36.1.2.1. Grain size and distribution

Larger and more uniformly graded grains have higher permeability.

Permeability refers to how easily fluids can move through a material. When the grains in a medium (like soil) are larger and more uniformly sized, there are fewer voids and more direct pathways for water to flow. This means that water can pass through the material more quickly, resulting in higher permeability. Essentially, if you imagine a sandpaper with coarse grains, water passes through it more easily than through fine-grained material, like clay, which has numerous small particles creating a lot of resistance.

Think of a busy highway versus a narrow street. A highway with more lanes (like larger grains) can handle more traffic (water flow) quickly and efficiently compared to a narrow street with fewer lanes (like fine grains), which would get congested easily.

More voids increase permeability.

The void ratio in a material is the ratio of the volume of voids (spaces) to the volume of solid particles. When there are more voids, it means there is more space for water to flow through. Thus, a higher void ratio can lead to increased permeability because these voids create pathways for water. However, the connectivity of these voids also matters. If the voids aren't well connected, water won't flow as freely, even if there are many voids.

Imagine trying to fill a sponge with water. If the sponge has lots of holes that are interconnected, water can easily pass through. If the holes are blocked or far apart, the water struggles to flow through, even if there are many holes.

Fully saturated soils have higher permeability.

Degree of saturation refers to the amount of water that fills the voids in a given material. When a soil is fully saturated, all the voids are filled with water, allowing for more efficient flow. This increases permeability as there are fewer air pockets, and the water can move more freely through the material. If the soil is only partially saturated, some voids contain air, which can impede water's movement, resulting in lower permeability.

Consider a wet sponge compared to a dry sponge. When the sponge is soaked (fully saturated), water can flow out quickly. However, when it's dry, it can't release water easily because air fills the gaps.

Higher viscosity reduces permeability.

Viscosity is a measure of a fluid's resistance to flow. A fluid with high viscosity, such as molasses, flows much slower than a fluid with low viscosity, like water. When water flows through soil, its viscosity affects how easily it can pass through the voids. If the viscosity is high, it results in a slower flow and, as a result, lower permeability. The resistance caused by the thicker fluid restricts its movement through the material.

Think of trying to pour syrup versus water through a straw. The syrup (high viscosity) flows slowly and creates a blockage compared to water (low viscosity), which flows freely.

More compacted soils have lower permeability.

Compaction refers to the process by which soil grains are pressed together. When soil is compacted, the grains have less space between them, which reduces the number of voids and decreases the pathways for water. As a result, the permeability of the soil decreases. Compaction can occur naturally over time or can be induced by construction activities or the weight of overlying materials.

Imagine walking on freshly dug soil versus packed soil. The freshly dug soil feels soft, allowing water to seep through easily, while the packed soil, like in a construction site, is hard and keeps water from flowing through effectively.

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