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宽级配弱固结土内细颗粒运移规律研究
Alternative TitleResearch on the Fine Particle Migration in Wide Grading and Poorly Consolidated Soil
Language中文
郭朝旭
Thesis Advisor崔鹏
2015
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Name博士
Degree Discipline岩土工程
Keyword宽级配弱固结土 细颗粒迁移 孔隙结构 网络模型 现象学模型
Abstract宽级配松散土体在降雨作用下,其土体内部的细颗粒发生迁移是形成泥石流的一个重要因素,并且该因素已经被一些学者所认知,然而细颗粒在降雨条件下是否发生迁移,以及如何迁移成为亟待解决的问题。现有的一些机制猜想,例如细颗粒运移会导致土体内部形成上层滞水面,上部土体过分饱和从而导致土体破坏;细颗粒运移会在土体内部形成水膜,水膜为坡体的破坏提供滑动面,加速土体破坏;以及细颗粒在土体内部的运移堵塞,溃决从而降低土体强度,使松散弱固结土发生破坏。这些机制是否能够描述松散弱固结土中细颗粒的运移导致土体破坏和泥石流形成过程,仍旧缺乏物理证据。 本文分别通过SEM扫描电镜实验、宽级配弱固结土大尺寸渗透实验和水槽实验来分析细颗粒迁移的环境——孔隙结构特征,一维迁移特征(垂直方向)和二维迁移特征,并归纳总结宽级配弱固结土的迁移机理,最后提供了基于宏观现象学和细观孔隙网络的数值模拟方法,对细颗粒迁移规律在数值方法上进行了探索,取得以下成果: (1) 通过SEM扫描电镜实验确定了宽级配土体连续断面的二维孔隙结构特征,即孔隙的半径,二维空间位置,孔隙数量等特征;通过将连续断面的孔隙扫描图像进行插值和重建,获得了真实的三维孔隙结构,并提出了三维连通度计算方法,对三维孔隙的连通性进行了判定;孔隙数量满足分形的定义,同时具有分形的自相似性质,理论上可以将细观尺度的规律推广到宏观尺度进行应用;分析了孔隙结构特征与颗粒级配、强度的关系,以及宽级配弱固结土体孔隙结构特征的影响因素与应用价值; (2) 通过大尺寸的渗透实验,发现了宽级配弱固结土在不同细颗粒含量条件下(8%~48%)的渗透系数与细颗粒含量、水头条件的关系;分析了细颗粒在二维不同渗透条件下的迁移规律,及对孔压、渗透系数和渗透液浓度的影响;利用稀土元素对细颗粒进行了示踪,发现渗透液中的细颗粒主要为底层土体在水流作用下冲刷所致,土体内部迁移沉积的细颗粒则主要以中上部为主,与孔隙水压力等数据对比可以验证宽级配土体中细颗粒迁移形成相对滞水面并引起孔压上升的现象及规律; (3) 利用水槽实验,水槽中的稀土元素示踪表明,雨强和底床坡度较小的情况下,细颗粒随优先流和渗透水流沿坡体内部向下迁移,增加渗透阻力,难以造成土体破坏;在雨强和底床坡度较大的情况下,细颗粒迁移造成相对滞水面,首先坡面形成浅层破坏,其次坡脚侵蚀造成剥落式破坏,最终大范围坡体破坏并形成泥石流。提出降雨诱发浅层破坏可以归纳为表层径流,壤中流和细颗粒迁移三个主要因素共同作用的结果; (4) 提出了宽级配土体内可移动细颗粒存在的三种状态:自由堆积于孔隙中,细颗粒相互粘结形成的块状结构和粘结在骨架颗粒上;细颗粒的受力包括粘附力(Fad),上举力(Fl),由流体形成的拖曳力(FD)和摩擦力(Ff)组成,其分离过程是其细颗粒在水动力作用下克服阻力的结果;细颗粒在土体内部(孔隙壁)沉积的机制可以分为惯性作用、大颗粒拦截作用、细颗粒沉淀作用、布朗扩散作用、水动力作用和筛滤效应; (5) 引入细颗粒迁移的宏观现象学模型,忽略细颗粒迁移过程,通过获取初始阶段不同高度的细颗粒浓度,迁移沉积量及孔隙压力数据,即可通过对细颗粒的沉积量、渗透出坡体的浓度及导致土体内部孔压的变化有较好的预测效果; (6) 引入孔隙网络模型可以模拟真实的孔隙结构数据,分析水流在孔隙网络中迁移过程,施加不同程度的细颗粒堵塞孔隙网格系数,可用于对含细颗粒流团的运动过程、孔隙网络节点压力的影响、迁移速度和时间的模拟;通过模拟可以发现由于细颗粒迁移堵塞效应导致压力陡增和优先流通道的形成现象。 本文对宽级配弱固结土中细颗粒迁移规律的研究和认识,可以完善科研工作者对现有细颗粒迁移导致泥石流起动物理过程的认知,并为泥石流防灾减灾提供重要的理论支持。
Other AbstractFine particle migration in wide grading and poorly consolidated soil is a main factor to trigger debris flow. It is well known by a lot of scholars. However, the problem about when and how the fine particles transport with water flow is still up in the air. In the current hypotheses, some people believed that the fine particles would form perched water table in soil, and over saturated soil in the up, and lead soil failure in the end; others regard the water film may form and provide the sliding face. Whether these mechanisms can accurately describe the process of fine particles migration is still lack of evidence. In this paper, SEM test, penetration test and flume test are carried out to investigate pore structure, fine particle migration mechanism for wide grading soil. Then, phenomenological model and pore network model are chose to simulate the fine particle migration and deposition. The achievement is as follows. (1) Through SEM tests, 2D pore structure of wide grading soil, including pore size, pore position in 2D, pore number and so on have been got from continuous cross-section images; And the continuous images can also be used to build 3D pore structure by interpolation. 3D connectivity calculation method has been proposed and applied to judge the pore structure of wide grading soil. The relationship between pore number and pore radius satisfies the function of power exponent, namely the definition of fractal. And it would have the characteristic of self-similarity from meso-scale to macro-scale. Additionally, the relationship between pore structure and particle grading and strength has been discussed. Influencing factors of pore structure and its application in soil strength and failure are analyzed at the same time. (2) The relationships between permeability coefficient and fine particle content (8~48%) and water head about wide grading and poorly consolidated soil is found by penetration tests. Meanwhile, with different conditions, fine particle migration in 2D, and its influences on pore water pressure, permeability coefficient, and concentration of fluid flowing out are considered and deeply studied. Rare earth element (REE) has been applied to track the fine particle transport. The results indicate that fine particles in the water flowing out are mostly washed out from lower part of soil column in penetration tests; fine particles transporting and depositing in the soil are mostly from upper and middle part. Moreover, comparing with pore water pressure data and grading analysis after tests, it can be clearly found that fine particles are firstly migrating in soil, then forming perched water table, and leading to pore water pressure increase in the end. (3) REE tracking fine particle in flume tests shows that when rainfall and bed gradient are all small, fine particle would transport along slope with preferential flow, and permeation resistance will increase, and no soil failure will happen; when rainfall and bed gradient are all large, fine particle migration forming relative perched water table in shallow layer, would firstly cause shallow failure on slope surface, then slope toe the regressive failure, and large soil failure or debris flow in the end. The essence of shallow failure is the interaction of surface runoff, subsurface flow, and fine particle migration. (4) It is proposed that fine and removable particles in wide grading soil has three states: depositing in the pore singly and freely, forming blocky structure, or bonding on skeleton particles. Supporting force of fine particles includes adhesive force (Fad), uplift force (Fl) and drag force (Fl) by water, and frictional force (Ff). Fine particle detaching from parent body is the result of resultant force over zero. And the mechanism of fine particle deposition can be divided into inertial impaction interception, sedimentation, Brownian diffusion, hydrodynamic action, and staining. (5) Phenomenological model which can neglect the exact fine pa
Document Type学位论文
Identifierhttp://ir.imde.ac.cn/handle/131551/15031
Collection山地灾害与地表过程重点实验室
Affiliation中国科学院成都山地灾害与环境研究所
Recommended Citation
GB/T 7714
郭朝旭. 宽级配弱固结土内细颗粒运移规律研究[D]. 北京. 中国科学院大学,2015.
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