水泥加固黏性土微观特征试验研究

通过电镜扫描试验与图像分析手段,对加载前、后不同土性及不同水泥掺量的加固黏性土微观结构进行研究,定量分析其微观孔径大小、孔隙形状系数以及孔隙定向角等参数变化的影响,并探索其与加固土体强度及承载机制的关系。研究结果表明:(1)水泥黏性加固土在加载前、后孔径分布和孔隙形状系数呈现出不同的规律,水泥掺量小(5%)的加固土加载后孔径减小,孔隙形状系数增大,宏观表现为剪缩;而水泥掺量高(20%)的加固土加载后整体孔径增大,孔隙形状系数减小,宏观表现为剪胀;(2)水泥黏性加固土体无论其水泥掺量的高低,在加载过程中,其颗粒或团粒均在内部应力作用下发生定向性的滑移或转动,从而导致孔隙长轴方向发生改变,孔隙定向角分布由均匀变为集中,孔隙定向性趋于显著。     

软土加固过程中微结构变化的分形研究

土体内部孔隙的特征及分布情况是土体微结构变化的内因,也是决定土体物理力学性质的主要因素。在软土性质发生改变时,孔隙的变化是最直接、最显著的。所以,对土体微观结构进行研究,应着重研究土中孔隙的变化。由于土中孔隙的复杂性及非确定性,很难用传统的几何方法对其研究和描述。为研究加固前后软黏土微结构的变化情况,在珠江三角洲某高速公路软土路基利用动力排水固结法加固的施工现场,取不同深度内加固前后土样进行压汞测试,用分形理论对压汞测试数据进行分析,探讨软土中孔隙的分布特征,在此基础上提出了土中孔径划分的方法,进而探讨动力排水固结法加固软土地基的微结构变化,建立了加固后土样的孔隙度分维数与土体固结度之间的关系,研究结果表明土中孔隙分布具有分形特征,用孔隙度分维数可以实现对地基加固程度的预测。     

基于扫描电镜和核磁共振技术的分散性土改良微观结构性变化研究

分散性土是一种在低含盐量水中易分散解体的特殊性黏土，这一特性导致了许多水利与岩土工程的失稳与破坏，分散性黏土在中国东北地区分布广泛。为了改良分散性黏土抗冲蚀能力，探索分散性土孔隙结构演化特征，分析木质素改良分散性土作用机理，以黑龙江省南部引嫩工程分散土为研究对象，通过分散性鉴别试验、扫描电镜(SEM)试验与核磁共振(NMR)试验对不同木质素掺量(0%～10%)的分散性、元素组成与微观结构进行测试与观察。试验结果表明：(1)木质素可有效改良分散土分散性，木质素掺量≥3%且养护龄期≥7 d即可有效加强土体抗冲蚀能力。(2)SEM图像分析与NMR试验具有较好的一致性，随着木质素掺量增加，土体中微小孔隙逐渐发育成大孔径孔隙，孔隙率呈先减小后增大的趋势，在冻融循环作用下，土体中大孔径孔隙呈增加趋势，对木质素掺量≤5%的改性土影响更为显著。(3)木质素中的Ca²⁺与土体中Na⁺发生离子交换反应，可吸附在土体表面并形成一层疏水层，提高土体的抗冲蚀能力，但木质素掺量过多，其自带的磺酸基相互吸附，导致土体黏度增加，难以压实，形成大孔缺陷。结果表明，适量的木质素...     

聚丙烯酰胺改性石灰固化吹填土固结特性研究

吹填土加固技术成为当前软基处理的一项技术难点。石灰等固化材料的掺加提升了吹填土的抗压、抗剪强度,但降低了其抗变形能力。通过添加不同掺量的聚丙烯酰胺,利用室内一维固结压缩试验探讨其对石灰固化吹填土的固结特性的影响,并对固结后的土样进行了压汞试验分析,探讨了微观孔隙特征与宏观变形特性的相关性。研究表明:聚丙烯酰胺可以有效改善石灰加固吹填土的抵抗变形能力,改善其脆性破坏的特点;聚丙烯酰胺掺加增强了石灰加固吹填土体的结构性特征,结构屈服应力较单一石灰添加提高了54%;聚丙烯酰胺的增加提高了石灰固化吹填土的固结系数,且掺加量越大,固结速率的提升幅度越大;聚丙烯酰胺、石灰的组合掺加可以改善吹填土固结后的孔径分布特征,相同石灰掺加量下,随着PAM含量的增加,加固土体的大直径孔隙的含量不断增多,渗透性增强,固结速率增大,微观孔隙特征与宏观变形特性保持了内在的一致性。     

基于图像处理的Q3黄土的微观结构变化研究

 黄土地质灾害的发生与黄土的微结构特征息息相关。针对黄土微结构图像定量分析中存在的问题,对原状及循环荷载作用后Q3黄土的微观结构特征进行研究。首先探讨SEM图像处理中的降噪和分割方法,将Lee图像增强算法应用于图像的降噪处理,将FCM聚类方法应用于灰度图像的分割,在此基础上计算循环荷载作用前后Q3黄土的孔隙面积、孔径、圆度、形态比等参数。结果表明：(1) 通过SEM图像表观特征的分析可知,在动应力的作用下,大孔隙数目减少,颗粒排列逐渐密实,接触关系也逐渐由点接触变为面接触;(2) 通过对孔隙面积、数量变化的分析可知,循环荷载产生的变形主要由于大孔隙破坏造成;(3) 通过对孔隙定向分布特征的研究可知,原状黄土的孔隙具有定向特性,在循环荷载的作用下,孔隙的定向性发生改变;(4) 通过对孔隙形状分布特征的研究可知,在动应力作用下,土颗粒发生转动,部分孔隙连通性受损。本研究可为揭示土体宏观受力的变形机制提供依据。     

碳化作用对水泥固化/稳定化铅污染土渗透特性的影响

采用人工配制铅污染土的方法制备不同水泥掺量和不同铅浓度的土样,开展碳化前后水泥固化/稳定化铅污染土的渗透试验,并分析试样的宏观渗透系数和微观孔隙结构之间的内在联系。研究结果表明,水泥掺量增加会降低试样的渗透系数;水泥掺量为7.5%时,碳化作用下渗透系数提高;水泥掺量为15%时,碳化作用下渗透系数降低。铅浓度越高,试样的渗透系数越高,而碳化作用又会增大试样的渗透系数。水泥掺量增加会显著降低试样的孔隙率;碳化作用使得试样中孔径小于0.1μm的孔隙增多,大于0.1μm的孔隙减少。     

考虑胶结作用的木质素固化粉土边界面塑性模型

为研究木质素固化粉土的应力–应变特性,通过无侧限抗压强度试验和微观结构分析,探讨木质素固化土的胶结特性。基于边界面塑性理论,引入硬化参数、应力剪胀参数和胶结破坏速率等参数,提出考虑胶结作用的木质素固化土边界面塑性模型,采用非相关联流动法则和改进映射法则描述土体的不同破坏模式,并阐述模型中各参数的意义及计算方法。根据室内固结试验和三轴压缩试验,对木质素固化粉土的应力–应变、应力剪胀和超孔隙水压力变化特征进行分析,并验证了本文所提模型的有效性。研究表明:木质素产生的胶结作用是土体工程性质改善的主要原因之一;12%掺量木质素固化土屈服应力和不排水抗剪强度较素土分别提高约90%和40%,高、低围压下土体应力剪胀特性不同,围压对超孔隙水压力的变化影响较大;通过试验验证了模型计算的准确性,该模型可描述土体在不同受力状态下的应变特征,具有原理简单,参数明确的特点,可为固化土应力–应变的数值计算提供相应的理论基础。     

水泥改良前后土体热物理参数试验研究

目前,软土地区地铁隧道端头加固往往采用化学加固后辅以人工冻结,关于典型土层水泥改良前后土体、含盐土体、含盐土层水泥改良前后土体的热物理和力学特性室内试验的研究还很少,缺少相关试验参数。结合南京地铁10号线过江隧道盾构始发工程,对始发端头典型的两种土质进行了水泥改良前后土体热物理参数的室内试验。试验表明:不同土质不同温度下导热系数和容积热容均随着水泥掺量和龄期的增大而减小,常温下的导热系数明显低于-10 ℃时的导热系数,常温下的容积热容明显高于-10 ℃时的容积热容;常温下不同土质的导温系数随水泥掺量的增大而线性增大,而-10 ℃时不同土质的导温系数随着水泥掺量的增大而缓慢减小,不同土质不同温度的导温系数均随龄期的增大而增大,常温下的导温系数明显低于-10 ℃时的导温系数。最后给出了数值分析时端头土体热物理参数的建议取值并经现场验证取值正确可行。     

滑带土环剪剪切面的微观观测与分析

 滑带土的残余强度一直是边坡工程研究的重点。由于剪切面微观结构在剪切前后的变化与宏观的土体强度存在必然的联系,所以针对一古滑坡滑动带的含砂黏土进行环剪试验,采用电镜扫描技术分析剪切前后微观结构的变化,探讨微观结构与土体宏观力学特征的关系。研究表明,含砂黏土的强度特征不同于一般黏性土,没有明显应变软化特性,粗粒含量对峰后强度有比较明显的影响。剪切过程会导致颗粒出现明显的定向性排列,微观结构发生改变,出现颗粒破碎、孔隙率提高,颗粒形状变狭长的现象,这与滑带土的宏观强度变化有密切的关系。另外,通过周长–面积法对剪切面颗粒形态的分形研究,发现剪切面土颗粒微观形态具有明显的分形特征,分形维数与滑带土的残余强度指标呈线性负相关关系。     

结构性粘土剪切带的微观分析(英文)

土体剪切带的形成与土体逐渐破损理论是当今国际力学界和岩土工程界十分关注的焦点。本文在结构性粘土三轴试验的基础上，首先讨论了剪切带形成的宏观力学条件及其倾角，接着借助于扫描电镜，并采用微观定量测试技术，对剪切带及其周围土体的微观结构进行了分析。结果表明：土体具有足够的结构强度是剪切带形成的条件之一，剪切带的倾角与Ｒｏｓｃｏｅ理论不相符；剪切带的厚度在宏观、微观上有差异；剪切带内外的土体，在孔径分布、孔隙比、定向度、各向异率等方面差别很大；剪切面上剪胀与剪缩并存，总体呈剪缩；带内土体亦强烈剪缩；土样宏观却呈轻微剪胀特性。这些成果既验证了沈珠江砌体理论，也为建立土体结构性指标，发展结构性模型及土体逐渐破损理论打下基础。     

Fibre-reinforced cemented soils compressive and tensile strength assessment as a function of filament length

This study aims to develop a dosage methodology based on tensile and compressive strength for artificially cemented fibre reinforced soils considering filament length. The controlling parameters evaluated were the fibre length (l), the cement content (C), the porosity (η) and the porosity/cement ratio (η/C_iv). A number of unconfined compression and split tensile tests were carried out in the present work. The results show that fibre insertion in the cemented soil, for the whole range of cement content studied, and the increase of reinforcement length improve unconfined compressive and split tensile strengths. It was shown that the porosity/cement ratio, in which volumetric cementitious material content is adjusted by an exponent (0.28 for all the fibre-reinforced and non-reinforced cemented soil mixtures) to end in unique correlations for each mixture, is a good parameter in the evaluation of the unconfined compressive and split tensile strength of the fibre-reinforced and non-reinforced cemented soil studied. Analysis of variance (ANOVA) performed on the results of a factorial experiment considering the effect of adjusted cement content, fibre length and porosity showed that all of these factors are significant in affecting the measured changes in split tensile and unconfined compressive strength values. Finally, unique dosage relationships could be achieved linking the unconfined compressive strength (q_u) and the split tensile strength (q_t) of the sandy soil studied with porosity/cement ratio (η/C_iv) and fibre length (l).     

Microstructure characteristics of cement-stabilized sandy soil using nanosilica

An experimental program was conducted to explore the impact of nanosilica on the microstructure and mechanical characteristics of cemented sandy soil.Cement agent included Portland cement type II.Cement content was 6% by weight of the sandy soil.Nanosilica was added in percentages of 0%,4%,8% and 12% by weight of cement.Cylindrical samples were prepared with relative density of 80% and optimum water content and cured for 7 d,28 d and 90 d.Microstructure characteristics of cementnanosilica-sand mixtures after 90 d of curing have been explored using atomic force microscopy(AFM),scanning electron microscopy(SEM) and X-ray diffraction(XRD) tests.Effects of curing time on microstructure properties of cemented sandy soil samples with 0% and 8% nanosilica have been investigated using SEM test.Unconfined compression test(for all curing times) and compaction test were also performed.The SEM and AFM tests results showed that nanosilica contributes to enhancement of cemented sandy soil through yielding denser,more uniform structure.The XRD test demonstrated that the inclusion of nanosilica in the cemented soil increases the intensity of the calcium silicate hydrate(CSH) peak and decreases the intensity of the calcium hydroxide(CH) peak.The results showed that adding optimum percentages of nanosilica to cement-stabilized sandy soil enhances its mechanical and microstructure properties.     

Effect of fiber-reinforcement on the strength of cemented soils

This study aims to verify the differences in the strength of an artificially cemented sandy soil with and without fiber reinforcement. The controlling parameters evaluated were the amount of cement, porosity, moisture content, and voids/cement ratio. A series of unconfined compression tests and suction measures were carried out. The results show that fiber insertion in the cemented soil, for the whole range of cement studied, causes an increase in unconfined compression strength. The UCS increased linearly with the amount of cement and reduced with the increase in porosity (η) for both the fiber-reinforced and unreinforced specimens. A power function fits well as the relation between unconfined compressive strength (UCS) and porosity (η). Finally, it was shown that the voids/cement ratio is a good parameter in the evaluation of the unconfined compressive strength of the fiber-reinforced and unreinforced cemented soil studied.     

Mechanical behaviour of a compacted well-graded granular material with and without cement

Cement additions improve the performance of granular soils. However, most literature examples of cement additions are in poorly graded sands, either to mimic the behaviour of sandstones or to accentuate the mechanical differences between cemented and uncemented soils. In this article, the behaviour of a well graded granular soil, used for base and sub-base of roads, was studied by doing triaxial tests on cemented and uncemented samples. Samples were compacted to achieve a dense fabric and tested at stresses commonly used in practice. Sieving was used to understand if breakage is important and to determine the grain size distributions of the samples after compaction and shearing. The results show that the addition of small percentages of cement greatly increase stiffness and dilation. Thereby, generating larger strengths; this is particularly important at low confining stresses in roads and parking areas, where this material is commonly used. At large strains, the results show that different Critical State Lines exist for both the uncemented and cemented soils. Each line has a different slope, which is believed to be the result of the evolution of the grain size distribution of the cemented soil. The normalised data indicate that a unique state boundary surface can be determined for all three tested soils.     

污水环境对水泥土力学性能的影响试验研究

由于多数地下水泥土工程直接与地下腐蚀性介质环境接触,必将导致水泥土材料的逐步劣化甚至失效破坏。以某市区工地附近明渠排放的污水作为侵蚀性介质,制作了不同水泥掺量的水泥土试件,通过对比试验,研究了污水环境和清水环境下不同水泥掺量、不同龄期的水泥土抗压强度和抗剪强度。结果表明,在污水或清水环境下,相同水泥掺量水泥土30 d 龄期的抗压强度几乎相等,随着龄期的增加其抗压强度均逐步增大,但污水环境下其抗压强度增长的幅度明显小于清水环境,90 d 后清水环境的水泥土抗压强度不再增长,而污水环境的抗压强度开始降低;污水环境和清水环境下的水泥土内摩擦角和黏聚力随龄期、水泥掺量的增加均逐步增大,污水环境下龄期90 d后的内摩擦角和黏聚力均开始降低。     

Triaxial shear behavior of a cement-treated sandegravel mixture

A number of parameters, e.g. cement content, cement type, relative density, and grain size distribution,can influence the mechanical behaviors of cemented soils. In the present study, a series of conventionaltriaxial compression tests were conducted on a cemented poorly graded sandegravel mixture containing30% gravel and 70% sand in both consolidated drained and undrained conditions. Portland cement usedas the cementing agent was added to the soil at 0%, 1%, 2%, and 3% （dry weight） of sandegravel mixture.Samples were prepared at 70% relative density and tested at confining pressures of 50 kPa, 100 kPa, and150 kPa. Comparison of the results with other studies on well graded gravely sands indicated moredilation or negative pore pressure in poorly graded samples. Undrained failure envelopes determinedusing zero Skempton抯 pore pressure coefficient 餉 ？0？criterion were consistent with the drained ones.Energy absorption potential was higher in drained condition than undrained condition, suggesting thatmore energy was required to induce deformation in cemented soil under drained state. Energy absorptionincreased with increase in cement content under both drained and undrained conditions.
2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.     

Fiber reinforcement effects on sand considering a wide cementation range

This paper describes laboratory drained standard triaxial tests conducted on artificially cemented Osorio sand specimens reinforced with randomly oriented discrete extensible polypropylene fibers. Cemented specimens were prepared with cement contents varying from 0% to 10% by weight of dry sand and cured for seven days. Fiber length and diameter were 24 mm and 0.023 mm, respectively, in the contents of 0% and 0.5% by weight of dry sand–cement mixture. Test results indicated that the addition of cement to sand increases stiffness, peak strength and brittleness. Both cement and fiber insertions affect dramatically the stress–dilatancy behavior of the sand. The fiber reinforcement increases peak strength just up to a certain cement content (up to about 5% in the present study), increases ultimate strength, decreases stiffness and changes the cemented sand brittle behavior to a more ductile one. The triaxial peak strength increase due to fiber inclusion is more effective for smaller amounts of cement, while the increase in ultimate strength is more efficacious when fiber is added to sand improved with higher cement contents. Peak strength envelopes indicate that the friction angle is about 46° for fiber-reinforced specimens containing up to 7% cement content, reaching 51.5° for higher cement contents. Cohesion intercept is drastically affected due to fiber addition to all cement contents, increasing for cement contents up to 4% and reducing for higher cement contents. It is important to make clear that the trends observed herein are relevant for the soil, cement and fiber type used in the present research and that further studies are necessary to generalize such findings.     

氯化镁对水泥土早期强度的影响研究

通过水泥加固不同含量的氯化镁污染土室内28 d抗压强度、X射线衍射（XRD）和微观扫描电镜试验（SEM）,研究了氯化镁对水泥土的早期强度影响机理。XRD分析结果表明：氯化镁参与反应生成的M-S-H、M-A-H、氯化钙结晶、轻质氯化镁结晶和水化氯铝酸钙等产物对水泥土产生分解与结晶的复合作用；SEM图象分析表明：氯化镁使水泥土的孔径分布发生改变,当氯化镁含量较低时,水泥土中的水泥水化产物与结晶物将颗粒连结在一起,形成较强的结构联结,使其孔隙率变小,对水泥土的强度增长有利；随着氯化镁含量增大,土颗粒周围的凝胶物被分解,使水泥土的孔隙率增大与胶结程度下降,对水泥土的强度不利。     

Ring shear characteristics of discontinuous plane

Residual shear strength is an essential parameter in evaluating the long-term stability of reactivated landslides in geotechnical engineering. According to previous studies, earthquake-induced landslides may occur on discontinuous planes, such as bedding planes, between weathered and unweathered mudstones having different cementation properties resulting from diagenesis. However, the shear behaviour at the contact surfaces between the cemented and the non-cemented soil layers has not yet been sufficiently investigated. The objective of this study is to elucidate the residual strength characteristics of artificial bedding planes that model the actual behaviour of slip surfaces occurring between two layers with different degrees of cementation. The experimental tests were conducted with a ring shear test apparatus. Additionally, in order to simulate the realistic mechanical behaviour of naturally cemented clay, artificial cementation bonds were created by adding a cementing agent at different ratios to clay slurry. A series of ring shear tests was performed under various conditions on one-layer non-cemented and cemented kaolin samples, respectively, as well as on two-layer specimens composed of one layer each of cemented kaolin and non-cemented kaolin. The test results showed that the residual friction angle of the two-layer combinations of non-cemented and cemented kaolin was approximately 33.6% lower than that of pure kaolin. In contrast, the residual friction angle of cemented kaolin may be as much as 6.2° greater than that of non-cemented kaolin. At cement ratios of up to 2%, the stress ratio of cemented kaolin increased as the shear displacement rate increased. As the cement content was increased beyond 2%, the degree of increase was not significant. These results suggest that the residual strength of cemented kaolin at cement contents greater than 2% is independent of the shear rate.