Compacted Sand-bentonite Mixtures Research Articles

Compacted sand–bentonite mixtures for the confinement of waste landfills

AbstractThis paper illustrates the results of an experimental study on sand–bentonite mixtures for their use as confinement barriers for solid waste landfills. The mixtures have been prepared parametrically varying the percentage of bentonite. The sample preparation method was established willing to simulate the compaction processes on site. In fact, the compacted samples were tested following two different stress-wetting paths representative of the possible stress and imbibition sequences occurring on a landfill confinement barrier. In the first case, the barrier comes into contact with rainwater before being subjected to the overloading stress induced by waste disposal, while, in the second case, the barrier is overloaded by the waste before being wetted by the leachate. The compressibility and permeability of the sand–bentonite mixtures were determined, in both cases, by oedometric compression tests. The experimental results are analysed and compared in order to evaluate the influence of the bentonite content on the mechanical and hydraulic behaviour of the mixture. Interpretation of the results is also accomplished with a micro-mechanical investigation of the mixtures fabric. Suitable compositions of sand and bentonite are finally proposed for the design of effective confinement barriers.

On the thermal hydro mechanical chemical behavior of bentonite sand mixture

The concept to high-level radioactive waste (HLW) repository generally involves anengineering-artificial barrier system, in which hardly compacted bentonite sand mixture blocks are placed carefully around the waste canister, and the canister is of stiffness in the design. Placed the bentonite sandmixture blocks, gradually receive hydration due to groundwater. Thermal-hydraulic-mechanical-chemical phenomena of compacted bentonite sand mixture after wetting, suction change and thermal effort are a critical parameter for buffered system This study proposes the influence of suction, relative humidity and thermal effort in thermal-hydraulic on deformation properties and mechanical properties for bentonite-sand mixture. A series of testing programs are prepared such as calibration test for changing of relative humiditywith temperatures, measurement of volume change involving heating and changing of suction, determination of unconfined compressive strength, for thermal performance triaxial compression tests for both unsaturated bentonite sand mixture and saturated bentonite sand mixture. The obtained results are further interesting, and summary provide as following. To suction increment incorporation with thermal heating produce induces obviously shrinkage of the bentonite sand mixture, and lead the decrement of strength. Elevated temperature confirm further reduction for maximum deviator stress using a developed triaxial compression apparatus that is regardless of unsaturated condition and saturated condition.

Saturated hydraulic conductivity of compacted bentonite–sand mixtures before and after gas migration in artificial seawater

To understand the self-healing property of an engineered barrier for radioactive waste disposal, the hydraulic conductivity of compacted bentonite–sand mixtures saturated with artificial seawater (SW) before and after gas migration was examined. Na- and Ca-bentonites were mixed with fine sand at a ratio of 70% bentonite in dry weight. Two aspects were considered during the experiment: the hydraulic conductivity of the specimen that was resaturated after gas migration and the distribution of water content immediately after gas migration to study gas migration pathways. The gas migrated through the entire cross-section of the specimen, and gas breakthrough occurred in the equilibrium swelling pressure range approximately. Subsequently, the gas flow rate reached a sufficient large value when the gas pressure was approximately twice the equilibrium axial pressure (the sum of swelling and confining pressures), which excluded the back pressure. Although the gas migration pathway was not visible when the specimen was observed immediately after gas migration, the water content distribution showed that several parts of the specimen with lower water content were connected in the direction of gas migration. After resaturation, the change in permeability was within a limited range—two to three times larger than that before gas migration for each type of bentonite in SW. This slight change suggests that gas migration creates a pore structure that cannot be sealed via crystalline swelling of montmorillonite in SW, even if highly compacted bentonite is used under a constant-volume condition.© 2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ ). • Gas migration of compacted bentonite–sand mixtures in seawater. • Saturated hydraulic conductivity after gas migration in seawater. • Breakthrough pressure increased along equilibrium axial pressure. • Low permeability of compacted bentonite did not change substantially.

Effect of a Water Phase on the Swelling Pressure and Water Retention of an Unsaturated Bentonite–Sand Mixture with Insignificant Osmotic Suction

AbstractCompacted bentonite–sand mixture is proposed widely as backfill in geological repositories for disposal of radioactive waste in many countries because this material has significant swelling capacity and low water permeability. Development of the swelling pressure of backfills upon hydration is related closely to the stability of the host rock in the geological repository. No systematic experimental studies have been carried out to explore the effect of a water phase on the swelling pressure and water retention of bentonite–sand mixtures with insignificant osmotic suction. The objective of the current study was to examine experimentally the influence of a water phase involving liquid water and water vapor on swelling pressure and water retention of a bentonite–sand mixture with insignificant osmotic suction. Swelling-pressure tests with suction control and water-retention measurements under constant-volume conditions were performed on the compacted bentonite–sand mixture with a dry density of 1.80 g/cm3. Osmotic and vapor equilibrium techniques were used to make identical specimens adsorb liquid water and water vapor, respectively. The experimental results showed that the water phase had almost no effect on the swelling-pressure patterns of the unsaturated bentonite–sand mixture upon hydration over a suction range from 27 to 3 MPa. The swelling pressure increased significantly with decreasing suction from 27 to 3 MPa, regardless of the mixture adsorbing either the liquid water or water vapor. Nevertheless, the water phase had a considerable impact on both the swelling pressure and water retention of the unsaturated bentonite–sand mixture upon hydration over the same suction range. For a given value of suction in the range above, the swelling pressure and the water content of the bentonite–sand upon adsorption of liquid water were greater than those upon adsorption of water vapor. The influence of the water phase on the swelling pressure and the water retention of the bentonite–sand mixture with insignificant osmotic suction is related mainly to the hydration or swelling mechanism of Ca-rich bentonite.

An assessment of the mechanical performance of a novel sand bentonite-glass fiber composite for the avoidance of catastrophic landfill failure

• Octahedral shear stress increased higher at the fiber aspect ratio than at the fiber dosage. • At peak and residual stress, bearing capacity factors improved significantly. • Negative excess pore waster pressure increased with an aspect ratio of 40. • Energy absorption capacity was the function of fiber dosage and fiber aspect ratio. Compacted sand bentonite (CSB) blend is one of the finest materials for landfill construction. CSB mixes with a strain-softening tendency may fail abruptly, increasing soil permeability. Water movement over the shear zone can induce flow slides, triggering a catastrophic collapse. Waste glass fiber was added to the CSB mixture (90:10) in various fractions (0.5 %, 1, and 1.5 %) and aspect ratios (l/d = 40, 80, and 120) to prevent the soil structure from collapsing. A full-scale consolidated undrained (CU) test was conducted to assess (i.e., c-phi-analysis) the load-carrying capabilities of waste fiber soil composite. The octahedral shear stress was increased significantly for both stress level (i.e., peak and residual stress). Positive pore water pressure (PWP) increased progressively after initial yield, while negative PWP reduced as the aspect ratio increased from 80 to 120. Fiber dosage and fiber aspect ratio improved bearing capacity factors (N c ,N q ,N ϒ ) and undrained bearing capacity. The EAC (Energy absorption capacity) was determined using a numerical integration approach through MATLAB (version14), and the use of waste fiber resulted in a considerable improvement. Several variables contributed to the principal stress and soil modulus. Accordingly, they were framed with all the variables and built a model that matched the experimental results well.

Effect of Various Aqueous Mediums on the Microstructure of Compacted Bentonite–Sand Mixture Characterized by X-ray CT Investigation

Compacted bentonite–sand mixture (CBM) is a kind of candidate buffer material of high-level radioactive waste (HLW) disposal in many countries. It is believed that the permeability of CBM is greatly related to its microstructure. The aim of this study was to search the effect of various aqueous mediums on the microstructure and pore characteristics of this buffer material. Permeation experiments and X-ray computerized tomography (X-ray CT) were used to explain the correlation between microstructure and permeability. Representative samples of CBM at a dry density of 1.7 g/cm3 were used. X-ray CT was used to study the CBM under the de-ionized water (DI) and three other aqueous medium conditions. After reconstruction with Dragonfly software, the pore characteristics and permeability of different solution-saturated samples were analyzed using AVIZO software, including pore size distribution (PSD), porosity, and connectivity. The results showed that the permeability coefficient of samples was NaOH > NaCl-Na2SO4 > Simulated Beishan groundwater > DI water, and the maximum swelling pressure of samples was NaOH < NaCl-Na2SO4 < Simulated Beishan groundwater < DI water. The permeability coefficient decreased with the increasing of maximum swelling pressure. Quantitative analysis indicated that the volume of interconnected pores increased owing to the infiltration of NaCl-Na2SO4 and NaOH.

Recycled waste tire fiber as a sustainable reinforcement in compacted sand–bentonite mixture for landfill application

A layer of compacted sand-bentonite (CSB) mixture is employed as a liner at the landfill. The formation of desiccation-induced shrinkage cracks can increase the leachate migration into groundwater. The usefulness of the liner system may be endangered if the liner material's strength is insufficient. To address this issue, the CSB barrier can be modified by incorporating with waste tire fiber. This study aimed to assess the undrained shear strength, consolidation, and desiccation-induced crack of mixtures of CSB and reinforced CSB. An analytical model was derived to assess the behaviour of tire fiber composite soil from the experimental result. Positive excess pore water pressure was reduced significantly by the inclusion of tire fiber in the consolidated undrained triaxial test results. By the addition of 15% tire fiber, the effective frictional angle was raised from 21.0° to 24.2°, while the effective cohesion component was increased from 15.0 kPa to 24.2 kPa at the same fiber content. The crack density factor and crack intensity factor of the mixture without tire fiber were much higher than those of the CSB with fiber. By the addition of tire fiber, time for 90% consolidation was reduced, while the coefficient of consolidation increased. Although the permeability increased marginally, but it was within the limiting value. An analytical model adequately captured the stress-strain and pore water pressure responses of all proportion of tire fiber, and the yield surface of the composite was expanded maximally with only 10% tire fiber.

Swelling pressure of a sodium bentonite in solutions of different suction

In this study, the swelling pressure of unsaturated, compacted bentonite-sand mixture was measured, which the all samples had experiment in dying-wetting history in suction changing. This test program used Kunigel V1 as sodium type. Constant volume swelling test was conducted out that swelling pressure was measured using a developed apparatus. A pressure sensor was mounted in the swelling test apparatus. Previously, the all samples deformed due to suction change (i.e. drying-wetting process in suction). Two types were prepared before applying suction that were initial unsaturated condition and saturated condition on same initial dry density. Swelling pressure and adsorbed water amount was monitored in the test, which lasted for one month.

Effect of groundwater chemistry and temperature on swelling and microstructural properties of sand–bentonite for barriers of radioactive waste repositories

Compacted bentonite–sand mixtures are commonly considered as the key materials for the construction of engineered barrier systems (EBSs) in deep geological repositories (DGRs) in Canada and other countries. These bentonite-based barriers can be simultaneously exposed to a wide range of chemical and thermal (nuclear waste heat decay) conditions of underground water. Thus, an understanding of the effects of groundwater chemistry and temperature and their interactions on the changes in the swelling properties as well as the microstructure of bentonite-based barriers is essential for assessing the durability and safety of an EBS in a DGR. This study experimentally investigates the impact of a Canadian deep underground water chemistry (C; total dissolved solids up to 300 g/L), temperature (T; 23 °C, 40 °C, and 80 °C), and the coupled effects of C and T on the swelling properties and microstructure of bentonite–sand barrier mixtures (30 to 70% by weight). The results show that the groundwater chemistry of southwestern Ontario (Canada) has a significant impact on the swelling characteristics of the examined bentonite–sand material. The swelling ability significantly decreases with respect to the groundwater chemistry. The mineralogical composition of the bentonite changes. Most of the Na-montmorillonite is mainly transformed to Ca-montmorillonite and traces of illite with a lower swelling capacity when the bentonite–sand barrier material is exposed mixed to saline water solutions. The temperature (alone) has a limited effect. However, the temperature–groundwater chemistry interactions significantly affect the swelling capacity of the studied bentonite–sand material. Future safety assessments and the final engineering design of engineered barriers for DGRs in Ontario should take into consideration the possibility of the long-term deterioration of bentonite–sand barriers from chemical attacks and/or thermochemical attacks induced by the high salinity of the groundwater and by simultaneous impact of the chemistry of the groundwater and the heat generated by the nuclear wastes, respectively.

Self-sealing behavior of compacted bentonite–sand mixtures containing technological voids

The self-sealing behaviors of compacted Na- and Ca-bentonite–sand mixtures were investigated by conducting swelling–permeability tests with specimens containing three types of technological voids. The technological voids, including an upper gap, a surrounding gap, and a center hole of the specimens at 0.03–17.65% of the total cell volume, were sealed by the swelling of the specimen. The vertical and lateral pressures became steady during the experiments, resulting in equal equilibrium pressure in the surrounding gap. The unequal equilibrium pressure in the vertical and lateral directions was measured when the upper gap and center hole were set. The lateral pressure tended to be higher than the vertical pressure. The residual compaction pressure likely explains the higher equilibrium lateral pressure. The coefficient of permeability depended on the average effective montmorillonite dry density in both Na- and Ca-bentonite–sand mixtures. This study proposed an effective Na-montmorillonite dry density, focusing on the high swelling potential of montmorillonite with exchangeable Na ions. The effective Na-montmorillonite dry density of the specimen, including the swollen part, exhibited a unique relation with the permeability after self-sealing, despite the different ionic types of bentonite. Under fully saturated condition, the distribution of water content throughout the specimen after self-sealing suggested that the dry density of the swollen part was lower than the initial specimen part, despite the presence of steady state pressures. It is most likely that the density distribution remained after self-sealing if external factors, including changeable boundary conditions and pore water chemistry, are not considered. Although the specimen after self-sealing exhibited the potential for heterogeneous density, its permeability was approximately evaluated by using the average effective Na-montmorillonite dry density.

Swelling pressures and hydration times in a clay seal

Underground repositories for nuclear fuel waste will eventually become filled, and access shafts will have to be sealed. In many international studies, the preferred sealant is a compacted sand–bentonite mixture in which montmorillonite in the bentonite swells as it absorbs groundwater and develops swelling pressure. Lowered porosities in the bentonite lead to reduced hydraulic conductivity. Sealing is most effective when the seal is saturated and swelling pressure has been maximised. The time required to reach complete hydration is important. This paper describes the development of swelling pressures during hydration of a field-scale shaft-sealing experiment at Canada’s underground research laboratory. Numerical modelling used the finite-element program Code_Bright and hydro-mechanical parameters from laboratory tests to determine time-dependent porosities, total pressures and degrees of saturation in the full-scale field project. Data from laboratory-scale ‘wetting’ tests on specimens of seal material were then used in models by Liu and Dueck and Börgesson to evaluate changes in swelling pressures with time. Six years of data from field instruments indicate that the seal was functioning as designed. In the future, when the seal becomes fully hydrated and reaches stress equilibrium, the modelling suggests that swelling pressures in the seal will be in the range of 0.43–0.86 MPa. This is broadly similar to the value of 0.8 MPa suggested by a separate series of laboratory swelling tests. Full saturation and maximum swelling pressure can be expected in 17–19 years after the seal has been completed.

Evaporation and shrinkage processes of compacted bentonite-sand mixtures

The deterioration of compacted bentonite-sand mixtures (CBSMs) after suffering desiccation increases the hydraulic conductivity of clay barriers, posing risks to the environment. This work compressed bentonite-sand mixtures (BSMs) to various dry densities in order to conduct the free-drying tests, focusing on the evaporation and shrinkage processes. The compaction constrained the permeability of the mixtures and resulted in the delay in evaporation and shrinkage of the CBSMs’ internal parts (compared to their external parts). Subsequently, the secondary shrinkage occurred (i.e., the smaller volumetric shrinkage after the shrinkage limit reached). Compaction constrained the plate-like smectite particles in being propped up and led them to orientate. This structure reinforced the CBSMs’ shrinkage property and contributed to their lower air entry value, shrinkage limit, and final void ratio. During the design and maintenance of compacted clay barriers, secondary shrinkage and the reinforced shrinkage property are proposed to be taken into consideration.

Hydro-Mechanical Properties of Sand-Bentonite-Glass Fiber Composite for Landfill Application

Hydro-mechanical characteristics of compacted sand-bentonite (SB) mixture gets affected significantly due to desiccation cracking and can cause a severe rise in hydraulic conductivity and differential settlement. Glass fiber can be added to the SB mixture to reduce the desiccation cracking. However, the addition of glass fiber to SB mixture can adversely affect its hydro-mechanical behavior. Present study was conducted to analyze the impact of the inclusion of the glass fiber on the hydro-mechanical behavior of SB mixture. Various experiments were performed on SB mixture added in a ratio of 70:30 and reinforced with 1% of glass fiber of the aspect ratio of 40, 80 and 120. Test results suggested that effective strength parameter, initial tangent modulus and energy absorption capacity increased; whereas, the swelling tendency decreased due to the addition of glass fiber. The results also indicated that the compressibility, hydraulic and shrinkage behavior of the mixture got impacted significantly due to the presence of the fiber.

Undrained performance of sustainable compacted sand-bentonite–glass fiber composite for landfill application

Abstract A compacted sand-bentonite composite is considered as a barrier material for the use at the landfill. If the strength of the barrier material is not adequate, then the actual operation of the liner system can be endangered. Worldwide environmental awareness and high dumping cost have influenced the possible use of glass fibers for landfill application. Since the composite possessed a lower shear strength, glass fiber can be added to enhance its shear strength. The purpose of the present investigation was to assess the effective shear strength parameters of compacted sand–bentonite mixture, mixed in a proportion of 80:20 and reinforced with glass fiber. Glass fiber with an aspect ratio of 40, 80, and 120 were added to sand-bentonite composite in a fraction of 0.5, 1, and 1.5%. Test result suggested that the negative excess pore water pressure increased for the composite with an aspect ratio of 40; however, it decreased gradually with an increase in the aspect ratio to 80 and 120 for any fiber concentration. At a fiber content of 1.5% with the aspect ratio of 80 and 120, the peak effective frictional angle was almost constant. The cohesion component enhanced moderately up to the aspect ratio of 80 for any fiber concentration and remained constant with a further increase in the aspect ratio from 80 to 120. Residual strength parameter of the composite improved significantly with the addition of glass fiber. Initial tangent increased up to 1% fiber content for any aspect ratio, and then reduced with the further addition of glass fiber. The effective principal stress at failure predicted using a mathematical model showed an excellent agreement between predicted and experimental values.

Swelling behavior of compacted bentonite-sand mixture during water infiltration

A laboratory-based water infiltration test was conducted with compacted Calcigel bentonite-sand mixture (50:50) at room temperature to mimic the transient hydration of a backfill material in nuclear waste repository conditions. The test was performed with an innovative column-type experimental device, which facilitated the continuous and non-destructive measurements of temperature, relative humidity, water content and total stresses in both axial and lateral direction at various preselected locations along the height of soil sample. The effect of groundwater geochemistry on the bentonite hydration process was not considered in this study. The distilled water was supplied from the bottom-end under 15-kPa hydration pressure for a period of 349 days to mimic the water ingress from the host rock. The test results highlighted the hydration-induced heterogeneity in the material and its effect on the lateral swelling pressure development along the height of soil sample. The axial swelling pressure measurements revealed the factors, which affect the stress-transfer mechanism between both the ends. The simultaneous measurements of relative humidity and water content indicated the porosity redistribution close to the hydration-end during the test.

Impact of glass fibre on hydromechanical behaviour of compacted sand–bentonite mixture for landfill application

Desiccation-induced moisture variations creates shrinkage cracks to the landfill liner and can increase the migration of leachates. In addition to this, if the strength of the material is not enough then the effective operation of the liner system may be threatened. Glass fibre can be used to reduce the shrinkage crack of a sand–bentonite mixture. However, addition of glass fibre may adversely affect the other geotechnical parameters of the mixture. Therefore, the main objective of this study is to investigate the effect on geotechnical properties of sand–bentonite mixture due to the inclusion of the glass fibre. In this purpose, tests were performed on sand and bentonite mixture (mixed in a proportion of 90:10) and added with 0.5%, 1% and 1.5% of glass fibre. Test results suggested that compression index decreased; whereas a maximum increase in the swelling index occurred due to the inclusion of 1% of glass fibre. Hydraulic conductivity and rate of consolidation slightly altered with the inclusion of glass fibre to the mixture; whereas peak and residual strength increased. Unconfined compressive strength (UCS) test result suggested that improvement factor, displacement ratio and normalized energy absorption capacity increased with the inclusion of glass fibre.

Geotechnical Characteristic of Compacted Sand Bentonite Mixture

Geotechnical Characteristic of Compacted Sand Bentonite Mixture

Hydraulic and mechanical characteristics of compacted sand–bentonite: tyre chips mix for its landfill application

Compacted sand–bentonite mixtures are generally placed at waste dumping places as a means of avoiding contaminant movement to reduce or remove the potential for groundwater pollution. Owing to their almost impermeable nature and high pollutant adsorption tendency, bentonite generally treated as a liner material to avoid subsurface contamination. Cracks are formed in the landfill liner due to the desiccation, large interlayer shrinkage and differential settlement of bentonite. Fibres are commonly added to sand–bentonite mixture to act as a reinforcing material to eliminate the development of tensile cracks. Waste tyre chips are derived from the tyre retreated technique, and it can be included as short fibre in sand–bentonite mixture for the use as landfill barrier material. Hydraulic conductivity and shear strength are the two important parameters of the liner material, which are considered for hydraulic, and stability analysis in landfill application. In this context, consolidation and consolidated undrained tests were performed to study the hydraulic and mechanical behaviour of sand–bentonite mixture mixed in a proportion of 80:20 and mixed with 0, 5, 10 and 15% of waste tyre chips. From the consolidation test, swelling pressure and swelling potential reduced maximal at 15% of tyre chip content. Hydraulic conductivity and negative pore water pressure altered significantly with the application of tyre chips. Effective internal angle of friction enhanced from 24.9° to 30.5° and critical state parameter improved from 0.738 to 1.429 with the presence of 15% of tyre chips. Test result suggested that maximum of 10% of tyre chips can be used as a landfill liner and 15% tyre chips would be benefited for landfill cover.

Evaluation of a sand bentonite mixture as a shaft/borehole sealing material

Abstract The mechanical and hydrological characteristics of compacted sand bentonite mixtures with bentonite contents ranging from 5 to 40% were investigated in the laboratory in order to assess their use as a waste isolation material and to select an optimum sand bentonite mixture. Laboratory tests included compaction, compaction permeability, unconfined compression and direct shear tests which led to a recommendation to select a mixture with a bentonite content of 30% for the isolation of underground geological waste disposal repositories. This study complements the previous studies of the authors of this manuscript by determining the mechanical and hydrological properties of sand bentonite mixtures that possess bentonite contents > 30% to determine the geotechnical properties (i.e., unconfined compressive strength, Young's modulus, cohesion and angle of internal friction) and the mechanical behavior of these relatively high levels of bentonite mixtures for the first time in the literature.

Hydromechanical modelling of the SEALEX experiments

Numerical modelling of coupled physical processes in bentonite–sand mixtures under the geological conditions is significant for designing and constructing sealing systems in deep underground repositories for highly radioactive nuclear waste. Within the framework of DECOVALEX 2015, Task A, this work presents the model validation of OpenGeoSys by numerical modelling of coupled hydromechanical (HM) processes in bentonite–sand mixtures. Parameters used in the HM model were determined by modelling the laboratory tests of the sealing experiment (SEALEX). Afterwards these parameters were applied for the modelling of a small-scale mock-up test considering the influence of technological gap and incidental fail of the seal in the sealing system. In order to investigate the availability of employing these HM parameters and numerical models directly to field predictions, the modelling results and measured data of an in situ SEALEX experiment were analysed comparatively. The modelling results reproduced well the main features in HM behaviour of the compacted bentonite–sand mixture, which denotes that the adopted HM models and parameters are adequate for describing the HM processes in the sealing system. It is necessary to take the elastoplastic behaviour and evolution of the permeability of bentonite–sand mixtures into account when using the adopted models to reproduce the HM processes of a sealing system.