Temperature effects on ageing properties and diffusivity of a HDPE GM in landfill.

High-density polyethylene (HDPE) geomembranes (GMs) play a crucial role in preventing the leakage and migration of pollutants. GM service life and ageing properties are the main concerns for the choice of materials. However, it is not clear how the mechanical properties and anti-fouling performance of geomembranes change with ageing time. To solve this problem, a HDPE GM was selected for testing under exposed air condition. The tests included oxidation induction time (OIT), melt flow index (MFI), tensile properties and diffusivity under four temperature conditions for 1½ years. The test results showed that the GM has higher OIT degradation rates. Stage I - depletion of antioxidants occurred at only 10 years for the GM, which was approximately 1/4 that of the GM-GSE. The GM engineering properties index showed the same changes as those of the GM-GSE. However, MI rapidly decreased with the incubation time. The molecular weight degradation of the GM was approximately 57% and far greater than that of GM-GSE after 15 months, but the tensile properties of the two GMs showed little change. The diffusion coefficient Di of GM increases gradually with the increase of temperature in methane and trichloromethane. Under the same conditions, the diffusion coefficient Di of the GM in methane is significantly higher than that in trichloromethane, indicating that the GM has better barrier to trichloromethane.

A simplified method for analysis of geosynthetic reinforcement used in pile supported embankments.

The inclusion of geosynthetic reinforcement in the piled embankment can help transfer loads to the piles and reduce total and differential settlements. In order to select the appropriate reinforcement material, the reasonable calculation of the deflection and tension is very important. Current design methods usually do not represent the true three-dimensional (3D) nature of the displacements, strains, and stresses of the geosynthetics, and the resulting error may be large and cannot be neglected in some cases. In this study, two- and three-dimensional finite element analyses were conducted to identify the behavior of geosynthetic reinforcement and investigate the accuracy of the assumptions made in the current design methods. Based on the numerical results, a new 3D deflected shape of the geosynthetic reinforcement was suggested, and then the corresponding governing equation was derived and solved based on the membrane theory. To investigate the validity of the proposed method, the predicted maximum deflection, deflection shape, and the developed tensile force of the geosynthetics have been compared with the experimental data collected from the literatures and finite element analysis results.

Study on the immobile water of saturated compacted clay-rich materials by oil seepage.

Immobile water is defined as a firmly bound thin film of water surrounding clay particles that somehow resembles solid features. The characteristics and amount of immobile water depend on pore water chemistry, the type of clay, and the contact area of water with particles. Seepage can occur only when the hydraulic gradient increases sufficiently. To date, no accurate method is available to determine the immobile water content. Therefore, an approach is proposed for direct measurement of the immobile water content, which applies vegetable oil to displace water in saturated compacted clay-rich materials. Under a high hydraulic gradient, the vegetable oil completely displaces the free porewater in the clay-rich materials, and the remaining residual water is assumed to be immobile water. By using carbon tetrachloride (CCl4) as an extracting agent, the oil in the permeated sample was subjected to extraction and distillation, and the total liquid content was determined by the oven-drying method. As a final step, the immobile water content in the sample was calculated. A high-speed centrifugation method was used to determine the immobile water content from the corresponding soil water potential, which accounted for approximately 634.3 kPa. It was found that the immobile water of the clay-rich materials was essentially equal to the bound water. The effective porosity of clay was obtained based on the immobile water content. By comparing the relationship between porosity, effective porosity and hydraulic conductivity, the effective porosity revealed an improved linear relationship with log (k). The test results show the oil seepage method to be useful and reliable for the quantification of the immobile water content of saturated compacted clay-rich materials.

Effects of pH and EC on the strength and permeability of plastic concrete cutoff walls.

At present, plastic concrete is widely used in anti-seepage engineering, and its strength and hydraulic conductivity are important indexes of plastic concrete cutoff walls. The pondus hydrogenii (pH) and electrical conductivity (EC) have an important effect on the strength and hydraulic conductivity of plastic concrete cutoff walls. However, it is not clear why the pH and EC are related to the strength and permeability of these walls. For this reason, plastic concrete composed of sand, cement, water, and bentonite was taken as the research object to study the influence of different mix ratios and curing ages on the unconfined compressive strength and permeability of plastic concrete. The pH and EC of the corresponding mix ratio and curing age were measured. The effects of the pH and EC on the strength and permeability of plastic concrete were studied, and the internal reason was explored from the product type and hydration reaction degree of plastic concrete. Furthermore, the quantitative relationship between pH and EC and the strength and hydraulic conductivity of plastic concrete was established. With increasing cement content, the strength, pH, and EC of plastic concrete increase continuously, while the hydraulic conductivity decreases. With increasing curing age, the strength increases, the hydraulic conductivity decreases, and both the pH and EC show a trend of first increasing and then decreasing. The pH value and electrical conductivity can reflect the product type and hydration reaction degree of plastic concrete, further affecting its strength and hydraulic conductivity. Under different dosage and curing age conditions, the pH showed three stages, namely, 11.7-12.27, 12.27-2.5, and 12.5-12.75, and there is a good power functional relationship among the EC and strength and hydraulic conductivity at each stage.

COD (glucose configuration) effects on the non-Darcy flow of compacted clay in a municipal solid waste landfill.

Clay liners play a critical role in preventing leachate leakage and pollutant migration from landfills through their low permeability and non-Darcy behavior during seepage, and such liners exhibit a threshold-gradient characteristic. Landfill waste may produce complex, highly concentrated leachates through chemical and biological degradation. The hydraulic conductivity and threshold gradient of a clay liner is affected by high leachate concentrations. Some scholars have suggested that chemical oxygen demand (COD) can be selected as a key indicator for pollution alerts and used to assess the environmental risk posed by municipal solid waste (MSW) landfill sites. To study the influence of leachate concentration on the permeability of compacted clay, the highest concentrations of organic pollutant COD (glucose configuration) were used as the target dialysate in this study. COD is abbreviation of chemical oxygen demand. A COD solution was prepared from dissolved glucose for the experiments. The results showed that as the COD concentration increased, the hydraulic conductivity increased and the threshold gradient decreased. The permeate viscosity and the soil-water characteristic curve were measured. As the COD concentration increased, the permeate viscosity increased and the bound water content decreased. By considering the COD concentration effects on permeate viscosity and intrinsic permeability and adapting a previously established empirical relationship between the threshold gradient and apparent fluidity (K/η), this study derived an equation for calculating the hydraulic conductivity and threshold gradient with changes in the COD concentration, and good predictions were obtained.