兰州大学机构库 >土木工程与力学学院
黄土非饱和增湿变形特性研究
Alternative TitleWetting-induced Deformation Behavior of Loess Soil
邵显显
Thesis Advisor张虎元
2018-04-01
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name博士
Keyword黄土 增湿变形 土水特征曲线 非饱和渗透性 结构性 微观机制
Abstract

传统的黄土湿陷性评价方法主要包括内室试验和现场试验,二者均是在黄土完全饱和的条件下进行的。实际工程中,黄土在含水量增加却未达到饱和状态时已发生了严重的附加变形,称之为增湿变形。近年来,随着黄土高原地区城市周边削山造地工程的加剧,一部分Q2、Q1黄土地层出露地表,或变得埋深较浅。而在一些沟谷区,出现了大量的黄土高填方工程。在这些不同地层的原状黄土,或者填方黄土上进行工程建设、修建地基,都会面临到不同程度的增湿变形问题。实际工程中黄土的饱和过程和增湿变形过程发展极快,难以监测,且增湿变形过程中的微观结构变化也难以准确测定,加剧了这一问题的研究难度。对此,本文以兰州市九州台全新世(Q4)黄土、晚更新世(Q3)黄土、中更新世(Q2)上部黄土、中更新世(Q2)下部黄土和早更新世(Q1)黄土为研究对象,一方面引入多梯度逐级浸水的方法进行黄土增湿变形试验,从而模拟黄土从初始含水率逐级增湿至饱和的动态变化过程;另一方面针对不同地层原状黄土和重塑黄土进行有关非饱和特性、增湿变形特性和增湿变形机理等的研究。

本文主要的研究内容和取得的研究成果包括:1、不同地层原状黄土的基本物理化学性质、微观结构 随着地层的由新到老,原状黄土的比重、含水率和塑限基本不变,干密度、液限和塑性指数增大;原状黄土中含量最多的矿物是石英,其次依次是是长石、云母和方解石,还有少量的白云石和绿泥石;原状黄土中含量最多的阳离子是Na+,其次依次是Ca2+、Mg2+和K+;含量最多的阴离子是 ,其次是Cl-。当地层由新到老变化时,原状黄土微观结构最明显的变化特征是:矿物碎屑颗粒的菱角变得越不明显,颗粒粒径略有减小;颗粒间的接触方式由点-点接触、点-面接触向面-面接触发展;集合体内孔隙含量变化不大,颗粒间孔隙含量减小甚至接近消失。2、结构性对黄土非饱和特性的影响 结构性对黄土土水特征曲线的影响为:当黄土的地层较新(或干密度较低)时,原状黄土与重塑黄土的孔径分布相近,使得结构性对黄土进气值的影响不明显;而原状黄土颗粒间的胶结作用降低了孔隙连通性,使原状黄土的残余含水量略大于重塑黄土。当黄土地层较老(或干密度较大)时,原状黄土颗粒间孔隙的减少使其进气值大于重塑黄土;由于胶结作用对孔隙连通性的影响被削弱,原状黄土和重塑黄土的残余含水量相差不大。结构性对黄土非饱和渗透系数的影响为:当黄土的地层较新(或干密度较小)时,原状黄土颗粒间的胶结物对其水力通道有着明显的阻碍作用,使原状黄土的非饱和渗透系数低于重塑黄土;而当黄土地层较老(或干密度较大)时,黄土颗粒之间接触极为紧密,原状黄土颗粒间的胶结物对其水力通道的阻碍作用不大,使得原状黄土和重塑黄土的非饱和渗透系数很接近。3、结构性对黄土增湿变形特性的影响 在水和压力的共同作用下,黄土原有的结构崩溃重组。由于原状黄土的孔隙分布更不均匀,原状黄土的微观结构相比与重塑黄土有更大的调整空间,孔隙比减小得更多,增湿变形量更大。由于结构性的影响,原状黄土的增湿变形临界孔隙比小于重塑黄土,且差值随着竖向压力的增大缓慢增大。即在竖向压力一定时,原状黄土更容易在较低的初始孔隙比条件下发生增湿变形。4、黄土的非饱和增湿变形特性 将黄土在一定竖向压力下发生增湿变形所需要达到的饱和度称为临界饱和度。原状黄土的增湿变形临界饱和度随地层越老而越大,随竖向压力增大而减小;重塑黄土的增湿变形临界饱和度随干密度增大而增大,随着竖向压力增大减小。当竖向压力一定时,不同地层原状黄土(或重塑黄土)在增湿变形阶段的孔隙比随着饱和度增大呈唯一的指数函数形式递减。原状黄土(或重塑黄土)增湿变形过程中的孔隙比变化路径与饱和度和竖向压力的大小有关,与黄土试样的地层或初始孔隙比无关。对于原状黄土和重塑黄土,在每一级竖向压力下都存在一个临界孔隙比,只有当黄土的初始孔隙比大于其临界孔隙比时,才会发生增湿变形。原状黄土和重塑黄土增湿变形的临界孔隙比都随竖向压力的增大呈指数函数形式递减。5、黄土增湿变形的微观机制 黄土增湿变形过程中,颗粒粒径、集合体内孔隙的孔径和体积不发生变化;而颗粒间孔隙的孔径和体积有减小趋势。黄土的增湿变形主要是由黄土颗粒间孔隙孔径的大小,以及颗粒间孔隙的孔径在水、压力作用下的减小程度所决定的。当重塑黄土内部含有大量集合体内孔隙时,有可能具有较低的干密度,间接证明该重塑黄土有发生增湿变形的可能。

Other Abstract

The traditional evaluation methodology of collapsibility of loess including single oedometer test and double oedometer test, are usually conducted on loess in saturated status. In practical engineering, additional deformation would generate before loess being wetted to saturation, which is call wetting-induced deformation. In recent years, some Q1 and Q2 loess layers are exposed or closed to the land surface by cutting loess hills to terraces, while a mount of high filled projected appears in gully regions. It probably occurs loess wetting-induced deformation by building constructions no matter on this intact loess layers or filled loess layers. In addition, the process of the loess saturation and wetting-induced deformation are so rapidly and difficult to be monitored. Meanwhile, the microstructure evolution during the wetting-induced deformation of loess is also hardly to measured accurately. These make the research for wetting-induced deformation behavior and mechanism become difficult. Thus, the paper conducted on the loess soil in Holocene (Q4) loess layer, Upper Pleistocene (Q3) loess layer, upper Middle Pleistocene (Q2) loess layer, lower Middle Pleistocene (Q2) loess layer and Lower Pleistocene (Q1) loess layer located in Jiuzhou Tai loess profile in Lanzhou city, and introduced a graded wetting collapse test to simulate the dynamic process of loess wetted from initial saturation degree to saturated condition. On the other hand, a serious of tests was conducted on natural loess in different layers and remolded loess with different dry densities to study the unsaturated character, wetting-induced deformation and wetting-induced deformation mechanism of loess.

In this paper, the main research contents and research results are as follows:1. The basic physical properties, chemical properties and microstructure of nature loess from different loess layers The specific gravity, water content and plastic limit are in constant, while the dry density, liquid limit and plasticity index increase when the age of loess develops from younger to older. The mineralogical compositions of the loess are predominantly quartz, feldspar, mica and calcite, with a small amount of dolomite and chlorite. The major cation in loess is Na+, secondly are Ca2+, Mg2+ and K+; The major anion in loess is , secondly are Cl-.The microstructure variation of loess from youngest loess layer to oldest loess layer is as follows: The angles of mineral grains are weakened and the particle size decreases; the type of particle connection develops from point-point contact to point-face point and even face-face contact; The volume of the intra-aggregate pores changes slightly, while the volume of inter-aggregate pores decreases significantly.2. The effect of loess structure on unsaturated character The effect of loess structure on soil-water characteristic curves is that: With respect to the condition that loess soil being in younger layer (or with lower dry density), the pore size distribution of natural loess and remolded loess are similar, so as to the air entry value of natural loess and remolded loess are close; while the existence of cement bonds between the particles of natural loess obstructing the connectivity between pores, so as to the residual water content of natural loess is slightly larger than remolded loess. With respect to the condition that loess soil is in older layer (or with higher dry density), the air entry value of natural loess is larger than remolded loess, depending on the volume of inter-aggregate pores of natural loess is smaller that remolded loess; while the residual water content of natural loess and remolded loess are similarly, because the effect of comment bonds on the connectivity between pores is reduced.The effect of loess structure on soil-water characteristic curves is that: With respect to the condition that loess soil being in younger layer (or with lower dry density), the existence of cement bonds between the particles of natural loess obstructing the connectivity between pores, so as to the unsaturated permeability coefficient of natural loess is slightly smaller that remolded loess. With respect to the condition that loess soil is in older layer (or with higher dry density), closely contact between particles of natural loess reduce the obstruction of cement bonds to pores, so as to the unsaturated permeability coefficient of natural loess and remolded loess are similarly.3. The effect of loess structure on wetting-induced deformation behavior Under loading and wetting, the initial microstructure of loess will collapse and then restructure, even for natural loess possess cement bonds between particles. The fabric of natural loess featuring a relatively nonuniform distribution of pores, so as to the microstructure adjustment, pore volume reduction and deformation of natural loess is larger than remolded loess under loading and wetting. In addition, depending on the structure difference between natural loess and remolded loess, the critical void ratio for wetting-induced deformation of natural loess is smaller than remolded loess, even the difference value increases with the increase of vertical pressure. In other words, natural loess enable to wetting collapse with a lower dry density when subjected to a constant vertical pressure. 4. The wetting-induced deformation behavior of loess The minimum saturation degree for remolded loess to begin wetting collapse is referred to as the critical collapse saturation degree in this study. For natural loess, the critical collapse saturation degree increases when loess changes from youngest loess layer to older loess layer, while it decreases as the vertical pressure increased. For remolded loess, the critical collapse saturation degree increased with increasing initial dry density, while it decreased as the vertical pressure increased.During the process of wetting collapse, the void ratio evolution of natural loess or remolded loess depends on the saturation degree and the vertical pressure, while it is unaffected by the initial dry density (or initial void ratio). The void ratios of specimens with various initial dry densities decreased exponentially along the same curve with an increase of the saturation degree when subjected to a constant vertical pressure.Under a constant vertical pressure, there is a critical void ratio, where the natural loess or remolded loess collapses after being wetted to saturation if its initial void ratio is larger than the critical void ratio. The critical void ratio of natural loess or remolded loess decreases exponentially when the vertical pressure increases.5. The wetting-induced deformation behavior of loess The size and volume of inter-aggregate pores decrease when loess collapse upon loading and wetting, while the particle size distribution, the size and volume of intra-aggregate pores change slightly. The dominant inter-aggregate pore size is the main factor for the collapsibility of loess upon loading and wetting. In addition, abundant intra-aggregate pores indirectly identify that the inter-aggregate pore structure in remolded loess is possibly unstable, which leads to the collapse of the soil.

URL查看原文
Language中文
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/226335
Collection土木工程与力学学院
Recommended Citation
GB/T 7714
邵显显. 黄土非饱和增湿变形特性研究[D]. 兰州. 兰州大学,2018.
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