Bentonite Sand Research Articles

Time-Dependent Deformation Characteristics of Unsaturated Sand–Bentonite Mixture under Drying–Wetting Cycles

AbstractThe main objective of this study was to investigate the primary consolidation and creep characteristics of two unsaturated sand–bentonite mixtures subjected to drying–wetting cycles. An ext...

Thermomechanical Analysis of Fiber–Bentonite-Based Mixtures as Buffer Material in an Engineered Nuclear Barrier

Buffer/backfill material is an important part of engineered barriers in deep geological repositories of high-level radioactive waste (HLRW). A good buffer material not only possesses good heat conductivity, expansion, and sealing characteristics, but also exhibits enough strength. Its thermomechanical performance can significantly influence the safety and stability of a HLRW repository. Polypropylene fibers were mixed with bentonite-based mixtures (bentonite–sand–graphite) to form a buffer material with high strength, good heat dissipation capacity, and strong isolation capacities. The effects of fiber content, fiber length, and sand particle size on the shear strength, swelling pressure, and thermal conductivity of fiber–bentonite-based (BSGF) mixtures were analyzed under given graphite and sand contents. Detailed analyses of the shear, expansion, and heat transfer mechanisms of BSGF mixtures were performed to understand the behavior. The results indicated that the strength of BSGF material improved significantly with the addition of fiber content, whereas swelling potential and thermal conductivity were reduced. With an increase in sand particle size, cohesion and maximum swelling pressure decreased, whereas internal friction angle and thermal conductivity increased. The optimum fiber content, fiber length, and sand particle size were determined based on the thermomechanical performance of BSGF mixtures. These findings help in the field application of buffer materials with different fiber contents, fiber lengths, and sand particle sizes.

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.

Performance evaluation of soil mixtures treated with graphite and used as barrier fill material for high-level radioactive waste repository

Buffer/backfill material is an important engineering barrier in a deep geological repository of high-level radioactive waste (HLW). Its thermo-hydro-mechanical (THM) performance is very important for the safe and stable operation of the HLW repository system. Natural graphite powder mixed with sodium bentonite forms a buffer/backfill material that can dissipate heat quickly and provide strong isolation. In this paper, the THM characteristics of bentonite–sand–graphite–polypropylene fiber (BSGF) mixtures, used as a buffer/backfill material, were studied through a series of laboratory tests. The influence of graphite and polypropylene fiber contents on thermal conductivity, swelling pressure, hydraulic conductivity, and strength properties of BSGF mixtures with different sand contents was analyzed. Experimental results indicated that the graphite content, the maximum graphite mesh number, and the initial dry density of bentonite–graphite mixtures influenced the thermal conductivity of bentonite–graphite mixtures. The addition of polypropylene fiber was found to enhance the shear strength and inhibit cracking without significantly affecting the expansivity, permeability, and thermal conductivity of the BSGF mixtures. This study provides a new buffer/backfill material that can improve the stability, functionality, and thermal efficiency of the HLW repository.

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.

Consolidation characteristics of a thermally cured sand–bentonite mixture

Consolidation behavior of clays and sand–clay mixtures is an important topic in environmental geotechnics, thermal geostructures and disposal of hazardous wastes. The purpose of this paper is investigation of the changes in consolidation behavior of a sand–bentonite mixture due to the pre-curing at elevated temperatures. For this reason, 1:1 mixture of sand–bentonite was used to prepare samples 100 mm in diameter and 200 mm in height. The specimens were cured at 40, 60 and 80 °C for 1, 3 and 5 days under a constant confining pressure in a modified thermal triaxial cell. Then, consolidation behavior of the prepared samples which have been named as cured samples was investigated. The results show that the cured samples shifted from a reconstituted state to a quasi-structured one. A distinct yield stress was observed in compression curves of cured samples which was not apparent for reconstituted ones. The preconsolidation pressure of cured samples increased with increase in curing temperature and curing time. Also, the coefficient of consolidation and permeability increased as temperature increased, while both values decreased with an increase in curing time. Moreover, a number of specimens were tested under cyclic thermal paths which clearly proved change in behavior of cured samples to a structured one compared to the samples without thermal curing.

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.

Water Retention Characteristics of Fly Ash–Bentonite Mix

The use of sand–bentonite mixes as hydraulic barrier in municipal and hazardous landfills are established in the literature. Environmental awareness and stringent rules on river conservation has restricted the free availability of construction sand making it non-viable for massive geo-environmental projects such as waste containment. Following the same philosophy for which sand was used in bentonite, this study propose to replace the former with fly ash. Apart from being an alternate material to sand, this proposition would ensure mass utility of waste fly ash for environmentally advantageous waste containment project. For this purpose, it is important to study the unsaturated characteristics of fly ash–bentonite mix, which is not reported in the literature. This study explores the influence of fly ash properties and its classification on the water retention characteristics curve (WRCC) of fly ash–bentonite mixes. The extent of variability in WRCC attributed to different sources of fly ash were quantified to investigate whether there is a possibility of generalizing the unsaturated behavior of fly ash–bentonite mixes. In addition, the influence of bentonite water content on the WRCC of mixes were studied and compared with the previously published results in the literature. It was found that at high suction (> 105 kPa), all the WRCC of FA–B mixes merges together and is mainly dominated by the water retention of bentonite only. It was also established from this study that any types of fly ashes irrespective of its source can be used in this application if compacted at optimum moisture content.

Influence of Fibers on Hydro-Mechanical Properties of Bentonitic Mixtures

This paper reports the influence of fibre inclusion on the hydro-mechanical properties of bentonitic mixtures at varying compaction states. Pond ash–bentonite and sand–bentonite mixtures are synthesized by adding 20% commercial bentonite to pond ash and sand respectively. Discrete polypropylene fibres ranging from 0 to 1% are added to the mixtures. The hydro-mechanical properties of specimens; compacted to either standard or modified Proctor density; are investigated for a wide range of molding water content varying from dry to wet side of optimum. Though the inclusion of fibre has insignificant effect on optimum moisture content and maximum dry density of mixtures, the compressive strength, failure strain, unit cohesion and frictional angle of the compacted specimens are found to increase whereas the volumetric shrinkage strains are found to decrease irrespective of the molding water content. Both unreinforced and reinforced specimens exhibited the maximum unconfined compressive strength at relative water contents of 90% and 80% when compacted to standard and modified Proctor density respectively. With an addition of 1% fibre, these values are found to increase by 2 to 3 times. The specimens compacted at dry of optimum did not show any significant variation in hydraulic conductivity whereas specimens compacted at wet of optimum exhibited an increased hydraulic conductivity with fibre content. However, specimens compacted at optimum moisture content showed an insignificant change in hydraulic conductivity with increase in fibre content. Furthermore, the increase in volumetric shrinkage strain with relative water content is reduced as the fibre content increases.

Shear behavior at interface between compacted clay liner–geomembrane under freeze-thaw cycles

Bentonite sand mixtures (BSMs) are commonly used in compacted clay liners (CCLs) in waste disposal facilities when the local soil is not suitable for use in the liners. Given that the temperature tends to vary drastically in many cold regions, it is important to have a good understanding of the interface shear behavior of composite liners that are affected by freeze-thaw (F-T) cycles. However, no studies on the shear behaviour at the interface between a compacted clay liner and high-density polyethylene (HDPE) geomembrane that are subjected to F-T cycles have been conducted in the literature. Therefore, an experimental investigation on the influence of F-T cycles on the shear behaviour at the interface between a BSM and smooth HDPE geomembrane is conducted in this study. The obtained results show that the shear strength at the interface is lower than that of the BSM itself. In addition, the shear strength decreases with increasing numbers of F-T cycles, and the total reduction in the interface shear strength of the samples without undergoing any F-T cycles is 23%, 15.2% and 11.4% under normal stresses of 150 kPa, 250 and 350 kPa, respectively (in comparison to the samples that are subjected to 10 F-T cycles).

Influence of bentonite type and producing method on hydraulic conductivity of sand–bentonite mixture

Sand-bentonite mixtures with bentonite content of 10-30% had been planned to handle low-level radioactive waste in Japan, because of its low permeability. Hydraulic conductivity of sand–bentonite mixture depends on the bentonite type, bentonite content, initial water content, and other factors. Given this background, falling head permeability tests were conducted on sand–bentonite mixture by varying the compaction energy for specimen preparation, initial water content (10–20%), and bentonite content (15– 30%). For these tests, the hydraulic gradient of 25-500 was set. Consequently, the hydraulic conductivities were 10-8 – 10-13 m/s for all tested conditions. Correlation between the hydraulic conductivity and the effective montmorillonite dry density (montmorillonite mass divided by the sum of montmorillonite, air, water volume), which is often used to correlate the hydraulic conductivity of bentonite, was found. Correlation was also found between the hydraulic conductivity and a new index designated as the effective montmorillonite wet density (sum of montmorillonite and water mass / sum of montmorillonite, air, water volume). Effective montmorillonite wet density reveals differences in the specimen structural distribution through consideration of the initial water content.

Bentonite slurry infiltration into sand: filter cake formation under various conditions

Experiments on bentonite slurry infiltration into saturated sand have been carried out in a modified laboratory set-up that provides a hydraulic gradient comparable to real tunnels. Test series 1 investigated the characteristics of mud spurt and filter cake formation during water–bentonite slurry infiltration, before and after removing the external filter cake formed on the original boundary between the slurry and the sand. Test series 2 examined the conditions of filter cake formation during water–bentonite–sand slurry infiltration. The experimental results of series 1 indicate that, in the second infiltration, a new external filter cake will be formed for low concentrations of slurries (40 and 50 g/l), whereas hardly any new external filter cake formed for high concentration of slurry (60 g/l) owing to the formation of an internal filter cake in the sand during the first infiltration. The formation of this internal filter cake meant that the infiltration distance was much smaller for the second infiltration than that in the first for 60 g/l slurry. The experimental results of series 2 with a water–bentonite–sand slurry showed no external filter cake formation, but depth filtration was observed for high-density slurry (1300 and 1500 kg/m3). For low-density slurries (1050 and 1100 kg/m3), a thicker external filter cake, but with a higher permeability, than that in series 1 was observed on the sand surface, because the cake consisted to a large extent of sand particles. Mechanisms of mud spurt and filter cake formation are discussed, with a new solution for mud spurt and the theory of depth filtration.

Compaction Behaviour and Grain Size Alteration of Sand -Clay Mix

Sand-bentonite mix is commonly used as liner material or backfill material in landfill sites. Bentonite belonging to group of montmorillionite clay provides higher density and less void ratio to lessen permeability of liner layer mixed with sand. This mix is inevitable and economical where soil nearby landfill site have sandy soil .Many literatures provided geotechnical aspects of using this mix.In this present study, the grain size distribution analysis and compaction of sand–bentonite mixture for 10%,20% and 30% bentonite mix is studied to propose economic and less permeable liner material with lower shrinkage. Locally available sand from different river , Odisha (India) was mixed with different proportions of commercial sodium bentonite. The following research output presented here elaborately show the effect of clay particle on compaction, Relative Density, Coefficient of uniformity,Coefficient of curvature, permeability by gradually adding commercial Sodium Bentonite 10% by weight upto 30%. The result show that there is a significant improvement in MDD and OMC of Sand-Bentonite mix along with decrease in void ratio. The grain size alteration eventually helped to increase the dry density and reduced void ratio will enhance the scope of this mix to be used as liner layer in landfill system.

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.

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.

ANALYZING THE CHARACTERISTICS OF LANDFILL LEACHATE ON EXPANSIVE SOIL AND ITS IMPACT ON LINER MATERIALS REINFORCED WITH DISCRETE RANDOM FIBERS

This research work is an insight into the various properties of liner materials utilized in landfill sites to control the leachate. Although expansive clay soil is inorganic, it is problematic. The soil owes this aspect to erratic fluctuations in its behavior due to the movement of water. Urbanization, increase in population and industries generate enormous amounts of waste in different forms. Disposal of these wastes in the landfill without any scientific manner creates a major problem to the surrounding soil and groundwater. In this study, an attempt was taken to identify the effect of soil properties and groundwater quality due to the continuous generation of leachate in the dumping sites. To prevent such an environmental problem, highly impermeable sand–bentonite mixture is used as a liner material in the waste disposal sites. Therefore, various mixtures of sand–bentonite were prepared by varying sand contents and different sieve sizes, i.e. 2–0.075[Formula: see text]mm; 2–0.425[Formula: see text]mm and 0.425–0.075[Formula: see text]mm were used. And also to improve the strength and hydraulic conductivity of the liner materials, discrete random fiber is mixed with various proportions to these mixtures and examined.

Experimental Study to Investigate the Effect of Backfilling Materials on Thermal Performance of Ground Air Heat Exchanger System

Abstract In ground air heat exchanger (GAHE) system, the heat transfer between air and underground soil largely depends on soil thermal properties and therefore, any improvement in soil thermal properties will shorten the pipe length required and the land area needed for its installation. The objective of the present study is to investigate the effect of different backfilling materials (low cost and locally available) on the thermal performance of GAHE system using a small-scale laboratory experimental setup. Seven different backfilling materials have been considered for the study and it was observed that after 6 h of continuous operation, the drop in air temperature was 6.2 °C at outlet section of pipe (2.4 m away from inlet) for the native soil. However, for sand–bentonite with graphite as a backfilling material (BFM), the drop in air temperature of 6.2 °C was obtained at a pipe length of 1.15 m only. Therefore, the use of sand–bentonite with graphite as a BFM reduces the pipe length of GAHE system by more than 50%. The study establishes the fact that the length of pipe and land area requirement for GAHE system can be substantially reduced by using thermally enhanced backfilling materials at the close vicinity of GAHE pipes.

Evaluation of thermal-mechanical properties of quartz sand–bentonite–carbon fiber mixtures as the borehole backfilling material in ground source heat pump

Ground source heat pumps (GSHPs) use sustainable technology and renewable geothermal energy to exchange heat between the ground and the interior of residential, industrial, and commercial buildings. The borehole grouting material is a key contributor to the stability and thermal transmission properties of GSHPs. To investigate the thermomechanical properties of quartz sand–bentonite–carbon fiber mixtures to assess their potential as a borehole backfilling material, the thermal needle and unconfined compressive strength (UCS) tests were conducted. The thermal conductivity of quartz sand–bentonite–carbon fiber mixtures increased with the sand particle size, and the thermal conductivity showed a parabolic relationship with bentonite fraction for all sand particle sizes. The thermal conductivity of the mixtures increased with increasing moisture content and degree of saturation, and increased linearly with increasing dry density. The optimal percentage of bentonite, by dry mass, was approximately 10–12%. A relatively high thermal conductivity (1.90–1.98 W/m·K) and an unconfined compressive strength of 124–200 kPa were considered acceptable for borehole backfilling material used for GSHP. The practicality of quartz sand–bentonite–carbon fiber mixtures as borehole backfilling material for GSHPs was verified using the TICA GSHP Design Program. This research confirms that quartz sand–bentonite–carbon fiber mixtures are viable new borehole backfilling material that enhance the thermal efficiency and reduce the costs of GSHPs.

Apparent salt diffusion coefficients for soil–bentonite backfills

Apparent diffusion coefficients, Da, were measured for two soil–bentonite (SB) backfills characteristic of those used in SB vertical cutoff walls for subsurface control of contaminant migration. The base soils for the backfills comprised either a natural lean clay or sand–bentonite mixtures. The base soils were mixed with 5% bentonite–water slurry to obtain a slump of 125 mm, resulting in total bentonite contents ranging from 4.76% to 7.31%. Values of Dafor sodium chloride were measured using a recently developed dialysis-leaching test method. The Davalues for the clay–bentonite and sand–bentonite backfills ranged from 2.5 × 10−10to 5.3 × 10−10m2/s and from 1.4 × 10−10to 8.1 × 10−10m2/s, respectively. Values of Dafor both backfills increased with increasing average salt concentration in the specimen (Cave). Values of Dadecreased by ≤50% with increasing backfill bentonite content. For all Cavevalues, the clay backfills exhibited lower Dathan the sand–bentonite backfills, likely due to additional fines from the lean clay. Results of this study enhance understanding of solute diffusion through SB cutoff walls, as well as support future use of the dialysis-leaching test method to measure diffusion properties of SB backfills.

Mixed metals migration through zeolite-amended backfills for vertical cut-off walls

Column tests were performed with two sand–bentonite backfills comprising 5·8% bentonite with 5% of one of two high-cation-exchange-capacity (CEC) zeolite amendments – clinoptilolite and chabazite upper bed (chabazite-UB). The backfill specimens were permeated with a mixed salt solution of 17·5 mM potassium chloride (KCl) and 10 mM zinc chloride (ZnCl2) to evaluate the extent of attenuation of two metals – potassium (K) and zinc (Zn). The specimens were characterised by low hydraulic conductivity (<1·0 × 10−9 m/s) and delayed breakthrough of potassium and zinc that was consistent with exchange of potassium (K+) and zinc (Zn2+) ions for the sodium (Na+) and/or calcium (Ca2+) ions initially occupying the exchange sites of the bentonite and zeolite components of the backfills. The results were consistent with those previously reported for the same backfills permeated with single-salt solutions of the same metals, indicating that the competition between potassium and zinc ions in the mixed salt solution had little effect on the metals migration. Overall, the metals attenuation was enhanced relative to that for the unamended backfill by a factor ranging from 2·0 for zinc with both zeolite-amended backfills to 3·7 for potassium with the chabazite-UB-amended backfill, further illustrating the benefit resulting from amending a sand–bentonite backfill with a small amount of high-CEC zeolite.