What Is Seepage ,Seepage Pressure , ?? Soil Mechanics

SEEPAGE

* Seepage is the movement of water through the soil under hydraulic gradient.

* The pressure that is exerted on the soil due to the seepage of water is called the seepage force or

seepage pressure.

* The problems posed by seepage are:

1. Loss of stored water by an earth dam

2. Settlement of water retaining structures resting on soil due to consolidation of soil underneath.

3. Uplift pressure on hydraulic structures exerted by percolating water.

4. Instability of soil bunds.

* If flow occurs in the downward direction, the effective pressure is increased. If flow occurs in the

upward direction effective pressure is decreased.

* If the seepage pressure becomes equal to the submerged weight of the soil, the effective pressure

reduces to zero. At this stage, the cohesionless soil losses its shear strength completely and its

particles have tendency to move up in the direction of flow. This phenomenon is known as quick

condition or quick sand.

The hydraulic gradient at which quick condition occurs is called critical hydraulic condition (ic

).

Mathematically

where G = sp.gr of soil particles and e = Void ratio

Laplace Equation

The two-dimensional flow of water through soil is governed by Laplace equation

= 0

where x is horizontal direction and z is vertical direction.

According to Darcy’s law,

vx = vz =

\ Laplace equation reduces to

= 0

or = 0

where f is hydraulic potential.

Flow Net

The graphical representation is given by two sets of curves intersecting at right angles. One set is the

flow lines which represents the path followed by percolating water through a saturated soil under

laminar conditions. Another set, known as equipotential lines represent lines connecting points of

equal potential. The head lost between two equipotential lines is known as potential drop Dh.

* The portion of a flow net bounded by two adjacent flow lines is known as flow channel.

* The properties of flow nets are:

1. Flow lines and equipotential lines meet each other at right angles.

2. No two flow lines cross each other.

3. No two flow or equipotential lines start from the same point.

4. Flow and equipotential lines are smooth curves.

Applications of Flow Nets

1. Determination of seepage: It is given by the expression.

q = kh1

where k = coefficient of permeability

h1 = Total hydraulic head causing the flow

Nf = Total number of flow channels

Nd = Total number of potential drops in a flow net

is known as shape factor

2. Determination of uplift pressure

Uplift pressure = hw gw

3. Seepage pressure: Seepage pressure = (h1 – Nd Dh) gw.

4. Exit gradient: It is given by

ie =

where Dh is the potential drop between two successive potential lines and L the average length of

the seepage.

Piping

When the uplift forces due to seepage on the downstream side of a hydraulic structure exceed the

submerged weight of the soil, the soil grains are dislodged and eroded. Gradually, it results into a

pipe shaped discharge channel and ultimately it may lead to a failure of the structure. This is called

piping failure.

Piping failure may be prevented by:

1. increasing the stress due to weight of the structure

2. increasing the creep length of flow of water

3. diverting the seepage water into filter wells.