2. SEEPAGE ANALYSIS
• Seepage- It is the flow of water under
gravitational forces through a permeable
medium.
• Flow of water takes place from a point of
high head to a point of low head. The flow
is generally laminar in nature.
• Seepage Velocity-The apparent velocity
with which groundwater moves through
the bulk of the porous medium.
3. Effective, Neutral and Total Stress
Effective, Neutral and Total Stress
Two types of stresses act within a saturated soil mass and
both of them make total stress. One is named effective
stress or intergranular pressure and the another is named
neutral stress or pore water pressure.
Neutral stress- stress carried by the pore water and it is
the same in all directions, also called ‘pore water pressure’
and is designated by u.
At a depth z below the water table :u = Ꝩw . z
Neutral stress is transmitted through the pore present in the
soil.Neutral stress does not bring any change either in void
ratio or in any engineering properties of soil. It acts equally
on all sides.
4. contd...
Effective Stress- difference between the total stress and
the neutral stress ; this is also referred to as the
intergranular pressure and is denoted by :
σ͞ = σ – u
Effective stress is transmitted from one grain to another
grain through their points of contacts in a soil mass.
Such stress is effective in changing the void ratio of soil
mass and in changing other engineering properties such
as shear strength etc.
Hydraulic Gradient :- Loss of head or dissipation of head
per unit distance of flow
It is denoted by i and is given by i = h/L
Seepage Pressure :- Seepage water flowing through a soil
mass exerts a force or pressure to the soil mass in the
direction of flow, known as Seepage pressure.
5. QUICK SAND CONDITION
When flow takes place in upward direction the
seepage pressure also acts in the upward
direction and the effective pressure is reduced. If
the seepage pressure becomes equal to the
pressure due to submerged weight of the soil
the effective pressure reduced to zero. In such a
case cohesionless soil looses its shear strength
and the soil particles have a tendency to move
up in the direction of flow. This phenomenon of
lifting of soil particles is known as Quick
condition, boiling condition or Quick sand
condition.
The soil that experiences quicksand condition
would lose shear strength and bearing capacity.
6. QUICK SAND CONDITION
When the soil particles are
in a state of critical
equilibrium the total
upward force at the bottom
of the soil becomes equal
to the total weight of the
soil particles above the
bottom section.
At critical condition,
(h+L).Aγw = γsat.L.A
h/L =ic = (G- 1)/(1+e)
7. How to Avoid Quicksand Condition?
It can be prevented by lowering the
water table at the site before
excavation or by increasing the
length of upward flow.
8. PHERATIC LINES
• Phreatic line also known as
the seepage line or saturation
line is an imaginary line which
separates saturated zone from
unsaturated zone in an earthen
dam. It is also known as
Hydraulic gradient line.
• Along the phreatic line, the
atmospheric pressure exists
(which is equal to zero) at the
face of phreatic line.
• Above the phreatic line there is
a negative hydrostatic pressure
in the dam section and below
which there is positive
hydrostatic pressure.
9. FLOWNETS
• When water flows through
the interconnecting voids of
a soil mass it follows a
certain path.
• Flownet is the graphical
representation of flow of
water thgrough the
interconnecting voids of the
soil mass.
• A flownets consists of two
groups of curves. These are
flow lines and equipotential
lines. These curves bear a
fixed relationship to each
other.
10. Some Definitions
• FLOW LINES--are the paths which water particle follow
in the course of seepage through a soil mass under
laminer flow comdition. Water flows from the point of
higher head to low head. Flow lines represent the pattern
of flow.
• EQUIPOTENTIAL LINES--are the lines formed by joining
the points of same head or potential on the flow lines.
• FLOW NETS -If we draw the flow lines and the
equipotential lines for a given flow of water through a
given soil we get a net like sketch which is known as flow
net.
• Flow Channels- The space between two adjacent flow
lines is called flow path or channel.
• Field- The figure formed on the flow net between any two
adjacent flow lines and adjacent equipotential lines is
refferd to as field.
11. Properties of Flow net:
1.Flow lines and equal potential lines intersect each other
at 90 degrees.
2.The areas bounded by the flow lines and equal potential
lines form approximate squares.
3.Flow nets must satisfy the boundary conditions of flow
field.
4.Quantity of water flowing through each flow channel is
the same.
5.The potential drop in any two consecutive equal potential
lines is same/constant.
6.Flow lines and equal potential lines are smooth curves.
7.Flow lines do show refraction at the interface between
two soils having different coefficient of permeability.
8. Darcy’s law is valid.
12. APPLICATION OF FLOWNETS
1.Estimation of seepage losses from reservoirs: It is
possible to use the flow net in the transformed space to
calculate the flow underneath the dam.
2.Determination of uplift pressures below dams: From the
flow net, the pressure head at any point at the base of the
dam can be determined. The uplift pressure distribution
along the base can be drawn and then summed up.
3.Checking the possibility of piping beneath dams: At the
toe of a dam when the upward exit hydraulic gradient
approaches unity, boiling condition can occur leading to
erosion in soil and consequent piping. Many dams on soil
foundations have failed because of a sudden formation of a
piped shaped discharge channel.
13. Determination of seepage:
For the seepage through a flow net for
isotropic condition
Total Discharge, q = k.h.Nf/Nd
where, k = co-efficient of permeability
h= head causing flow
Nf= Total numbers of flow channels
Nd= Total numbers of potential drops.
