PURULIA GOVERNMENT ENGINEEERING
COLLEGE
NAME: SHANKHADEEP SHAI
ROLL NO: 35001319045
SEMESTER: 7th
DEPARTMENT: CIVIL ENGINEERING
BATCH: 2019-23
SUBJECT: HYDRAULIC STRUCTURE [CE(PE)701C]
TOPIC: SEEPAGE ANALYSIS FOR CONSTRUCTION OF
EMBANKMENT DAMS
Seepage in Embankment Dam:
Seepage in Embankment Dams and seepage control
are important aspects of dam design and construction.
Seepage flow in the dam is defined as the movement of water
from the upstream side of the dam to the downstream side
through embankment of below the foundation or base.
The seepage problem is most commonly found in earthen
embankment dams. This will directly affect the stability of
embankments, sloughing of slopes due to the rising of pore
water pressure, and also causes internal erosion which may
further lead to piping. Failure to prevent seepage in earthen
dam could lead to Slide Failure of Earth Dams.
Causes of Seepage in Earth Dam:
The seepage in dams primarily depends on the
properties of soil such as the plasticity of soil, the gradation,
the degree of compaction, etc. Plastic clay with a plasticity
index less than 15 mostly (for well compacted too) venerable
to seepage.
Also, poorly graded soil most often shows less resistance to
seepage. It is also considered that the poor design practices
will permit the seep of water through the embankment and
then leads to the discharge to the downstream side.
Seepage Analysis:
The seepage analysis in the earth dam is carried out to analyse
the phreatic line, the pore pressure within the dam or in its
foundation, the exit gradient at the downstream face of the
dam, and the amount of seepage flow that may pass through
the dam’s cross-sections.
Seepage analysis is also used to evaluate the efficiency of
various seepage prevention methods and to select suitable
seepage prevention methods for the particular dam. In most of
the cases, the seepage analysis is done by various analytical
methods (Dupuit’s method, Casagrand’s method, and
conformal mapping method), Experimental method
(Permeability tank), Numerical methods (Finite element
analysis, Finite-difference models. Numerical methods are
now commonly adopted in seepage analysis instead of an
experimental method for deciding control methods, because of
its ease of analysis, and its takes only less time.
Flow Nets:
Flow nets are the graphical representation of water movement
through the soil and it is used to analyze the uplift water
pressure and seepage amount. The flow nets are comprised of
flow lines and equipotential lines, which is a graphical
representation of Laplace’s equation.
From the above figure, the Phreatic Line (top parabolic line)
meets at point A which is at the downstream slope of the dam.
Below the line, the slope is saturated and water flowing. In
this condition the following
problems can happen:
The high piezometric
pressure can develop I
the downstream slope
which later leads to
slope instability.
It causes high seepage flows and later causes internal
erosion and piping.
To solve these problems, drainages are introduced in the
bottom of the dam.
Phreatic line:
It is the top flow line of saturated soil mass is called the
phreatic line. Below the phreatic
line, the seepage takes place.
The hydrostatic pressure is acts
below the phreatic line. This is
not an equipotential line. In the earth dam, the phreatic line
shapes like a parabola.
Problems due to Seepage in Earth Dams:
The stored water in the dam always seeks a way out. So there
are higher chances to seep the water through small pores,
previous strata, joints, fissures, cracks, etc. These risks are
mostly found in earthen dams. Considering all these factors, it
is recommended to include instrumentation to measure
piezometric pressure and seepage flows in the design stage.
The three basic problems due to seepage are given below:
Piping
Piping happens when the seepage water carries out soil
particles while moving out through embankment or
foundation. When it continuously happens, pipes or channels
will form inside the soil mass and initiates the seepage flows
at a higher rate. Later on, this may lead to the failure of the
dam.
Heaving and slope instability
The excess seepage causes instability of slopes in the
foundation and sometimes the seepage forces to develop in
the foundation soils immediately downstream of the dam, may
cause heaving.
Excess water loss
It is obvious that the seepage is a loss of water. And it
happens when the soil strata or foundations are pervious. But
proper stability management against piping and heaving water
loss will not pose problems to the dam.
Seepage control measures in Earth Dams:
Providing drainage filters is the best method to prevent
seepage. Filters are provided for the free discharge of water
and prevent piping and heaving. Drainages are used to reduce
the pore water pressure inside the embankment
and foundation soil.
A multi-layered drain system that consists of graded of both
fine and coarse materials is adopted in the earth dam in order
to prevent the seepage of fine materials from the embankment.
Various kinds of drains which commonly used in earth dams
are given below:
Rock Toe
In this arrangement stone size which varying from 15 cm to
20 cm is arranged in the downstream toe end of the dam. It is
arranged graded in layers which consist of fine sand, coarse
sand, and gravel as shown in the fig below. The height of the
rock toe usually kept between 15 to 30% of the reservoir
height.
Horizontal filter
Horizontal filter extends from
downstream side of the dam to inside
at a distance of 25% to 100% from
toe to centre line of the dam. In
common case height equal to three times the height of the
dam is sufficient.
Chimney Drain
The horizontal drainage is better in
bringing down the phreatic line
down and also causes stratification
in a horizontal direction. When large-scale stratification
occurs horizontal filters are efficient. To solve this issue a
vertical filter is placed along with the horizontal filter, which
carries out the seeping water effectively as shown in the
figure. This arrangement is termed as chimney drain.
Impervious cut off
A vertical cut off which made of concrete or sheet pile
extended up to the depth of impervious foundation. If the
depth is large partial cutoff is provided (up to lesser depth).
Relief wells to prevent Seepage in Earth Dams:
When large scale seepage happens through pervious
foundation relief wells are adopted. The relief wells are
used for relieving uplift pressure in the previous
foundation. It is usually located at the downstream end of
the dam.
Design criteria for Drainage filters:
The drainage filters are to be designed in such a way that
neither the foundation nor embankment material should not
enter into the drain to form a clog. The permeability or size of
the filter material should sufficient to carry water freely and
without any blockage. According to the Terzaghi equation, the
filter materials should satisfy the following equations,
D15 offilter D 15 offilter
< 4 ¿ 5<
D15 ofbasematerialls D 15 ofbasematerialls
The embankment soil or foundation soil which is surrounded
by the filter materials is called base materials. When the ratio
of D15 of filter to D85 of base materials is less than 4 to 5, the
base material is prevented from passing through the filters.
Similarly, when the ratio of D15 of filter materials to D85 of the
base material is greater than 5, the seepage forces in the filters
are controlled up to the permissible magnitude.
