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.     

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。     

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...     

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。     

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...     

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).     

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.     

Membrane behavior of bentonite-amended compacted clay

The performance of landfill liners can be enhanced if they exhibit a semipermeable membrane behavior, which restricts the migration of contaminants. Consequently, enhancing the membrane property of clays used for liners is becoming increasingly important. As bentonite has already proven to be an excellent additive for improving the membrane behavior of clay, the hydraulic conductivities and membrane behaviors of a locally available clay, known as Fukakusa clay, amended with different amounts of dry bentonite (5%, 10%, 15%, and 20%) were herein evaluated. The chemico-osmotic efficiency coefficient, ω, was obtained under different concentration differences of KCl solution (0.5, 1, 5, 10, and 50 mM) for specimens in rigid-wall cells. The test results show that the ω of unamended clay is very low and can be ignored unless amended with bentonite, and that hydraulic conductivity k is suitably low (1.58×10⁻⁹ m/s). Additionally, ω decreased as the KCl concentration increased, which is consistent with the theory that increasing concentration causes progressively greater shrinkage of the diffuse-double layers of the clay particles. Furthermore, the mechanisms that influence the membrane behavior are discussed from the viewpoints of the diffuse-double layer and the interparticle pore size with the assistance of SEM.     

Investigation of effects of temperature cycles on soil-concrete interface behavior using direct shear tests

The intermittent operation of the ground source heat pump connected to thermo-active geo-structures (e.g. energy piles) results in cyclic thermal loading on the soil-structure interface. To investigate the effects of cyclic thermal loading on soil-structure interface properties, a conventional direct shear device was modified by replacing the bottom shear box with a concrete plate (with smooth and rough surfaces) that has embedded aluminum tubes to heat and cool the interface. A series of tests were performed with interface temperatures of 4.5, 22.5, and 42.5 °C, respectively. The constant normal stresses of the direct shear tests were 27.6, 41.4, and 100 kPa. The tests were conducted both under cooling and heating conditions with thermal cycle numbers of 0.5 and 10.5. The tests were conducted at a shearing rate of 3 mm/min. The effects of water content changes on the shear strength of soil-concrete interface was also investigated by performing tests with soil water content ranging from 15% to 19%. The responses of soil-concrete interface subjected to temperature change and cycles and different water contents are presented in this paper.     

Numerical thermo-hydro-mechanical modeling of compacted bentonite in China-mock-up test for deep geological disposal

The China-mock-up test is to evaluate the performance of the compacted Gaomiaozi(GMZ) bentonite under coupled thermo-hydro-mechanical(THM) conditions in deep geological disposal.A numerical study of the test is conducted in this paper.The principal THM characteristics of the bentonite are presented at first.A THM model is then presented to tackle the complex coupling behavior of the bentonite.The model of Alonso-Gens is incorporated to reproduce the mechanical behavior of the bentonite under unsaturated conditions.With the proposed model,numerical simulations of the China-mock-up test are carried out by using the code of LAGAMINE.The time variations associated with the temperature,degree of saturation,suction and swelling pressure of the compacted bentonite are studied.The results suggest that the proposed model is able to reproduce the mechanical behavior of the bentonite,and to predict moisture motion under coupled THM conditions.     

Swelling pressure of compacted MX80 bentonite/sand mixture prepared by different methods

Compacted bentonite/sand mixture have been considered as possible sealing/backfilling material in the deep geological disposal of high-level radioactive nuclear waste. The swelling pressure of the compacted bentonite/sand mixture is of significance in the design of the geological repository, which requires good consistency between data obtained by laboratory and field measurement. In this work, a series of swelling pressure tests were performed on compacted MX80 bentonite/sand specimens prepared by methods commonly adopted in laboratory (As-compacted, Transferred) and those mimicking the real block manufacturing process (Trimmed, Inserted). Results shown that with identical dry density (especially when the dry density was larger than 1.70 Mg/m³), largest swelling pressure was obtained in specimens prepared by method Inserted, followed by method As-compacted, Transferred and Trimmed. The distinct difference between the swelling pressure could be largely attributed to the effects of residual post-compaction lateral stress. More interestingly, specimens prepared by methods Trimmed and Transferred followed a similar swelling pressure-dry density relationship. From this point of view, method Transferred without causing mass loss, change of bentonite content and possible technological voids effect was recommended in lieu of method Trimmed for specimen preparation in laboratory.     

Sealing performance of compacted block joints backfilled with bentonite paste or a particle-powder mixture

Technology gaps are inevitable parts of the buffer barriers in the high-level radioactive waste (HLW) repositories. The mechanical properties, permeability and water-solute-radionuclide migration of buffer barrier are different when considering the gaps. The present study was designed to investigate the sealing effect of joint between the compacted blocks with two types of sealing materials, a bentonite paste (P) and a bentonite particles and powder mixture (P/P) both from the macro- and micro-aspects.A series of laboratory test was conducted to study the hydro-mechanical properties of bentonite blocks after backfilling the joint between blocks. Both the intake and outflow water volume, and the pressure on radial and axial direction were monitored during the hydration. Following the permeability test, the joint’s sealing was evaluated by a final dry density, water content distribution and microstructure evolution. The results showed that the axial and radial pressure of blocks backfilled with bentonite P and bentonite P/P mixture were higher than that of blocks with a blank joint. Pressure preferentially increased in the low-stress areas (the joint areas), contributing to sealing of the joint. The hydraulic conductivities for backfilled bentonite blocks were almost on the same order of magnitude as to intact bentonite blocks, revealing a clear sealed hydraulic behavior. The final dry density decreased in the block area and increased in the joint area after hydration, forming a transition zone with a certain width. A smaller difference was found between the block and joint area in the backfilled block. The mercury intrusion porosimetry (MIP) test and scanning electron microscope (SEM) test showed that backfill decreased the numbers of inter-aggregates pores ranged between 8 and 15 μm compared with that in the blank joint. The comprehensive data showed that a joint backfilled with a bentonite P/P mixture has a higher swelling capacity, lower permeability, and denser structure compared with joint backfilled with bentonite P. The results suggested that joint backfilling improved the integrity of the blocks and enhanced the swelling property and impermeability compared with that of the non-backfilled blocks. Both bentonite P and bentonite P/P has satisfactory sealing performance of compacted block joints, with better effect of the later.