The main components of an earth dam are as follows : 1. Impervious core 2. Pervious shell 3. Filter 4. Rock toe 5. U/S slope protection 6. D/S slope protection 7. Cutoff core shouldnot be less than 3 m and its height should be 1 m more than the maximum water levelin the reservoir. The upstream pervious zone provides free drainage during sudden drawdown. , Usually following types of filters are provided : (1) Toe filter (2) Horizontal drainage filter (blanket) (3) Chimney drains Such a filter is sometimes known as inverted filter or reverse filter. Rock toe keeps the phreatic line well within the section and also facilitates drainage. The following measures are taken to protect the slope. (1) Rock riprap (2) Concrete pavement (3) Steel facing (4) Bituminous pavement (5) Precast concrete blocks Cut off is required to (1) reduce loss of stored water through foundation and abutments (2) Prevent sub surface erosion by piping. Cutoff may be provided in the following ways : • by providing concrete cutoff wall • by providing cutoff trench filled with impervious material • by driving sheet pile • by curtain grouting

1. EMBANKMENT DAM
DESIGN OF HYDRAULIC STRUCTURE
MODULE 1
LECTURE 2 ON 10 45
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prepared by V.H.KHOKHANI, ASSISTANT
PROFESSOR, DIET.
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2. COMPONENTS OF EARTHEN
DAM
The main components of an earth dam
are as follows :
1. Impervious core
2. Pervious shell
3. Filter
4. Rock toe
5. U/S slope protection
6. D/S slope protection
7. Cutoff
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3. 1/8/2014
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4. Impervious core :
The impervious core at the centre
checks seepage through the dam. It
is constructed of clay, silt, silty
clay or clayey silt. At the top of top
width of impervious core should
not be less than 3 m and its
height should be 1 m more
than the maximum water level
in the reservoir.
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PROFESSOR, DIET.
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5. Pervious shell :
The pervious shell gives stability to the
dam. It consists of sand, gravel or a
mixture of these materials.
The upstream pervious zone provides
free drainage during sudden
drawdown. The down stream
pervious zone acts as a drain to
control the phreatic line.
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PROFESSOR, DIET.
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6. FILTER Filter zones are invariably
provided in all earth dams.
They are constructed of pervious
materials. Provision of filter
zones at d/s side serves the
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PROFESSOR, DIET.
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7. following functions :
 to drain off the water seeping
through earth dam safely.
To reduce the pore water pressure
in the downstream portion of the
dam.
 To keep the phreatic line within the
body of the dam.
To check the piping by checking
migration of the particles.
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8. Usually following types of
filters are provided :
(1) Toe filter
(2) Horizontal drainage filter
(blanket)
(3) Chimney drains
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9. Filter criteria :
Generally, a multilayered filter is
provided in which each Subsequent
layer becomes increasingly coarser
than the previous one. Such a
filter is sometimes known as
inverted filter or reverse filter.
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10. The soil to be protected is
known as base material.
 Basic requirements of filter material
are :
(i) The filter material should be
sufficiently fine and properly graded
so that the voids of the filter are small
enough to prevent base material
particles from penetrating and
clogging the filter.
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11. • (ii) The filter material should be
sufficiently coarse and
pervious compared to the base
material so that the incoming
water is rapidly removed without
any appreciable build up of
seepage forces within the filter.
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12. (iii) The filter material should be
coarse enough not to be carried
away through the drainage pipe
openings.
(iv) The filter layer should be
sufficiently thick to provide a
good distribution of all particle
size throughout the filter and to
be able to carry the seepage
discharge.
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13. The Filter material should satisfy the
following two criteria.
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14. Rock toe :
At the d/s end of an earth dam a rock
toe is provided by arranging 15 to 20 cm
size stones. The height of rock toe is
usually kept 30 to 40% of the reservoir
head.“
Rock toe keeps the phreatic line well
within the section and also facilitates
drainage.
It also prevent failure of d/s of the dam
due to piping or sloughing.
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15. u/s slope protection :
The u/s slope should be protected against
the wave action and the d/s slope should
be protected against rain.
The following measures are taken to protect
the slope.
(1) Rock riprap
(2) Concrete pavement
(3) Steel facing
(4) Bituminous pavement
(5) Precast concrete blocks
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PROFESSOR, DIET.
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17. Cut off :
Cut off is required to
(1) reduce loss of stored water
through foundation and
abutments
(2) Prevent sub surface erosion
by piping.
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18. • As per IS : 8826-1978
recommendations :
(1) The alignment of the cut off
trench should be fixed in such
a way that its center line
should be within the u/s base
of the impervious core and it
should be keyed into rock.
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19. (2) The bottom width of cutoff
trench may be fixed taking
following factors into
considerations :
• Provide sufficient working space
to carry curtain grouting.
• Provide sufficient working space
for compaction equipment.
• Provide safety against piping.
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20. Cutoff may be provided in the
following ways :
• by providing concrete cutoff wall
• by providing cutoff trench filled
with impervious material
• by driving sheet pile
• by curtain grouting
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prepared by V.H.KHOKHANI, ASSISTANT
PROFESSOR, DIET.
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