Embankment lecture 3

EMBANKMENT
DAM
• LECTURE 3
• DESIGN OF HYDRAULIC
STRUCTURE
1/9/2014

PREPAREDBY V.H.KHOKHANI, ASST.
PROFESSOR, DIET

1

FAILURES OF EARTHEN
DAM

Hydraulic failures .... 40%
Seepage failures…….. 30%
Structural failures .... 30%

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Hydraulic failures .... 40%
(1) Overtopping
(2) Erosion of u/s slope by
waves
(3) Erosion of d/s slope by
wind and rain
(4) Erosion of d/s toe
(5) Frost action
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(1) Overtopping
the design flood is under estimated.
spillway capacity is not adequet
spillway gates are not properly
operated
free board is not sufficient
excessive settlement of the
foundation and dam
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(2) Erosion of u/s slope by
waves
The waves developed near the top water
surface due to the winds, try to notch out
the soil from the upstream face and may
even, sometimes, cause the slip of the
upstream slope.
Upstream stone pitching or riprap should,
therefore, be provided to avoid such
failures.
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Wave Erosion

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(3) Erosion of d/s slope by
wind and rain
• In the absence of proper slope
protection the downstream
slope of an earth dam may get
eroded due to rain or high
winds.

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• The rainwater flowing down the
slope; may result in the formation
of 'gullies' on the
downstream slope thus
damaging the dam which may
generally lead to partial failure of
the dam or in some cases it may
cause complete failure of the dam.
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This can be avoided by proper
maintenance, filling the cuts
from time to time during rainy
season,
by grassing the d/s slope and
by providing proper berms at
suitable heights so that water
has not to flow for considerable
distance.
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Berms on d/s Slope

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Toe erosion may occur due to two
reasons :
 erosion due to tail water
 erosion due to cross
currents that may come
from spillway buckets.
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• The toe erosion can be
avoided by providing a
downstream slope
pitching or a riprap upto
a height just above the
normal tail water depth.

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• Side walls of the Spillway
(called diaphragm walls)
must be of sufficient height
and length, so as to prevent
the possibility of the cross
currents towards the
earthen embankment.
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Frost action :
• If the earth dam is located at a place
where the temperature falls below
the freezing point, frost may form in
the pores of the soil in the earth
dam.
• When there is heaving, the cracks
may form in the soil. This may lead
to dangerous seepage and
consequent failure.
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Seepage failures :
Seepage failures may occur due to the
following causes :
(1) Piping through the foundation
(2) Piping through the dam
(3) Sloughing of d/s toe

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Piping through the foundation :
When highly permeable cavities or
fissures or strata of coarse sand Or
gravel are present in the foundation
of the dam, water may starts seeping
at a huge rate through them. This
concentrated flow at a high gradient
may erode the soil.
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• This leads to increased flow of
water and soil, ultimately resulting in
a rush of water and soil, thereby
creating hollows below the
foundation. The dam may sink into
the holloWs so formed, causing its
failure.
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Piping through foundation

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Piping through the dam :
• Piping is the progressive
backward erosion which may
be caused through earth dam or
its foundation, by the water
seeping through the dam or its
foundation. It begins at a point
where the water seeping
through the dam emerges on
downstream side of the dam.
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Piping through the dam :

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Piping in the dam may occur due to the
following causes :
Poor construction control - dam is not
properly compacted,
poor bond between the successive
layers of the embankment or between
the embankment material and the
foundation.
Burrowing animals - like crocodiles or
musk rats, may dig holes in the dam.
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Presence of roots, pockets of gravel or
boulders in the dam.
Differential settlement - cracks may
develop in the dam due to differential
settlement in the foundation of the
dam.
Soluable salts - if there are soluable
salts in the soil, they get leacked out due
to which hollows are created in the soil,
which may lead to failure.
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3. Sloughing of d/s toe :
• Sloughing is a process in which the
soilmass falls down after saturation. It
is a type of local slope failure. The
sloughing of the d/s toe of the dam
occurs under the reservoir full
condition. The failure due to sloughing
starts when the d/s toe becomes
saturated and eroded, producing a
small slump or miniature slide
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• The miniature slide leaves a
relatively steep face which becomes
saturated by the seepage from the
reservoir and slump again, forming
a more unstable surface.
• The process is continues till the
remaining portion of the dam is too
thin to withstand the horizontal
water pressure, leading to the
sudden failure of the dam.
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Structural failures :
• Structural failures in earth dams
are generally shear failures
leading to sliding of the tents or
the foundations.

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Structural failures in the earth dams
are of the following types :
(1) u/s and d/s slope failures due to
construction pore pressures
(2) u/s slope failure due to sudden
drawdown
(3) D/s slope failure due to steady
seepage
(4) Foundation slide due to spontaneous
liquefaction
(5) Failure due to earthquake
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(6) Failure by spreading
(7) Slope protection failures
(8) Failure due to damage caused
by borrowing animals
(9) Failure due to holes caused by
leaching of water soluable salts
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u/s and d/s slope failures due to
construction pore pressures :
• When an earth dam is constructed of
relatively impervious soil, the
drainage extremely slow. The pore
water pressure develops during and
immediately after construction,
especially if the rate of construction
is relatively fast.
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• The pore water pressure upto 100%
of the vertical stress due to weight of
soil at that point has beer observed
in the field.
• The slope may fail during
construction if it is not designed for
such a high pore water pressure.

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(2) u/s slope failure due to sudden
drawdown :
The most critical condition for the u/s
slope occurs during the sudden
drawdown of the reservoir, when the
reservoir is suddenly emptied,
the hydrostatic force acting along the u/s
slope at the time of full reservoir is
removed,
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• During this condition the pore water
pressure in the soil is not fully
dissipated due to low permeability of
the slope and consequently the shear
strength of the soil is reduced and It
may fail.

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Sudden draw down

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(3) D/s slope failure due to steady
seepage :
• Critical condition for d/s slope occurs
when the reservoir is full and
percolation is at its maximum rate.
The direction of seepage forces tend
to decrease stability.

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There are two types of d/s slides :
deep slides
shallow slides
The pore water pressure acting on the
soil below the phreatic line causes a
reduction in the shear strength of the
soil, the failure of d/s slope may occur.

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(4) Foundation slide due to
spontaneous liquefaction :
If the foundation consists of fine
sand silt in loose condition,
liquefaction may occur
when the dam is subjected to
vibrations. In liquefaction, a large
part of the weight of the overlying
soils mass is momentarily carried by
the pore water pressure
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there is very little intergranular (or
effective) shear strength, which
depends upon the intergranular
pressure is reduced to almost zero
and
the soil flows like a liquid in the
downward direction in the
foundation.
Hence these slides are termed as
flow slider or liquefaction slides.
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(5) Failure due to earthquake :
Most of the failures due to
earthquake have occurred only with
respect to those
which were constructed before 1920
with inferior design details and
construction methods. '
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Some of the damages which may be
caused to earth dams due to
earthquake as under :
(a) Longitudinal cracks at the top of
the dam or in the core.
(b) Liquefaction of loose and
saturated soil mass in the lower
portion of the dam.
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(c)- Settlement of the crest of the dam
thus reducing the freeboard and
increase the possibility of
overtopping.
(d) Shear failure at the base of the
dam.
(f) Rock slide from the side hills into
the reservoir.
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(6) Failure by spreading :
• Failures by spreading have been
observed only in connection with fills
located above stratified deposits
that contain layers of soft clays.
Outstanding failures due spreading
are,
• Lafagets Dam in California (1928)
• Marshall Creek Dam in Kansas (1927)
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(7) Slope protection failures :
Slopes are generally protected by riprap
over a layer of gravel or filter
During a heavy storm, the waves on the
surface of the reservoir beat repeatedly a
the slope just above the reservoir level.
The wave energy is dissipated in
turbulent action on and within the rocks
of the riprap layer.
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• These waves have two effects :
(a) The waves may pass through the
voids of riprap and may wash away
the filter layer, exposing the
embankment to wave erosion.
(b) If the average size of rock
comprising riprap is not heavy, it
may be washed out of the layer by
the hydraulic forces generated by the
waves.
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(8) Failure due to damage caused by burrowing
animals :
• Burrowing animals like muskrats burrow into
embankment either to make homes or to dig
passage from one pond to another. If many
muskrats are involved, their holes may
dangerously honeycomb a small earth dam,
making it extremely weak.
• Sometimes squirrels also dig holes in the
embankment. Due to these holes piping failure of
dam will occur.
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(9) Failure due to holes caused by
leaching of water soluable salts :
• Leaching of water soluble salts such as,
gypsum, iron oxide, etc. from
embankment and foundation soils may
cause cavities, which may lead to the
excess settlement of the dam.
Moreover, these salts may get deposited
in the toe filter may choke it, thus
causing drainage problems.
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Criteria for safe Design of Earth Dam :
For the safe design of an earth dam the following
basic criteria should be satisfied 1. No overtopping :

• The dam should be safe against overtopping
during occurrence of the worst flood
• An adequate free board should be provided.
• A suitable allowance should be made in the
height of the dam to account settlements.
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2. No seepage failure :
• The phreatic line should remain well
within the d/s face of the dam so that
sloughing of the downstream face
occurs.
• Seepage through the body of dam,
foundations and abutments should be
control! against suitable measures.
• The dam and foundation should be
safe against piping failure.
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• 3. No structural failure :
• • The u/s slope should be safe against sudden
drawdown conditions.
• • The d/s slope should be safe during steadyseepage conditions.
• • The d/s and d/s slopes should be safe during
and immediately after construct
• • The foundation shear stresses should be
within the safe limits.
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• 4. Slope protection :
• • The u/s slope should be protected against
erosion by waves.
• • The d/s slope should be protected against
erosion due to rain and wind.
• 5. The portion of the dam d/s of the impervious
core should be properly draine
• 6. The dam should have an economic section. As
far as possible locally available mate
• should be used to reduce the cost.
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• Section of an Earth Dam :
• The design of an earth dam essentially
consists of determining such a cross section
• the dam which when constructed with the
available materials will fulfill its required
• tion with adequate safety. Thus there are two
aspects of the design of an earth dam.

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• (1) to determine the cross section of the dam.
• (2) to analyse the stability of the proposed
cross-section.
• The cross section of an earth dam depends on
the following factors :
• (1) Foundation conditions.
• (2) Availability of materials.
• (3) Physical properties of various materials.
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• (4) Method of construction and degree of
construction control.
• (5) Types of earth moving machinery.
• (6) Diversion methods and construction schedule.
• (7) Climatic conditions affecting the placement
moisture control and subsequent
• moisture changes.
• Safety factor with respect to seepage.
• Safety factor with respect to stability.
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• On the basis of these factors the cross section
of an earth dam may be determined
• involves the consideration of the following
parameters.
• I. Crest width
• Free board
• u/s and D/s slopes
• Settlement allowance
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•
•
•
•
•
•

Casing or outer shells
Central impervious core
Cut-off trench
Downstream drainage system
width : *
crest width (i.e. top width) of an earth dam
depends upon the following factors :
• Nature of the embankment materials
• Height of the dam
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•
•
•
•

• Importance of the dam
• width of the roadway on the top of the dam
• Practicability of construction
• Protection against earthquake forces

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The following empirical formulae are
commonly used to determine the top
bidth (a)
of the earthdam.

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• In general, the crest width varies from 6 to 12
m, the larger values are for
• higher and more important dams. In no case,
the crest width should less than 4 m, for
• maintenance work.

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• 2. Free board :
• Free board is the difference in the elevation of
the crest of the dam and the still
• water level in the reservoir. Sufficient free board
must be provided so that there is no
• possibility of the dam being overtopped.
• The freeboard may be classified as :
• • Normal freeboard
• • Minimum freeboard
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• The freeboard above the Full Reservoir Level
(FRL) is known as normal freeboard
• The freeboard above the Maximum Water Lelvel
(MWL) is known as minimui
• freeboard.
• According to IS : 10635-1983, the freeboard
should be equal to the sum of wave
• run-up and wind set-up. The normal free board
as well as minimum freeboard should
• not be less than 2 m.
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The values of freeboard, for various
heights recommended by U.S.B.R. are
given
in Table-2.1.

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• 3. u/s and D/s slopes :
• The upstream and downstream slopes of the dam
depend upon the type of material,
• foundation conditions, the height of the dam,
and many other factors. No specific values
• can tye given for the slopes. The general practice
is to select the side slopes on the basis
• of the experience gained with similar dams
elsewhere and to check the stability of selected
• slopes.
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fable-2.2 : Side slopes for Earth Dams
(as per Terzaghi)

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• Settlement allowance :
• Settlement of an earth dam may be caused by
consolidation of the soil mass in the
• and in the foundation. It depends on the
character of the soil in the dam and the
• ition and the method of construction used.
• h is the usual practice to consturct the earth
dams to a somewhat greater height than
• •squired height to have a suitable settlement
allowance
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• Generally, a provision of allowance of 1
to 2% of the height of the dam is made.
However, in case
• of height greater than 30 m, an extra
allowance of 1% is made to account for
• lement due to earthquake. The extra
height is provided in the form of a
longitudinal
• over the top of the dam.
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• outer shells :
• function of casing or outer shell is to impart
stability and protect the core. The
• pervious materials, which are not subject to
cracking on direct exposure to
• are suitable for casing.
• 2.3 gives recommendations for suitability of
soils used for earth dams as per
• MB26-1978.
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• 6. Central impervious core :
• A zoned section is usually preferred for an earth
dam. The core may be vertical
• inclined. An impervious core is provided at the
centre of the zoned sections to contr
• the loss of water by seepage through the dam.
Thus the loss of head is localised in t"
• core of the dam and, therfore, the phreatic line
does not cut the d/s face of the dam a~
• cause sloughing of the d/s face.
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• The minimum safe thickness of the central
impervious core depends on the follow1
• factors.
• Tolerable seepage loss.
• Maximum width that will permit proper
construction.
• Types of the materials for the core and the shells.
• Design of the proposed filter layers.
• Precedent on similar projects.
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• It should be noted that the shear strength of core
materials is always lesser than
• rest of the embankment. Hence a thinner shell is
preferable from the point of vieW
• stability. However, a thick core has more resistance to
piping. The governing criteria
• that the thickness of the core at any elevation is not
less than the height of the embanl
• at that elevation so that average hydraulic gradient is
less than unity. The width of the
• at the crest of the dam should be a minimum of 3 m to
permit economical placement
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• compaction of impervious embankment material by
construction equipments. The top
• of the core should be atleast 1 m above the MWL to
prevent seepage by capillary syphon:
• • Suitability of Material for Core :
• The soils having high compressibility and higher liquid
limit are not suitable as;
• are prone to swelling and formation of cracks. Soils
having organic contents are also
• suitbale. Table 2.3 gives the suitability of soils for
impervious core of zoned earth
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• Table 2.4 gives recommendations based on IS :
8826-1978 regarding suitability of soils
• for construction of core for earth dams in
earthquake zones

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Table-2.4 : Suitability of Soils for
Construction of
Core of Earth Dam in Earthquake
Zones

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• Cut-off trench :
• To reduce seepage through the foundation
and to avoid piping failure, a cut of wall
• usually provided when the foundation is
pervious to moderate depth. The centre line
• the cut-off wall is kept parallel to the centre
line of the dam. The side slopes of the
• :h are usually 1.5 : 1 to 1 : 1 [Fig. 2.19(a)]
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• Cut-off is required to
• (1) reduce loss of stored water through
foundation and abutments *
• (2) Prevent sub surface erosion by piping.
• As per IS : 8826-1978 recommendations :
• (1) The alignment of the cut off trench should be
fixed in such a way that its central
• line should be within the u/s base of the
impervious core and it should be keyed
• into rock.
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• (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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• Cut-off 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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• Sheet pile cut-off :
• A sheet pile cut-off can be used in place of a
cut-off trench [Fig. 219(a)]. However,
• if the foundation strata consists of soil mixed
with boulders, the sheet piles would bend
• and will not be effective.
• .

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• Impervious blanket :
• If the foundation is impervious to a very large
depth, a positive cut-off wall up
• the impervious stratum is not practicable. In
that case, an u/s impervious blanket is provi
• on the river bed as an expansion of the central
impervious core of the dam

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• 8. Downstream drainage system :
• 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
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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•
•
•
•

Usually following types of filters are provided :
(1) Toe filter
(2) Horizontal drainage filter (blanket)
(3) Chimney drains

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• Iter criteria :
• Generally, a multilayered filter is provided in which each
subsequent layer becomes
• asingly coarser than the previous one. Such a filter is sometimes
known as inverted
• r or reverse filter. The soil to be protected is known as base
material. Basic
• luirements 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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• (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.
• (iii) The filter material should be coarse
enough not to be carried away through the
• drainage pipe openings.
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• (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.
• The Filter material should satisfy the following
two criteria.

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• 2.9 Cross-section of Earth Dam for Different Situations :
• The section of a zoned earth embankment dam should be
so selected that av
• materials are utilized to the maximum and limited quantity
of other required materi
• imported to the site. •
• • Available materials for dam :
• 1. Gravel or coarse sand is available along with clayey silt
• 2. Only fine gravel or coarse sand is available ,
• 3. Only silty clay is available

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• General Foundation Conditions :
• (a) Foundations impervious to a large depth
• (b) Foundation pervious to moderate depth,
after with impervious strata is av *
• (c) Foundations pervious to large depth.

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Case-1 : Gravel or coarse sand is available
along with clayey silt :
(a) Foundations impervious to a large
depth :

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b) Foundation pervious to a moderate
depth, after which impervious strata is
available :

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(c) Foundation pervious to a great depth
:

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