1018 whose combined effect defines total suction. Matric suction and osmotic suction are defined as described by Aitchison (1964): • Matric suction represents the equivalent suction obtained by comparing the partial pressure of water vapor in equilibrium with soil water to that of water vapor in equilibrium with a solution that has the same chemical composition as the soil water. • Osmotic suction is defined as the equivalent suction determined by comparing the partial pressure of water vapor in equilibrium with a solution of identical composition to the soil water and the partial pressure of water vapor in equilibrium with pure, free water. The mathematical relationship among the different components of soil suction can be expressed as follows: Õ =(Ç 5 − Çm)+ í Where Ç 5 is the pore-air pressure (kPa), Çm is the pore-water pressure (kPa) and í is the osmotic suction (kPa). Matric suction is defined as the difference between pore-air pressure and pore-water pressure (Ç5−Çm) and can be determined using either direct or indirect measurement techniques. Direct methods involve measuring negative pore-water pressure, while in field conditions the pore-air pressure component is typically equal to atmospheric pressure. 2.3 Soil-Water Characteristic Curve (SWCC) The Soil-Water Characteristic Curve (SWCC) offers a conceptual framework that relates the mass and/or volume of water in a soil to the energy state of the water phase. It has been widely used as an interpretive model based on fundamental capillary concepts to explain how water is distributed within soil voids. Additionally, factors such as soil plasticity, particle size distribution, and void ratio are commonly incorporated into the interpretation of laboratory-measured SWCC data. When using a soil-water characteristic curve (SWCC) to calculate soil suction, it is essential to know the curve’s parameters. SWCCs typically have a sigmoidal shape, which can be described using three fitting parameters, commonly labeled “a”, “n”, and “m”. The “a” parameter usually represents a suction value slightly higher than the soil’s air-entry value (AEV). The “n” parameter indicates how quickly the soil desaturates once suction surpasses the air-entry point. In some models, “m” parameter is included to provide additional flexibility in fitting the SWCC data. Additionally, the saturated gravimetric water content (2E) must also be determined (Fredlund et al., 2012). In this study, the Fredlund & Xing model (1994) was used to analyze the behavior of filtered tailings under unsaturated flow conditions. This model allows the soil-water characteristic curve (SWCC) to be described by fitting parameters that define how volumetric water content varies with suction in porous materials. The Fredlund & Xing model can be represented mathematically as: ® (Õ) = s p (®E) (:Å ([ +ç Õ k é$))- Where Õ is the suction (kPa), ® (Õ) is the volumetric water content at suction, ®E is the saturated volumetric water content, [ is the natural number (2.71828), k,Å,ï are model fitting parameters and sp is a correction coefficient that allows a progressive decrease in water content at high suction forcing the function through a water content of zero at one million kPa.
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