Desiccation-Induced Volumetric Shrinkage of Compacted Foundry Sand Treated with Cement Kiln Dust

Foundry sand, an industrial waste, was treated with up to 12 % cement kiln dust content at comparative energy levels of British standard light, West African standard or “intermediate” (WAS) and British standard heavy (BSH) efforts at molding water contents ?2, 0, 2, 4 and 6 % of optimum moisture content. Samples were extruded from the compaction molds and allowed to air dry in the laboratory in order to assess the effect of desiccation-induced shrinkage on the material for use as a hydraulic barrier in waste containment application. Results recorded show that volumetric shrinkage strain (VSS) values were large within the first 5 days of drying; VSS values increased with higher molding water content, water content relative to the optimum moisture content. VSS generally increased with higher initial degree of saturation for all compactive efforts, irrespective of the level of cement kiln dust (CKD) treatment. A compaction plane of acceptable zones for VSS based on the regulatory value is ≤4 %. The influence of CKD treatment generally showed a decrease in the desiccation-induced volumetric shrinkage strain with increasing CKD content. This is largely due to the pozzolanic input of CKD. Finally, only the BSH compactive effort gave successful results of volumetric shrinkage strain at CKD treatment content of between 4 and 8 %, while 12 % CKD content produced successful volumetric shrinkage strain results at WAS and BSH compactive effort, respectively.     

Desiccation-induced Shrinkage in Compacted Lateritic Soils

Specimens prepared from three lateritic soil samples were subjected to drying under laboratory conditions. Volumetric shrinkage strains were measured at the end of the drying period. Results of this study indicate that, for the lateritic soils tested, volumetric shrinkage strains are influenced by soil composition and compaction conditions. Volumetric shrinkage strain increased with higher compaction water content. The influence of compaction water content on measured volumetric shrinkage strain was more pronounced in specimens with higher fines content. A regression equation was developed from the data to estimate volumetric shrinkage strain given the compaction water content relative to optimum, plasticity index, fines content and compactive effort.     

Hydraulic Conductivity of Compacted Granite Residual Soil Mixed with Palm Oil Fuel Ash in Landfill Application

Laboratory tests were carried out on compacted granite residual soil treated with 0 to 15% Palm Oil Fuel Ash (POFA), with a view to evaluate its hydraulic conductivity for its application in landfilling. The Soil–POFA mixtures were compacted using both Standard and Modified Proctors compactive efforts at 2% dry of Optimum Moulding water Content (?2%), at Optimum Moulding water Content (0%), at 2 and 4% on the wet side of Optimum Moulding water Content (+2 and +4%). The samples were permeated with water and the effect of moulding water content; compactive effort and POFA content were examined. The samples that met the minimum threshold of 1 × 10^?9 m/s were used in plotting the acceptable zones criterion at various POFA mixtures. The results gave indications of reduction in the hydraulic conductivity values, with increase in compactive efforts, moulding water content and POFA content up to about 10%. This was the most suitable soil–POFA mixture for the hydraulic application.     

Influence of the initial water content and dry density on the soil–water retention curve and the shrinkage behavior of a compacted clay

The paper presents the results of an experimental study on the effects of the initial water content and dry density on the soil–water retention curve and the shrinkage behavior of a compacted Lias-clay. The initial conditions after compaction (initial water content and initial dry density) have been chosen on the basis of three Proctor tests of different compaction efforts. According to the eight chosen initial conditions clay samples have been compacted statically. The relation between total suction and water content was determined for the drying path starting from the initial conditions without previous saturation of the specimens. A chilled-mirror dew-point hygrometer was used for the suction measurements. For the investigation of the shrinkage behavior cylindrical specimens were dried to desired water contents step-by-step without previous saturation. The volume of the specimens was measured by means of a caliper. Based on the test results the influence of different initial conditions on the soil suction and the shrinkage behavior is analyzed. The soil–water retention curves obtained in terms of the gravimetric water content are independent of the initial dry density. At water contents above approximately 11–12.5% a strong influence of the compaction water content is observed. At smaller water contents, the soil–water retention curve is independent of the compaction water content. The results of the shrinkage tests show that the influence of the compaction dry density on the shrinkage behavior is negligible. Similar to the drying behavior of saturated samples a primary and a residual drying process could be distinguished. The primary drying process is strongly influenced by the initial water content. In contrast, the rate of the volume change of the residual drying process is unaffected by the initial water content.     

Shrinkage and desiccation cracking in bentonite–sand landfill liners

Data are reported on the shrinkage and desiccation cracking exhibited by bentonite-enhanced sand mixtures (BES) upon air-drying. Mixtures containing 10 and 20% bentonite by dry weight, compacted at moisture contents ranging from 8 to 32%, were investigated. Hydraulic conductivity data for BES specimens saturated and tested immediately after compaction, and for similar specimens that had no visible damage after air-drying, are also presented.

All the mixtures exhibited volumetric shrinkage upon air-drying with the amount of shrinkage increasing with increasing moisture content during compaction. At any initial moisture content mixtures containing 20% bentonite shrink more than those containing 10% bentonite, but the shrinkage is insensitive to the compactive effort. Compacted beds of BES containing 10 and 20% bentonite exhibit no visible desiccation cracking as the top surface is dried when compacted at 15 and 14% moisture content, respectively, and only minor cracking when compacted at initial moisture contents of 20 and 15%, respectively. For the range of mixtures tested, it appears that cracking only occurs when BES undergoes more than about 4% volumetric shrinkage when air-dried. The saturated hydraulic conductivity of intact BES specimens is unaffected by a drying episode prior to testing.     

Evaluation of bagasse ash treated lateritic soil as a potential barrier material in waste containment application

Laboratory tests were conducted on a reddish-brown lateritic soil treated with up to 12 % bagasse ash to assess its suitability in waste containment barriers applications. Soil samples were prepared using four compaction energies (i.e. reduced Proctor, standard Proctor, West African Standard or ‘intermediate’ and modified Proctor) at ?2, 0, 2 and 4 % moulding water content of the optimum moisture content (OMC). Index properties, hydraulic conductivity (k), volumetric shrinkage and unconfined compressive strength (UCS) tests were performed. Overall acceptable zones under which the material is suitable as a barrier material were obtained. Results recorded showed improved index properties; hydraulic conductivity and UCS with bagasse ash treatment up to 8 % at the OMC. Volumetric shrinkage strain increased with higher bagasse ash treatment. Based on the overall acceptable zone obtained, an 8 % optimal bagasse ash treatment of the natural lateritic soil makes it suitable for use in waste containment barrier application.     

Delineation of compaction criteria for acceptable hydraulic conductivity of lateritic soil-bentonite mixtures designed as landfill liners

In current geoenvironmental practice, design engineers usually require that soil liners in waste landfills be compacted within a specified range of water content and dry unit weight. This specification is based primarily on the need to achieve a minimum dry unit weight for factors controlling the performance of compacted soil liners most especially the hydraulic conductivity, k. In this study, lateritic soil treated with up to 10% bentonite, prepared at various compaction states (dry of optimum, optimum and wet of optimum moisture content) was compacted with four compactive efforts (i.e., the reduced British Standard Light, British Standard Light, West African Standard, and British Standard Heavy) to simulate the range of compaction energies expected in the field. Prepared soil mixtures were permeated with water and specimens that yielded the permissible limit of k????1?×?10^?9?m/s were enclosed in an envelope (known as the acceptable zone) on the water content?Cdry unit weight curve. It was observed that compaction conditions resulting in moisture content slightly wet of optimum led to the lowest values of k and that the shapes and boundaries of the acceptable zones gradually increased in extent, shifting to wet side of optimum moisture content as the bentonite content increased to 10%. This approach provides good control over the quality of compacted soils and has great potential for field application.     

非饱和重塑黏土渗透性试验研究

工程中常常遇到非饱和土渗流问题,其渗透性是非饱和土的重要研究内容。采用常规压力板仪详细研究了重塑黏土在3种不同击实功和5种不同击实含水率下的土-水特征曲线,并用变水头法测量了相应的饱和土渗透系数。结合土水曲线和饱和渗透系数,基于Van Genuchten模型和Fredlund模型,详细探讨了重塑非饱和重塑黏土在不同击实条件下的渗透性。结果表明,土样饱和渗透系数随着击实含水率的增大而减小,且随着基质吸力的增大土样渗透性差别逐渐减小。当试样吸力达到1 MPa时,可认为不同击实含水率下试样的渗透性基本相同;在轻型击实和简化轻型击实下,试样渗透性很相似。重型击实功下试样的渗透系数和轻型击实下土样渗透系数的比值 随着吸力的增大而逐渐增大,在饱和状态时比值约为1/100,当吸力达到1 MPa时比值逐渐稳定在1/10右。     

Strength characteristics of compacted pond ash

Strength properties of compacted ash layers depend to a great extent on the moulding conditions. This paper focuses on the effects of compaction energy and degree of saturation on strength characteristics of compacted pond ash. The pond ash sample, collected from the ash pond of Rourkela Steel Plant (RSP), was subjected to compactive energies varying from 357 kJ/m³ to 3488 kJ/m³. The optimum moisture content and maximum dry densities corresponding to different compactive energies were determined by conventional compaction tests. The shear strength parameters, unconfined compressive strengths (UCS) and California bearing ratio (CBR) values of specimens compacted to different dry densities and moisture content were assessed and reported. The effects of compaction energy and degree of saturation on shear strength parameters i.e. unit cohesion (c_u) and angle of internal friction (Ф) values and also the UCS values are evaluated and presented herein. The results indicate that the dry density and strength of the compacted pond ash can be suitably modified by controlling the compactive energy and moulding moisture content. The strength achieved in the present study is comparable to the good quality, similar graded conventional earth materials. Hence, it may be safely concluded that pond ash can replace the natural earth materials in geotechnical constructions.     

Quantification of desiccation cracks using X-ray tomography for tracing shrinkage path of compacted expansive soil

Acta Geotechnica - Compacted expansive soils undergo large volumetric changes during wet–dry cycles owing to the seasonal moisture fluctuations, and during drying the shrinkage is accompanied...     

Potential Application of Lateritic Soil Stabilized with Cement Kiln Dust (CKD) as Liner in Waste Containment Structures

This study evaluates the applicability of residually derived lateritic soil stabilized with cement kiln dust (CKD), a waste product from the cement manufacturing process as liner in waste repositories. Lateritic soil sample mixed with 0–16 % CKD (by dry weight of the soil) was compacted with the British Standard Light, West African Standard and British Standard Heavy compaction efforts at water contents ranging from the dry to wet of optimum moistures. Geotechnical parameters such as Atterberg limits, compaction characteristics, hydraulic conductivity, unconfined compressive strength and volumetric shrinkage strain were determined. Results indicate that the plasticity index, the maximum dry unit weight and hydraulic conductivity together with the volumetric shrinkage decreased with increased amount of CKD while the optimum moisture content and unconfined compressive strength increased with higher CKD content for all the efforts. When measured properties were compared with standard specifications adopted by most environmental regulatory agencies for the construction of barrier systems in waste containment structures, the resulting values showed substantial compliance. Besides developing an economically sustainable liner material, the present study demonstrated effective utilization of an industrial by-product otherwise considered as waste by the producers, in addition to a systematic expansion in the use of the lateritic soil for geotechnical works.     

Tropical residual soil as compacted soil liners

A series of laboratory tests was conducted on a tropical residual soil, which is widespread and readily available over a considerable part of Peninsular Malaysia, to assess whether it could be compacted as hydraulic barriers in waste disposal landfills. Index properties, swelling potential, cation exchange capacity (CEC), compaction characteristics, and hydraulic conductivity of the soil indicate that it is inorganic, very plastic, inactive (activity <0.75), moderately expansive (modified free swell index is about 3.06), and of fair attenuation capacity (for inorganic contaminants). For hydraulic conductivity measurement, the soil was compacted in rigid-wall permeameter moulds at a variety of water contents and compactive efforts and then permeated with de-aired tap water. The results of hydraulic conductivity tests illustrate that hydraulic conductivity lower than 1×10^–7 cm/s can be achieved using a broad range of water contents and compactive efforts, including water contents dry of optimum. Its shrinkage and strength properties show that it has minimal potential to shrinkage and has adequate strength to support the overburden pressure imposes by the waste body. These findings suggest that the residual soil can be potentially utilized as compacted soil liner material.     

非饱和击实粉土的强度和屈服特性研究

由于吸力量测技术的困难,对京―九线粉土在不同含水率下击实并进行三轴不排水剪切试验,研究其强度和屈服特性,为非饱和击实粉土力学模型的建立提供依据。试验结果表明：该击实粉土在剪切过程中,在最优含水率干侧击实的土样具有较高的强度,应力-应变关系曲线呈软化趋势,土样先剪缩后剪胀,湿侧击实的土样具有较低的强度,应力-应变关系曲线呈硬化趋势,土样不断剪缩;随着试样击实含水率的增加,击实粉土的强度和屈服应力不断减小,水对土体具有一定的软化作用;不同击实含水率下,土样在q-p平面内破坏时的屈服轨迹为近似平行的斜直线,含水率对临界状态线的斜率没有影响;含水率对击实粉土抗剪强度的贡献表现为土体的似黏聚力的增加,似黏聚力与含水率之间呈乘幂关系,得出以含水率为变量的击实粉土强度计算公式,可在实际工程中推广应用。     

干湿循环下南阳膨胀土的土水和变形特性

对初始状态相同南阳膨胀土试样进行1～6次干湿循环,选取其中的1、3、6次循环后的试样,采用饱和盐溶液蒸汽平衡法测定其含水率、孔隙比、饱和度等与吸力的关系,并对比分析干湿循环对南阳膨胀土的持水能力影响。在1～6次干湿循环过程中,以环刀为参照物拍摄每次烘干后试样上表面照片,用数字图像处理提取图像中的裂隙与收缩面积,以此分析试样烘干过程中裂隙与收缩与干湿循环次数的关系。试验结果表明,干湿循环过程中相同吸力的试样含水率略降低、孔隙比略增大、持水能力略降低,烘干过程中试样收缩面积增大,裂隙展开面积增大,但上述性质变化幅度都随干湿循环次数增加而减小。试验成果为研究膨胀土的持水能力和湿胀干缩变形性质随干湿循环的变化规律提供实测数据。     

Cyclic Swell–Shrink Behaviour of a Compacted Expansive Soil

Laboratory cyclic swell–shrink tests were carried out on compacted expansive soil specimens to study in detail the effect of changes in shrinkage pattern on the swell–shrink behaviour of compacted expansive soils. Compacted soil specimens were allowed to swell and either shrank fully or partially shrank to several predetermined heights in each cycle. The tests were carried out at a surcharge pressure of 50 kPa. The test results revealed that shrinkage of compacted saturated soil specimens to predetermined height in each shrinkage cycle provides similar conditions as that of the controlled suction tests with an increasing number of swell–shrink cycles. The water content of soil specimens and hence soil suction was found to remain nearly constant for each pattern of shrinkage. For soil specimens equilibrated to a given swell–shrink pattern, suction at the end of shrinkage cycles was changed from a higher suction to a lower suction, and also from a lower to a higher suction. The experimental results showed that there may be an immediate equilibrium state attained by the soil in terms of swell–shrink potential if suction at the shrinkage cycles was less than the past suction; otherwise, the equilibrium state was accompanied by fatigue of swelling. The volumetric deformation of the soil specimen subjected greater shrinkage was found to be much larger than the corresponding vertical deformation. The compressibility index of microstructure, κ_m, was determined for several shrinkage patterns. It is shown that κ_m is heavily influenced by suction at the end of shrinkage cycles.     

干湿循环作用下红黏土收缩特征研究

以贵州某高速公路沿线红黏土为研究对象,开展了干湿循环作用后红黏土的收缩特性试验研究,探讨了干湿循环次数和初始含水率对红黏土收缩性状的影响.试验结果表明：同一循环次数内,各试样的环向收缩率在6～8 h时达到稳定,而轴向线缩率则需更长的稳定时间;随着初始含水率增加,各试样的初始轴向线缩率减小,而初始环向收缩率增加;28%含水率试样具有相近的最终轴向变形和环向变形量,而34%含水率的试样环向变形量高于轴向变形量;初始含水率变化不大,但体缩率的变化率却明显不同,说明干湿循环过程使得土体内部结构变得不稳定,但颗粒间相互作用的本质未发生明显变化;干湿循环作用对重塑红黏土最优含水率位置的基本无影响,这说明其未能改变黏土集聚体内对结合水的最大吸附能力.通过压汞试验得到了不同干湿循环作用后试样的孔隙分布情况,随着干湿循环次数的增加小孔径尺寸和分布密度改变较小,而大孔径尺寸减小,分布密度也在减小.干湿循环作用对聚集体内孔径的影响大于对微聚体内孔隙影响,与所测得的宏观收缩规律相吻合.     

Influence of Cyclic Wetting Drying on Collapse Behaviour of Compacted Residual Soil

The climatic zones where residual soils occur are often characterized by alternate wet and dry seasons. Laboratory studies of earlier workers have established that the alternate wetting and drying process affects the swell-shrink potentials, water content, void ratio and particle cementation of expansive soils. The influence of cyclic wetting and drying on the collapse behaviour of residual soils has not been examined. This paper examines the influence of alternate wetting and drying on the collapse behaviour of compacted residual soil specimens from Bangalore District. Results of such a study are useful in anticipating changes in collapse behaviour of compacted residual soil fills. Experimental results indicated that the cyclic wetting and drying process increased the degree of expansiveness of the residual soils and reduced their collapse tendency. Changes in the swell/collapse behaviour of compacted residual soil specimens from wetting drying effects are attributed to reduction in water content, void ratio and possible growth of cementation bonds.     

基于DIC技术的三趾马红土表面干缩裂纹扩展与自愈规律

新近系三趾马红土黏粒含量高,失水干燥易开裂形成干缩裂纹。为研究干密度对三趾马红土干燥开裂行为影响,利用自制干燥装置开展压实三趾马红土脱湿试验,并采用数字图像相关(DIC)方法分析其表面干缩裂纹扩展与自愈规律。结果表明:(1)不同初始干密度试样干燥过程中均出现开裂现象,其表面干缩裂纹演化过程可分裂纹萌生、裂纹扩展、裂纹自愈、裂纹稳定4个阶段;小干密度试样表面干缩裂纹网络展布密集均匀,而大干密度试样表面开裂以“宽大稀疏”裂纹为主;(2)试样表面应变场变化能够较好反映试样表面干缩裂纹演化过程,蓝色压应变区代表试样失水整体产生的体积收缩,红色应变条带代表试样表面受拉而产生的干缩裂隙;试样干燥过程表面蓝色压应变区不断扩大,红色拉应变条带区逐渐变小,表明表面裂纹受到试样整体收缩牵拉而产生“自愈”收缩。且干密度越小,试样干缩过程表面红色拉应变网络越密集,干缩裂纹自愈程度越高;(3)试样表面应变场变化表明各等级干缩裂纹均出现自愈现象,但主裂纹的主应变减少量最大,自愈程度最高,对试样整体裂纹自愈起主导作用;试样初始干密度越小,各等级裂纹的峰值应变越大,自愈时的应变缩减量越高,表明试样失水收缩特征越显著。研究成果对进一步揭示压实黏土干燥裂纹的复杂演化过程机理研究提供新参考。     

黏性土干/湿过程中土结构演化特征研究进展

干湿变化是自然界中土体必然经历的过程,对土体工程性质有重要影响。系统掌握干/湿过程中土结构演化特征,对深入认识土体宏观力学性质有重要意义。基于国内外大量文献资料,着重总结了黏性土在干燥过程、湿化过程和干湿循环过程中微观结构的演化特征,得到了以下几点主要认识:（1）含水率是影响土体微观结构的关键因素之一,在最优含水率干侧制备的土样呈典型的团聚体结构,孔隙分布曲线具有双峰特征,而在湿侧则呈相对均匀的基质结构,孔隙分布曲线呈现单峰特征;（2）在干燥过程中（吸力增加）,到达缩限之前,土体积的减小主要由大的宏观孔隙收缩所致。在不同的吸力区间内,主要受影响的孔隙尺寸是不同的;（3）在湿化过程中（吸力减小）,团聚体内的小孔隙和团聚体间的大孔隙都逐渐增大,且以团聚体内的孔隙增大为主,孔隙结构的演化特征与侧限条件密切相关;（4）在干湿循环过程中,土结构变化并不是完全可逆的,土体产生的累积收缩/膨胀形变量主要来自于宏观孔隙,随着干湿循环次数的增加,土体的体变特性会达到一个平衡状态,可用弹性孔隙比进行描述。除此以外,还总结了土结构观测的常规技术方法,包括SEM、ESEM、MIP和CT技术等。最后针对土结构研究现状,提出了今后的研究重点和方向,主要包括制样新方法、观测新技术、湿化过程的微观结构以及微观结构参数与宏观力学模型相结合等。     

Osmotic Swelling and Osmotic Consolidation Behaviour of Compacted Expansive Clay

Compacted clay soils are used as barriers in geoenvironmental engineering applications and are likely to be exposed to salinization and desalinization cycles during life of the facility. Changes in pore fluid composition from salinization and desalinization cycles induce osmotic suction gradients between soil–water and reservoir (example, landfill/brine pond) solution. Dissipation of osmotic suction gradients may induce osmotic swelling and consolidation strains. This paper examines the osmotic swelling and consolidation behaviour of compacted clays exposed to salinization and desalinization cycles at consolidation pressure of 200 kPa in oedometer assemblies. During salinization cycle, sodium ions of reservoir fluid replaced the divalent exchangeable cations. The osmotic swelling strain developed during first desalinization cycle was 29-fold higher than matric suction induced swelling strain of the compacted specimen. Further, the diffusion controlled osmotic swelling strain was 100-fold slower than matric suction-driven swelling process. After establishment of ion-exchange equilibrium, saturated saline specimens develop reversible osmotic swelling strains on exposure to desalinization cycles. Likewise the saturated desalinated specimen develops reversible osmotic consolidation strains on exposure to cycles of salinization. Variations in compaction dry density has a bearing on the osmotic swelling and consolidation strains, while, compaction water content had no bearing on the osmotic volumetric strains.