Use of shear wave velocity for assessing engineering properties of compacted bentonite after swelling.

The characteristics of compacted bentonite after swelling determine the long-term stability of barrier systems. Due to the fact that the current stress level is the most important variable in determining the performance of engineered geosystems, this study aims to investigate the stress states and the consequent change in engineering properties of compacted bentonites after swelling. A series of vertical and horizontal swelling pressure tests were performed for compacted bentonites with varying initial dry unit weights at varying pore fluid concentrations. The compacted bentonite samples after swelling were loaded to investigate the changes in lateral stress and deformability. In addition, the shear wave velocity was continuously measured during and after swelling processes. The results of this study demonstrate that the swelling pressure increased with increasing dry unit weight of tested materials and decreasing pore fluid concentrations. The changes in lateral stress and void ratio of compacted bentonite after swelling were only measurable when the applied vertical stress was greater than the swelling pressure, reflecting that the swelling pressure cancels out the externally applied stress. Most notably, this study reveals that the initiation and termination of the swelling process and the change in engineering properties of compacted bentonite after swelling can be determined by measuring shear wave velocity.

Synthesis and Evaluation of Engineering Properties of Polymer-Coated Glass Beads.

Modern construction projects are often challenging, which has increased the demand for innovative materials that ensure improved safety, durability, and functionality. To explore the potential of enhancing soil material functionality, this study synthesized polyurethane on the surface of glass beads and evaluated their mechanical properties. The synthesis of polymer proceeded according to a predetermined procedure, where the polymerization was confirmed through analysis of chemical structure by Fourier transform infrared spectroscopy (FT-IR) and microstructure observation by a scanning electron microscope (SEM) after complete synthesis. The constrained modulus (M) and the maximum shear modulus (Gmax) of mixtures with synthesized materials were examined by using an oedometer cell equipped with bender elements under a zero lateral strain condition. Both M and Gmax decreased with an increase in the contents of polymerized particles due to a decrease in the number of interparticle contacts and contact stiffness induced by the surface modification. The adhesion property of the polymer induced a stress-dependent change in M but was observed to have little effect on Gmax. Compared to the behavior of the rubber-sand mixtures, polymerized particles show the advantage of a smaller reduction of M.

Thermal conductivity of dry fly ashes with various carbon and biomass contents.

Recently, sustainable energy portfolios have added biomass combustion and coal/biomass co-combustion as alternative fuel sources for generation of electricity. Fly ashes that result from combustion of biomass or its co-combustion with coal contain relatively high contents of unburned carbon, while increasingly stringent air quality regulations have also increased the residual carbon content in fly ash produced by coal combustion alone. While previous studies documented the mechanical and chemical behavior of fly ash relatively well, the thermal characteristics of those fly ashes have not been well studied. Therefore, this study evaluated the thermal conductivity of fly ashes with varied carbon and initial biomass contents to quantify the impact of unburned carbon particles and biomass-fired fly ash on thermal conductivity. Observed results demonstrated that the thermal conductivity of fly ashes almost linearly decreased as biomass content increased while the variation of thermal conductivity of fly ashes caused by unburned carbon content was relatively low. In addition, the thermal conductivity of fly ashes was lower than that of natural soils mainly because of the microporous structures of fly ash particles. The trend of thermal conductivity of fly ashes as a function of dry density was consistent with that of natural soils, due to the similar mineralogy of fly ash with that of natural soils. The developed stepwise regression model indicated that the porosity and the specific gravity was the most critical factor in predicting the thermal conductivity of fly ash.

Effect of pH Variations on the Yield Stress of Calcium Bentonite Slurry Treated with pH-Responsive Polymer.

The pH-responsive polymers, such as polyacrylamide (PAM), show distinct conformational states according to the pH of their environmental groundwater. Therefore, the interactions between clay-polymer and polymer-water molecules, which determine the yield stress of bentonite-polymer composites, can be affected by the pH of groundwater. This study aims to evaluate the effect of pH variation on the yield stress of calcium bentonite treated with PAM. The yield stresses (τy) of untreated and PAM treated clays were measured with varying volume fractions of solid (VF = 10-23%) and under varying pH conditions (pH = 7.6-9.6). In addition, the zeta potential was measured for both untreated and treated clays to figure out the change in the surface charge of the mineral surface due to PAM treatment. The results of this study demonstrate that τy for treated clay is higher than that for untreated clay at a given VF, because van der Walls attraction dominates electrostatic repulsion in the case of treated clay. Due to the change in conformational states of PAM and the consequent change in surface charge that comes with varying pH, the pH-dependent change in τy of treated clay is significantly different from that of untreated clay.

Engineering Characteristics of Chemically Treated Water-Repellent Kaolin.

Water-repellent soils have a potential as alternative construction materials that will improve conventional geotechnical structures. In this study, the potential of chemically treated water-repellent kaolin clay as a landfill cover material is explored by examining its characteristics including hydraulic and mechanical properties. In order to provide water repellency to the kaolin clay, the surface of clay particle is modified with organosilanes in concentrations (CO) ranging from 0.5% to 10% by weight. As the CO increases, the specific gravity of treated clay tends to decrease, whereas the total organic carbon content of the treated clay tends to increase. The soil-water contact angle increases with an increase in CO until CO = 2.5%, and then maintains an almost constant value (≈134.0°). Resistance to water infiltration is improved by organosilane treatment under low hydrostatic pressure. However, water infiltration resistance under high hydrostatic pressure is reduced or exacerbated to the level of untreated clay. The maximum compacted dry weight density decreases with increasing CO. As the CO increases, the small strain shear modulus increases, whereas the effect of organosilane treatment on the constrained modulus is minimal. The results indicate that water-repellent kaolin clay possesses excellent engineering characteristics for a landfill cover material.

Engineering Behavior and Characteristics of Wood Ash and Sugarcane Bagasse Ash.

Biomasses are organic materials that are derived from any living or recently-living structure. Plenty of biomasses are produced nationwide. Biomasses are mostly combusted and usually discarded or disposed of without treatment as biomass ashes, which include wood and sugarcane bagasse ashes. Thus, recycling or treatment of biomass ashes leads to utilizing the natural materials as an economical and environmental alternative. This study is intended to provide an environmental solution for uncontrolled disposal of biomass ashes by way of recycling the biomass ash and replacing the soils in geotechnical engineering projects. Therefore, in this study, characteristic tests of wood and sugarcane bagasse ashes that are considered the most common biomass ashes are conducted. The test of chemical compositions of biomass ashes is conducted using energy dispersive X-ray spectroscopy (EDS), and Scanning Electron Microscope (SEM), and heavy metal analysis is also conducted. Engineering behaviors including hydraulic conductivity, constrained modulus and shear modulus are examined. Also, coal fly ash Class C is used in this study for comparison with biomass ashes, and Ottawa 20/30 sands containing biomass ashes are examined to identify the soil replacement effect of biomass ashes. The results show that the particle sizes of biomass ashes are halfway between coal fly ash Class C and Ottawa 20/30 sand, and biomass ashes consist of a heterogeneous mixture of different particle sizes and shapes. Also, all heavy metal concentrations were found to be below the US Environmental Protection Agency (EPA) maximum limit. Hydraulic conductivity values of Ottawa 20/30 sand decrease significantly when replacing them with only 1%-2% of biomass ashes. While both the constrained modulus and shear modulus of biomass ashes are lower than Ottawa 20/30 sand, those of mixtures containing up to 10% biomass ashes are little affected by replacing the soils with biomass ashes.