Effect of stress on water retention of needlepunched geosynthetic clay liners

Geosynthetic clay liners (GCLs) are placed at the bottom of waste disposal facilities where they hydrate from the subsoil and eventually from a hydraulic head on geomembranes (GMs) defects. Predicting hydration behavior of GCLs requires knowledge of the water-retention properties of the GCL along wetting paths. Given that GCLs could be subjected to different ranges of vertical stresses that are induced by the weight of the supported waste, the confining stress could affect water-retention properties of GCLs and should be investigated. To do so, a laboratory methodology to establish the water-retention curves (WRCs) of needlepunched GCLs under stress was undertaken. Various constant vertical stresses corresponding to different weights of the supported waste were applied to GCL specimens placed in controlled-suction oedometers. Suction values were selected so as to mimic a wetting path from the initial dry state to zero suction. Suction was controlled by using controlled suction techniques with controlled humidity imposed by a saturated saline solutions and using the osmotic technique with polyethylene glycol (PEG) solutions. Measurements were undertaken on oedometer systems as to apply confining stresses and have been complemented by standard saturated oedometer swelling tests. The data obtained confirm that increasing the stress on to the GCL results in less, albeit faster, water uptake, which could emphasize on recommendations about rapidly covering GCLs after they are placed at the bottom of a waste disposal facilities. Finally, the potential validity of the state-surface concept, which was developed in unsaturated soil mechanics, is discussed using van Guenuchten's and Fredlund and Xing's equations for water retention curves.     

等向应力条件下非饱和原状黄土增湿渗水特性试验研究

渗水系数是非饱和土固结分析的关键参数之一。用自制的非饱和土三轴剪切渗透仪,在无应力及不同等向应力下分别对不同和相同孔隙比的原状黄土进行了直接向试样分级注水使吸力逐渐减小的增湿渗透试验,分析了孔隙比,应力与饱和度及吸力对增湿渗水系数的影响,对比分析了无应力与应力作用下的渗水特性,提出了可以考虑应力与饱和度或吸力影响的非饱和原状黄土增湿渗水的渗透性函数。研究结果表明:孔隙比和应力对渗水系数与饱和度关系及渗水系数与吸力关系皆有影响,吸力较大时对后者几乎没有影响,可近似归一。无应力及不同应力条件下,只要孔隙比相同,则渗水系数与饱和度或吸力关系相同;相对渗水系数与吸力的关系不能归一,同一吸力对应的相对渗水系数随孔隙比的减小或应力的增大而增大,而相对渗水系数与饱和度及吸力比(吸力与脱气值之比)关系皆可以归一。v G-M模型不适用于描述常孔隙比下原状黄土的渗水系数;提出的渗透函数可以预测一定等向应力作用下增湿过程中饱和度增大及吸力减小时原状黄土的渗水系数,预测结果与试验结果吻合较好。     

A novel transient gravimetric monitoring technique implemented to GCL osmotic suction control

A modified osmotic suction control technique for monitoring apparent transient weight changes was successfully adapted to the wetting and drying paths of geosynthetic clay liners (GCLs). Reasonable control was possible, enabling suction equilibrium to be achieved without disruption to the test. The results provide unique insight into the time-dependent changes in water retention properties and the semi-permeable membrane behaviour of the bentonite component in GCLs. The stages of suction equilibrium, related to the tri-modal pore structure of GCLs and the point of capillary break, could also be monitored. While the osmotic method has been traditionally used to control matric suction (up to 10 MPa) in soils, the overall results presented in this paper indicate that its application for total suction control in GCLs is largely due to the membrane behaviour of their bentonite component. Furthermore, because of capillary break between the GCL and the osmotic solution at the water entry (or residual) suction value of a GCL, an upper limit of 2.8 MPa suction is recommended for the application of the osmotic method to measure the water retention properties of GCLs.     

等向压缩应力条件下原状黄土的吸湿持水特性

用改装的非饱和土三轴剪切渗透仪,在不同等向压缩应力作用下对天然状态的原状黄土进行了分级浸水试验,分析了应力对吸湿持水曲线的影响,提出了可以直接考虑等向压缩应力影响,以饱和度及含水率与吸力关系表征的持水特性模型。研究结果表明:应力对饱和度与吸力关系的影响较大;对含水率与吸力间关系的影响与吸力大小有关,当吸力大于阈值时几乎没有影响,可近似归一,当吸力小于阈值时影响较大。随着应力增大,脱气吸力值增大,饱和含水率减小,且可分别用线性函数及对数函数描述之。不同应力下,饱和度及含水率比(含水率与饱和含水率之比)与吸力比(吸力与脱气吸力值之比)关系皆可以归一,且可用提出的模型描述。该模型直接把等向压缩应力作为变量,与以应力作用下孔隙比作为变量的持水曲线模型相比,更便于实际工程应用。模型对饱和度及含水率与吸力关系的预测结果与试验结果吻合较好。     

侧限条件下增湿时湿陷性黄土的变形及持水特性

 用改装的非饱和土直剪仪在无应力及净竖向应力作用下对湿陷性黄土进行分级增湿试验,测量增湿过程中变形及吸力的变化,分析增湿前孔隙比(无应力时)及净竖向应力对增湿时湿陷性黄土的变形及持水特性的影响,探讨无应力与净竖向应力作用时增湿持水特性之间的关系,提出力水耦合作用下孔隙比与吸力关系的表达式及以饱和度与吸力关系表征的持水特性模型。研究结果表明：力水耦合作用下,孔隙比与归一化吸力关系可用对数函数来描述,可以用孔隙比的变化来反映净竖向应力对持水特性的影响。无应力及净竖向应力作用时,只要孔隙比相同,则饱和度与吸力间关系相同。吸力小于阈值时,增湿前孔隙比和力水耦合作用所致孔隙比的变化对含水率与吸力间关系皆有较大影响;大于阈值时,几乎没有影响,且可用同一幂函数描述,据此提出考虑孔隙比变化的修正V-G模型,该模型可以预测无应力及净竖向应力作用时湿陷性黄土的增湿持水特性,预测结果与试验结果吻合较好。     

膨胀土增湿过程中吸力–孔隙比–含水率关系

通过不同初始孔隙比条件下的土水特征试验及增湿试验,研究了膨胀土的土水特征曲线拟合参数及体积膨胀曲线拟合参数与初始孔隙比的关系,采用曲面拟合法建立了孔隙比与重量含水率及初始孔隙比的关系曲面、孔隙比与吸力及初始孔隙比的关系曲面、重量含水率与吸力及初始孔隙比的关系曲面、体积含水率与吸力及初始孔隙比的关系曲面。试验结果表明,在重量含水率(或吸力)–初始孔隙比–孔隙比坐标系中的体变曲面由饱和部分及非饱和部分组成;在增湿过程中,曲面由非饱和区进入饱和区的转折点对应的重量含水率随着初始孔隙比的增大而增大,转折点对应的吸力随着初始孔隙比的增大而减小;在吸力–初始孔隙比–重量含水率或体积含水率坐标系中,与特定初始孔隙比对应的土水特征曲线是纵坐标恒定的平面曲线;在吸力–孔隙比–重量含水率或体积含水率坐标系中,与特定初始孔隙比对应的土水特征曲线是纵坐标在变化空间曲线,它能同时表示初始孔隙比的影响及试验过程中孔隙比的变化。     

Water retention curves of a geosynthetic clay liner under non-uniform temperature-stress paths

This paper presents a novel suction-controlled chamber that permits the determination of the full water retention curves of geosynthetic clay liners (GCLs) under non-uniform temperature-stress paths. It investigates field conditions encountered in brine ponds (low confining stress settings) and heap leach pads (high confining stress settings) during construction and operation stages. Consequently, the analysis of the moisture dynamics in a GCL was defined under the wetting path (construction) and drying path (operation). High vertical stresses were found to facilitate a more rapid water uptake as capillarity is established faster than at low, confined stresses. In general, the drying curves increase the water desorption over the suction range investigated due to the low water viscosity caused by high temperatures. The wetting of the GCL at 20 °C and drying at 70 °C under either low, confined stress (2 kPa) or high confining stress (130 kPa) shows a reduction in the volumetric water contents. Furthermore, on the drying path, the coupled effect of elevated temperature and high confining stress accelerates water desorption leading possibly to potential desiccation.     

Hysteresis of the water retention curves of geosynthetic clay liners in the high suction range

This paper examines two needle-punched geosynthetic clay liners' water retention behaviour at high suction ranges using the vapour equilibrium technique where super-saturated salt solutions controlled the relative humidity. This study shows that the bentonite form and its mineralogy affect the absorption/desorption of GCLs and their corresponding water retention curves. In particular, a granular bentonite-based GCL was found to absorb more and release less water than a powdered bentonite-based GCL due to its higher montmorillonite content and larger pores. The water retention curves of both GCLs exhibited very little hysteretic behaviour at high suction. Repeated wetting-drying cycles shifted the WRCs of both GCLs slightly downward with minimal impact on their degree of hysteresis.     

非饱和黄土初始剪切模量与孔径分布试验研究

土的初始剪切模量G_0是地震、爆破响应中一个非常重要的土动力学参数,对预测地表沉降、土工建筑物周围土体变形等具有重要作用。黄土在中国及世界各地分布广泛,且现场常处于非饱和状态。对非饱和黄土的压实试样进行了吸力控制的共振柱试验以及压汞试验,研究非饱和黄土初始剪切模量G_0与孔径分布的关系。试验表明:干湿循环过程中,试样的G_0随吸力的增大而增大,再随吸力的减小而减小。主要原因是试样内孔隙密度最大的孔径大小随吸力的增大而减小,试样内大孔隙减少,小孔隙增多,导致毛细水所占比例增加,土颗粒接触更加紧密。同一吸力作用下,湿润段试样的小孔隙比例比干燥段大,故毛细水作用更显著,从而湿润段试样的G_0大于干燥段。     

改性木素磺酸钙高效减水剂GCL1-3A性能研究

通过对木素磺酸钙 (以下简称木钙 )普通减水剂进行改性 ,并对改性工艺进行优化 ,研制出一种高效减水剂GCL1 3A。该减水剂保留了木钙普通减水剂价格低廉、来源广泛、坍落度损失小等优点 ;使减水率从原来的 9.4%提高到 2 0 % ;2 8天混凝土的抗压强度比从原来的 10 2 %提高到 12 6 % ,达到了高效减水剂的标准。研究结果揭示了GCL1 3A的减水作用机理是由于对水泥颗粒良好的润湿作用、吸附润滑作用及在水泥颗粒表面形成了较强静电空间斥力。     

Soil-water characteristics of Gaomiaozi bentonite by vapour equilibrium technique简

Soil-water characteristics of Gaomiaozi (GMZ) Ca-bentonite at high suctions (3–287 MPa) are measured by vapour equilibrium technique. The soil-water retention curve (SWRC) of samples with the same initial compaction states is obtained in drying and wetting process. At high suctions, the hysteresis behaviour is not obvious in relationship between water content and suction, while the opposite holds between degree of saturation and suction. The suction variation can change its water retention behaviour and void ratio. Moreover, changes of void ratio can bring about changes in degree of saturation. Therefore, the total change in degree of saturation includes changes caused by suction and that by void ratio. In the space of degree of saturation and suction, the SWRC at constant void ratio shifts to the direction of higher suctions with decreasing void ratio. However, the relationship between water content and suction is less affected by changes of void ratio. The degree of saturation decreases approximately linearly with increasing void ratio at a constant suction. Moreover, the slope of the line decreases with increasing suction and they show an approximately linear relationship in semi-logarithmical scale. From this linear relationship, the variation of degree of saturation caused by the change in void ratio can be obtained. Correspondingly, SWRC at a constant void ratio can be determined from SWRC at different void ratios.     

Influence of polymer enhancement on water uptake,retention and barrier performance of geosynthetic clay liners

This paper explores the influence of polymer enhancement on water uptake and retention by geosynthetic clay liners (GCLs) across a wide suction range (up to 10⁶ kPa), including the low suction regime (0.1–10 kPa) typically omitted in past studies. The suction measurement methods used enabled elucidation of water uptake and retention behaviour through the framework of GCL pore structures and their corresponding suction regimes. Polymer enhanced GCLs (PE-GCLs) have high maximum water uptake, and both the water entry and air expulsion values tend to be high. Due to high swelling, the onset of geotextile confinement for PE-GCLs was observed at high suctions. The impact of polymer becomes more apparent when the bentonite achieves a pseudo-two-layer interlayer hydration state at a suction of about 40 MPa (RH = 75%). The hydration mechanism for the polymer fraction in bentonite is unique to the specific polymer type, polymer dosage, and manufacturing process. The water retention behaviour at the low suction range is caused by the in-filling of geotextile pores, bentonite swelling and extrusion, and polymer water adsorption. Insights from this study can form the basis for developing a more suitable bimodal generalised model for fitting the water retention curves of GCLs.     

高吸力下高庙子钙基膨润土的土水-力学特性

高庙子钙基膨润土具有明显的湿胀干缩特性,吸力变化会引起持水状态和孔隙比的变化。用蒸汽平衡法测得高吸力段（3～287 MPa）高庙子钙基膨润土的土水特性和变形特性。分析比较了吸力路径、试样初始孔隙比、应力加载历史以及土体结构对土水特性、变形及吸力应力的影响。研究表明,脱湿和吸湿过程中孔隙比的差异会引起体积含水率和饱和度的滞回。饱和度与吸力间关系受试样初始孔隙比和孔隙结构影响较大,与应力加载历史无直接关系。由吸力引起的吸力应力随吸力的增大而非线性增大,并在较高吸力时趋向于一定值。吸力应力受吸力路径、初始孔隙比以及孔隙结构的影响。     

考虑孔隙水盐分效应的人工软黏土工程特性与本构模型

中国江苏北部连云港地区海相软土蒙脱石族矿物含量相对较高,且在海陆交互环境沉积形成,沉积过程中具有孔隙水盐分较高的特点,但在后沉积过程中由于地表和地下淡水入侵,会使孔隙水盐分发生变化。工程设计一般依据既有地下水环境中土体参数,未能考虑盐分变化后土体参数变化,会造成工程安全系数的冗余度不足,进而导致工程风险。为了解决江苏北部地区基础设施运营中,孔隙水盐分变动环境下软黏土地基长期性状与工程性质变异问题,需要深入了解矿物成分、孔隙水盐分在软黏土物理和力学特性中的作用机制。由于天然软黏土的矿物成分、孔隙水成分差异较大,在黏土矿物盐敏性的研究和当地地下水盐分测试基础上,试验材料采用矿物成分均一的商用高岭土与膨润土组成的人工黏土作为研究对象,配制不同浓度的NaCl溶液作为孔隙水,模拟孔隙水盐分变化。通过液塑限试验、常规固结试验和固结不排水剪切试验,发现对于含蒙脱石矿物的人工黏土,液限w_L、压缩指数C_c和回弹指数C_s随着孔隙水盐分的增加而减小,而内摩擦角随着孔隙水盐分的增加而增加;对于高岭石矿物为主的人工黏土,液限w_L、压缩指数C_c、回弹指数C_s和内摩擦角都基本不随孔隙水盐分的增加而改变。将孔隙水盐分效应采用渗透吸力进行表征,在试验规律、修正剑桥模型和BBM模型认知的基础上,建立了考虑盐分效应的人工软黏土本构模型,并对比了计算结果与试验结果,两者在趋势上较为接近,验证了本构模型的实用性,可为近海岩土工程设计提供参考。     

脱湿速率影响下的膨胀土工程性状与持水特征初探

通过控制恒温恒湿箱的温湿度工作参数,实现饱和膨胀土在不同脱湿速率条件下的状态变化,借助收缩试验和非饱和三轴试验,研究脱湿速率对膨胀土收缩特性、强度特性与持水特性的影响规律。试验结果表明,膨胀土的收缩系数随脱湿速率的减小而变大,并先呈现线性脱湿过程,后呈现非线性脱湿特征;在相同含水率条件下,膨胀土的吸附强度亦随脱湿速率的减小而增大,尤在含水率和围压较低时更趋明显。结合吸附强度与吸力关系的乘幂函数表达式,利用收缩系数和吸附强度反算出不同脱湿速率下的膨胀土"双线性"土水特征曲线,发现不同脱湿速率时曲线下降段接近平行趋势,脱湿速率越小,膨胀土的进气值越大,相同含水率时的吸力越高。     

Consolidation behavior of an unsaturated silty soil during drying and wetting

In this work, the effect of hysteresis phenomenon on the consolidation behavior of an unsaturated silty soil was investigated through a program of experimental tests. Compacted samples were prepared by the slurry method and experimental tests were carried out in a double-walled triaxial cell. Consolidation tests were conducted by the ramping method at suctions of 0, 100, 200, 250 and 300 kPa on drying and wetting paths of the soil water characteristic curve. The results show that the paths of specific water volume and specific volume are not consistent during stabilization in either condition (drying or wetting). In addition, the yield stress for the wetting path is higher than that for drying. The trend of variations of the specific water volume during loading is similar to the consolidation curves for different suction. For both conditions of drying and wetting, the slope and intercept of the virgin line due to variations of specific volume and specific water volume are a function of suction. While their values decrease with increasing suction, these values are higher for the dry path than wetting.     

非饱和土湿吸力与含水率的定量关系研究

以立方松散堆积方式为例,从理论上推导并给出了湿吸力与非饱和土含水率的定量关系表达式,开展了不同含水率状态下重塑细粒砂性土的吸力测试,对试验结果进行了曲线拟合和分析,理论计算和试验拟合结果均表明：对于特定的非饱和土,湿吸力随含水率先增大而后减小,即存在确定的界限含水率。这与“在一定的土体含水率变化范围内,非饱和土,尤其是非饱和砂性土的强度随含水率的增加而先增大后减小”的事实完全相符,这再次证明了湿吸力,而非基质吸力,才是非饱和土有效应力的重要组成部分。     

干密度对非饱和黄土基质吸力的影响

为了研究起始湿度、天然密度以及竖向荷载对吸力特性的影响,对西安原状黄土进行不同起始湿度、密度条件下的常含水量压缩试验,提出无应力作用下同时反映初始含水率和天然干密度的基质吸力表达式。研究结果表明:天然干密度、初始含水率、应力相关的干密度对基质吸力的影响程度都与试样的起始湿度有关,起始湿度小于试样塑限含水量时,影响都比较大,起始湿度大于试样塑限含水量时影响都很小。净竖向荷载对吸力特性的影响与起始湿度、天然密度有关,净竖向荷载小于等于400 kPa时,对吸力的变化影响很小,可不考虑净竖向荷载影响,可用未受荷载作用时的SWCC曲线来描述原状试样的吸力特征;净竖向荷载大于400 kPa时,对吸力的变化影响较为显著。视起始湿度和天然密度的不同,净竖向应力相关的干密度增大引起基质吸力非单调增减或单调下降,天然干密度的增大引起基质吸力单调上升。     

Time Effects on Total Suction of Bentonites

This paper presents a study on time effects on total suction of bentonite-based materials under constant water content conditions. Three different types of bentonite (i.e., MX80, Calcigel I, and Calcigel II) and a natural expansive clay (i.e., London Clay) were used. Total suction was measured using chilled-mirror hygrometer technique. The measurements were performed on specimens aged 1 hour, 6 hours, 1 day, 2 days, 1 week, 2 weeks, 1 month, and 2 months. The results show that the true equilibrium state for two types of bentonite (i.e., MX80 and Calcigel I) used is time dependent. Total suction increases with time and this behaviour is controlled by the characteristics of the bentonites. Total suction of the other specimens (i.e., Calcigel II and London Clay) is not affected by curing time. The changes in total suction with time as the specimen's age are attributed to non homogeneity of the water content distribution in the micro- and macropores of the bentonites and inaccuracy of the device used.     

On the chemo-thermo-hydro-mechanical behaviour of geological and engineered barriers

An overview of the recent findings about the chemo-hydro-mechanical behaviour of materials used for both geological and engineered barriers in nuclear waste disposal is presented, through some examples about the natural Boom Clay (BC) and compacted bentonite-based materials. For the natural BC, it was found that compression index identified from both oedometer and isotropic compression tests is similar and the compressibility of BC from the Mol site is higher than that of BC from the Essen site; the shear strength of Mol BC is also higher than that of the Essen BC, suggesting a significant effect of carbonates content; the thermal volume change is strongly overconsolidation ratio (OCR) dependent—low OCR values promote thermal contraction while high OCR values favour thermal dilation; the volume change behaviour is also strongly time dependent and this time dependent behaviour is governed by the stress level and temperature; the effect of pore-water salinity on the volume change behaviour can be significant when the smectite content is relatively high. For the bentonite-based materials, it was found that thermal contraction also occurs at low OCR values, but this is suction dependent—suction promotes thermal dilation. Under constant volume conditions, wetting results in a decrease of hydraulic conductivity, followed by an increase. This is found to be related to changes in macro-pores size—wetting induces a decrease of macro-pores size, followed by an increase due to the aggregates fissuring. The presence of technological voids can increase the hydraulic conductivity but does not influence the swelling pressure.