Collapse behavior and microstructural evolution of loess soils from the Loess Plateau of China

Loess soils are characterized by metastable microstructure, high porosity and water-sensitivity. These soils have always been problematic soils and attracted attention from researchers all over the world. In the present study, three loess soils extracted at various depths from the Loess Plateau of China, i.e. Malan(Q_3), upper Lishi(Q_2~2) and lower Lishi(Q_2~1) loess soils, were studied. Single oedometer-collapse tests were performed on intact loess specimens to investigate the collapse behavior of three loess soils. The microstructure and chemical composition of each loess before and after collapse test were characterized using scanning electron microscopy(i.e. SEM) and energy dispersive spectroscopy(i.e. EDS) techniques. The microstructural evolution due to wetting collapse was interpreted qualitatively and quantitatively in terms of the pore morphology properties. The results suggest that:(1) the collapse potential of each loess may rise again after a round of rise and drop, which could be failure of the new-developed stable structure under quite high vertical pressure. It implies that loess may collapse even if it has collapsed.(2) Q_3, Q_2~2 and Q_2~1 loess have different types of microstructure, namely, granule, aggregate and matrix type of microstructure, respectively.(3) The microstructural evolution due to loading and wetting is observed from a granule type to an aggregate type and finally to a matrix type of structure. The variations in distributions of pore morphology properties indicate that collapse leads to a transformation of large-sized pores into small-sized pores, re-orientation and remolding of soil pores due to particle rearrangement.(4) A porous structure is essential for loess collapse; however, the non-water-stability of bonding agents promotes the occurrence of collapse under the coupling effect of loading and wetting.

Processes and mechanisms of multi-collapse of loess roads in seasonally frozen ground regions: A review

Usually, the collapsible loess widely distributed across the world can serve as a type of foundation soil that meets its strength requirement after dense compaction and elimination of collapsibility. However, many problems such as cracks and differential settlement still occur in loess roads in the seasonally frozen ground regions after several years of op- eration. Many studies have demonstrated that these secondary or multiple collapses primarily result from the repeated freezing-thawing, wetting-drying, and salinization-desalinization cycles. Therefore, we conducted a research program to （1） monitor the in-situ ground temperatures and water content in certain loess roads to understand their changes, （2） study the effects of freezing-thawing, wetting-drying, and salinization-desalinization cycles on geotechnical properties and micro-fabrics of compacted loess in the laboratory, and （3） develop mitigative measures and examine their engineered effectiveness, i.e., their thermal insulating and water-proofing effects in field and laboratory tests. Our results and advances are reviewed and some further research needs are proposed. These findings more clearly explain the processes and mechanisms of secondary and multiple collapse of loess roads. We also offer references for further study of the weakening mechanisms of similar structural soils.

Review of collapse triggering mechanism of collapsible soils due towetting

Loess soil deposits are widely distributed in arid and semi-arid regions and constitute about 10% of land area of the world.These soils typically have a loose honeycomb-type meta-stable structure that is susceptible to a large reduction in total volume or collapse upon wetting.Collapse characteristics contribute to various problems to infrastructures that are constructed on loess soils.For this reason,collapse triggering mechanism for loess soils has been of significant interest for researchers and practitioners all over the world.This paper aims at providing a state-of-the-art review on collapse mechanism with special reference to loess soil deposits.The collapse mechanism studies are summarized under three different categories,i.e.traditional approaches,microstructure approach,and soil mechanics-based approaches.The traditional and microstructure approaches for interpreting the collapse behavior are comprehensively summarized and critically reviewed based on the experimental results from the literature.The soil mechanics-based approaches proposed based on the experimental results of both compacted soils and natural loess soils are reviewed highlighting their strengths and limitations for estimating the collapse behavior.Simpler soil mechanics-based approaches with less parameters or parameters that are easy-to-determine from conventional tests are suggested for future research to better understand the collapse behavior of natural loess soils.Such studies would be more valuable for use in conventional geotechnical engineering practice applications.

Numerical study on settlement of high-fill airports in collapsible loess geomaterials:A case study of Lüliang Airport in Shanxi Province,China

Foundation settlement is of great significance for high-fill engineering in collapsible loess areas.To predict the construction settlement of Lüliang Airport located in Shanxi Province,China,a plane strain finite element method considering the linear variation in the modulus,was carried out in this paper based on the results of geotechnical tests.The stress and deformation of four typical sections caused by layered fill are simulated,and then the settlement of the high-fill airport is calculated and analyzed by inputting three sets of parameters.The relative soft parameters of loess geomaterials produce more settlement than the relatively hard parameters.The thicker the filling body is,the greater the settlement is.The filling body constrained by mountains on both sides produces less settlement than the filling body constrained by a mountain on only one side even the filling thickness is almost the same.The settlement caused by the original subbase accounts for 56%−77%of the total settlement,while the fill soils themselves accounts for 23%−44%of the total settlement,which is approximately consistent with the field monitoring results.It provides a good reference for predicting the settlement of similar high-fill engineering.

Static load test and load transfer mechanism study of squeezed branch and plate pile in collapsible loess foundation

As a special geological phenomenon, the character of collapsible loess foundation is collapsible when penetrated by water. This character leads to the soil losing load bearing capacity largely and may lead to foundation failure. Pile is a popular foundation used in collapsible loess. The squeezed branch and plate pile is a new type of pile developed in recent years and has not be used in a project before. In this paper three squeezed branch and plate piles are tested in collapsible loess after immersion processing. The results may be used for reference in similar construction project, and to provide theoretical references for de- signing of the squeezed branch and plate piles in engineering practice.

Microstructure evolution of loess under multiple collapsibility based on nuclear magnetic resonance and scanning electron microscopy

In recent years, the acceleration of urbanization in loess areas has prompted frequent dismantling and reconstruction of old urban areas. Demolition and reconstruction of buildings involve multiple collapses of the foundation. To study microstructure evolution of loess under multiple collapsibility, this paper selects undisturbed loess samples from Guyuan, Northwest China for multiple compression tests. Using nuclear magnetic resonance(NMR) imaging and scanning electron microscopy(SEM) as auxiliary methods, a combination of qualitative and quantitative analyses was used to study the microstructure of loess samples before and after various number of collapses under different pressures. Results show that the loess does undergo multiple collapse under 200 kPa pressure. Pore is an important reason for loess collapse. The initial collapse comes primarily from the compression of macropores and mesopores, and the second collapse comes primarily from mesopore compression. The compression process of loess can be roughly divided into two stages. First, under the action of dissolution and compression, the relative displacement of soil particles occurs. Macropores and mesopores are destroyed first, generating small pores. Second, with increasing pressure and times of collapses, pore compression gradually transforms into small pore compression. During the first collapse, particle aggregates disintegrate due to water and pressure. However, with increasing times of collapses, the contact relationship between particles gradually changes from the point contact to face contact. Loess particles tend to gradually become rounded. The study of the microstructure provides the possibility to further reveal the mechanism of multiple collapsibility of loess.

Evaluation and Analysis of the Effect of Lignin Amelioration on Loess Collapsibility

The road subgrade and road surface in collapsible loess area are prone to many engineering diseases such as uneven subgrade settlement,insufficient bearing capacity of soaked foundation,collapse and instability of sub-grade side slope due to the special properties of loess.As an environment-friendly,low-cost soil modifier with good adhesion and chelation properties,lignin has been considered to be used in highway subgrade construction.In order to explore the effect of lignin on loess,the compressive and collapsible properties of modified loess with different lignin contents were analyzed based on consolidation compression test.The improvement mechanism of lignin on loess collapsibility was studied by means of infiltration test and SEM test.The results show that lignin fibers can promote the agglomeration of loose particles and form a network structure in the soil particle pores,enhance the cementation strength between particles and soil skeleton,and reduce the permeability of loess.With the increase of lignin fiber content,the improvement degree of loess collaps ility shows a trend of first increasing and then decreasing.When the lignin fiber content is 2%,the effect is the best,and the improved loess ollapsi-bility is eliminated.

Argentinean Collapsibility Loess. Characterization and Evaluation on the Constrained Modulus

Loessic soil in the north-west of Argentina, which consists of silt and silty clay with reduced content of fine sand, has collapsible characteristics. This means that by increasing the moisture content close to the liquid limit value, the loess soil＇s macro porous structure breaks experiencing large volumetric deformations. The collapse pressure and soil constrained modulus are fundamentals parameters for the characterization of these soils and the study of solutions to geotechnical problems. In this work we study the loess from the north-west region of our country, especially the Santiago del Estero＇s plain, based on numerous field tests and laboratory tests in order to correlate the modulus and collapse pressure from double-odometer test with the blow count from SPT （standard penetration test）. It also analyzes the influence of these parameters on moisture content, void ratio and the presence of salts and calcareous concretions in soils and discusses the validity of these correlations as well as those proposed by other authors.

Experimental study on improving collapsible loess with cement

The collapsibility of loess ground can directly affect stability of subgrade. Therefore, how to adopt practical technical measures to reduce or eliminate its collapse deformation is an important content in foundation design in collapsible loess zone. Selecting collapsible loess from Fuxin-Chaoyang highway in Liaoning, the authors conducted a series of tests for improving loess with cement. The loess in different water content was mixed with the cement in varying proportions, unconfined compression strength for the samples at four different curing periods were tested, and the relationships of improved soil strength among cement mixture ratio and curing periods were analyzed. When the curing periods are certain, the strength of loess increases along with the mixture ratio increases; when the cement mixture ratio is 5%-15%, the scope of increases is quite obvious; when the mixture ratio is greater than 15%, the tendency of intensity increases turns slow. When the mixture ratio for the specimen is certain, the intensity of the test specimen increases along with the curing period increases, the intensity grows obviously in 28 days, and the growth rate is small in 28-90 days, the intensity tends to be steady in the curing period of 90 days.

Experiment Study of Dynamic Compaction Applied in Collapsible Loess

The collapsibility of loess, which can be effectively eliminated by the dynamic compaction, does great harm to the safety of constructions. The effect of the dynamic compaction is evaluated through the contrast and analysis of the physical and mechanical properties of the collapsible loess before and after dynamic compacting. The compacting effect can be divided into three phases along the depth, and the most effective improved depth is from 3 to 8 m.

The Research on Treating Collapsible Loess by Down Whole Deep Compaction and Cement Fly-ash Gravel

The treatment of loess foundation is always difficult.The analysis of its advantages and mechanism of treating loess foundation by CFG,on the base of project geology,through construction example,we suggest the compound plan by both DDC and CFG.The tests illustrates that the down hole deep compaction and cement-fly ash-gravel are effective foundation treatment method to eliminate the collapsibility of loess,increase the bearing capacity and improve the behavior of composite foundations.

Constructing Large Capacity Power Plant on Collapsible Loess Stratum with Huge Thickness

1 Preface In the northern and northwestern parts of China, quite a large portion of area, approximately 630,000 km^2, is covered by loess and loess-liked soils. The loess thickness ranges from several meters to several hundred meters along the river’s terraces to those geomorphologic plateaus. In geology, "China Loess" has become a geologic term, because the loess in China has evolved with the widest distribution and greatest thickness in the world, and is also a typical and significant deposit in Quaternary Period.

Influence of Collapsible Loess on Foundation and its Treatment Strategy

The properties of collapsible loess are complex.The self-gravity of overlying soil,sei gravity stress and additional stress act together,which will damage the soil structure and lead to the deformation of the soil structure.Collapsible loess is widely distributed in Northwest and Northeast China.A series of problems caused by its structural characteristics will affect the quality of foundation construction.Therefore,construction enterprises need to deeply study the foundation treatment measures of collapsible loess,so as to avoid the uneven settlement after the construction of collapsible yellow soil foundation.This paper analyzes from the judgment and classification of collapsible loess,studies the impact of collapsible loess on building fbxmdation construction,and explores the specific construction treatment measures of collapsible loess,in order to promote the effective application of foundation construction.

古土壤层间富水对黄土场地湿陷性的影响

黄土地层中往往交替分布着多层红色古土壤层,由于其硬度相对较高,遇水不易湿陷,对黄土地层湿陷量的测量结果有很大的影响。然而,对于古土壤对湿陷量的控制作用目前还缺乏相关研究,导致对湿陷地层湿陷量的科学取舍缺乏理论依据。为此该文统计了黄土高原区的浸水试验结果,分析了不同区域湿陷量室内外差异特征,并以西安地区两个试验场地的大型浸水试验为研究对象,进行了不同试验条件下土体中水分的扩散、含水率的变化、土压力变化以及累计湿陷量等测量工作。最终结果表明,古土壤层的存在阻碍湿陷进程,在阻止水分下渗的同时,阻碍深部地层湿陷量传递至地表,使得湿陷量实测值与计算值之间的差异与古土壤层数呈现正相关的趋势。该文将为普遍存在古土壤的黄土地层自重湿陷机理的研究提供参考。

湿陷性黄土区输水隧洞盾构施工风险评估

为了提高针对湿陷性黄土区输水隧洞盾构法施工安全评估的可靠性,从人员管理、机械材料、施工技术、施工环境4个方面构建较为完善的施工安全风险评估指标体系;利用灰色-DEMATEL方法对中心度进行处理得到风险指标权重;以风险发生概率和风险损失程度两个维度为基础构建二维云评估模型,其中发生概率由主、客观方法相结合来确定,损失程度通过改进雷达图法从人员伤亡、经济损失、工期延误、功能缺陷和环境影响5个方面来综合确定。将该评估模型运用到引汉济渭工程X输水隧洞盾构法施工项目的安全评估中。结果表明:该项目施工风险评估等级为Ⅱ级,施工安全风险程度较小,评价结果符合工程实际,验证了该评估模型的可行性和有效性。研究成果可为湿陷性黄土区类似输水隧洞盾构法施工安全风险评估提供借鉴和参考。

渭北黄土台塬区水库水下岸坡稳定形态预测模型研究

黄土地区水库塌岸预测仍以卡丘金法等经验图解法为主,此类方法预测结果的准确性取决于对岸坡剖面形态的准确描述。为了提高黄土地区水库塌岸宽度预测的准确性,选取渭北黄土台塬区选择典型水库进行现场调查,对原河道岸坡形态与蓄水后岸坡形态进行对比分析,在此基础上建立了水下岸坡形态预测模型。结果表明,渭北黄土台塬区水库岸坡塌岸稳定后,水上岸坡呈直立状,高度可达30 m以上,水下岸坡呈曲线形,受水深和岸坡高度共同影响,塌落物可能露出水面。在此基础上基于对数螺线方程建立了水下岸坡形态预测模型,并与经典图解法所用直线型岸坡进行对比,误差分析结果表明采用对数螺线方程进行水下岸坡形态预测时堆积体积误差为4.50%~39.70%,均值为12.64%,而直线型岸坡的预测误差为25.75%~124.69%,均值为75.69%。即采用对数螺线方程可以更好的测水下岸坡形态及水下堆积量。相关研究成果对黄土台塬区水库塌岸预测方法的改进,以及黄河流域的渭北黄土台塬区的环境保护与高质量发展具有实际意义。

黄土堆填场地转化为泥石流物质过程--以兰州市碱水沟为例

黄土地区人工堆填场地是近几年兰州市拓展城市用地空间的一个新举措,在场地边缘形成较多堆填边坡,成为参与泥石流的潜在物质。以兰州市碱水沟“4•19”泥石流灾害为例,结合粒径分析试验、扫描电镜图像及野外入渗试验,探讨其特征及运行模式,并提出相应的治理建议。结果表明:1)黄土地区人工堆填体颗粒粒径<1.000 mm的物质约占11.4%~55.2%,有利于泥石流启动;2)以2个相距170 m的泥位调查断面做比较,下游断面流量增加2.5倍,说明入渗引起的黄土湿陷能促进泥石流物质的转化率;3)入渗试验表明:黄土堆填场地渗透性强,低水头条件下8 min左右达到稳定入渗状态,高水头条件下12 min左右达到稳定入渗状态,能在较短的时间内使土壤软化,影响人工堆填场边坡稳定,提高泥石流物质转化速度;4)黄土地区人工堆积体场地诱发的泥石流流动模式复杂、多变,兼具波状流和间歇流的特点。在此基础上,笔者提出过程管理和重点防护相结合的防治模式。

靖边Q_(3)砂质黄土湿陷特征及其微观机制研究

沙漠-黄土高原过渡带的砂质黄土遇水后会产生明显的湿陷变形,广泛分布于陕北靖边的长庆油田工程遭受砂质黄土湿陷危害严重,揭示砂质黄土湿陷特性及湿陷机制是长庆油田工程建设迫切需求。为此,以陕北靖边沙漠-黄土高原过渡区Q_3砂质黄土为研究对象,通过室内基本物理性质、X射线衍射和湿陷试验,研究砂质黄土基本物理特性和物质组成,厘清湿陷特性、影响因素及其规律。在此基础上,通过扫描电子显微镜试验和孔隙及裂隙图像识别分析手段,探究砂质黄土湿陷前后的微观结构,孔隙大小分布、方向频率及丰度变化,从微观角度揭示砂质黄土湿陷机制。研究表明:靖边Q_(3)砂质黄土具有湿陷性,且湿陷系数随着轴向压力的增加先增加后减小,随着干密度、含水率的增加而逐渐减小,轴向压力为150 kPa时湿陷系数达到峰值;靖边Q_(3)砂质黄土以石英、钠长石、白云母以及方解石为主,颗粒形态多以棱角状亦或次棱角状为主,具有架空排列结构和架空孔隙,以点-点接触为主,黏粒胶结物多分布在骨架颗粒接触处。靖边Q_(3)砂质黄土的架空孔隙结构崩塌是砂质黄土湿陷变形的本质,为湿陷提供主要空间,黏粒胶结物包裹少量碎屑颗粒形成的凝块结构产生的沉降变形为砂质黄土湿陷变形提供了增量。研究成果为长庆油气田工程建设区砂质黄土湿陷性评价提供了数据依据。

水库黄土岸坡塌岸预测模型及参数敏感性研究

塌岸宽度预测是水电工程重要研究问题之一。现有的经验图解法基于塌岸最终稳定坡形进行塌岸宽度预测,难以反映塌岸的过程、特征,且对岸坡岩土体的物理力学性质考虑也较少。基于此,研究提出了基于极限平衡理论的黄土岸坡塌岸宽度预测模型,确定了模型关键参数取值,并对参数敏感性进行了分析。结果显示,基于极限平衡理论进行预测塌岸宽度时应选取合理的水下堆积系数,水下堆积系数随岸坡高度和水深的增加而增加。而岸坡角度对塌岸宽度的影响较小,且主要影响首次塌岸宽度,对后续塌岸宽度的影响不大。随着岸坡坡度的增大,塌岸宽度呈波动上升趋势。同时,塌岸参数敏感性分析结果显示,参数敏感性排序由大到小依次为岸坡高度、水下岸坡内摩擦角、水下岸坡黏聚力、水上岸坡重度、水深与岸高比、水上岸坡黏聚力、岸坡坡度、水上岸坡内摩擦角、水下岸坡重度,这与岸坡稳定性的参数敏感性分析结果基本一致。研究结果对黄土岸坡塌岸宽度预测具有重要意义。

湿陷性黄土地基中桩基负摩阻力计算新方法

负摩阻力对湿陷性黄土地基中桩基的承载变形性状具有重要影响,现行桩基规范及黄土规范推荐的负摩阻力计算方法不能反映黄土桩基负摩阻力的实际性状。对近30年来在黄土地区开展的钢筋混凝土灌注桩现场浸水试验实测数据进行统计,分析黄土浸水完成后桩身中性点深度、桩身最大负摩阻力深度及负摩阻力系数与桩长径比的关系,通过线性拟合得到了中性点深度比、最大负摩阻力深度比及负摩阻力系数与桩长径比的经验表达式,提出湿陷性黄土地基中桩基负摩阻力计算的新方法。新方法用三角形表示负摩阻力沿深度的分布,能反映负摩阻力从桩顶向下先逐渐增大、达到最大值后随深度增加逐渐减小、最终在中性点处减小为零的分布特征。将提出的方法、桩基规范及黄土规范方法的计算结果与实测结果进行对比,结果表明,桩基规范预测的中性点深度比优于黄土规范,黄土规范预测的最大下拉荷载优于桩基规范,而用新方法计算的负摩阻力最接近实测结果。