As per Darcy’s Law in saturated soil

Concepts:

Darcy’s Law states that velocity of flow through porous media is directly proportional to hydraulic gradient for any given saturated soil under steady laminar flow conditions.

 If the rate of flow is Q through the cross-sectional area (A) of the soil mass, Darcy's Law can be expressed as

V ∝ i or V = K × i

Q = K × i × A

k  = permeability of the soil
i = dh/dl

dh = difference in total heads
dl = length of small soil element

Darcy’s law assumes the flow must be laminar i.e. flow of water through soil voids should have very less velocity. This only possible in case of soils having small but large no of voids. Therefore, it is applicable for soils such as fine saturated sands, clays, silts etc. It is not applicable for soils having large size voids like gravel, boulder etc because in these soils water is drained immediately due to action of gravity and hence, not possible to maintain laminar flow in these soils.