A fractal-based model for soil water characteristic curve over entire range of water content

et al. 2021

Water

The unsaturated hydraulic characteristics of waste soil are an essential basis for predicting and evaluating leachate migration and distribution in landfills. The saturated water content and permeability coefficient were measured, and a multi-step drainage monitoring experiment was conducted indoors at different dry densities, particle sizes, and degradation ages. Single and dual permeability models were adopted to determine the unsaturated hydraulic characteristic parameters of waste. Results show that dry density and particle size are the key factors affecting the saturated water content and permeability of waste. A single degradation age has little effect on it. Respectively, the saturated water content has a linear relationship with dry density, and permeability has an exponential relationship with dry density under limited experimental data. The overflow numerical inversion method can accurately obtain the unsaturated hydraulic characteristic parameters of wastes and summarizes the values of the unsaturated hydraulic characteristic parameters of wastes with different attributes in the literature and the results of this study. The dual-permeability model performed significantly better than the single-permeability model for water movement, suggesting that a dual-domain description is required for water flow in landfills.

Section: Introductionmentioning

confidence: 99%

Determination of Unsaturated Hydraulic Properties of Seepage Flow Process in Municipal Solid Waste

Chai

et al. 2021

Water

“…The residual volumetric water content θ r can be obtained by the four-parameter VG model (Table 4). When the value of the residual volumetric water content rate is ignored, the fractal-based SWCC model considering capillary flow can be transformed into the same equation reported by Jin et al [26] and Xu [61]. Obviously, the SWCC with a matric suction range of ψ e to ψ r is affected by capillary force, which is associated with micro-pore structure of soils and its POSD curve follows the geometric fractal theory.…”

Section: Modified Vg Model In Terms Of Cumulative Porementioning

confidence: 97%

“…Tao et al [25] established a SWCC prediction model considering fractal characteristics of pores based on the fractal theory. Jin et al [26] proposed a fractal SWCC model of soil in the full suction range, which can reflect the film flow in the high suction section under dry conditions. The internal pore structure and POSD of the soil are sensitive to the dryingwetting history, which in turn affects the shape of the SWCC.…”

Section: Introductionmentioning

confidence: 99%

Influence of Drying–Wetting Cycles on the Water Retention and Microstructure of Residual Soil

Xian

et al. 2022

Geofluids

Due to frequent changes in the humid and hot environment, the residual soil with a particle-size distribution (PSD) from gravel to clay experiences multiple drying–wetting cycles. The pressure plate test and nuclear magnetic resonance (NMR) spectroscopy were used to investigate the influence of drying–wetting cycles on the soil–water characteristic curve (SWCC) and pore-size distribution (POSD) of undisturbed residual soil. The results showed that the water-holding capacity of the residual soil decreased as the number of drying–wetting cycles increased and gradually stablilized, and then the van Genuchten (VG) model was found to perform well on the SWCC during the drying–wetting processes. The NMR results indicated a double-pore structure, and the porosity of the residual soil as well as the internal water content increased smoothly with more drying–wetting cycles. The obtained POSD curve of soil implied that drying–wetting cycles had a more obvious effect on small pores and macro-pores than on micro-pores and meso-pores. Theoretical calculations evinced that the product of the matric suction and relaxation time should be constant at a constant temperature. However, the experimental results did not effectively reflect such a relation between the matric suction and relaxation time. A modified VG model based on the cumulative pore volume was utilized to describe the POSD under drying–wetting cycles. Subsequently, the proposed Rational2D surface equation was used to accurately reflect the internal relationship between the SWCC and POSD curve under different numbers of drying–wetting cycles. Moreover, the fractal model for the SWCC derived from the capillary theory confirmed that the matric suction had a strong linear relationship with the relative volumetric water content in the log-log scale. Also, the fractal dimension can be approximated as a constant, because its attenuation is small with more drying–wetting cycles.

“…e soil-water characteristic curve (SWCC) describes the relationship between matric suction and water content (saturation) and can be used as an indirect index for understanding the shear strength [1], permeability, and deformation characteristics of the soil [2].…”

Section: Introductionmentioning

confidence: 99%

Effects of Contact Angle on the Hysteresis Effect of Soil‐Water Characteristic Curves during Dry‐Wet Cycles

Tao

Advances in Civil Engineering

et al. 2021

The hysteresis characteristics of soil-water characteristic curves (SWCCs) under dry-wet cycling conditions are very important for understanding unsaturated soil properties, so it is crucial to propose an accurate and efficient method for predicting the hysteretic behaviors of SWCCs. To this end, this paper investigates the hysteresis characteristics of SWCCs in the full suction range of seven kinds of Hunan red clay with different initial dry densities by combination of the pressure plate method, the paper filter method, and the saturated salt solution method. It is found that there are, respectively, strong and weak hysteresis zones in the drying and wetting SWCCs under dry-wet cycling conditions. By combining this feature and based on the drying curve, the soil volume and contact angle changes during the drying and wetting processes are employed to predict the hysteretic behaviors of SWCCs. To verify the validity of the prediction method, the predicted curves of the samples with different initial dry densities are compared with the measured curves. The results show that in the strong hysteresis zone, the hysteresis characteristics of the drying and wetting SWCCs are mainly resulted from the changes in the soil pore structure; in the weak hysteresis zone, the hysteresis characteristics are mainly influenced by the changes in the receding and advancing contact angles corresponding to the drying and wetting processes. The Young–Laplace theory is used to transform the changes of contact angle during the drying and wetting processes into the proportional relationship k of matric suction, and the corresponding wetting curve is obtained by smoothing the drying curve. It is found that the prediction effect in the high suction part (the strong hysteresis zone) is better than that in the weak hysteresis zone, which confirms that the hysteresis effect of SWCCs in the high suction part is influenced by the contact angle. Our proposed method can greatly reduce the test period and has a significant practical application value, which provides a new idea for the prediction of SWCCs under dry-wet cycling conditions.