Slit and mucky soil are mainly composed of solid phase matter including tiny clay particles, organics, oxides, and water. Correspondingly, soft soil is small in particle size and large in specific surface area, and the electric charges assemble densely on particle surface. Under the interaction of particle-water-electrolyte system, the adsorbed bound water film formed on particle surface is an important microscopic physical factor that remarkably influences the engineering characteristics of soft soil. The thickness of bound water changes with mineral composition, property and concentration of porewater, and temperature and so on, leading to the change of macroscopic engineering characteristics—plasticity, rheology, seepage and strength. The research of engineering characteristics of soft soil is still focus on macroscopic stage mainly, which is unable to reveal the mechanism and essential law of its inferior properties. Base on experimental study, this dissertation analyzes the deformation and seepage characteristics of tiny-particle soft soil and its corresponding microscopic mechanism.Deformation and seepage characteristic tests of soft soil were conducted with different combination of microscopic parameters such as mineral composition and proportion, specific surface area, cation exchange capacity, pore size and distribution and so on. Base on the theory of diffused double layer, the relationship between deformation and seepage parameters and microscopic parameters are shown quantificationally, the intrinsic relation of them is analyzed, and the micromechanism that influence the soft soil characteristics is explained. Finally, the microscopic seepage models of circular and parallel-plate pore are established, which are confirmed reasonably and effectively by test results.The outline of the research tasks and achievements of this dissertation are listed as follows:(1) Microscopic experiments—mineral composition and proportion, particle size, specific surface area(SSA), cation exchange capacity(CEC), micropore scale and distribution—were carried out, and the particle surface potential was figured out by surface charge density. The test results showed that the SSA and CEC vary significantly among different mineral particles, resulting in different surface charge density, and the mineral crystalline structure is the most important factor that leads to these distinctions. The particle surface potential decreases with the augmentation of ion concentration of pore water. The scale of pore in soft soil is generally ofμm level, and the characteristic of pore scale distribution relates to mineral composition, particle size and internal structure of soil.(2) Research of the strength and plastic deformation features of soft soil with different mineral composition, ion concentration of pore water and water content were carried out. According to the test results, the determinant of soft soil strength is frictional and cohesive effect among mineral particles, which can be changed by the variety of bound water film and results in the change of strength properties. Especially, the main micromechanism of low Atterberg limits and unusual strength properties about saline soil is the micro-electric field effect that changes the properties of bound water by soluble salt ions. (3) The influence of clay mineral, organics, oxides and water content on the rheology of soft soil was analyzed by the combination of direct shear creep test and microscopic parameter test. The results showed that the composition factor of soft soil rheology is presented by different influence degree of different ingredients, and the micromechanism that influence soft soil rheology is generated by the change of thickness and characteristics of bound water film, strong-bound water and weak-bound water are the main and secondary factors respectively that influence the soft soil rheology.(4) In order to research the micro-electric field effect and micro-scale effect on tiny-particle soil seepage, the permeability tests on artificial and natural soil were conducted with different electrolyte ion concentrations. And the mercury-intrusion tests were carried on to study the seepage-consolidation characteristics of soil during consolidation. The test results indicated that the micropore seepage is affected significantly by micro-electric field produced by particle surface charges, and the seepage characteristic is influence by the change of porewater viscosity and effective pore size, and‘boundary slippage’of micropore seepage provides a reasonable explanation for the abnormal phenomena during the tests—the permeability coefficient increases with the reduction of hydraulic gradient, and the pore scale and distribution characteristics are important factors that affect the seepage-consolidation properties of soil.(5) The microscopic seepage models of circular and parallel-plate pore are established and confirmed reasonably and effectively by test results. The influence of micro-electric field on porewater viscosity and electric field force of moving ions are taken into consideration in the models, and the fluid motion equations are derived from circular-shaped and parallel-plate-shaped pore. The relationship of theoretical permeability coefficient and systematic microscopic parameters is verified by test results. In addition, the permeability equations prove that Darcy law is one of the special cases of the models.更多还原