Compacted Bentonite Specimens Research Articles

Apparatus for swelling deformation of compacted bentonite utilizing multi-ring moulds for evaluation of dry density and water content distribution

In the geological disposal of high-level radioactive waste, bentonite buffer materials are required to have self-sealing property, which is the ability to fill gaps by swelling deformation between the buffer material and the waste containment vessel or surrounding bedrock. Self-sealing property was evaluated based on the pressure generated after the gap filling under the assumption of constant dry density and water content in the specimen and buffer material. On the other hand, it takes tens to hundreds of years for the transitional period until the buffer contacts with groundwater, fills the gap, and the internal state converges to a uniform state. To assess changes more accurately in the condition of the buffer material, it is necessary to evaluate time courses in the density and water content of the buffer material because of groundwater infiltration and swelling deformation. From these, the authors developed an experimental apparatus for swelling deformation of bentonite, which consists of a mould with 2 mm-thick stacked rings. It is aimed to evaluate the distribution of dry density, water content, and degree of saturation inside the specimen by cutting the specimen with a 2 mm thick ring after the end of experiment. In this paper, results of preliminary experiments to confirm the applicability of this apparatus to swelling deformation test of bentonite was firstly described. Then, experiments were conducted on compacted bentonite specimens using this apparatus, and changes in the dry density and water content distribution inside the specimens were evaluated in terms of swelling deformation behaviour.

Experimental investigation on infiltration behavior of sodium ion in compacted bentonite

Because of its high swelling capacity and low hydraulic conductivity, bentonite has been selected as the candidate buffer material in deep geological disposal project for handling high-level radioactive waste. In Japan, seawater infiltration is an important issue in the evaluation of buffer material, as disposal facilities are under discussion for siting in coastal areas [1]. Sodium ion, which is the main ion in seawater, has profound effects on the properties of bentonite, such as hydraulic conductivity and swelling behaviours etc. [2][3]. These changes may be closely related to the infiltration behavior of sodium ion in bentonite. T In this study, for observing the infiltration behavior of sodium ion in compacted bentonite, infiltration tests were conducted on compacted bentonite specimens with thickness of 2 mm, where the specimens were wetted by NaCl solutions with a concentration range of 0 to 1 mol/L. In the testing program, wetting time, specimen dry densities and initial water contents were varied to investigate their effects on sodium ion infiltration. After the infiltration test, final water contents of specimens were measured and leached cations were obtained using benzyltrimethylammonium chloride (BTM) method [4].
 Figure 1 shows the results of leached cations and final water content of specimens with 1.5g/cm3 initial dry density for different wetting time. It is found that under the 0.1mol/L NaCl solution case, there is almost no sodium ion infiltration in compacted bentonite with a dry density of 1.5g/cm3 as shown in Figure 1(a). For the case of 0.5mol/L and 1mol/L NaCl solution, the leached sodium cation from specimens increases with the increase of concentration of supplied NaCl solution under the same wetting time. For other exchangeable cations (i.e., Mg+, Ca2+ and K+), there is no appreciable difference among the varieties of NaCl solutions and wetting time. For the final water content measurement results, it is found from Figure 1(b) that the water content increasing speeds in 0.5mol/L and 1mol/L NaCl solutions are significantly faster than that in 0.1mol/L NaCl solution.

Pore fluid chemistry effects on homogenization of compacted bentonite specimen with technological voids

Pore fluid chemistry effects on homogenization of compacted bentonite specimen with technological voids

Impact of iron corrosion on swell pressure of compacted barmer bentonite induced with thermal history

The bentonite is compacted around the steel canister containing high-level waste to ensure the sealing of the canister by blocking the entry of harmful radiations emitted due to the reactions of active radionuclides. However, the canister continuously exhibits a high temperature (150 °C–200 °C) due to radioactive waste decay over a long time, which creates a thermal history on the surrounded compacted bentonite. Corrosion can generate changes in the bentonite buffer, which comes in direct contact with the canister and may lead to a change in its swelling ability. This study investigates the influence of corrosion products on the swell pressure of compacted bentonite specimens subjected to thermal history. Two Bentonite samples (B1 and B2) from the Barmer district of Rajasthan, India, were mixed with corrosion products and compacted at dry densities 1.5 Mg/m3, 1.75 Mg/m3 and 2.0 Mg/m3. These specimens were subjected to thermal histories at 110 °C and 200 °C and saturated with distilled water. The decrease in the swell pressure values was observed with the corrosion product and the increase in temperature of induced thermal history—comparatively, more time required for saturation in the presence of corrosion products than in its absence. The alteration in montmorillonite minerals might have occurred under the induced thermal history and corrosion reaction.

Insight on the Void Ratio-Suction Relationship of Compacted Bentonite during Hydration.

Investigation on swelling characteristics of buffer/backfill materials during hydration is an important issue in the design of artificial barriers in high-level radioactive waste (HLW) disposal repositories. In this work, for clarifying the characteristic of void ratio–suction relationship for compacted bentonite on hydration path, suction-controlled swelling deformation tests under constant vertical stresses 0.001~40 MPa were carried out on compacted bentonite specimens. Four different types of void ratio-suction curves indicated that swelling-collapse behavior under hydration depends on suction and over-consolidation ratio (OCR), based on which the swelling index was defined. Then, equations were proposed for describing the swelling-collapse characteristic of void ratio–suction curves. Simulation results of suction-controlled swelling deformation tests show that the different types of the hydration deformation curves could be well described by the proposed equations. Obviously, the proposed equations could be used for description and prediction of swelling characteristics of compacted bentonite during hydration, which is also of great importance for the safety assessment of the HLW repositories.

Insights into gas migration in saturated GMZ bentonite using the RCP technique

In a deep geological repository for disposal of high-level radioactive waste, understanding of gas migration behavior in bentonite materials is one of the most important issues for safety assessment of the repository. This study tries to provide insights into gas transport properties in saturated bentonite tested under a constant-volume condition using the residual capillary pressure (RCP) technique. In this regard, a test setup was developed and gas injection tests were conducted on saturated GaoMiaoZi (GMZ) bentonite specimens using the RCP technique. Among which, some specimens that experienced initial gas breakthrough processes were re-saturated and re-submitted to conduct the subsequent gas injection tests. During the tests, gas permeability and gas breakthrough pressures were measured. Gas migration mechanism at lower gas injection pressures was analyzed and the influence of repetitive breakthroughs on sealing efficiency of saturated GMZ bentonite was discussed. Results show that as dry density increases from 1.3 to 1.7 g/cm3, gas breakthrough pressure increases from 1.37 to 8.56 MPa. Explanations to this conclusion could be that with increasing dry density, the proportion of large pores decreases and the strongly adsorbed water on the surface of minerals increases, leading to harder capillary displacement of pore water and thus a higher gas breakthrough pressure. Meanwhile, both diffusion and advection could play significant roles during the gas migration process at lower injection pressures, while advection could gradually become the dominant transport mechanism as gas injection pressures increase to higher values. Finally, after experienced a gas breakthrough process, the breakthrough pressure of compacted bentonite specimen might not recover to its initial value by re-saturating the specimen, indicating that degradation of sealing capacity of the bentonite materials induced by gas breakthrough process is irreversible.

Effect of aging on shear strength of compacted GMZ bentonite

The shear strength characteristics of compacted bentonite preferred as feasible buffer/backfill component is essential to the mechanical stability of the multi-barrier system in the context of nuclear waste disposal. This paper presents an investigation on the potential aging-induced changes in shear strength and microstructure of compacted GMZ bentonite at a dry density of 1.68 Mg/m3 and various water contents. For this purpose, direct shear and Mercury Intrusion Porosimetry (MIP) tests were correspondingly performed, as well as the soil-water retention tests. Statically compacted bentonite specimens were kept at constant volume and water content conditions for different periods of aging time up to 90 days, prior to experimental investigations. Results showed that the shear strength decreased with aging time, with a more noticeable trend during the first 30 days. Aging effect on shear strength was less pronounced for specimens at water content of 24%. Significant changes in microstructure after aging were observed, characterized by an increase in tiny pores (< 6 nm) and a decrease in inter-aggregate pores. The former phenomenon is related to the interlayer hydration as water redistribution is expected to occur, while the latter one is attributed to the exfoliation of clay particles that fill and reduce the inter-aggregate pores, which accounts for the dry density effect on soil-water retention curves (SWRC). Aging-induced changes in microstructure are governed by the water uptake from the non-interlayer space acting as reservoir into the interlayer pores, thereby promoting a further clay layer hydration and an arrangement of clay fabric. As such, both the effective degree of saturation (mainly considering the capillary water) and total suction decreased with aging, as well as the Bishop-type effective stress, thereby resulting in a decrease in the shear strength of compacted GMZ bentonite.

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

With the interests to see specimen thickness (hsp) effect on apparent swelling pressure (ps) of compacted bentonite and capture characteristics of ps development during wetting bentonite specimens (ps evolution, hereafter), a series of tests to measure ps of compacted bentonite specimens with a dry density (ρd) range of 0.99–1.76 Mg/m3 and a specimen thickness (hsp) range of 2–10 mm were conducted. Test results suggest: (1) ps evolution generally has common characteristics with four feature points chronologically: peak (tp, pp), valley (tv, pv), initial equilibrium (tei, pei) and an equilibrium (teq, peq), where, tp, tv, tei and teq represent time (t) and pp, pv, pei, and peq denote ps of the four feature points, respectively; (2) within the tested range, hsp effect is generally minor on ps at feature points, and pp, pv, pei, and peq have well correlations with specimen final dry density (ρdf); (3) time scale for specimens with different hsp can be well unified by a time coefficient χsp (=hsp/√t).

Method for measuring the saturated permeability coefficient of compacted bentonite at temperatures exceeding 100 °C

In deep geological repositories for high-level radioactive waste, the bentonite buffer layer is considered an important artificially engineered barrier between nuclear waste canisters and the host rock. Low permeability requirements of the bentonite must be met to prevent groundwater infiltration and radionuclide leakage. During the repository operation, gases will inevitably be generated through various physicochemical processes around the canister, and the continuous accumulation of these gases will result in a high-pressure state in the repository. In addition, the bentonite buffer layer may experence temperatures exceeding 100 °C because of the continuous release of decay heat from the nuclear waste in the canister, which can also affect the permeability of the bentonite buffer layer. Hence, it is necessary to obtain the saturated permeability coefficient of compacted bentonite at temperatures exceeding 100 °C. However, the conventional methods fail to consider the pressure effect, because when the temperature exceeds 100 °C under standard atmospheric pressure, free water will boil violently to form steam, which hinders the measurement of the saturated permeability coefficient of bentonite. Therefore, an innovative measurement method for the saturated permeability coefficient of compacted bentonite at temperatures exceeding 100 °C was proposed in this study, based on the relationship between the boiling point of water and the pressure. As the boiling point was increased by raising the pressure, the saturated permeability coefficients at 25 °C and 105 °C were sequentially measured for the same compacted bentonite specimen with the dry density of 1.65 g/cm3 to verify the feasibility of the proposed method. The results at 25 °C showed that the difference between the permeability coefficients measured by the proposed method and the conventional method was no more than 1 × 10−13 m/s. Thus, the proposed method can be applied to measure saturated permeability coefficients at temperatures exceeding 100 °C. Furthermore, the results at 105 °C indicated that it was more reasonable to calculate the permeability coefficients according to the amount of water seepage at the outlet. The proposed method can provide technical support for future studies on the effect of temperatures exceeding 100 °C on permeability and allow evaluation of the long-term performance of bentonite buffer layers in deep geological repositories.

Movement of water in compacted bentonite and its relation with swelling pressure

A testing procedure was proposed to study water movement in compacted bentonite and the development of swelling pressure (ps) when compacted bentonite specimens were wetted. In this procedure, a multi-ring mold was introduced for ps measurements, after which the specimen was sliced for X-ray diffraction to find movement of water in the interlayer space of montmorillonite. Results revealed a relation between four phases of ps development and evolution of four states of interlayer water molecule arrangement of montmorillonite (L): when ps reached its first peak in phase I, L moved from 1 row water arrangement (1w) to at least 2w; when ps decreased and re-increased in phases II or III, L moved from 2w to at least 3w; and when ps reached a steady state in phase IV, L = 3w. The w distribution in the compacted bentonite was also measured as water absorption time increased. Based on those results, the global water movement was estimated in terms of diffusivity (D) following a method employing Boltzmann transform. Results of comparisons implied that D calculated using this method matched experimental data well and the method was rather easily handled.

Variation of swelling pressure, consolidation characteristics and hydraulic conductivity of two Indian bentonites due to electrolyte concentration

The hydro-mechanical properties of the buffer material play an important role in the design of deep geological repositories. This paper presents the effect of various electrolyte concentrations on swelling pressure, consolidation characteristics and hydraulic conductivity of two different Indian bentonites. Constant volume swelling pressure tests on compacted bentonite specimens as well as consolidation tests on saturated bentonite specimens were carried out by supplying 0, 0.1, 0.5 and 1.0 N NaCl and CaCl2 solutions. The change in morphology and microstructure of bentonite specimens were examined by Scanning Electron Microscope (SEM) and XRD analysis. The hydraulic conductivity was calculated based on consolidation test results. At a dry density of 1.4 Mg/m3 both swelling pressure and hydraulic conductivity of divalent bentonite were found to be approximately 2.9 and 7.3 times higher than that of monovalent bentonite when saturated with deionized water. The electrolyte solutions altered the morphology, microstructure and hydro-mechanical properties of the monovalent bentonite significantly; however, the effect was found to be negligible for divalent bentonite. An empirical equation is proposed to predict the hydraulic conductivity of the bentonites. A good correlation between experimental and predicted hydraulic conductivity of bentonites is obtained. The divalent bentonite was found to be better suitable engineered barrier material in the electrolyte environment, whereas monovalent bentonite was found preferable at low salinity conditions.

Volume change behavior of saturated compacted GMZ bentonite and slurry during cyclic loading-unloading processes

In order to investigate the volume change behavior of saturated GMZ bentonite during cyclic loading-unloading processes, oedometer tests with cyclic loading-unloading process were conducted on saturated compacted GMZ bentonite and slurry specimens. A significant hysteretic loop was observed in compression curve of saturated compacted GMZ bentonite during cyclic loading-unloading process. Compression curve of the slurry specimen presented a bilinear shape. Under smaller stresses (<0.5~2MPa), the unloading-reloading loop was insignificant. However, under higher stresses (>0.5~2MPa), the unloading-reloading loop was significant. The unloading-reloading loop could be explained by the competition between the mechanical and physico-chemical effects on the microstructure changes. When stress was higher than 10MPa, compression curve of the slurry gradually closed to and coincided with the compression curve of the compacted bentonite specimen. Therefore, normal consolidation line of GMZ bentonite could be determined based on the overlapped part of the compression curves.

Effect of aging on thermal conductivity of compacted bentonites

The thermal conductivity of compacted bentonite is one of the most crucial parameters for the safe operation of high-level radioactive waste (HLW) repositories where bentonite can act as a feasible buffer. In this study, the potential effects of aging on the thermal conductivity of Gaomiaozi (GMZ07) and Wyoming (MX80) bentonites were investigated experimentally. The bentonite specimens were prepared under different conditions, such as varying water content and dry density, through the static compaction method in a special mould. The water content of the compacted bentonite specimens was kept constant during curing periods of 0, 5, 15, 30, 60, and 100 d under constant volume conditions, and the thermal conductivity of the two bentonites was then measured using the thermal probe method. The test results showed that the thermal conductivity decreased with increasing aging time for both of the compacted bentonites, with a trend that decreased significantly at the early aging periods and then tended to be constant when the aging time exceeded 60 d. The effects of aging are more pronounced for specimens with higher dry densities and water contents. Mercury intrusion porosimetry (MIP) test results confirmed that the reduction in thermal conductivity with aging time could be attributed to smectite hydration within the bentonites during the aging process. With this hydration, part of the soil water moves into the interlayer spaces of smectite crystals, resulting in a decrease in the pore water outside the bentonite particles, which decreases the thermal conductivity of the bentonites.

Influence of temperature on swelling deformation characteristic of compacted GMZ bentonite-sand mixtures

Laboratory swelling deformation tests were carried out on compacted GMZ bentonite and bentonite-sand mixtures with 30% and 50 % sand contents at 20, 40, 60, 80 and 90 °C with infiltration of distilled water. Influence of temperature, initial dry density, and quartz sand content on the swelling deformation characteristic of compacted bentonite specimens was analyzed. Results indicate that the swelling deformation process is accelerated, and the maximum swelling strain increases with the increase in temperature, while the maximum swelling strain tends to be stable with increasing temperature. In the meantime, the temperature effects depend on both of the sand content and the initial dry density of the specimens, the increases of the maximum swelling strain induced by increasing temperature, are enlarged by increasing sand content or initial dry density. Adding of quartz sand to bentonite not only influences the integrality of bentonite specimen, but also increase the microfissuring in area on quartz sand, which are advantageous to the heat transfer, leading to the increase of swelling deformation capacity of the specimen. The increased dry density relatively increases the bentonite content, so the swelling property is enhanced. However, no change on mineral composition of bentonite was observed when temperature was changed from 20 to 90 °C.

Effect of NaCl Solution on Swelling Characteristics of Bentonite with Different Diffuse Double Layers

Soil particles generally possess negative ions, and form an electronic field nearby the particles, which results in generating diffuse double layers on the surface of the particles. There is no diffuse double layer at dry state, and it develops diffuse double layers first when wetted by solution. This paper investigates the effect of NaCl solution on swelling characteristics of bentonite with different initial water contents by performing a series of wetting tests. The wetting test results indicate that when the initial water content is less than 10%, the NaCl solution has no effect on the swelling. When the initial water content is larger than 10%, the swelling strain decreases with increase in the NaCl solution concentration. The nuclear magnetic resonance (NMR) tests on compacted bentonite specimens were performed. The test results indicate that the strongly bound water content is 10%, which is similar to that by other method. The NMR tests on full wetting specimens, whose initial water content is less than 10%, show that the NaCl solution has no effect on NMR tests results. When the initial water content is larger than 10%, the proportion of curves from the NMR tests decreases with increase in the NaCl solution concentration. The NMR tests results indicate that the swelling occurred mainly in the intra-aggregate rather than in the inter-aggregate.

Ultrasonic measurement in variation of a degree of saturation in compacted bentonite

Bentonite, which is a kind of clay, is considered as a candidate for a buffer material in a geological repository of the high-level radioactive waste. Bentonite could provide long-term isolation of the radioactive waste because of its high adsorption capacity, low water permeability and good swelling property. In the context of long-term safety of disposal management, understanding groundwater behavior in bentonite buffer is one of the important issues, related to evaluating its long-term performance. In order to understand water behavior in compacted bentonite, the ultrasonic measurement was focused on. However, Ultrasonic wave propagation in compacted bentonite has not been understood well. Therefore, in this study, high spatial measurement system with Laser Doppler Vibrometer was developed in order to observe the ultrasonic wave propagation in compacted bentonite specimen. Ultrasonic wave propagation in compacted bentonite surface was measured. As a result, ultrasonic wave reflection was observed on the boundary between two different degrees of saturation. Thus, the capability of the developed system for investigation of the wave propagation in compacted bentonite is confirmed.

Response of Compacted Bentonites to Thermal and Thermo-Hydraulic Loadings at High Temperatures

The final disposal of high-level nuclear waste in many countries is preferred to be in deep geological repositories. Compacted bentonites are proposed for use as the buffer surrounding the waste canisters which may be subjected to both thermal and hydraulic loadings. A significant increase in the temperature is anticipated within the buffer, particularly during the early phase of the repository lifetime. In this study, several non-isothermal and non-isothermal hydraulic tests were carried on compacted MX80 bentonite. Compacted bentonite specimens (water content = 15.2%, dry density = 1.65 Mg/m3) were subjected to a temperature of either 85 or 150 °C at one end, whereas the temperature at the opposite end was maintained at 25 °C. During the non-isothermal hydraulic tests, water was supplied from the opposite end of the heat source. The temperature and relative humidity were monitored along predetermined depths of the specimens. The profiles of water content, dry density, and degree of saturation were established after termination of the tests. The test results showed that thermal gradients caused redistribution of the water content, whereas thermo-hydraulic gradients caused both redistribution and an increase in the water content within compacted bentonites, both leading to development of axial stress of various magnitudes. The applied water injection pressures (5 and 600 kPa) and temperature gradients appeared to have very minimal impact on the magnitude of axial stress developed. The thickness of thermal insulation layer surrounding the testing devices was found to influence the temperature and relative humidity profiles thereby impacting the redistribution of water content within compacted bentonites. Under the influence of both the applied thermal and thermo-hydraulic gradients, the dry density of the bentonite specimens increased near the heat source, whereas it decreased at the opposite end. The test results emphasized the influence of elevated temperatures (up to 150 °C) on the thermo-hydro-mechanical response of compacted bentonites in the nuclear waste repository settings.

Effect of temperature on swelling pressure and compressibility characteristics of soil

Clays are being used for several construction purposes particularly as waste isolation barrier and undergo several temperature and hydraulic condition during operational period. Clays may undergo large compression during the particular loading condition. Bentonite is highly swelling clay containing high amount of montmorillonite mineral is considered as barrier and backfilling material for high level radioactive waste disposal repositories. The present study investigates the effect of temperature on swelling pressure and compressibility behavior of divalent rich Indian bentonite (liquid limit =139%) from Bikaner, Rajasthan and effect of temperature on compressibility behavior of local soil (liquid limit =35%) from Rourkela. A new oedometer was designed and developed in-house to carry out consolidation and swelling pressure tests at various temperatures. Swelling pressures tests on compacted bentonite specimens of targeted dry density 1.6Mg/m3 were conducted under constant volume condition for the temperature range between 25 and 90°C. Consolidation tests at various temperatures between 25 and 90°C for both soils were carried out using distilled water as inundating fluid. The study revealed that with increase in temperature the swelling pressure and compressibility index of bentonite increased. However, effect of temperature on compressibility behavior of local soil was found to be minimum.

Diffusion behavior of selenite in purified bentonite

Selenium (Se) is an important element for assessing the safety of high-level waste disposal. In this study, the diffusion coefficients of selenite in purified bentonite were investigated. A Japanese purified sodium bentonite, Kunipia-F, which contains approximately 99 wt% montmorillonite, was used in this experiment. Bentonite powder was compacted into cylinders (diameter, 10 mm; height, 10 mm; dry density, 0.8–1.6 Mg m−3). Each compacted bentonite specimen was inserted into an acrylic resin column and saturated with 0.01 M–1.0 M of NaCl (aq.) for 30 days. One side of the bentonite was spiked with 10 μL of tracer solution containing 0.13 M Na2SeO3 and it was kept at 10, 25, 40, or 55 °C for 1–29 days. The apparent diffusion coefficients of selenite in compacted bentonite were 2.5 × 10−11 to 1.9 × 10−13 m2 s−1 and decreased with the increase in dry density. The ionic strength showed no significant effect on the apparent diffusion coefficient of selenite for bentonite densities higher than 1.4 Mg m−3. However, the apparent diffusion coefficients for 1 M NaCl were several times larger than those in 0.1 or 0.01 M NaCl for densities lower than 1.2 Mg m−3. This increase in diffusion coefficient would be caused by a change in anion accessible porosity. The apparent diffusion coefficient would be explained by Archie's law and is proportional to exponentiation of porosity. It is desired to measure exact anion accessible porosity. The activation energy of selenite in purified bentonite was 20 ± 4 kJ mol−1 and was similar to the activation energies of oxo-anions in free water. This also indicates that selenite diffuses in free pore water in the bentonite.

High-density bimodal bentonite blends for hydraulic sealings at the Ibbenbüren coalmine

AbstractThe two high-pressure water-retaining dams at the Ibbenbüren coalmine in Münsterland (Germany) have to perform reliably under the induced tension caused by further exploitation of the current mining area. The load-bearing and the sealing functions of the new barriers were separated and new sealing materials were developed. An innovative multilayer sealing system of bentonite and sandwiched equipotential layers (SANDWICH) supporting homogeneous swelling and sealing, independent of formation water (Nüesch et al., 2002), was applied in this project. A testing program of strain-controlled swelling pressure tests on compacted bentonite specimens and on a bentonite/sand mixture was conducted to ensure an adequate potential for swelling-pressure development.The measurements under constant volume for dry densities between 1.45 g/cm3 and 1.67 g/cm3 showed an evolving swelling pressure between 1.04 and 1.8 MPa for 100% bentonite samples. Straincontrolled oedometer tests for zero strain and step-wise applied strain up to 2% revealed that a sufficient magnitude of swelling pressure existed at maximum applied strain.