The document discusses the design of embankment dams. It defines embankment dams as dams constructed of natural materials like earth or rockfill. It describes the different types of embankment dams including homogeneous dams, zoned dams, and diaphragm dams. It also discusses important design considerations for embankment dams like controlling seepage, providing internal drainage, and ensuring the shear strength of the soil is sufficient to resist failure. Pore water pressure in saturated soils is identified as an important factor that reduces the effective stress and shear strength of soils in embankment dams.

1. EMBANKMENT DAMS
MODULE II
DESIGN OF HYDRAULIC
STRUCTURE

2. INTRODUCTION
• An embankment dam, as defined earlier, is one
that is built of natural materials. In its simplest
and oldest form, the embankment dam was
constructed with low-permeability soils to a
nominally homogeneous profile.
• The section featured neither internal drainage nor
a cutoff to restrict seepage flow through the
foundation. Dams of this type proved vulnerable
associated with uncontrolled seepage, but there
was little progress in design prior to the
nineteenth century.
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3. INTRODUCTION
It was then increasingly recognized that, in
principle, larger embankment dams required
two component elements.
1. An impervious water-retaining element or
core of very low permeability of soil, for
example, soft clay or a heavily remoulded
‘puddle’ clay, and
2. Supporting shoulders of coarser
earthfill(or of rockfill), to provide structural
stability
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4. DESIGN AND ANALYSIS
• Although the loads acting on the concrete
gravity dam (discussed SOON) is the same
acting on the embankment dam, the
method of design and analysis of the two
differ considerably.
• This is mostly because the gravity dam
acts as one monolithic structure, and
it has to resist the destabilizing forces with
its own self weight mainly.
• Failure to do so may lead to its
topping, sliding or crushing of some of the
highly stressed regions.
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5. An embankment dam, on the other
hand, cannot be considered monolithic.
It is actually a conglomerate of
particles and on the action of the various
modes, which are much different from
those of a gravity dam.
Hence, the design of an embankment dam
is done in a different way than that of a
gravity dam. In fact, the design procedures
are targeted towards resisting the failure
of an embankment dam under different
modes, which are explained in the next
section.
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6. Embankment dam
types
• An embankment dam, whether made of
earth completely or of rock in-filled with
earth core, has a trapezoidal shape with
the shoulder slopes decided from the
point of stability against the various
possible modes of failure,
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7. • The top crest is kept wide so as to accommodate
roadway In order to check the seepage through
the body of the dam, a number of variations are
provided. For earthen embankment dams, these
range from the following types:
1. Homogeneous dam with toe drain
2. Homogeneous dam with horizontal blanket
3. Homogeneous dam with chimney drain and
horizontal blanket
4. Zoned dam with central vertical core & toe drain
5. Zoned dam with central vertical core, chimney
filter and horizontal blanket
6. Zoned dam with inclined core, chimney filter and
horizontal blanket
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8. Embankment dam
types
EARTHEN DAM
ROCKFILL DAM
Earth dams have been
built since the early
days of civilization.
They are constructed
mainly from earth (or
soil). Fig..
Rockfill dams are
constructed mainly
from rockfill or pieces
of rocks. Fig. 2.1(1
They require somewhat
stronger foundations
as compared to earth
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dams,
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10. RESERVOIR
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EARTHEN
DAM
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13. For example, The
Nurek dam across
river Vakhsh in
U.S.S.R, which is
the second highest
dam in the world
is an earth dam
and it is 300 m
high
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but the foundation need
not be as strong as
those for gravity
dams.
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14. Composite earth and rockfill
dams :
These dmas are constructed from
both earth and rockfill. Fig.
The Rogun dam across river
Vakhsh in U.S.S.R. is the
highest dam in the work
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15. which is a composite earth and
rockfill type embankment dam
antf it is 335 m high.
In India, Tehri dam across
river
Bhagirathi
is
the
highest dam which is also a
compositie earth and rockfill dam
type embankment dam and it is
261 m high.
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16. Embankment Classified
Earthen
embankment
dam
Rockfill dam
Composite
dam
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17. EARTHEN EMBANKMENT
CLASSIFIED
METHOD OF
CONSTRUCT
ION
HYDRAULIC
FILL
ZONED TYPE
DIAPHRAM TYPE
SEMI
HYDRAULI
C FILL
EAR
THE
N
DAM
FOUNDAT
ION
AVAILABL
E
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ROLLED
FILL
HOMOGENE
OUS TYPE
ONLY PERVIOUS
MATERIAL
AVAILABLE
ONLY IMPERVIOUS
MATERIAL
AVAILABLE
BOTH IMPERVIOUS AND
PERVIOUS MATERIAL
AVAILABLE
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19. Method Of Construction
• Rolled fill
HOMOGE
NEOUS
DAM
DIAPHR
AM DAM
ZONED
EMBANKM
ENT DAM
• Hydraulic fill
• Semi hydraulic fill dam
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20. Rolled fill
• A rolled filled dam is the one which is
constructed in successive, mechanically
compacted
layers.
The
material
(sand, clay, gravel, etc.) is transported from
the borrow Pits to the dam site by trucks or
scrapers.
• It is then spread within the dam section by
Bulldozers to form layers of 15 to 45 cm
thickness. Each layer is then thoroughly
compacted bonded with the preceding layer
by means of power operated rollers of proper
design weight.
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21. Usually sheep foot rollers and heavy
pneumatic-tyred rollers are used. For
proper compaction the moisture
content of the material should be near
that for OMC (Optimum Moisture
Content), for which required quantity
of water is sprinkled on each layer of
soil during compaction.
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22. Hydraulic fill
• In the case of hydraulic fill dam the
transportation of the material from
borrow pits the dam site as well as its
placement in final position in the dam is
done by water.
• In this case at the borrow pits the material
is mixed with water to form slush which is
transported through flumes or pipes and
deposited near the faces of the dam.
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23. • The coarser material of the slush stay near
the faces of the dam while the finer'ones
moves towards centre and get deposited
there.
• This would provide a dam section with
shoulders of coarse-free draining material
and an imperious central core of fine
grained material
• as clay and silt. In this method no
compaction is required.
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24. HYDRAULIC FILL DAM
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25. Semi-hydraulic fill dams :
In semi-hydraulic fill dams, the coarse
material is dumped from trucks as in
case of rolled fill dam without use of water.
Some of dumped material is then moved to
its final position in the dam by action of
water.
For this jet of water are directed on
dumped fill which causes finer material
from fill near faces of dam to be washed
away.
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26. The finer material moves towards the
central position of the dam and is
deposited there thus forming an
impervious central core while the
coarser material stays near the face of
dam.
However absence of proper jetting
action the dumped fill faces of dam may be
more dense and impervious than the
material immediately below it which may
result in failure.
In this case no compaction is done.
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27. SEMI HYDRAULIC FILL DAM
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28. ROLLED FILL DAM
HOMOGENEOUS
DAM
ZONED
EMBANKMENT
DAM
DIAPHRAM
DAM
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29. HOMOGENEOUS DAM
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33. HOMOGENEOUS TYPE DAM
A purely homogeneous type earth dam is
composed of a single kind of
material, dams are usually built of
impervious or semi-impervious soils, but
several successful have been built of
relatively pervious sands and sand
gravel mixture.
These dams are used only for low to
moderate heights
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34. A purely homogeneous section has been
replaced by a modified homogeneous
section
in which internal drainage system in the
form of a horizontal drainage blanket
or rock toe or a combination of both
is provided.
By providing drainage system, dams with
much steeper slopes may be designed
and also the phreatic line (or top seepage
line) is kept well within the body of the
dam.
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35. Zoned type dam :
• A zoned embankment dam is composed of
more than one kind of material. This is most
common type of a rolled fill dam in which a
central impervious core is flanked by zones of
materials considerably more pervious.
• The pervious zones enclose, support and
protect the impervious core. The upstream
pervious zone provides free drainage during
sudden drawdown and the downstream
pervious zone acts as a drain to control the
phreatic line
• The impervious core checks the seepage.
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36. PERVIOUS
SHELL
Transition
Filter
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39. The pervious zones may consists of
sand, gravel, cobbles or rock or
mixtures of these materials while the
core consists of impervious soil such as
clay, silt, etc.
In general, dam is considered as
zoned embankment type dam if the
horizontal width of impervious core
at any elevation equals or exceeds the
height of the dam above in the dam and
is not less than 3 m.
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41. Diaphragm embankment type :
In a diaphragm type earth dam the bulk of
the dam is constructed of pervious
materials, (i.e. sand, gravel or rock) and a
thin core usually known as diaphragm of
impervious material is provided to check
seepage.
As such these dams are also sometimes
known as thin core dams
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42. The position of the diaphragm may
vary from a central vertical core to a
blanket directly on the u/s face of the
dam and in between these extreme
positions
there
may
be
several
intermediate
positions
of
the
diaphragm in which an inclined
diaphragm sloping upstream may be
provided.
The inclined diaphragm is also sometimes
known as burried blanket.
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43. Diaphragm type dam : -
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44. • Scan p.n.modi page no 448
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45. Shear strength of Soils :
Coulomb's law :
The resistance offered by a soil mass against
the shearing forces, is known as shear
strength of the soil. In order to ensure
that the foundation soil and the soil of
the body of the dam are stable, we
must ensure that the shearing strength
of this soil mass is sufficiently greater than
the shear stresses developed within the soil
mass by external forces.
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46. According to Coulomb's law, the shear
strength of soil mass is provided by two
factors:
(1) Cohesion between the soil grains.
(2) Internal friction between the soil
particles.
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47. The Coulomb's law for shear strength is
given by :
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48. Void ratio (e) :
Porosity (n) :
Bulk unit weight (y to yb) :
Dry unit weight (YD) :
Saturated unit weight (Ysat) :
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49. Submerged density (ysub or y') :
Degree of saturation (Sr) :
When soil is fully saturated, voids are
completely filled with water. There is no
air,
Sr = 1
When soil is fully dry, voids are completely
filled with air. There is no water, Sr = 0
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50. Pore water pressure and its
significance in the desing of
Earth Dams :
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51. Saturated sediments are more
likely to undergo mass wasting
because
pore
pressure
decreases the frictional force.
in
a
saturated
sediment
the
weight of all the
water
above
produces a pore
pressure
that
tends to push the
grains apart
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52. the effective
normal stress
is the lithostatic
stress (from the
weight of the
sediments in
grain on grain
contact) minus
the pore pressure
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53. total effective pressure:
y1
y
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54. • Soil mass contain some voids or pores
which are partly or fully filled with
water, let us consider a soil mass
below the water table BB as shown
in fig.
• The soil mass below water table BB
is fully saturated and the voids are
fully filled up with water . Thus soil
below water table is subjected to
hydrostatic uplift.
• Hence as explained above
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55. Now as we know total normal pressure =
density x height = p =wh
At level CC
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56. • It, therefore, become evident that the
effective normal stress is much less than
the total normal stress, as a part of the
total stress gets consumed by water as
pore pressure.
• The pore water pressure acts equally in
all directions. It does not press the soil
grains in its frictional resistance, that is
why, it is called a neutral stress.
• When the pore water pressure is
considered, the Coulomb's law will become
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57. This clearly indicates that the
shear strength of soil gets
reduced due to the presence
of pore pressure. The pore
water pressure gets developed
in the body of the earthen
dam when seepage takes
place through the body of the
dam, thus reducing the shear
st
strength of the soil
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