Darcy’s law is valid — 

Concept:

For the flow of water through soil, the flow is laminar if Reynold’s number is less than 1.

Darcy’s Law:

The flow of free water through soil is governed by Darcy’s law. As per this law, for laminar flow through homogenous soil, the velocity of flow (v) is given as

ν = ki

where, k = coefficient of permeability, i = hydraulic gradient

ν = discharge velocity or superficial velocity.

Validity of Darcy’s law:

It is valid when the flow through the soil is laminar. This assumption was based on the fact that there is no head loss associated when the water flows through soil due to the formation of turbulent rollers.

Note:

For the flow of water through soil, the flow is laminar if Reynold’s number is less than 1.

• In fine-grained soil, flow is laminar.
• In coarse-grained soil, the flow becomes turbulent when the size of particles is very large i.e. in gravel range.

Thus the assumed soil mass must be homogenous and fully saturated for the validity of Darcy’s law.