89 Figure 1 - Schematic of a filtered tailings facility (ICOLD 181) 2.2. Unsaturated Soil Mechanics Unsaturated soils are typically described as systems composed of three phases: solid particles, pore water, and pore air. However, a more accurate representation includes a fourth phase, known as the air–water interface or contractile skin (Fredlund and Morgenstern, 1977). Compaction is a mechanical process aimed at increasing soil density by expelling air from its pore spaces. This technique is extensively applied in geotechnical engineering for the construction of infrastructure such as roads, dams, landfills, airfields, foundations, hydraulic barriers, and for ground improvement purposes. Laboratory compaction curves obtained from standardized tests are commonly used to guide field compaction control. The primary goal of these tests is to identify the optimum moisture content that will produce the maximum dry density, as well as to evaluate how soil properties change as the water content varies from optimum conditions (Fredlund & Rahardjo, 1993). Soil suction can be quantified based on the relative humidity at the water surface and is referred to as total suction. This suction consists primarily of two components: matric suction and osmotic suction. In addition, a component associated with adsorptive forces between water and solid surfaces has been identified (Yong & Warkentin, 1966), although this adsorptive suction is difficult to isolate and measure. Most geotechnical engineering problems can be effectively addressed by considering matric and osmotic suction, 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: =( − )+ Where is the pore-air pressure (kPa), is the pore-water pressure (kPa) and is the osmotic suction (kPa). Matric suction is defined as the difference between pore-air pressure and pore-water pressure ( − ) 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.
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