5.4. Increased compactive effort reduces permeability.

1. Increased dry density reduces the void space, thereby reducing permeability.

When soil is compacted, the particles are pushed closer together. This process increases the dry density of the soil, which means there are fewer spaces (or voids) between the particles. As a result, the pathways that water could take through the soil are limited. Hence, higher density leads to lower permeability, meaning that water cannot pass through the soil as easily.

Imagine a jar filled with balls of different sizes. If you fill the jar tightly with smaller balls, there will be very little space left for air. This is similar to compacted soil, where the limited space means water has a harder time moving through it.

2. At the same density, soil compacted dry of optimum is more permeable.

Soil has an optimum moisture content where it achieves maximum density during compaction. If soil is compacted at a lower moisture content (dry of optimum), it has more voids for water than soil compacted at the optimum level. Therefore, even though the density may be the same, the arrangement of particles in dry soil allows water to flow through more easily compared to wet soil.

Think of packing a suitcase. If you pack it tightly with clothes that are dry, you can fit more in without leaving space. But if you try to pack in wet clothes, they clump together and leave spaces less filled, making it harder to zip the suitcase. In soil terms, this means drier soil can still let water pass relatively easily.

3. At the same void ratio, soil with bigger particle size is more permeable.

Void ratio refers to the amount of void space compared to the volume of soil particles. When soils have the same void ratio but consist of larger particles, the pathways for water flow become wider. This means that larger particles create bigger gaps for water to pass through, thus increasing permeability compared to soils made of smaller particles.

Imagine trying to run water through two different types of sand: one is made of fine particles (like flour) and the other has larger grains (like small pebbles). The larger grains create bigger spaces between them, allowing more water to flow through quickly, whereas the finer sand would clog and slow down the water flow.

4. Increased compactive effort reduces permeability.

Compactive effort refers to the amount of force applied to compact soil. When more effort is used, it forces the soil particles even closer together. This further reduces the void spaces between them, thereby decreasing the capacity of the soil to allow water to seep through. Essentially, the more you compact, the denser the soil becomes, which means water has even fewer pathways available to escape.

Consider stepping on a sponge. Initially, when you press down lightly, the sponge retains some of its porous structure. But if you push really hard, the fibers tighten, and it becomes difficult for water to flow through. Similarly, applying more force during soil compaction closes up spaces and makes it harder for water to permeate.

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