The property of the soil by which it permits the flow of fluid through it is called permeability of the soil. The porosity in the soil material is contributed by the presence of interstices within it. The property of permeability emerges when these interstices are connected, bringing a pathway for the movement of fluids. Generally, the soil is porous and permeable in nature.

A soil with high porosity has high permeability. A soil with a smaller value of permeability is categorized as impervious. Here some basic features related to soil permeability are explained.

## Need to Study Permeability

Permeability is one of the most important engineering properties of the soil that is a solution for a number of engineering problems encountered in construction. Some of them are:

- Settlement of foundation and buildings
- Seepage below the earth structures
- Seepage through the earth structures
- The Yield of the wells
- Control of Hydraulic Stability of masses
- For designing filter in hydraulic structures in order to prevent piping

## Total Head and Hydraulic Head

The flow of water through the soil is dependent on the permeability of the soil and the hydraulic head causing the flow. Figure-1 below shows two vessels A and B with water, that is connected by means of a small tube which contains soil sample. The length of the tube be ‘L’.

The total head at any point in a flowing fluid is the sum of elevation head(z), pressure head and the velocity head. The flow of water in soils are extremely slow, hence the velocity head is neglected. The elevation head is the distance from the datum line to the point about which the hydraulic head is determined. The pressure head is determined by means of a piezometer whose tip is placed at the point at which the head is determined.

From the figure-1 above, the total heads at point 1, 2 and 3 are determined and tabulated below.

Point | Elevation Head(z) | Pressure Head (Piezometer Reading) | Total Head |

1 | -z_{1} | h_{1} | h |

2 | -z_{2} | h_{2} | h’ |

3 | -z_{3} | h_{3} | 0 |

From the total head determined, ‘h’ is called as the hydraulic gradient. Hydraulic head is defined as the difference in elevation of the water levels at the entry and exit points of the soil mass under consideration. The hydraulic gradient is defined as the loss of head per unit length of the flow of fluid through the soil. It is also called the effective head. The hydraulic gradient is represented by ’i’, which is given by the formula:

**i = h/L**

Where, h = hydraulic head; L= Length of the soil specimen

## Darcy’s Law and Coefficient of Permeability

Darcy’s law studied the laminar flow of fluid in a homogeneous soil profile and demonstrated that the velocity of flow (v) is directly proportional to the hydraulic gradient (i). i.e.

**v =ki**

Where k is a constant called coefficient** of permeability.** This velocity of flow is also called as **superficial velocity or discharge velocity.**

If the discharge velocity (v) is known, the discharge (q) can be obtained as:

**q = vA = kiA**

Where ‘A’ is the cross-sectional area of soil including both the solids and voids.

When, the hydraulic gradient is equal to 1, k = v;

Hence, the coefficient of permeability can be defined as the velocity of flow occurring in the soil for a unit hydraulic gradient. The unit of coefficient of permeability is mm/sec or cm/sec or m/day;

## Coefficient of Permeability for Different Soil Types

The coefficient of permeability is dependent on the particle size, structure of soil mass, void ratio, properties of water, the shape of the particle, water impurities, adsorbed water, etc.

Also Read: Factors Affecting Permeability of Soils

Sl.no | Type of Soil | Coefficient of Permeability (mm/sec) | Drainage Properties |

1 | Clean Gravel | 10^{1} to 10^{2} | Very Good |

2 | Coarse and Medium Sand | 10^{-2} to 10^{1} | Good |

3 | Fine sand, loose silt | 10^{-4} to 10^{-2} | Fair |

4 | Dense silt, clayey silts | 10^{-5} to 10^{-4} | Poor |

5 | Silty clay and clay | 10^{-8} to 10^{-5} | Very Poor |