When a soil deposit consists of a number of horizontal layers having different permeabilities, the average value of permeability can be obtained separately for both vertical flow and horizontal flow, as kVand kH respectively.
Consider a stratified soil having horizontal layers of thickness H1, H2, H3, etc. with coefficients of permeability k1, k2, k3, etc.
For vertical flow
The flow rate q through each layer per unit area is the same.
Let i be the equivalent hydraulic gradient over the total thickness H and let the hydraulic gradients in the layers be i1, i2, i3, etc. respectively.
where kV = Average vertical permeability
The total head drop h across the layers is
Horizontal flow
When the flow is horizontal, the hydraulic gradient is the same in each layer, but the quantity of flow is different in each layer.
The total flow is
Considering unit width normal to the cross-section plane,
