What is Seepage Pressure?
When water flows through soil pores a viscous friction is exerted on it which causes transfer of energy between soil and water. This water pressure applied on soil which tends it to percolate is called as seepage pressure.
It is generally represented by notation : ‘Ps’
Seepage Pressure Formula (Ps) :
Where,
h = hydraulic head
L = length over which head is lost
i = hydraulic gradient
γw = unit weight of water
Also, Seepage Force (Fs) is given by :
Where,
A = total cross sectional area of soil mass
Seepage force per unit volume is given by :
This seepage pressure will generally act in the direction of flow and the effective pressure in mass of soil is given by :
For downward flow it is given as,
For upward flow it is given as,
Quick Sand Condition
Quick sand condition is a phenomena which is caused by seepage. When the flow of water takes place in upward direction, this condition occurs.
Because of the seepage pressure acting in the upward direction the effective pressure is reduced when the flow of water takes place in upward direction.
When seepage pressure and the weight of submerged soil becomes equal the effective pressure becomes zero.
Particularly in this case the cohesionless soil looses it shear strength and soil particles tends to move in the direction of flow. This overall phenomena due to which lifting of soil occurs is known as quick sand condition or sand boiling condition.
Moreover during the period of this phenomena effective pressure becomes zero.
………. or
But We have,
Thus,
The Hydraulic gradient of this condition is called as critical hydraulic gradient. Thus, quick sand condition is the particular condition of flow which takes place when effective pressure reduced to zero at the time of upward flow.
Also Read : Three Phase System of Soil (Diagram)
Also Read : Soil Stabilization Methods & Necessity
Also Read : Permeability of Soil- Definition, Properties, Darcy’s Law
Also Read : Residual Soil and Transported Soil