Earthen Dam

EARTH DAM
(WATER RESOURCES ENGINEERING – II)
UNIT – III
Rambabu Palaka, Assistant ProfessorBVRIT

Learning Objectives
1. Types of Earth Dam
2. Causes of Failure
3. Criteria for Safe Design
4. Section of an Earth Dam
5. Seepage Analysis
6. Seepage Control Measures

Types of Earth Dams
Depending upon method of construction,
1. Rolled Fill Dam
a) Homogeneous Embankment Type
b) Zoned Embankment Type
c) Diaphragm Embankment Type
2. Hydraulic Fill Dam
 In this type of dams, successive layers of moistened or damp soils are laid one
over the other.
 They are then thoroughly compacted and bonded with the preceding layer by
means of power-operated rollers of proper design and weight.
In this Hydraulic Fill dams, the construction, excavation, transportation of the earth
is done by hydraulic methods.
Method:
 Outer edges of the embankments are kept slightly higher than the middle portion
of each layer.
 During construction, a mixture of excavated materials in slurry condition is
pumped and discharged at the edges. This slurry of excavated materials and water
consists of coarse and fine materials.
 When it is discharged near the outer edges, the coarser materials settle first at
the edges, while the finer materials move to the middle and settle there.
 Fine particles are deposited in central portion to form a water tight central core.
 In this method, compaction is not required.

Homogeneous Embankment Type Dams
 Constructed with uniform and homogeneous materials.
 Suitable for low height dams (up to 10m).
 Usually constructed with soil and grit mixed in proper ratios.
 The seepage action of such dams are not favorable,
therefore, for safety in case of rapid drawdown, the upstream
slope is kept relatively flat (3:1)
 Homogeneous section is modified by constructing rock toe at
the downstream lower end and providing horizontal filter drain.

Zoned Embankment Type Dams
 Made up of more than one material
 Outer shells are made of pervious, freely draining materials
 Central portions called core or hearting made from materials
which are relatively impervious.
 The thickness of the core wall is made sufficiently thick to
prevent leakage of water through the body of the dam.
 A suitable drainage system, in the form of horizontal drain or
a rock toe is also provided at downstream side.

Diaphragm Embankment Type Dams
 Constructed with pervious materials, with a thin impervious
diaphragm in the central part to prevent seepage of water.
 Impervious diaphragm may be made of impervious clayey
soil, cement concrete or masonry or any impervious material
The main difference in zoned and diaphragm type of dams
depend on the thickness of the impervious core or diaphragm.
 The thickness of the diaphragm is not more than 10 m.

Causes of Failure
1. Hydraulic Failures  40%
2. Seepage Failures  30%
3. Structural Failures  30%a) Piping
b) Sloughing
a) Overtopping
b) Wave Erosion
c) Toe Erosion
d) Gullying
a) Upstream slope failure due to sudden
drawdown
b) Failure by excessive pore pressure
c) Downstream slope failure by sliding
d) Failure due to settlement of foundation
e) Failure by sliding of foundation
f) Failure by spreading

Failure due to Tree
roots Over
Topping
Toe
Erosion
Gulling
Piping
Sloughing
Foundation
Settlement
Seepage through
Foundation
Downstream
Sliding

Criteria for Safe Design
1. To avoid overtopping failure, spillway of adequate capacity
and sufficient freeboard must be provided
2. The seepage line is well within the downstream face so that
no sloughing of the slope takes place
3. The upstream and downstream slopes are flat enough to
be stable with the materials
4. The shear stress induced in the foundation should be less
than the shear strength of the foundation material. For this
purpose, the embankment slopes should be sufficiently flat.
5. The upstream and downstream faces are properly protected
against wave action
6. There should not be any possibility of free passage of water
through the embankment or through the foundation
7. The foundations, abutments, and embankment must be
stable for all conditions of operation
8. The dam as a whole should be earthquake resistant

Section of an Earth Dam
1.Top Width
2.Free Board
3.Casing or outer shells
4.Central Impervious Core
5.Cut-off Trench
6.Downstream drainage system

Top Width
1. The top width of the dam depends upon:
a) Construction material
b) Height of structure
c) Roadway
2. Empirical Formulae:
a) B = Z/5 +3  for very low
b) B = 0.55 Z1/2 + 0.2 Z for lower dams (<30m)
c) B = 1.65(Z+1.5)1/3  for higher dams (>30m)
Where Z = Height of dam
3. A minimum width of 6m is required for maintenance so that small trucks can
operate on it
4. The berm may be provided for the dam, which are more than 10 m in height.
Minimum berm width may be kept as 3 m.

Free Board
Free board is vertical difference between the horizontal crest of the embankment
and reservoir level.
Normal Free Board  Top of the crest to normal reservoir level
Minimum Free Board  Top of the crest to Maximum Reservoir Level during floods
The USBR suggests:
Minimum of 2 m and maximum of 3 m over maximum flood level

Casing or Outer Shells
1. The function of casing is to impart stability and protect the
core.
2. The relatively pervious materials, which are not subjected to
cracking on direct exposure to atmosphere, are suitable for
casing.
3. IS: 8826 – 1978 provides recommendations for suitability of
soil used for earth dams

Central Impervious Core
1. The core provides impermeable barrier within the body of the dam.
2. Impervious soils are generally suitable for the core (IS 1498 -1970). However
soils having high compressibility & liquid limit, and having organic contents may
be avoided, as they are prone to swelling & formation of cracks.
Following guidelines are recommended for design of core (for Small Dams)
1. The core may be located either centrally or inclined upstream.
2. The minimum top width should be kept 3 m
3. The top level of the core should be fixed at 0.5 m above MWL.
4. The side slopes may be kept 0.5:1 and 1:1.
5. Thickness of core at any section shall not be lesser than 30% (preferably not less
than 50 percent) of maximum head of water acting at that section.

Cutoff Trench
1. To reduce loss of stored water through foundations and abutments
2. To prevent sub-surface erosion by piping.
The following guidelines may be adopted for design of cut off.
1. The cut off shall be located such that its centre line should be within the base of
impervious core and should be upstream of centre line of dam.
2. The positive cut off should be keyed at least to a depth of 0.4 metre into
continuous impervious sub stratum or in erodable rock formation.
3. A minimum bottom width of 4.0 metre is recommended.
4. Side slopes of at least 1:1 or flatter may be provided in case of over burden while
1/2:1 and 1/4:1 may be provided in soft rock and hard rock respectively.
5. The cut off in the flanks on either side should normally extend up to the top of
impervious core.

Downstream Drainage System
To ensure safety of dam, it is very important to handle the seepage water in the dam
so as to maintain the original particles of soils in their place.
The measures commonly adopted for safe disposal of seepage water through
embankment dams are;
1. Toe drain  installed in oldest homogenous dams to prevent softening at d/s
2. Horizontal filter  used in moderate high dams
3. Inclined or vertical filter (chimney filter)  used in higher homogeneous dams
Purpose:
 To reduce pore pressure in the downstream portion of the dame
 To control piping failure
Generally, a multi-layer filter or inverted filter is provided in which subsequent layer
becomes increasing coarser than the previous one.
According to Terzaghi, the filter material should fulfill the following criteria:
1. D15 of filter / D85 base material > 4 and < 20
2. D15 of filter / D15 base material < 5
3. The gradation curve of the filter material should be nearly parallel to the gradation
curve of the base material.
Note:
D15 is permeability protection limit,
D85 is piping predicting limit

Seepage Analysis
Objective:
To estimate the quantity of seepage (rate of leakage) through dam using Flow Net
Assumptions:
1. Soil is homogeneous (Coefficient of permeability is constant everywhere)
2. Soil is isotropic (Coefficient of permeability is same in all directions)
3. Size of pore spaces do not change
4. Darcy’s law is valid (Flow is laminar) Q=kiA
5. Soil is completely saturated (Degree of saturation is 100%)
6. Hydraulic boundary conditions at entry and exit are known
7. During flow, volume of soil and water remains constant

Flow Net
A flow net is a graphical representation of the paths taken by water in passing
through soil.
Characteristics of Flow Net:
1. Flow lines represent flow paths of particles of water
2. Flow lines and equipotential line are orthogonal to each other
3. The area between two flow lines is called a flow channel
4. The rate of flow in a flow channel is constant (∆q)
5. Flow cannot occur across flow lines
6. An equipotential line is a line joining points with the same head
7. The velocity of flow is normal to the equipotential line
8. The difference in head between two equipotential lines is called the potential
drop or head loss (∆h)
9. A flow line cannot intersect another flow line.
10. An equipotential line cannot intersect another equipotential line

Flow Net for an Earth Dam
Top Flow Line or Phreatic Line
Flow Line
Equipotential Line
Field
Phreatic Line is a seepage line separating saturated and unsaturated zones

Where k’ = Equivalent Permeability of the transformed field

Plotting of Phreatic Line
In order to draw flow net, it is first essential to find the location and shape of Phreatic
line or top flow line separating Saturated and Unsaturated Zones.
Phreatic line can be located by
1. Graphical Method or Casagrande Method
2. Analytic Method
3. Experimental Method

1. Draw Arc taking C as
centre and CF as Radius
Discharge through the body of dam, q = k s
(for Isotropic Soils)
2. Draw Directrix and
Find Focal Distance S
3. Draw Parabola
curve
(y2= 2sx + s2)

Seepage Control Measures
Seepage Control measures are required to prevent adverse effects of water
percolating through embankment and its foundation.
1. Embankment Seepage Control
a) Toe Filter
b) Horizontal Drainage Filter
c) Percolating Filter d/s of toe
d) Embankment Zoning
e) Chimney Drains extending upwards to embankment
2. Foundation Seepage Control
a) Impervious Cutoff
b) Upstream Impervious Blanket
c) D/s Seepage berms
d) Drainage Trenches
e) Relief Wells

0.3H to 0.4H
H
0.25L to L
L
To keep phreatic line well within the section of embankment and also facilitates drainage

Previous Questions
1. Describe the phreatic line and its importance in an earth dam. Explain
the graphical method of drawing flow net in an earth dam.
2. Describe the procedure of determining phreatic line through
homogeneous earthen dams provided i). with a horizontal filter ii).
without a horizontal filter.
3. Discuss various causes of failure of earth dams
4. Explain methods of construction of earth dams. Also write merits and
demerits of earth dam
5. Write briefly about overtopping, wave erosion and piping phenomenon in
earth dams
6. Write Hydraulic Failures of earth dams

Previous Questions
7. Significance of pore pressure in relation to earthen dam construction
8. Slope protection in earthen dams
9. Filters in earthen dams
10.How do you decide the section of a zoned earth dam if the following
materials are available: i). silty clay ii). fine gravel or coarse sand
11.Explain the necessity of slope protection in earthen dams. How do you
protect upstream and downstream slopes of an earthen dam?
12.Explain with neat sketches how you would carry out the stability analysis
of an earth dam.

Previous Questions
13. An earthen dam has to be constructed to store a maximum depth of 12
m of water over river bed consisting of coarse sand and gravel up to a
depth of 3 m below river bed followed thereafter by hard and sound rock.
Clay soil is available in plenty in the vicinity of the river. Draw and detail a
suitable section of the dam at the river bed.
14. Illustrate with neat sketches the following parts of an earthen dam and
state their functions briefly
i) Rock toe ii) horizontal drainage blanket iii) cut-off iv) Rip-rap
15. Describe Swedish Circle Method of Slope Stability Analysis for the
upstream side of an earth dam under sudden drawdown condition

Reference
Chapter 10
Irrigation and Water Power Engineering
By Dr. B. C. Punmia,
Dr. Pande Brij Basi Lal,
Ashok Kr. Jain,
Arun Kr. Jain

Earthen Dam

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