Hydraulic Conductivity Of Mixtures Research Articles

Hydraulic behaviour of sand-biochar mixtures in water and wastewater treatment applications

Interest in using biochar in environmental engineering applications has substantially increased with strong evidence emerging for its applicability in water and wastewater treatment, the containment of hazardous gases, carbon dioxide sequestration and the filtration of microplastics. Understanding the hydraulic behaviour of sand-biochar mixtures is critical for designing biochar-based water and wastewater filter systems. We present an investigation on the impact of the biochar microstructure on the hydraulic conductivity of sand-biochar mixtures, focusing on where there are gaps in existing knowledge on how the biochar affects the hydraulic properties. A dual-porosity concept for sand-biochar mixtures is introduced and included in four existing constitutive models for the hydraulic conductivity of soils.We have experimentally investigated the hydraulic properties of sand-biochar mixtures for uniformly graded sand with three representative grain sizes, and one type of hardwood biochar in the UK market. Based on a series of constant-head permeability tests, the hydraulic conductivity of the mixtures with biochar content from 0 to 50% has been measured. We used randomly generated spherical 3D models representing the same particle distribution of sand biochar systems to characterise the biochar's intraparticle porosity and tortuosity. By employing the dual-porosity concept and effective porosity instead of the total porosity, the accuracy of the four models' predictions was considerably improved when compared with the experimental measurements. We show that by excluding biochar's intraparticle porosity, the hydraulic conductivity prediction accuracy can be significantly improved, thus providing a more accurate prediction of the hydraulic behaviour of the soil-biochar filters.

Engineering properties of GMZ bentonite-sand as buffer/backfilling material for high-level waste disposal

This paper reports on the engineering properties of various mixtures of GMZ bentonite and sand, as regards their effectiveness as buffer/backfilling material. The sand ratio in the various test specimens was 0, 20, 30, 40 and 50%. Compaction tests were conducted to determine the optimum water content and maximum dry density of the bentonite-sand mixtures. A series of hydraulic and mechanical tests was conducted to assess Atterberg limits, the soil water characteristic curve, hydraulic conductivity, shear strength, swelling pressure and swelling strain of the bentonite-sand mixtures. The hydraulic conductivities of the mixtures, that had sand ratio varying from 0 to 30%, remained consistent and were low enough to prevent the flow of groundwater through the vitrified waste. The water holding capacity, shear strength and swelling capacity decreased slightly, but that could be partly offset by increasing the effective dry density and average dry density of the buffer/backfilling material. The Atterberg limits decreased by mixing in additional sand, which could offer improved the workability of buffer/backfilling material. It was suggested that the most effective sand ratio of GMZ bentonite-sand mixtures, which are available in China, should not be more than 30%.

Characterization, Hydraulic Conductivity and Compatibility of Mixtures of Tropical Soil and Bentonite for Barrier Usage Purpose

Compacted tropical soils have great potential to be used in barriers, once some technical issues are correctly addressed, including low hydraulic conductivity and compatibility to the disposed fluids. This paper involves a laboratorial study of the hydraulic conductivity of mixtures of a tropical soil sample and bentonite and also their compatibility to different chemical solutions. The experimental program consisted of (1) sample characterization, (2) hydraulic conductivity determination using a triaxial cell, and (3) compatibility assessment with different chemical solutions (HNO3 , NaOH, NaCl, and ethanol) using the modified Atterberg Limits. The characterization results revealed important changes in the index properties of the tropical soil sample as the result of the bentonite addition, especially in the plasticity, cation exchange capacity and free swelling. The hydraulic conductivity, in turn, experimented significant decreasing as the proportions of bentonite and confining pressure increased. The compatibility results showed a significant reduction in the plasticity of the samples when subjected to the chemical solutions, especially for the salt solution. In general terms, the addition of bentonite reduced the compatibility of the samples.

Permeability characteristics of pumice-bentonite mixtures in comparison with sand-bentonite mixtures

Environmental pollution is a significant concern around the landfill sites, waste repositories and other similar wastewater storage and treatment basin and ponds. Pumice is an abundant natural material in Kayseri, Turkey and therefore, permeability characteristics of pumice-bentonite mixtures were investigated in comparison with commonly used sand-bentonite mixtures. Four different bentonite contents (15, 20, 25 and 30%) and three grain size ranges (2.00 to 1.00; 1.00 to 0.50; 0.50 to 0.25 mm) of pumice and sand were used to determine the hydraulic conductivity of mixtures. Since it takes long time to determine the hydraulic conductivity of fine grained mixtures, empirical equations were developed between permeability and easily-determined geotechnical characteristics such as dry unit weight, moisture content, and swell potential of such mixtures. Pumice-based mixtures yielded identical permeability values for the same bentonite contents and size ranges. Overall evaluation of regression equations with regard to mean absolute error (MAE) and regression coefficients showed that while dry unit weight yielded the least MAE and the highest R2 values for pumice-based mixtures, swelling potential yielded the best estimations for sand-based mixtures. Bentonite content of ≥ 30% is recommended for both pumice and sand-based mixtures with grain sizes between 2.00 and 0.50 mm and ≥25% for mixtures with grain sizes less than 0.50 mm. Key words: Pumice, bentonite, mixtures, permeability.

Controlling factors of the swelling of various bentonites and their correlations with the hydraulic conductivity of soil-bentonite mixtures

Abstract The controlling factors of the swelling of bentonites and its effects on the hydraulic conductivity of soil-bentonite mixtures were investigated using fifteen types of bentonites with a wide range of swelling capacity. The free swell of the bentonites increased linearly with increasing exchangeable sodium percentage below 30%, and almost remained constant beyond 30%. The free swell of the bentonite was correlated with the clay fraction, whereas, no correlation was observed with the methylene blue index and cation exchange capacity. Multiple regression analysis indicated that the variation in the free swell of the bentonites can be explained in terms of their specific surface area and exchangeable sodium percentage. A positive correlation was found between the free swell of the bentonites and the liquid limit of the bentonites and soil-bentonite mixtures. The hydraulic conductivity of the soil-bentonite mixtures at the void ratio corresponding to the maximum dry density decreased from 5.5 × 10 − 8 to 5.3 × 10 − 9 cm/s with increase in the free swell from 6 to 20 mL/2 g, but remained almost constant beyond 20 mL/2 g. The hydraulic conductivity of the mixtures was found to be a function of the liquid limit of both bentonites and the soil-bentonite mixtures. It was also observed that the hydraulic conductivity of the mixtures containing Japanese bentonites meets the Japanese standards for engineered barriers.

Prediction of compressibility and hydraulic conductivity of soil-bentonite mixture

A mixture of bentonite and locally available soil is used as a clay liner material in the waste disposal site. Due to the high swelling and low hydraulic conductivity capacity, bentonite is preferred as a liner material. Since bentonite is available naturally its mineralogical composition and hence, its ability to swell and to provide a low value of hydraulic conductivity to the clay liner varies from source to source from which it has been procured. In this paper, a generalized approach is presented to predict the compressibility and hydraulic conductivity characteristics of the soil-bentonite mixture with the minimum input parameters. Four different methods to predict compressibility and three different methods to predict hydraulic conductivity of mixture has been discussed. Using the initial parameters such as the liquid limit, void ratio corresponding to the liquid limit water content, and overburden pressure the compressibility and hydraulic conductivity can be predicted effectively using these methods.

Hydraulic conductivity and consolidation response of mixtures of mine waste rock and tailings

Hydraulic conductivity and consolidation behaviour are examined for an alternative mine waste disposal technique. One type of waste rock, one type of tailings, and mixtures of the same waste rock and tailings were tested in bench-scale and column studies. Specimens 150 mm in diameter were tested for hydraulic conductivity by falling-head method alternated with one-dimensional consolidation, specimens 300 mm in diameter were tested for one-dimensional consolidation response, and specimens 1 m in diameter were tested for self-weight consolidation behaviour in 6 m high columns. Deformation of mixtures under one-dimensional loading was similar to that of waste rock alone and much less than that of tailings alone. Hydraulic conductivity of mixtures was similar to that of tailings alone and independent of specimen diameter. Mixture behaviour was attributed to a homogeneous structure including waste rock in particle-to-particle contact and a continuous, saturated matrix of tailings. The results indicate that mixing waste rock and tailings can produce dense deposits with values of hydraulic conductivity that are orders of magnitude lower than those of waste rock alone, thereby limiting fluxes through the waste and the associated potential for acid rock drainage. Mixture deposits will also have less consolidation-related settlement than tailings, thus improving stability and aiding reclamation.

Utilization of bentonite-embedded zeolite as clay liner

Bentonite-embedded zeolites (BEZ) with different bentonite content were developed for possible use as barrier material, as an alternative to bentonite-embedded sand (BES). Adsorption characteristics of Na-bentonite, Ca-bentonite and natural zeolite; cation exchange capacity (CEC) of Na-bentonite and natural zeolite; volumetric shrinkage, compaction characteristics; and hydraulic conductivity of the mixtures were investigated. Adsorption isotherm and CEC results indicate that the lead (Pb) adsorption capacity of natural zeolite is almost one-third of Na-bentonite. Considering the zero adsorption capacity of sand, the practical implication of high CEC is the remarkable savings in terms of both materials used and space where the waste is disposed. Furthermore, volumetric shrinkage and compaction results reveal that the optimum moisture content of BEZ is far less than the moisture content that can create 4% volumetric shrinkage, which is the upper limit for liner materials. Hydraulic conductivity tests on BEZ with 10% and 20% bentonite content show that the hydraulic conductivity of both mixtures are less than 1×10 −9 m s −1, which meets the common regulatory requirements. Moreover, the test results reveal that variations in hydraulic conductivity of the mixtures with different stress conditions are negligible for practical purposes.

Hydraulic conductivity of compacted bentonite–sand mixtures

In the absence of impervious natural soils, compacted mixtures of bentonite and sand have been used to form barriers to fluids, and the focus of this paper is on the hydraulic conductivity behaviour of these compacted mixtures. Results of laboratory investigations are presented to show the influences of bentonite–sand content, compaction water content, system chemistry, and changes of system chemistry. Some of the findings are as follows: (i) low hydraulic conductivity requires continuity of the bentonite matrix within the mixture, and this in turn requires both adequate bentonite content and adequate bentonite distribution (i.e., mixing); (ii) in well-compacted mixtures containing up to 20% bentonite in dry mass, sand forms the load-supporting framework and gives the mixtures dimensional stability at the macro level (crack resistance); and (iii) the hydraulic conductivity of mixtures containing freshwater, high-swell bentonite, when permeated with a strong saline solution, increased by only small amounts, suggesting that the fabric of bentonite enclosed within the sand framework was little influenced by this change of system chemistry. Key words : mixture, bentonite, sand, permeability, laboratory study.

Effect of Exchangeable Potassium on the Hydraulic Conductivity of Smectite-Sand Mixtures

AbstractChanges in hydraulic conductivity of smectite-sand mixtures (using four reference smectites) as a function of the concentration (0.01, 0.003, 0.002, 0.001 M Cl- and distilled water) and potassium adsorption ratio (of 2, 4, and 6) of the percolating solution were measured. Swelling and dispersion of the clays were evaluated from the changes in hydraulic conductivity of the mixture and from the clay concentration in the effluent.The effect of exchangeable potassium percentage (EPP) on the hydraulic conductivity of the smectites depended on the charge density of the clays. The effect of potassium at EPPs <20 on the hydraulic conductivity of smectites having high charge density was negligible. Conversely, the hydraulic conductivity of smectites having low charge density (smectites from Wyoming and Belle Fourche, South Dakota), changed markedly when leached with dilute solutions as the EPP of the clay increased. The dispersive effect of exchangeable potassium on low-charge smectites was similar to that of exchangeable sodium. The low hydration energy of the K+ cations, coupled with the strong electrostatic attraction forces between platelets of smectites with high charge density account for the “inefficiency” of K+ in dispersing these smectites.

Crushed aggregate—bentonite mixtures as backfill material for repositories of low- and intermediate level radioactive wastes

The candidate backfill materials for the repositories of low- and intermediate-level radioactive wastes consist of three components: crushed rock aggregate, finely ground rock aggregate and bentonite. The hydraulic conductivity of mixtures with 15% of sodium bentonite is approximately 5 · 10 -9 m/s based on the laboratory tests. The sweling potential for the same material varies between 20 and 60 kPa depending mainly on the salinity of the groundwater.