**What is Darcy’s Law?**

Darcy Law states that flow velocity through permeable medium is directly proportional to hydraulic gradient. Normally, the steady laminar flow velocity through permeable soil is represented by Darcy’s law.

If the discharge of flow is Q through c/s area A of the soil mass, Darcy’s law is represented by the following equation.

V ∝ i or V = Ki

Now, Q = A x V or V = Q/A

So, Q/A = Ki

Q = K x i x A

Where, Q = Rate of flow, K = Permeability of soil, V = velocity of flow, A = cross sectional area of soil mass

i = hydraulic gradient = dh/dl,

dh = difference in total head

dl = length of soil sample

**Limitation of Darcy’s Law**

Darcy’s assumes flow should be laminar, which means water flow through voids of soil should have very low velocity. This is possible when soil has very fine grain particles. So, it is suitable for soil such as fine sand, silts and clay etc.

Darcy’s law is not suitable for large grained soil such as gravel, boulders etc because in these types of soil, water drains instantly because of gravity action and large voids and hence, it is not possible to manage laminar flow in these types of soil.

**Refference :**

- Soil Mechanics And Foundation Engineering (geotechnical Engineering) : Book Author – K.R Arora