# Permeability of soil and it’s measurement

The permeability of soil is soil property which describes quantitatively, the ease with which water flows through that soil. Soil consists of discrete particle. The void spaces between the particles are interconnected. This interconnected void space behave like complex tube network. Water flows in this complicated tube network. Resistance to flow of water is much less when soil has large voids.

A clayey soil with flocculated structure is more pervious than the same soil with dispersed structure.

Darcy established the relation between $\Delta{h}, L$ and $A$, hence, said flow occurring per unit time is directly proportional to head causing the flow ($\Delta{h}$) and area of cross-section of the soil sample and inversely proportional to length of soil sample.

$$q \propto \frac{\Delta h}{L}A$$

where q is- units of volume/time. The constant of proportionality also changes with the soil. Thus, it is the function of soil property. Named as coefficient of permeability of the soil $k$

$$q=k \frac{\Delta h}{L}A$$

Since $\frac{\Delta h}{L}$ is hydraulic gradient $i$,

$$q=kiA$$

dividing both side by A,

$$q/A=v=ki$$

Darcy’s law can be expressed in both the form. $v$ is referred as superficial velocity of flow. It is so called because, flow is through the pores in the cross-section and not through entire cross section.

Hence, permeability of a soil can be defined as the superficial velocity flow under unit hydraulic gradient. Actual velocity of water flowing through pores is called seepage velocity $v_s$

## Measurement

Coefficient of permeability can determined in three ways:

1. Laboratory test methods
2. Field test
• Unconfined flow pumping test
• Confined floe pumping test
3. Empirical methods

## Factors affecting permeability

Soil type can make the big difference in the permeability of soil.

1. Grain size of soil
2. Effect of permeant
3. Effect of void ratio
4. Other factors