Anisotropic swelling behaviour of unsaturated compacted GMZ bentonite hydrated under vertical stresses

Bulletin of Engineering Geology and the Environment - Bentonite has been considered as a potential buffer/backfill material for construction of engineering barriers in deep geological...     

Swelling pressure characteristics of compacted Chinese Gaomiaozi bentonite GMZ01

Gaomiaozi bentonite (GMZ01) has been decided upon as the first option for use as buffer/backfill materials in the deep disposal of high-level radioactive waste in China. The basic functions of the materials used in the waste repositories request among others a sufficient swelling pressure and low hydraulic conductivity in order to provide long-term stability to the barrier system under environmental pressure and behavior of the waste loads. As such, it is necessary to investigate the influence of initial dry density on the swelling properties of Gaomiaozi bentonite (GMZ01) in order to achieve better design of buffer/backfill materials. In this study the swelling pressure of GMZ01 has been studied and analyzed by multi and one-step wetting constant volume tests with five different dry densities (1.15, 1.35, 1.50, 1.60 and 1.75 mg/m³). Results show that swelling pressure changes with time nonlinearly, while there is a linear relationship between time/swelling pressure and time. Curves of swelling pressure and the amount of absorbed water varying with time can be classified into typical phases. For the GMZ01 tested here, the initial dry density is an important factor influencing the swelling pressure. The results show that there is an exponential relationship between swelling pressure and dry density. Moreover, comparison was done between the experimental swelling pressure results of used GMZ bentonite in this study and other bentonites cited in literature: (i) other GMZ׳s and (ii) different types of bentonites proposed as buffer/backfill materials (i.e., MX80, Kunigel, Montigel, and Calcigel). The effect on the microstructure of the density and the wetting under the constant volume condition (after the swelling pressure test) has been investigated by studying the results of pore size distribution for GMZ01 by using the mercury intrusion porosimetry (MIP) test and the environmental scanning electron microscope (ESEM) photos. Finally, two different theoretical concepts were used to estimate the swelling pressure (the modified DDL and thermodynamics approaches). The results of the two methods show that the swelling pressure results compare relatively well with the experimental data for the GMZ bentonite.     

Swelling of unsaturated GMZ07 bentonite at different temperatures

Bulletin of Engineering Geology and the Environment - A series of wetting tests on unsaturated compacted GMZ07 sodium bentonite, taken from the Gaomiaozi area, Inner Mongolia, China, was performed...     

Pore fluid chemistry effects on the swelling behavior of compacted GMZ bentonite with an artificial annular gap

Bulletin of Engineering Geology and the Environment - Deep geological disposal has been adopted by many countries around the world as a method for disposal of high-level radioactive waste. During...     

Aging effects on the swelling behavior of compacted bentonite

Bulletin of Engineering Geology and the Environment - Due to its favorable low permeability, high swelling potential, and high retention capacity for radionuclides, the compacted bentonite has been...     

Constitutive modeling for compacted bentonite buffer materials as unsaturated and saturated porous media

Bentonite has remarkable swelling characteristics and low permeability that enhance the stability of the buffer materials in repositories for the geological disposal of radioactive waste. It is necessary to apply reliable numerical simulation techniques to assess the safety and mechanical stability of repositories over a long period of time. Having a constitutive model that can describe the mechanical behavior of bentonite is key to such numerical simulations. This paper proposes an elasto-plastic constitutive model to describe the changes in the mechanical properties of bentonite due to saturation in the progress of a repository becoming submerged under groundwater. In the proposed model, the swelling index is formulated as a function of the degree-of-saturation to express not only the swelling behavior, but also the dependency of the degree-of-saturation on the dilatancy characteristics. The montmorillonite content is used as a material parameter to determine the normal consolidation line. The experimental results of swelling volume and swelling pressure tests in previous literature are shown to have been satisfactorily predicted by the proposed model.     

温度对GMZ膨润土的膨胀性能影响研究

在25℃,40℃,60℃和90℃的蒸馏水中对高庙子(GMZ)膨润土进行了膨胀性能试验。由于在试验条件下渗透膨胀占主导地位,GMZ膨润土的膨胀性能随温度的升高而增大,其中最大膨胀率随温度线性增加,而膨胀力随温度呈指数增加。压实GMZ膨润土的膨胀性能可用em=Kp^D-3表示。N2吸附试验结果表明温度对GMZ膨润土的表面分维D基本没有影响。在渗透膨胀条件下,采用扩散双层模型计算发现膨胀系数K与温度T_c之间存在线性关系,并且通过GMZ膨润土和Bikaner膨润土的膨胀试验进行了验证。将K–T_c线性关系与e_m–p分形关系相结合,提出了温度作用下GMZ膨润土膨胀性能的一种简便定量评价方法。     

高庙子膨润土GMZ001三向膨胀力特性研究

为研究高庙子钠基膨润土GMZ001的三向膨胀力特性,改进了三向胀缩仪,扩大了该仪器膨胀力的测量范围,进行了一系列不同干密度和不同初始吸力的三向膨胀力试验。研究结果表明:①在所研究的干密度、含水率范围内,竖向(平行于压实方向)膨胀力、水平(垂直于压实方向)膨胀力均主要与干密度有关,初始吸力对其没有明显影响。②水平膨胀力与竖向膨胀力之比随干密度的增大而减小;当干密度较小时,竖向膨胀力与水平膨胀力基本相等;当干密度大于1.6 g/cm3时,水平膨胀力与竖向膨胀力之比变化很小,其值稳定在0.78左右。③初始吸力、干密度均会显著影响膨胀力随时间变化曲线的形状;对于高吸力高干密度试样,其膨胀力–时间关系曲线在中间一段出现"平台"。④相同初始吸力下,膨胀力变化速率随干密度的增大有所增加,但不同干密度试样的膨胀力变化速率的差异随时间逐渐缩小;相同干密度下,尽管初始阶段高吸力试样的膨胀力发展更快,但一段时间后低吸力试样的膨胀力变化速率会比高吸力试样的大。⑤膨胀力最终平衡时间随干密度增大有所增加,而初始吸力对平衡时间影响不大。     

干湿循环作用下非饱和土的持水特性研究

以干湿循环作用下非饱和土的持水特性为研究对象,通过采用滤纸法、非饱和固结仪法,针对三种初始干密度的试样,研究了土—水特征曲线的变化特征,依据试验结果,得出了一些有意义的结论。     

常含水率下非饱和高庙子膨润土加砂混合物的水力-力学性质

对非饱和高庙子膨润土与砂混合物击实样进行常含水率下的不排水等向压缩和不排水三轴剪切试验,研究非饱和高庙子膨润土与砂混合物的水力-力学特性,拟模拟深层地质处置工程中缓冲/回填材料在不排水状态下受力时的性状。试验研究表明:在不排水等向压缩和不排水三轴剪切试验中,固结应力和剪应力的增大引起非饱和土试样孔隙比减小,饱和度增大,吸力减小;在不排水剪切试验中,应力-应变曲线呈外凸的形状,试样发生体积收缩和侧向膨胀变形,净围压对试样的初始刚度、不排水剪切强度和变形有较大的影响。     

高压实膨润土孔隙结构特征研究进展

在详细阐述高压实膨润土孔隙结构组成及其界限孔径确定方法的基础上,全面总结了处置库近场条件下孔隙结构演化规律及其对水力特性影响的研究成果.结果表明:高压实膨润土具有由晶层间孔隙、层叠体间孔隙、集合体间孔隙组成的三孔结构,而描述本构关系时通常简化为由宏观孔隙和微观孔隙构成的双孔结构.双孔结构中宏、微观孔隙界限孔径的确定方法...     

干湿循环对纤维膨胀土特性影响的试验研究

为了研究干湿循环对膨胀土和聚丙烯纤维膨胀土强度的影响,进行了大量的试验研究。试验结果表明,膨胀土和纤维膨胀土随着干湿循环次数的增加,裂隙逐渐发展,土样的整体性遭到破坏,抗剪强度也随着循环次数的增加而降低;纤维可以增加膨胀土强度,提高土体的整体性。膨胀土和纤维膨胀土的粘聚力随着循环次数增加而降低的趋势一致,表明纤维是以强...     

Time- and density-dependent microstructure features of compacted bentonite

Pre-compacted bentonite bricks are often considered as sealing/backfill elements in deep geological repositories for high level radioactive waste. A good understanding of their microstructure changes upon hydration is essential as the microstructure changes are directly related to the macroscopic hydro-mechanical behaviour. In this study, the microstructure features of the compacted MX80 bentonite used as a sealing material in a field experiment were characterized by means of both mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Emphasis was put on the effects of final dry density (density after swelling) and hydration time. The results obtained show that the changes in soil porosity upon swelling are mainly due to the increase in large-pores of about 50 µm diameter and medium-pores of 1 µm diameter. In addition, the microstructure changed over time due to the water re-distribution that occurred among each level of pores: the volume of both the large-pores and small-pores decreased along as the volume of the medium-pores increased. A uniform microstructure can be then expected in the long term. Furthermore, it was observed that the higher the final dry density, the slower the microstructure changes.     

非饱和土石混合料的初始吸力

土石混合材料广泛应用在土石坝、堤防、路基等工程中,而这些土工结构的边坡稳定性是工程上关注的问题。在非饱和土石混合料的边坡稳定分析时,需要知道边坡的负孔隙水压分布。现场直接测试吸力（气压为零时吸力是负孔隙水压的绝对值）成本高、精度有限。因此在研究土石混合料的初始吸力特性基础上,估算初始吸力分布不失一种可行方法。本文对不同初始物理状态下土石混合材的击实试样进行初始吸力测试,分析了土石混合料的物理状态与初始吸力的关系。试验结果表明：土样的饱和度比含水率与初始吸力有更显著的关系;在饱和度相同的情况下,土样的干密度较大的土样有较大的初始吸力;土样的初始吸力随着净压力的增大而减小;在干密度、饱和度相近的情况下,土石比越大初始吸力越大;相同的击实能量下,土石比大的试样干密度小。     

Characterization of water retention behavior of cracked compacted lateritic soil exposed to wet-dry cycles

Bulletin of Engineering Geology and the Environment - Exploring the long-term mechanism of hazard formation and identifying prospective hazard areas is critical for human survival and the risk...     

Effects of specimen thickness on apparent swelling pressure evolution of compacted bentonite

With the interests to see specimen thickness (h_sp) effect on apparent swelling pressure (p_s) of compacted bentonite and capture characteristics of p_s development during wetting bentonite specimens (p_s evolution, hereafter), a series of tests to measure p_s of compacted bentonite specimens with a dry density (ρ_d) range of 0.99–1.76 Mg/m³ and a specimen thickness (h_sp) range of 2–10 mm were conducted. Test results suggest: (1) p_s evolution generally has common characteristics with four feature points chronologically: peak (t_p, p_p), valley (t_v, p_v), initial equilibrium (t_ei, p_ei) and an equilibrium (t_eq, p_eq), where, t_p, t_v, t_ei and t_eq represent time (t) and p_p, p_v, p_ei, and p_eq denote p_s of the four feature points, respectively; (2) within the tested range, h_sp effect is generally minor on p_s at feature points, and p_p, p_v, p_ei, and p_eq have well correlations with specimen final dry density (ρ_df); (3) time scale for specimens with different h_sp can be well unified by a time coefficient χ_sp (=h_sp/√t).     

Behavior and characteristics of compacted expansive unsaturated bentonite-sand mixture

Limited studies dealt with the expansive unsaturated soils in the case of large-scale model close to the field conditions and therefore,there is much more room for improvement.In this study,expansive(bentonite-sand(B-S) mixture) and non-expansive(kaolin) soils were tested in different water contents and dry unit weights chosen from the compaction curve to examine the effect of water content change on soil properties(swelling pressure,expansion indices,shear strength(soil cohesion) and soil suction)for the small soil samples.Large-scale model was also used to show the effect of water content change on different relations(swelling and suction with elapsed time).The study reveals that the initial soil conditions(water content and dry unit weight) affect the soil cohesion,suction and swelling,where all these parameters slightly decrease with the increase in soil water content especially on the wet side of optimum water content.The settlement of each soil at failure increases with the increase in soil degrees of saturation since the matric suction reduces the soil ability to deform.The settlement observed in B-S mixture is higher than that in kaolin due to the effect of higher swelling observed in B-S mixture and the huge amount of water absorbed which transformed the soil to highly compressible soil.The matric suction seems to decrease with elapsed time from top to bottom of tensiometers due to the effect of water flowing from top of the specimen.The tensiometer reading at first of the saturation process is lower than that at later period of saturation(for soil sample B-S3,the tensiometer #1 took 3 d to drop from 93 kPa to 80 kPa at early stage,while the same tensiometer took 2 d to drop from 60 kPa to 20 kPa).     

考虑干湿循环作用的膨胀土渠道边坡破坏机理研究

膨胀土渠道由于季节性的通水和停水,其边坡实际处于干湿循环的状态,非常容易引起边坡失稳。采用离心模型试验对干湿循环作用下的膨胀土渠道边坡稳定性进行了探讨,研究了渠道边坡的破坏机理。研究发现,由于渠道的反复通水和停水,渠道边坡土体经历了多次干湿循环,每次循环后边坡土体裂缝逐渐扩展,渠水不断入渗,饱和区逐渐扩大。渠基土的膨胀性越强,每次干湿循环后的裂隙扩展越严重,渠水入渗作用也越强,渠道越容易破坏。研究还发现,由于通水和停水造成的渠道干湿循环破坏模式主要表现为渠坡整体发生浅层失稳破坏,这一破坏模式与前人研究发现的牵引式滑坡具有较大不同之处。     

Microstructure and anisotropic swelling behaviour of compacted bentonite/sand mixture

Pre-compacted elements(disks,torus) of bentonite/sand mixture are candidate materials for sealing plugs of radioactive waste disposal.Choice of this material is mainly based on its swelling capacity allowing all gaps in the system to be sealed,and on its low permeability.When emplaced in the gallery,these elements will start to absorb water from the host rock and swell.Thereby,a swelling pressure will develop in the radial direction against the host rock and in the axial direction against the support structure.In this work,the swelling pressure of a small scale compacted disk of bentonite and sand was experimentally studied in both radial and axial directions.Different swelling kinetics were identified for different dry densities and along different directions.As a rule,the swelling pressure starts increasing quickly,reaches a peak value,decreases a little and finally stabilises.FOr some dry densities,higher peaks were observed in the radial direction than in the axial direction.The presence of peaks is related to the microstructure change and to the collapse of macropores.In parallel to the mechanical tests,microstructure investigation at the sample scale was conducted using microfocus X-ray computed tomography(μCT).Image observation showed a denser structure in the centre and a looser one in the border,which was also confirmed by image analysis.This structure heterogeneity in the radial direction and the occurrence of macro-pores close to the radial boundary of the sample can explain the large peaks observed in the radial swelling pressure evolution.Another interesting result is the higher anisotropy found at lower bentonite dry densities,which was also analysed by means of μCT observation of a sample at low bentonite dry density after the end of test It was found that the macro-pores,especially those between sand grains,were not filled by swelled bentonite,which preserved the anisotropic microstructure caused by uniaxial compression due to the absence of microstructure collapse.     

Modelling the effects of test conditions on the measured swelling pressure of compacted bentonite

In the current concept of repositories for radioactive waste disposal, compacted bentonite and bentonite-based materials with low hydraulic conductivity are being used as engineered barriers to inhibit the migration of radioactive nuclides. To ensure low hydraulic conductivity, the swelling characteristics of compacted bentonite are also important. However, previous studies have shown that swelling pressure measurements vary considerably and that this variation may be attributable to the differences in testing apparatuses. This variability increases the uncertainty in the design of the facilities. Thus, in this study, an improved stress–strain model for bentonite materials during the saturation process is proposed. The validity of the model is confirmed by comparing the experimental results obtained using several test methods with the simulated results from the model. Consequently, it is found that the swelling pressure test results of compacted bentonite, which are affected by several factors, such as the stiffness of the test cell, the height of the specimen, and the initial degree of saturation, can be numerically simulated using the proposed model. Thus, the effects of these factors on the test results can be evaluated quantitatively using the proposed model.