CEng 7033 - Design and Analysis
of Embankment Dams:
Seepage Analysis and Seepage
Control in Embankment Dams
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SEEPAGE ANALYSIS
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The seepage analysis in the earth dam is carried out to analyze:
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the
the
the
the
phreatic line,
pore pressure within the dam or in its foundation,
exit gradient at the downstream face of the dam, and
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:
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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).
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SEEPAGE ANALYSIS
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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
The fundamental aspects of the finite element Modelling are:
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the defining of the geometry,
dividing the model into appropriate regions,
defining material properties, and
specification of the appropriate boundary conditions.
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Addis Ababa University, Addis Ababa Institute of Technology, School of Civil and Environmental Engineering
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2D and 3D Flow of Water through Soils
 To derive the equation of continuity of flow, consider an elementary soil prism at
point A (Figure 7.1b) for the hydraulic structure shown in Figure 7.1a. The flows
entering the soil prism in the x, y, and z directions can be given from Darcy’s law as
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Addis Ababa University, Addis Ababa Institute of Technology, School of Civil and Environmental Engineering
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2D and 3D Flow of Water through Soils
 The flows entering the soil prism in the x, y, and z directions can be given from
Darcy’s law as
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Addis Ababa University, Addis Ababa Institute of Technology, School of Civil and Environmental Engineering
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2D and 3D Flow of Water through Soils
 The respective flows leaving the prism in the x, y, and z directions are
 For steady flow through an incompressible medium, the flow entering the
elementary prism is equal to the flow leaving the elementary prism. So,
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3
Addis Ababa University, Addis Ababa Institute of Technology, School of Civil and Environmental Engineering
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2D and 3D Flow of Water through Soils
 Substituting Equations 7.1 through 7.6 in the preceding equation we obtain
 For two-dimensional flow in the x-z plane, Equation 7.7 becomes
 If the soil is isotropic with respect to permeability, kx = kz = k, and the continuity
equation simplifies to
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Addis Ababa University, Addis Ababa Institute of Technology, School of Civil and Environmental Engineering
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2D and 3D Flow of Water through Soils
 The solution of any differential equation requires knowledge of the boundary
conditions.
 The boundary conditions for most ‘‘real’’ structures are complex, so we cannot
obtain an analytical solution or closed form solution for these structures.
 We have to resort to approximate solutions, which we can obtain using numerical
methods such as finite difference, finite element, and boundary element. We can
also use physical models to attempt to replicate the flow through the real
structure.
 Solution techniques for Laplace’s equation:
 Flownet sketching – approximate method, sketching technique is simple
 Analytical methods – The Laplace equation has analytical solutions for flow
domains with simple boundary conditions.
 Numerical Methods - Finite difference method and Finite element
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4
FLOWNET FOR
EMBANKMENT DAMS
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Construction of flownet
for embankment dams
is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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FLOWNET FOR EMBANKMENT DAMS
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Construction of flownet for embankment dams is difficult, but possible
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SEEPAGE CONTROL IN EMBANKMENT DAMS
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Seepage control is the most important consideration in the design of earth
dams.
The high potential energy of upstream water has to be dissipated in a safe
manner.
The objective in designing a new dam, or when assessing an existing dam,
is to be satisfied that the dam has adequate measures to control seepage
and potential internal erosion in the dam and its foundation so that:
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Pore pressures in the dam and the foundation are such that there is an adequate
margin of safety against slope instability.
erosion, which might progress to form a pipe and breach the dam, will not occur.
This applies to piping in the embankment, in the soil or rock foundation, from
the embankment to the foundation and the foundation into the downstream
zones of the embankment.
These objectives are achieved by zoning of the dam, provision of filters and
high seepage discharge capacity zones and other embankment design and
foundation treatment measures. Figure 10.16 shows some of the measures
which are used.
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SEEPAGE CONTROL IN EMBANKMENT DAMS
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The seepage controlling features consist of:
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Measures to control seepage through the dam (e.g., filters, zoning)
Measures to control seepage through the foundation (e.g., cut-off wall)
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SEEPAGE CONTROL IN EMBANKMENT DAMS
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The design features are:
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A: Low permeability “core’’ of the dam, to limit seepage through the dam.
B: Chimney filter drain, which intercepts seepage through the dam and controls
internal erosion of the core and pore pressures in the dam, if the downstream
zone C is low permeability.
C: Downstream zone, which maintains stability and may, if permeable compared
to the core and filter, control pore pressures.
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SEEPAGE CONTROL IN EMBANKMENT DAMS
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The design features are:
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D: Horizontal filter drain – which controls exit gradients, erosion of the
foundation and pore pressures (but may actually increase the amount of
seepage because the gradients are increased).
E: Upstream low permeability blanket which increases the seepage path, reduces
seepage, and seepage exit gradients.
F: Slurry trench (or other types of cutoff) which will reduce (or almost stop if
fully penetrating to a low permeability base) seepage and seepage exit gradients
(usually in soil or extremely weathered rock).
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SEEPAGE CONTROL IN EMBANKMENT DAMS
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The design features are:
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G: Grouting to reduce seepage and seepage exit gradients. Grouting is usually in
rock, since grouting in soil is usually costly and ineffective.
H: Pressure relief wells – control exit gradients where confined aquifers in the
foundation reduce the effectiveness of the horizontal drain. 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.
I: Weighting berm – which improves downstream slope stability and overcomes
the potential for liquefaction or “blow-up’’ or heave of the foundation (with H).
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SEEPAGE CONTROL IN EMBANKMENT DAMS
Controls
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SEEPAGE CONTROL IN EMBANKMENT DAMS:
Drainage Filters
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Providing drainage filters is the best method to prevent seepage.
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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.
Various kinds of drains which are commonly used in earth dams are given
below:
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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.
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SEEPAGE CONTROL IN EMBANKMENT DAMS
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Various kinds of drains which commonly used in earth dams are given
below:
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Horizontal filter: Horizontal filter extends from downstream side of the dam to
inside at a distance of 25% to 100% from toe to center line of the dam.
Chimney Drain: The horizontal drainage is better in bringing down the phreatic
line and also causes stratification in a horizontal direction. When large-scale
stratification occurs horizontal filters are inefficient. 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.
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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Seepage beneath a dam on a permeable soil (or permeable weathered rock)
foundation should be allowed to exit in a controlled manner into a horizontal
drain.
The horizontal drain may consist of a single layer of Zone 2A filter, or 3
layers of Zone 2A and 2B as shown in Figure 10.17. In both cases Zone 2A
should be designed to act as a filter to control erosion of the soil from the
foundation into the drain.
The filter design criteria detailed in previous lecture should be used.
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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The 3-layer drain incorporates the layer of high permeability Zone 2B
to ensure that the drain has sufficient discharge capacity.
It is good practice to design the horizontal drain to have sufficient
capacity to discharge the flow entering the drain from the dam
foundation and from the vertical drain without the phreatic surface
rising into the low permeability fill (see Figure 10.20).
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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If the horizontal drain has insufficient capacity, the phreatic surface
will rise into the downstream low permeability fill as shown in Figure
10.21, reducing the stability of the downstream slope.
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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Cedergren (1972) gives a design method for estimating the discharge
capacity of a horizontal drain without pressurization based on:
Where:
k1 =permeability of the drain material (m/sec)
h=vertical thickness of the drain (m)
L1 =length of the drain (m) as shown in Figure 10.22b.
q=discharge capacity per metre width of drain (width is measured across river) (m3/m/sec).
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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The capacity of the vertical drain is seldom a critical issue because the
quantity of seepage through the earthfill is small and the vertical drain
width is dictated by construction factors. However its capacity should be
checked by:
Where:
k2 =permeability of vertical drain (m/sec)
h2, L2 (m) are as shown in Figure 10.22(a)
w=width of drain (m).
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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When checking the design of the horizontal drain the following should
be noted:
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Water from the abutments will flow towards the lowest part of the drain before
flowing out the toe of the dam. Hence the required flow capacity per unit width
in this area will be greater than the average flow per unit width under the dam.
If a three-layer filter drain is used, Zone 2B will dominate the discharge capacity.
Conservative estimates of foundation permeability should be used for the design
of the horizontal drain, since failure to provide adequate capacity can lead to
failure of the dam, or the requirement for a stabilizing berm if the problem is
recognised.
Similarly, conservative estimates of the horizontal drain permeability should be
used, and care taken to ensure the drain is not contaminated by soil from the
foundation during construction. Generally it is good practice to provide a threelayer drain, so there is a large discharge capacity.
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HORIZONTAL AND VERTICAL DRAINS IN THE
EMBANKMENT
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The downstream toe should be designed to ensure the drain outlet is not blocked
by erosion of soil off the embankment or the abutment. It is wise to provide a
berm at the toe of the dam above the drain to collect any material eroded off the
downstream face of the dam (as shown in Figure 10.23).
Place the filter drain in contact with the higher permeability strata in the
foundation. So, as shown in Figure 10.23, the lower permeability sandy clay-silty
sand has been removed.
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CONTROL OF FOUNDATION SEEPAGE AND
INTERNAL EROSION AND PIPING BY CUTOFFS
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As shown in Figure 10.16, seepage through a permeable foundation
can be reduced by constructing a low permeability cutoff through the
permeable material. This may consist of:
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Cutoff trench filled with earthfill;
Slurry trench;
Diaphragm wall using rigid or plastic concrete;
Contiguous or intersecting bored piles;
Sheet pile wall;
Grout curtain.
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CONTROL OF FOUNDATION SEEPAGE AND
INTERNAL EROSION AND PIPING BY CUTOFFS
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As shown in Figure 10.16, seepage through a permeable foundation
can be reduced by constructing a low permeability cutoff through the
permeable material. This may consist of:
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Cutoff trench filled with earthfill;
Slurry trench;
Diaphragm wall using rigid or plastic concrete;
Contiguous or intersecting bored piles;
Sheet pile wall;
Grout curtain.
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Addis Ababa University, Addis Ababa Institute of Technology, School of Civil and Environmental Engineering
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