Voids in a soil mass give rise to permeability and when a soil is permeable, water can seep through. This phenomenon of water flowing through the soil is called seepage.

__Seepage pressure__

Water exerts a pressure on the soil through which it percolates. This pressure is known as seepage pressure. It is produced due to the resistance or frictional drag of water flowing through the soil and it acts in the direction of flow.

If ‘h’ is the hydraulic head or head lost which causes the water to flow through a soil mass of thickness ‘L’, then seepage pressure (ps) developed is given by –

ps = 𝛾_{w}h [𝛾_{w} = Sp.weight of water]

ps = 𝛾_{w}.(h/L).L

ps = 𝛾_{w}.i.L [i = hydraulic gradient]

ps = iL 𝛾_{w}

.. Total seepage force (Fs) = ps.A [X-sec area of soil over which seepage pressure acts]

= iL𝛾_{w}A

The above force is uniformly distributed throughout the volume of the soil mass.

.. Seepage force pr unit volume = = i𝛾_{w}

Depending upon the direction of flow, the seepage may increase or decrease the vertical effective pressure of soil.

If flow occurs in the downward direction, the effective pressure is increased and if it occurs upwards, the effective pressure is decreased.

.. effective pressure (σ’) in a soil mass subjected to seepage pressure is given by – σ’ = 𝛾_{sub}.L±p_{s} [𝛾_{sub} = Submerged unit wt. of soil mass]

[+ve sign for seepage in downward direction & -ve for seepage in upward direction]

__Importance of seepage Analysis__

The seepage pressure is responsible for the phenomenon known as quick sand and is of vital importance in the stability analysis of earth pressure subjected to the action of seepage.

__Assumption in seepage flow analysis__

- The soil is fully saturated.
- The soil particles and water are incompressible.

- The flow is laminar and Darcy’s law is valid.

- The soil layer is pervious.
- The quantity of water entering into the soil element is same as the quantity of leaving water.

__Quick sand condition__

When water flows in an upward direction through the soil, effective pressure = = 𝛾_{sub}.L- p_{s}

If ‘p_{s}’ equals the pressure due to submerged weight of soil, the effective pressure reduces to zero. In such a case cohesion less soil loses all its shear strength and bearing capacity and the soil particles tend to be lifted up along with the flowing water. This phenomenon is termed as quick sand condition or quick condition or boiling condition or quick sand.

It may be noted that quick sand is not a type of sand but a flow condition occurring within cohesion less soil when its effective pressure is reduced to zero due to upward seepage pressure.

Thus during quick condition –

P_{s} = 𝛾_{sub}.L

Or, i.L.𝛾_{w} = 𝛾_{sub}.L

Or, i = i_{c} = 𝛾_{sub}/𝛾_{w} = (G-1)/(1+e)

The hydraulic gradient at which quick sand occurs is called the *critical hydraulic gradient*.

- Flow net

The network framed by the two sets of curves like flow lines and equipotential line is called *flow net.*

The path which a particle of water follows in its course of seepage through a saturated soil mass is called *flow line.*

Every strip between two neighboring flow lines is called *flow channel.*

Along each flow line, there will be different head of water. A line connecting all points of equal head is called *equi-potential* *line.*

Every section of a flow channel between two successive equipotential lines is called *field.*

- Properties of flow net

- Flow lines and equipotential lines meet at right angles.
- Flow lines never cross each other.

- Equipotential lines never cross each other.

- The fields are almost square.
- Same quantity of water flows through each channel.
- Same potential drop occurs between the successive equip-potential lines.

- Smaller is the field, greater will be the hydraulic gradient.
- Flow lines and equi-potential lines are smooth curves.

- Application of flow net

Flow net can be utilized for the following purposes –

- Determination of seepage.
- Determination of hydraulic pressure.

- Determination of seepage pressure.

- Determination of exit gradient.