# Permeability of Soil – Darcy’s law – Limitation, Coefficient of Permeability, Importance of Study of Seepage Analysis

4th April 2018Contents

## Permeability of Soil

**Permeability is defined** as the property of a porous material which permits the passage or seepage of water (or other fluids)through its interconnecting voids.

A material having continuous voids is called permeable. **Gravels** are highly permeable, while **stiff clay** is the least permeable.

**4 Importance Of Study Of Seepage Analysis**

The **study of seepage** of water through soils is important for the following engineering problems:

- Determination of
**rate of settlement**of a saturated compressible soil layer - Calculation of
**seepage**through the body of earth dams, and stability of slopes. - Calculation of
**uplift pressure**under the hydraulic structure and their safety against piping - Groundwater flow towards wells and drainage of soils.

**DARCY’S LAW **

The law of flow of water through the soil was first studied by **Darcy**. This law states that **the rate of flow or discharge per unit time (q) is proportional to the hydraulic gradient (i)**.

This law only for **laminar flow** conditions in a saturated soil mass. If q is the discharge per unit time through a soil mass of cross-sectional area ‘A’ under hydraulic gradient ‘i’.

**Then, q ∞ i.A or, q= K.i.A **

Where,

K is a constant, known as the **coefficient of permeability**.

If a **soil sample** of length ‘L’ and cross-sectional area ‘A’ (measured perpendicular to the direction of flow) is subjected to a differential head of water (h1-h2), then

**Limitation Of DARCY’S LAW**

**Darcy’s law** is valid only for laminar flow condition in the soil mass.

**Coefficient Of Permeability**

The **coefficient of permeability** is defined as the average velocity of flow that will occur through the total cross-sectional area of the soil mass under a unit hydraulic gradient. It is denoted by ‘K’ Its unit is the same as the unit of velocities such as Cm/sec or m/sec or m/hr, etc.

*Read Also:*

What is Uniformity Coefficient(Cu) And Coefficient of Curvature(Cc) of Soil