膨胀粘土填筑条件的控制

本文介绍了我国中南地区常见的一种以含伊利石为主的、具有中等偏高膨胀性的粘土的基本特性和用这种土在室内制备不同压实含水量和容重的试样置于固结仪中进行试验,以测定其受荷膨胀和膨胀压力的成果。经研究认为:为了控制压实的膨胀粘土的膨胀势,必须兼顾填筑容重和填筑含水量,即前者要比按标准击实方法求得的容重低一些,而后者则要较高于标准最优值,以保证不致发生较大的膨胀。文中还讨论了压实的膨胀粘土结构对膨胀性的影响。     

Swelling behaviour of expansive soils with recycled geofoam granules column inclusion

Structures founded on expansive soils experience large uplift pressure due to the high swelling nature of these soils. In this investigation, an effort is taken to reuse the waste expanded polystyrene (EPS) beads to form geofoam granules column (GGC) and quantify the swelling behaviour of expansive soil with and without GGC inclusion. Several swell tests were carried out in statically compacted soil specimen with uniform thickness of 100?mm placed in a large scale one dimensional consolidation apparatus which can accommodate the California bearing ratio (CBR) mould. Attempts were made to ascertain the performance of GGC inclusion in expansive soil by varying diameters of GGC (25?mm, 40?mm, 50?mm and 75?mm), density of formed GGC (15?kg/m³ and 20?kg/m³) and two placement conditions of soil samples (by varying moisture content). Tests results were analysed which showed that the percentage of swell, swelling pressure and the time rate of swell decreases upon inclusion of GGC and significant reduction is noticed for lesser GGC density. Further, the mechanism of GGCs influence in control swelling of expansive soil is explained with the help of soil-GGC interaction.     

Zu Verformungsverhalten und Rissgefährdung einer mineralischen Abdichtung im Deponiebau – infolge gekoppelter zyklischer hydraulischer und mechanischer Beanspruchung

Deformation behaviour and crack initalisation of clay liner related to landfills – due to cyclic hydraulic and mechanic actions. Settlement and swell/shrink behaviour of clay liner under influence of environment are major topics of many research studies focussing expansive soil behaviour related to landfill applications. Depending on magnitude of surcharge pressure acting on the liner and wetting‐drying behaviour of the soil, settlement, together with swelling and shrinkage, may result in cracks or contrastly may close initial cracks induced by mechanical compaction in the field. In this paper, an analysis of the effects of field swelling and shrinkage of a compacted polymer‐enhanced bentonite‐sand mixture is presented. The analysis performed was based on field suction measurement data. Two different approaches; namely, single‐valued effective stress concept and two‐independent stress state variable approach for unsaturated soils were adopted in the analysis and comparison is presented in this paper.     

The unique relationship between swell percent and swell pressure of compacted clays

Expansive soils exhibit high volumetric deformations, posing a serious threat to the stability of structures and foundations. However, measurement of swelling properties is time consuming and requires special and expensive equipment. This study made an attempt to investigate the relationship between these parameters and easily obtained soil properties using various clay mineral mixtures to obtain soils in a wide range of plasticity indices. Free swell percent was correlated to clay percent, water content, dry unit weight, plasticity index, liquidity index and cation exchange capacity using multiple regression analyses. A very high (R = 0.94) fit was also found for a proposed relationship between the percent swell and swell pressure values for samples having a swell pressure ≤300 kPa. It is concluded that the proposed equations offer a rapid and inexpensive substitute for laboratory testing of swell percent/swell pressure in the preliminary stages of site investigations.     

膨胀土路基石灰改良试验研究

膨胀土是一种特殊性质的土,不同的掺灰率对膨胀土性质的改变也不同。文章对膨胀土进行掺石灰试验研究,探讨掺石灰对膨胀土的胀缩性与强度的影响规律,对比分析了膨胀土改良后的最佳含水量、最大干密度、无侧限抗压强度、CBR等指标与不同掺灰率之间的关系,确定了膨胀土的最佳掺灰率,试验结果对同类工程具有参考意义。     

Analyse du comportement d’un sol argileux sous sollicitations hydriques cycliques

Expansive soils swell and shrink regularly when subjected to moisture changes. Clayey soils are available worldwide and are a continual source of concern causing substantial damage to civil engineering structures. Cyclic expansion and shrinkage of clays and associated movements of foundations may result in cracking and fatigue to structures. In France, the damage caused by this phenomenon was estimated to be more than 3.3 billion euros in 2002 (Vincent in 3^ème conférence SIRNAT-Forum des journées pour la Prévention des Risques Naturels, Orléans, janv. 2003) and the Paris region is one of the most affected. The objective of this study is to investigate the swell–shrink behaviour of a natural clayey soil considered to be responsible for a lot of damage observed on buildings in the Paris region, and thus contributing to the characterisation and understanding of expansive clayey soils. The studied soil, Argile verte de Romainville, is a lagoonal-marine deposit and is part of the Paris Basin Tertiary (Oligocene) formations (Fig. 1). It is a clayey soil sampled in the eastern region of Paris. The mineralogical and geotechnical properties of the soil are presented in Table 1. The soil contains quartz (15–20%), carbonates (12–20%) and traces of mica and feldspars. X-ray diffraction showed that carbonates are essentially dolomite and the clay minerals are dominantly illite, kaolinite and a small amount of smectite (Fig. 2). A grain size analysis shows that the clay content (<2 μm) varies between 78 and 80%. The study of its microstructure by means of the scanning electron microscope indicates that the clayey soil has structural elements oriented in the direction of bedding. The structure of the sample generally consisted of dense and continuous clay matrices with very limited visible pore spaces (Fig. 3). At its natural water content (w = 25%), the soil shows mainly a unimodal pore size distribution with an average pore radius of 0.07 μm and a very limited porosity with radii larger than 10 μm (Fig. 4). To assess the effect of suction on the simultaneous changes in void ratio and degree of saturation under zero external stresses, drying–wetting tests are performed on the natural samples. The osmotic technique (Polyethylene glycol solutions) and various salt solutions are used to control the suction values ranging from 1 to 300 MPa. Once equilibrium is reached at the given suction, the samples are weighed and their volume is measured. A synthesis of the drying–wetting paths is given on Fig. 5. The swelling potential of the soil is evaluated using both indirect (or empirical methods Tables 2 and 3) and direct methods. Swell percentage and swell pressure of the soil are measured in a conventional oedometer apparatus according to ASTM (D 4546-85). The test specimens are 70 mm in diameter and the height varies between 12 and 24 mm. The swell percentage is measured under a nominal pressure of 0.7, 2.0 and 6.3 kPa. Swelling pressure of the soil is measured by the conventional consolidation test method (free swell and load, ASTM D 4546-85 method A) and by a constant volume method (ASTM D 4546-85 method C). The test parameters and results for each specimen are given in Tables 4 and 5, and on Fig. 7. Cyclic swell–shrink tests are carried out on similar samples taken from the same monolith. A scheme that permits the study of the clayey soil behaviour at the extreme states of wetting and drying is chosen. The test begins by wetting the samples at their natural moisture content and density. When swelling is stabilized, the water is removed from around the samples and they are dried in an oven maintained at 45°C until the vertical deformation (shrinkage) is stabilised and are then rewetted and so on. Some experiments are stopped at different swelling phases for microstructural study of the soil. The test parameters of the specimens are given in Table 9 and the results are shown in Figs. 9 and 10. The evolution of the microstructure during wetting and drying cycles is investigated using scanning electron microscope and mercury intrusion porosimetry. Observations are made only on soil specimens taken at the end of the swelling phase of the selected cycles. In order to preserve the microstructure, the specimens are cut in small pieces, frozen by liquid nitrogen and finally sublimated. The results of the drying–wetting path including the water retention curve are shown on Fig. 5. The results show that on the drying path (in the void ratio versus water content plane) the soil first follows nearly the saturation line and then, as the water content decreases, the void ratio tends towards a constant value. A shrinkage limit of w = 14.5 % and a corresponding suction value of 15 MPa is deduced from this path. An air entry value of 10 MPa is obtained from degree of saturation versus suction curve. The wetting path shows that the wetting–drying path is reversible for suction values higher than 60 MPa. The different indirect methods used to assess the swelling potential of the Argile verte de Romainville show a general agreement with respect to its swelling potential ranging from high to very high (Table 3). Examination of the free swell test results shows that the Argile verte de Romainville exhibits swell percentage in the range of 15–26% and that its degree of swelling depends on the initial conditions (water content, dry density) and the applied load (Table 4). The higher the water content and the applied load, the lower the swell percentage. A specimen taken parallel to the bedding plane shows similar values of swell percentage with a steep volume change versus time curve indicating an anisotropy of permeability. The two direct methods used to assess the swelling pressure of the Argile verte de Romainville give different values (Table 5). The values obtained by the constant volume method are relatively close and are about 700 kPa. Lower values varying between 360 and 540 kPa are obtained by the conventional consolidation test (free swell-consolidation). This indicates that besides the initial conditions, the swelling pressure is strongly dependent on the stress path followed. The results obtained from the wetting–drying cycle tests show that the magnitude of the first swell cycle is controlled by the initial water content, the maximum deformation occurring on the second cycle and the stabilization of swelling deformation from the third cycle (Figs. 9, 10). Furthermore, the experimental data indicate that upon repeated wetting and drying, the swelling rate of the soil becomes faster, which is explained by an increase in permeability of the soil due to the development of preferential flow paths (micro cracks) on drying. With an increasing number of cycles, a permanent increase in the volume of the samples is observed. This suggests that the swelling–shrinkage behaviour of expansive soils is not completely reversible. Mercury intrusion porosimetry analysis and SEM observations before and after different numbers of cyclic swelling indicate that the swelling–shrinkage cycles are accompanied by a continual reconstruction of the soil structure (Figs. 11, 12). The mercury intrusion porosimetry results show that with an increasing number of wetting–drying cycles the pore volume and the average diameter of the pores increase progressively (Fig. 11). Larger modifications are observed in the pores with radius in the range of 0.1–5 μm. SEM observations also show further destruction of large aggregates and disorientation of structural elements as the number of cycles increases (Fig. 12). After the fifth cycle, the soil original structure is totally lost and a disoriented homogeneous and loose structure with more homogeneous pore spaces is observed (Fig. 12d).      

自由膨胀率试验存在的人为影响因素分析

自由膨胀率用于对膨胀土的初判,适用于粒径小于0.5mm的粘性土,是反映粘性土膨胀性的指标之一,对判别膨胀土有较大参考价值。该试验所需仪器设备简单,操作方便且试验周期短,便于进行大量室内试验,是目前国内测定粘性土胀缩特性时采用较多的试验项目。按照试验步骤,综合参考文献资料及反复试验验证,全面分析了该试验各个环节可能出现的问题及对自由膨胀率的影响。     

典型红粘土与膨胀土的对比试验研究

通过室内试验对广西贵港红粘土、湖北荆门弱膨胀土与中膨胀土的物理力学性质指标、原状样脱湿过程中的强度变化、击实样泡水前后强度变化以及脱湿吸湿性能等方面进行了对比试验研究。结果表明：3种土原状样脱湿过程中的强度指标在土体裂隙性与基质吸力双重因素的作用下表现出完全不同的变化规律；由于红粘土与膨胀土膨胀性能上的差异，不同含水量的击实样泡水后的干密度峰值与加州承载比(CBR)峰值所对应的原击实样含水量比最优含水量有不同程度的增大；土体因矿物成分的差异而表现出明显不同的脱湿、吸湿速率。广西贵港红粘土与荆门膨胀土虽然在一些物理力学指标上具有相似之处，但其力学特性与水敏性特征具有明显的差异，在实际工程中应给予充分重视。     

膨胀土的自由膨胀比试验研究

自由膨胀率是膨胀土判别与分类的一项重要指标,由于自由膨胀率的测试人为干扰因素较多,自由膨胀率的科学性和可靠性被广泛质疑,因此,寻求一种人为干扰因素少,又能反映膨胀土本质的合理而适用的指标具有重要意义。为此,引进了一种新的试验方法即自由膨胀比试验,膨胀比是10g烘干土样分别在盛有蒸馏水和煤油(或CCl4)的标准量筒(通常为50mL)中沉积稳定时的体积之比。阐述该指标的意义和试验方法,系统开展自由膨胀比试验和膨胀土物理力学指标试验,试验表明:相对自由膨胀率而言,自由膨胀比与阳离子交换量、比表面积、蒙脱石含量具有更好的相关性,能较好反映膨胀土的胀缩特性;将自由膨胀比与公路规范中常用的指标液限、塑性指数、标准吸湿含水量进行对比分析,发现自由膨胀比作为膨胀土判别与分类指标具有更高的可信度,同时自由膨胀比与这些指标相关关系也较好,可将其结合起来用于膨胀土的判别分类中。     

Prediction of expansive soil swelling based on four micro-scale properties

A comprehensive study of expansive soil behavior includes understanding the surface phenomena of clay particles within the soil matrix. This research studies four micro-scale properties of four remolded expansive soils––matric suction, pH, surface conductance and percentage of montmorillonite––in order to predict soil swelling. An approach to approximate surface conductance is presented. Linear regression analyses were undertaken in an attempt to predict percent swell and swell pressure based on each of these micro-scale properties. Matric suction was found to be the most accurate predictor of the swelling behavior of the studied soils, which were initially compacted at optimum moisture content. Surface conductance, which is a combination of specific surface area, cation exchange capacity and cation mobility, also gave good predictions, except for one soil with high acidity.     

非饱和膨胀土抗剪强度的试验研究

膨胀土是一种特殊的非饱和土，经典的土力学理论在膨胀土问题中己显得无能为力。因此，用非饱和土力学理论来研究膨胀土问题在理论和实际两方面都具有重大意义。在非饱和土抗剪强度理论中，吸力的量测在工程实际中仍没有一种简单易行的方法。基于这种实际情况，试图通过其他间接的途径来代替吸力的量测，以确定非饱和土的吸附强度。对于膨胀土这种典型的非饱和土，膨胀力是其很重要的性质之一，它的大小受含水量的影响很大；另一方面，膨胀土的抗剪强度也随含水量的变化而不断地变化。进行了大量的膨胀力试验和抗剪强度试验，以确定膨胀土的膨胀力与吸附强度是否有一定的关系。通过对黑山土和梅山土的重塑试样试验得到的试验数据分析发现：膨胀力和含水量之间存在良好指数关系；粘聚力的对数和内摩擦角均随含水量的增大线性减小；非饱和膨胀土的吸附强度与膨胀力之间存在较好的线性关系，并在此基础上优化了非饱和膨胀土抗剪强度公式。     

改良膨胀土的干湿循环特性试验研究

干湿循环作用对改良膨胀土的工程性质存在重要的影响,而目前这方面的研究成果很少。以掺石灰和粉煤灰改良的典型合肥膨胀土为研究对象,通过系统的室内试验,研究在干湿循环作用下,改良膨胀土的膨胀性、界限含水量、颗粒分布以及无侧限抗压强度等方面的变化规律,深入探讨干湿循环作用对改良膨胀土工程性质的影响。结果表明,改良膨胀土的膨胀量、液限、塑性指数、黏粒含量随干湿循环次数的增加而增大,而塑限、粉粒含量以及无侧限抗压强度则随干湿循环次数的增加而减小。     

Effect of different types of wetting fluids on the behaviour of expansive soil during wetting and drying

This paper presents the results of an experimental investigation into the mechanical behaviour of an expansive soil during wetting and drying cycles. The experimental tests were conducted in a modified oedometer under two different surcharge pressures (10 and 20 kPa). During the tests, the samples were inundated with different types of wetting fluids (distilled water, saline water and acidic water). The volumetric deformation, void ratio and water content of the samples were determined during cycles of wetting and drying. The results show that the swelling potential increases with an increasing number of wetting and drying cycles. The effect of the distilled water on the swelling potential is not the same as that of the saline water or the acidic water, particularly for different surcharge pressures. The variations in void ratio and water content show that, at the equilibrium condition, the wetting and drying paths converge to nearly an S-shaped curve. This curve consists of a linear portion and two curved portions, and the majority of the deformation is located between the saturation curves of 90% and 40%.     

非饱和膨胀土变形和强度特性的三轴试验研究

为了研究吸力变化对非饱和膨胀土变形和抗剪强度特性的影响,利用三套新研制的双压力室非饱和土三轴仪,进行一系列吸湿试验、等吸力压缩固结试验和等吸力剪切试验。试验土样取自鄂西北的中膨胀性土,采用静力压实方法制备试样。试验成果表明:该非饱和膨胀土在低围压吸湿过程中的体变性状呈明显的屈服特性,因此验证了Barcelona膨胀土本构模型中SD屈服包线的存在。在等向压缩固结过程中,该非饱和膨胀土的屈服应力随吸力增加而增大,而屈服后的压缩系数随吸力增大而减少,表明吸力对土体具有硬化作用。该非饱和膨胀土的有效内摩擦角不随吸力变化而变化,吸力对抗剪强度的贡献(似凝聚力)随吸力呈非线性增加,吸力对该膨胀土抗剪强度的贡献明显高于压实高岭土和砂性土。     

基于电阻率法的膨胀土吸水膨胀过程中结构变化定量研究

针对非饱和黏性土结构定量研究中存在的困难以及现有的土的结构性研究方法所存在的不足,引入能同步反映土的微结构变化的电阻率方法,探讨土的电阻率方法在黏性土微结构定量研究中的应用。通过膨胀土击实样的无荷膨胀试验,同步测定土样的竖向电阻率与横向电阻率,计算出土的电阻率结构性参数指标,包括土的竖向与横向结构因子、平均结构因子、各向异性系数与平均形状因子的大小,通过研究土样吸水膨胀过程中的土的电阻率结构性参数指标的变化规律,建立描述膨胀土微结构变化的电阻率评价方法。试验结果表明,土的电阻率方法具有连续、快捷与测试方便等优点,其结构性参数指标可有效用于土的微结构变化定量研究。     

压实膨胀土的膨胀变形规律与计算模式

以陕西安康压实膨胀土为对象,通过不同初始干重度、初始含水率和上覆压力的一系列膨胀量试验,得到了人工压实膨胀土的膨胀变形随以上3种因素变化的规律,并总结提出了考虑初始含水率、初始干重度和上覆压力3种因素的耦合变化,满足工程精度要求的膨胀量计算模式,并对该计算模式进行了验证,证实了其正确性。结果表明:在实际工程中,可以通过有限的几组膨胀量试验得到计算模式的参数,用文中提出的计算模式直接计算任意初始干重度、初始含水率的土体在任意压力下的膨胀变形量,具有重要的实用价值。     

影响有荷载膨胀率的因素分析

根据击实膨胀土在不同的荷载、含水量和压实度下的膨胀变形试验 ,分析了击实膨胀土在 3个因素下的膨胀变形规律 ,并通过考虑交互作用的正交试验设计 ,找出 3个因素的重要性大小 ,且两两无交互作用 ,从而推导出膨胀土地基的膨胀量的计算公式 .     

干湿循环过程中膨胀土的胀缩变形特征

为了了解干湿循环过程中膨胀土的胀缩变形特征,分别开展了两组干湿循环试验。在控制吸力干湿循环试验中,吸力控制范围为 0.4 ～ 262 MPa ,采用了两种吸力控制方法,分别为渗析法（吸力＜ 4 MPa ）和蒸汽平衡法（吸力＞ 4 MPa ）,当每一级吸力达到平衡时,测量试样对应的含水率和体积;在常规干湿循环试验中, 采取了两种干缩路径,分别为全干燥和部分干燥,并测量试样在每次干湿循环过程中的轴向变形及循环结束后的含水率。结果表明： 在脱湿和吸湿过程中,试样孔隙比随吸力变化可分 3 个典型阶段：大幅变化阶段（ 0.4 ～ 9 MPa ）、过渡阶段（ 9 ～ 82 MPa ）和平缓阶段（ 82 ～ 262 MPa ）; 当吸力大于 113 MPa 时,试样的胀缩变形基本可逆,而当吸力小于 113 MPa 时,试样的胀缩变形 表现出明显的不可逆性,且不可逆程度随吸力的减小而增加。试样在常规干湿循环过程中的胀缩变形随循环次数的增加逐渐趋于稳定;胀缩特征受干缩路径的影响非常明显,全干缩路径中测得的膨胀率高于部分干缩路径,膨胀速率随干湿循环次数的增加而增加;试样在干湿循环过程中的膨胀率大小在一定程度上取决于吸湿能力。     

击实膨胀土的循环膨胀特性研究

主要研究了干湿循环对击实膨胀土胀缩特性的影响。研究结果表明,击实膨胀土的胀缩变形并不是完全可逆的,随干湿循环的发展,膨胀土的膨胀速率加快,绝对膨胀率总是增大而相对膨胀率则降低。这种变化在第Ⅱ、Ⅲ级循环时最明显,第Ⅲ级循环后便趋于稳定。这些变化特性主要是粘粒集聚、微结构改变的结果。     

Suitability of swelling and collapse theory proposed based on virgin compression surface

This study investigates the suitability of the swelling and collapse theory proposed based on a constitutive virgin compression surface (VCS) developed within the modified Monash Peradeniya Kodikara (MPK) framework. The modified MPK approach incorporates net stress, void ratio, and moisture ratio as state variables to interpret the swelling-collapse behavior of unsaturated soils. The soil selected for this research was an expansive Quaternary age basaltic residual clay located in Victoria, Australia. The experimental program included testing on unsaturated compacted clay specimens and clay specimens stabilized with lime at the optimum lime content (OLC). The OLC was obtained based on the swelling potential. Static compaction tests were conducted on untreated and lime-treated samples to establish the VCS and to propose the relationship between moisture ratio and net stress. Next, 1-D compression and consolidation laboratory tests were carried out to investigate and compare the mechanism of collapse and swelling based on well-established suction-based theories and the moisture content-based approach of this research. The swelling and collapse response obtained through both theories were very close which verified the suitability of the moisture content-based approach, being the Modified MPK framework. This study also investigated the application of suction-controlled experimental data extracted from the relevant literature within the modified MPK framework. The novelty of this study is proposing and validating a framework for conversion of the results between two different partial theories to interpret the volumetric behavior of expansive clay. Finally, a new method is proposed to estimate the swelling-collapse state of a sample through wetting without the need to establish the VCS.