What is permeability of soil?

Soil permeability is a complex property influenced by up to 13 key factors that essentially relate to the soil’s structure and composition. Soils are composed of particles with interconnected voids, which may be filled with air or water. When these voids are occupied by water, the soil is considered saturated. If only some of the voids contain water, the soil is partially saturated. Water movement occurs through these voids rather than through the soil particles themselves, and this flow is driven by pressure differences, known as hydraulic head, or by suction from plant roots during transpiration.

• Soil type: The first most important factor is the soil type itself since each of them have different permeabilities. Sand, silt, and clay–each differ from each other in the amount of water that can pass through them. Sandy soils generally exhibit high permeability, while clayey soils have low permeability.
• Particle size and distribution: The second factor is the size, shape, and texture of soil particles and how water interacts with them. Larger particles, such as sand, create larger pore spaces, facilitating easier water flow, whereas smaller particles, like clay, have tighter spaces that restrict flow.
• Void ratio: The void ratio, or the volume of voids in the soil, dictates the space available for water movement. Void ratio and permeability are directly proportional; in that a higher void ratio indicates greater permeability.
• Degree of saturation: The amount of water occupying the voids impact the soil’s permeability. Fully saturated soils may have reduced permeability due to the lack of air-filled voids, while partially saturated soils allow for more dynamic water movement.
• Compaction: Soil compaction reduces the void ratio by decreasing the volume of void spaces, thereby lowering permeability. While compaction is beneficial for increasing soil strength and stability, it can correspondingly also stop drainage and water infiltration.
• Stratification: The layering of soil, whether from construction or natural deposition, determines permeability. In layered soils, water tends to flow more horizontally than vertically. In a soil deposit with a silt layer between sand and clay, the silt’s permeability can be 10 times that of the clay and one-tenth of the sand’s. This variation impacts the soil’s overall behaviour. Other soils, like sand and clay, also show this difference in permeability. Knowing these differences is necessary for designing safe structures for dams and embankments.
• Organic matter: Organic matter works by modifying soil structure and composition, impacting its hydraulic conductivity and permeability. By forming stable aggregates, organic matter enhances soil porosity and pore connectivity, leading to increased permeability and improved drainage. In clay-rich soils, organic matter flocculates clay particles, creating a more open and permeable soil fabric that facilitates water infiltration and percolation.
• Pore connectivity: Referring to the degree to which pores are interconnected, in a porous medium, leading to water flow. In earthen dams, the connectivity of pores dictates the rate of water seepage while in case of landfills, it restricts or eases the movement of leachate and gases.
• Hydraulic conductivity: Denoted by (K) is the flow rate per unit area divided by the hydraulic gradient in the direction of flow. Hydraulic conductivity is essential for predicting water flow in various soil types at different water potentials. 
• Soil texture: The mix of sand, silt, and clay in the soil influences all the other properties of the site, itself. Right from compaction, load bearing capacity, to degree of saturation, variations in texture alters the mechanics of the project.
• Presence of cracks or macropores: Cracks and large pores can significantly increase permeability by providing direct pathways for water to flow through the soil.
• Soil temperature: Temperature changes the viscosity of water and the soil’s physical properties, influencing permeability through thermal expansion and contraction. Warmer temperatures increase permeability by reducing water viscosity. In civil engineering projects, such as embankments and landfills, temperature fluctuations impact drainage and stability. In cold climates, freezing temperatures cause soil to expand, reducing permeability, leading to waterlogging and structural issues.
• Depth of soil layer: Permeability also varies with depth due to changes in soil composition and compaction. Deeper layers tend to have different permeability characteristics compared to surface layers.