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Dams
Definition of Dams
Advantages and Disadvantages of Dams
Classification of Dams
Types of Dams
Dams 1
What is a Dam?
A dam is a structure built across a stream,
river or estuary to retain water.
Dams are made from a variety of
materials such as rock, steel and wood.
Dams
Dams
Structure of Dam
Toe
Heel
Sluiceway
Spillway
Freeboard
Gallery
Definitions
Heel: contact with the ground on the upstream side
Toe: contact on the downstream side
Abutment: Sides of the valley on which the structure of
the dam rest
Galleries: small rooms like structure left within the dam
for checking operations.
Diversion tunnel: Tunnels are constructed for diverting
water before the construction of dam. This helps in
keeping the river bed dry.
Spillways: It is the arrangement near the top to release
the excess water of the reservoir to downstream side
Sluice way: An opening in the dam near the ground
level, which is used to clear the silt accumulation in the
reservoir side.
Advantages of Dam
Irrigation
Water Supply
Flood Control
Hydroelectric
Recreation
Dams gather drinking water
for people.
Dams help farmers bring
water to their farms.
Dams help create power
and electricity from water.
Dams keep areas from
flooding.
Dams create lakes for people
to swim in and sail on.
Disadvantages of Dam
Dams detract from natural settings, ruin
nature's work
Dams have inhibited the seasonal migration
of fish
Dams have endangered some species of fish
Reservoirs can foster diseases if not properly
maintained
Reservoir water can evaporate significantly
Some researchers believe that reservoirs can
cause
earthquakes
Three Gorges Dam
Three Gorges Dam
Three Gorges Dam
Type: Concrete Gravity Dam
Cost: Official cost $25bn - actual
cost believed to be much higher
Work began: 1993
Due for completion: 2009
Power generation: 26 turbines on
left and right sides of dam. Six
underground turbines planned for
2010
Power capacity: 18,000
megawatts
Reservoir: 660km long,
submerging 632 sq km of land.
When fully flooded, water will be
175m above sea level
Navigation: Two-way lock system
became operational in 2004.
One-step ship elevator due to
open in 2009.
Three Gorges Dam
Sluice Gates
Three Gorges Dam
Shipping Locks
Shipping Locks
Hoover Dam
Hoover Dam
Location: Arizona and Nevada, USA
Completion Date: 1936
Cost: $165 million
Reservoir Capacity:1.24 trillion cubic feet
Type: Arch/ Gravity
Purpose: Hydroelectric power/flood control
Reservoir: Lake Mead
Materials: Concrete
Engineers: Bureau of Reclamation
The Hoover Dam is a curved gravity dam. Lake Mead
pushes against the dam, creating compressive forces
that travel along the great curved wall. The canyon
walls push back, counteracting these forces. This
action squeezes the concrete in the arch together,
making the dam very rigid. This way, Lake Mead can't
push it over.
Today, the Hoover Dam is the second highest dam in
the country and the 18th highest in the world. It
generates more than four billion kilowatt-hours a
year, that's enough to serve 1.3 million people!
Classification of Dams
Storage Dam
Detention Dam
Diversion Dam
Coffer Dam
Debris Dam
Classification based on function
Classification of Dams
Storage Dam:
1. To impound water to its upstream side During
periods of excess and deficient supply
2. Reservoir or lake is formed
3. Irrigation , water power generation etc
Classification based on function
Classification of Dams
Detention Dam:
1. Water is stored during floods and release
gradually @ safe rate
2. Ist type: water is stored & then released
3. 2nd type: water is not released,
water seeps in pervious banks
Water level in well rises
Lift irrigation is possible
4. Seeping may be sufficient that surface water
Classification based on function
Classification of Dams
Diversion Dam:
1. Raise water level in river & thus provides head
for carrying or diverting water into canals e.g.
weir or barriage
Classification based on function
Classification of Dams
Coffer Dam
Classification based on function
Classification of Dams
Debris Dam:
1.detention dams are constructed across
tributary carrying large silt and sediments
2. Debris dams traps the sediments and thus to
exclude the sediments to flow to the main
reservoir formed on main river.
Classification based on function
Classification of Dams
Classification based on hydraulic design
Classification based on material of construction
Overflow Dam/Overfall Dam
Non-Overflow Dam
Rigid Dam
Non Rigid Dam
Classification of Dams
Classification based on structural behavior
Gravity Dam
Arch Dam
Buttress Dam
Embankment Dam
Gravity Dam
Gravity dams are dams which resist
the horizontal thrust of the water
entirely by their own weight.
Concrete gravity dams
are typically used to
block streams through
narrow gorges.
Gravity Dam
Cross Section Plain View
Material of Construction:
Concrete, Rubber Masonry
Gravity Dam
ADVANTAGES
• External forces are resisted by weight of dam
• More strong and stable
• Can be used as overflow dams also with spillway feature
• Highest dams can be made as gravity dams cuz of its high
stability
• Specially suited for heavy downpour; slopes of earthen dams
might get washed away
• Less maintenance required
• Gravity dam does not fail suddenly but earthen dams
Gravity Dam
DISADVANTAGES
• Can be made only on sound rock foundation
• Initial cost is high
• Takes more time to construct if materials are not available
• Requires skilled labour
Arch Dam
An arch dam is a curved dam
which is dependent upon arch
action for its strength.
Arch dams are thinner and
therefore require less
material than any other
type of dam.
Arch dams are good for sites
that are narrow and have
strong abutments.
Arch Dam
Cross Section Plain View
Material of Construction:
Concrete
Arch Dam
Curved in plan
Carried load horizontally to its by arch action
Balance of water load is transferred to the foundation by cantilever
action
Advantages
adapted in gorges where length is small in proportion to
height
dam require less material
can be made in moderate foundation cuz of load
distribution as compared to gravity dams
Arch Dam
Curved in plan
Carried load horizontally to its by arch action
Balance of water load is transferred to the foundation by cantilever
action
Disadvantages
require skilled labor
speed of construction is slow
require strong abutments of solid rock of resisting arch
thrust
Buttress Dam
Buttress dams are dams in which the
face is held up by a series of
supports.
Buttress dams can take many
forms - the face may be flat or
curved.
Buttress Dam
Cross Section Plain View
Material of Construction:
Concrete, Timber, Steel
Buttress Dam
• Retain water between buttress
• Less massive than gravity Dams
• Ice pressure: ice tends to slide over the inclined U/S
so this factor is unimportant
• when future increase in reservoir; Future extension is
possible by extending buttress and slab
• Power house can be made B/W buttress; thus
reducing cost
• Can be designed to accommodate moderate
movement of foundation without any serious damage
Buttress Dam
Disadvantages
skilled labor requirements
deterioration of u/s as very thin concrete face
Embankment Dam
Embankment dams are massive
dams made of earth or rock.
They rely on their weight to
resist the flow of water.
Embankment Dam
Cross Section Plain View
Material of Construction:
Earth, Rock
Earth and rockfill Dam
• Made of locally available soil & gravels
• Can be made on any type of available foundation
• Can be constructed rapidly
• Cheaper
• Future consideration can be made (raising height)
Disadvantages
• Vulnerable to damage by floods
• Cannot be used as overflow dams
not suitable where heavy downpour is more common
• High maintenance cost
Types of Dam
Factors governing selection of types of dam
A Narrow V-Shaped Valley : Arch Dam
(top width of valley less than ¼ th of
height)
A Narrow or Moderately with U-Shaped
Valley : Gravity/Buttress Dam
A Wide Valley : Embankment Dam
Rolling plain: earth dam
Topography-Valley Shape
Types of Dam
Factors governing selection of types of dam
Solid Rock Foundation : All types
Gravel and Coarse Sand Foundation :
Embankment/Concrete Gravity Dam
Silt and Fine Sand Foundation :
(earth)Embankment/ low concrete Gravity
Dam but not rockfill
Clay foundation: earth dams
Geology and Foundation Condition
Types of Dam
Factors governing selection of types of dam
• if large spillway capacity is required;
overflow concrete dam
• if small spillway capacity is required;
earth dam with separate site for spillway
• In case of earth dam, where no other site
of spillway is available, Earth dam with
central concreting of spillway may be
preferred
Spillway size and location
Types of Dam
Factors governing selection of types of dam
Climate conditions
Availability of construction materials
Environmental considerations
Overall cost
General considerations
Communication road link, rail roads
Locality: free from mosquitoes as labor
and staff colonies are constructed
Gravity Dam
Gravity Dam
Forces on Gravity
Dam
1. Weight of the dam
2. Water pressure
3. Uplift pressure
4. Wave pressure
5. Earth and Silt pressure
6. Earthquake forces
7. Ice pressure
8. Wind pressure
9. Thermal loads.
Dams 1
Gravity Dam
• Dead load = weight of concrete or masonry or both +
weight of such appurtenances as piers, gates and
bridges.
• Unit weight of concrete (24 kN/m3)
• For convenience, the cross-section of the dam is
divided into simple geometrical shapes.
• Thus the weight components W1, W2, W3 etc. can be
found along with their lines of action.
Gravity Dam
Forces on Gravity Dam
Gravity or weight of dam
W
When W = Weight of dam
 = Specific weight of material
 = Volume of dam
Weight of Dam
Gravity Dam
Forces on Gravity Dam
Water Pressure
• Water pressure on the upstream face is the main
• destabilizing (or overturning) force acting on a gravity dam.
• Tail water pressure helps in the stability.
• The water pressure always acts normal to the face of dam.
Gravity Dam
Forces on Gravity Dam
Free-body diagram of cross
section of a gravity dam
Gravity Dam
Forces on Gravity Dam
Water Pressure
Gravity Dam
Forces on Gravity Dam
Water Pressure
Gravity Dam
Forces on Gravity Dam
Water Pressure
the weight of water is found in two parts
PV1 and PV2 by dividing the trapezium ABCD
into a rectangle BCDE and a triangle ABE.
Thus the vertical component PV = PV1 + PV2 =
weight of water in BCDE + weight of water in
ABE.
The lines of action of PV1 and PV2 will pass
through the respective centroids of the rectangle
and triangle.
Gravity Dam
Forces on Gravity Dam
Uplift Pressure
Water has a tendency to seep through the pores and
fissures of the material in the body of the dam and
foundation material, and through the joints between the
body of the dam and its foundation at the base.
The seeping water exerts pressure.
The uplift pressure is defined as the upward pressure of
water as it flows or seeps through the body of dam or its
foundation.
Gravity Dam
Forces on Gravity Dam
Uplift Pressure
A portion of the weight of the dam will be supported on the upward pressure of
water; hence net foundation reaction due to vertical force will reduce.
The area over which the uplift pressure acts has been a question of investigation
from the early part of this century.
One school of thought recommends that a value one-third to two-thirds of the
area should be considered as effective over which the uplift acts.
The second school of thought, recommend that the effective area may be taken
approximately equal to the total area.
Gravity Dam
Forces on Gravity Dam
uplift Pressure
Gravity Dam
Forces on Gravity Dam
uplift Pressure
in case of
drainage
gallery
Gravity Dam
Forces on Gravity Dam
Silt Pressure
IS code recommends that
a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360
kg/m3, and
b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925
kg/m3.
The gradual accumulation of significant deposits of fine sediment, notably silt, against the face of the dam
generates a resultant horizontal force, Ps.
a) Submerged unit weight of silt
b) Angle of internal friction
c) Height to which silt is deposit
Gravity Dam
Forces on Gravity Dam
Ice Pressure
Ice is subjected to expansion and contraction due to temperature variations
􀀹magnitude of forces varies b/w 250 kN/m2- 1500kn/m2 applied to the face of dam
over the anticipated area of contact of ice with the face of dam.
􀀹The problem of ice pressure in the design of dam is not encountered in India except,
perhaps, in a few localities.
Gravity Dam
Forces on Gravity Dam
Wind Pressure
Wind pressure does exist but is seldom a significant factor in
the design of a dam.
Wind loads may, therefore, be ignored.
Gravity Dam
Forces on Gravity Dam
thermal Pressure
􀀹Even the deflection of the dam is maximum in the morning and it goes on reducing to a
minimum value in the evening.
􀀹Measures for temperature control of concrete in solid gravity dams are adopted during
construction.
􀀹 Thermal are not significant in gravity dams and may be ignored.
Gravity Dam
Forces on Gravity Dam
Wave Pressure
The upper portions of dams are subject to the impact of waves.
􀀹Wave pressure against massive dams of appreciable height is usually of little
consequence.
􀀹The force and dimensions of waves depend mainly on the extent and configuration of
the water surface, the velocity of wind and the depth of reservoir water.
􀀹The height of wave is generally more important in the determination of the free board
requirements of dams to prevent overtopping by wave splash.
􀀹 An empirical method has been recommended by T. Saville for computation of wave
height hw (m), which takes into account the effect of the shape of reservoir and wind
velocity over water surface rather than on land by applying necessary correction.
Gravity Dam
Forces on Gravity Dam
Wave Pressure
Wind velocity of 120 km/h over water in case of normal pool condition and
80 km/h over water in case of maximum reservoir condition should generally be assumed for
calculation of wave height if meteorological data is not available.
􀀹Sometimes the following Molitor’s empirical formulae are used to estimate wave height
H= height of waves in meters
V= wind velocity in km/h
F=straight line of water expanse in km
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Seismic coefficient represents max. earthquake acceleration values
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Horizontal· acceleration causes two forces:
(1) Inertia force in the body of the dam, and
(2) Hydrodynamic pressure of water.
• Inertia forces(energy required to move or acc. The object):
The inertia force acts in a direction opposite to the acceleration
imparted by, earthquake forces and is equal to the product of
the mass of the dam and the acceleration.
• For dams up to 100 m height ,
• at the top of the dam
• horizontal seismic coefficient =1.5 times seismic coefficient αh
then reducing linearly to zero at the base
• It causes an overturning moment about the horizontal section
adding to that caused by hydrodynamic force.
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Dams 1
Dams 1
Gravity Dam
Forces on Gravity Dam
Earthquake forces
Dams 1
Dams 1
Dams 1
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Value generally varies b/w 2 to 3
Compression or crushing
•
• ∑V= total vertical force
• b= base width
• e= eccentricity of the resultant force from the centre of
the base
• The positive sign will be used for calculating normal
stress at the toe, since the bending stress will be
compressive there, and
• negative sign will be used for calculating normal stress at
the heel.
Allowable compressive stresses of dam material= 3000kN/m2
If pmin exceeds this , dam may fail by crushing
If pmin comes out –ve, or e>b/6, tension will produce
pmin pmax
Cc- normal stress
Cbc- bending stress
Dams 1
Dams 1
Principal stress at u/s or at heel
Dams 1
A dam will fail in sliding at its base, or at any other level, if the
horizontal forces causing sliding are more than the resistance
available to it at that level.
The resistance against sliding may be due to friction alone, or
due to friction and shear strength of the joint.
Externat horizontal forces < shear resistance
Or μΣV/Σ H > 1
sliding
This represents Factor of safety and is always > 1
Dams 1
Reservoir is full, three forces act on dam i.e P, W and U and the resultant of
these forces pass through the outer most middle third point.
If C=0 i.e no uplift considered
If C=0 i.e no uplift considered
Dams 1
Dams 1
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Multiple step method of design
of gravity dam
• For economical design, dam is divided into
various zones
Dams 1
Zone 1:
• ice sheet exist
• Controlled by free board
• Width is determined by economical and
practical consideration
Dams 1
Zone 2
• u/s and d/s remain vertical
• For full reservoir case, resultant force
passes thru outer third point of plane CC1
• Empty reservoir case, resultant force lies
within middle third
Dams 1
Zone 3
• u/s face vertical and d/s face inclined
Dams 1
Zone 4
• Upstream face begins to batter such that
line of resultant lie along corresponding
extremities of middle third
• Plane ee1 is governed by criteria such that
maximum inclines pressure @ d/s toe for
reservoir full condition equal to allowable
limit
• Design of zone 4 ; by dividing zone into
number of blocks till bottom zone 4 is
reached
Dams 1
Zone 5
• d/s slope is flattened so that maximum
inclined pressure @ d/s toe under
reservoir full condition remains within
working stress i.e. resultant of forces lies
within middle third
Dams 1
Zone 6
• Conditions of designed are determined by
maximum pressure@ both u/s and d/s
faces under reservoir empty and full
conditions.
• Line of resultant under both conditions lie
well within middle third
Dams 1
Zone 7
• Max compression at d/s toe exceeds
working limit
• Zone is usually eliminated
• If height of Dam is so large that it exceeds
zone 6, various changes are made in
upper zones so that height lies till zone 6.
e.g. using superior materials or height of
the dam is reduced
Embankment Dam
Earth-Fill Embankment Dam
A earth-fill dam in
Australia.
Embankment Dam
Rock-Fill Embankment Dam
Embankment Dam
Earth Dams:
• most simple and economic (oldest dams)
• built of natural materials.
• constructed with low-permeability soils to a nominally homogeneous
profile (single material)
• 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. 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
Embankment Dam
Homogeneous Embankment Dam
Embankment Dam
simple zoning
finer more cohesive soils placed adjacent to the impervious core
element
coarser fill material towards either face.
Central core checks the seepage
Transition zone prevent piping through cracks (that may develop
in the core)
Outer zone gives stability to the central impervious fill
Clay with fine sand as material of impervious core
Coarse sand gravel as outer shell
Transition filters are provided in between these 2 zones when
there is abrupt change in zones
Embankment Dam
Zone-Based Embankment Dam
Embankment Dam
Diaphragm Earth Dam
Thin impervious core(diaphragm) surrounded by earth or rockfill
Core made up of impervious soil, concrete, steel timber etc
Water barrier to prevent seepage through the dam
Core rest on impervious foundation material to avoid excessive
underseepage
Embankment Dam
Diaphragm Earth Dam
Embankment Dam
Types:
1.Homogeneous embankment type
2.Zoned embankment type
3.Diaphragm type
Embankment Dam
Seepage calculations in embankment dams
Location of phreatic line
Phreatic line, also variously also called as saturation line, top flow line, seepage line,
etc. is defined as the line within a dam in a vertical plane
section below which the soil is saturated and there is positive hydraulic
pressure.
On the line itself, the hydrostatic pressure is equal to atmospheric
pressure, that is, zero gauge pressure.
Above the phreatic line, there will be a capillary zone in which the
hydrostatic pressure is negative
Since the flow through the capillary zone is insignificant, it is usually neglected and
hence the seepage line is taken as the deciding line between the saturated soil below
and dry or moist soil above in a dam section.
Embankment Dam
Seepage calculations in embankment dams
The flow of the seepage water below the phreatic line can be approximated by the
Laplace Equation
∂2 φ/ ∂x 2 + ∂ 2 φ/ ∂y 2 = 0
where φ=k*h is the velocity potential, k= permeability of soil; h= head causing flow
the streamlines are perpendicular to the
equipotential lines
Embankment Dam
Homogeneous dam with horizontal drainage filter
Embankment Dam
It is assumed that the phreatic line which emanates at P, meets the horizontal
drainage blanket at B and is, for most of its downstream part, a parabola (first
proposed by A. Casagrande).
This curve is termed as the Base Parabola and is assumed to have its focus at
A, the upstream edge of the horizontal drainage blanket.
The Base Parabola, on its upstream part is assumed to meet the reservoir
water surface at a point P0 that is 0.3L upstream of P, as shown in Figure 49.
In order to obtain the Base Parabola, one has to consider P0 as the centre, and
draw an arc A-R, with the radius equal to P0-A. The point R is on a horizontal
line at the same elevation of the reservoir surface. From point R, a
perpendicular is dropped on to the top surface of the horizontal drainage
blanket to meet it at a point C.
Knowing the focus, the directrix and the point P0, a parabola can be drawn,
which gives the Base Parabola shape. It may be recalled that point B is mid
way of points A and C. At its upstream point, however, the parabola has to be
modified such that it takes a curve upwards and meets the point P with the
gradient of the phreatic line being perpendicular to the dams upstream face.
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Dams 1
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components
• Core
• Casing or shell
• Cutoff
• Slope protection measure
• Internal drainage system
• Surface drainage
components
Dams 1
Dams 1
Drainage system
• The conventional types of seepage control and drainage
features generally adopted for the embankment dam are:
• a) Impervious core,
• b) Inclined/vertical filter with horizontal filter,
• c) Network of inner longitudinal drain and cross drains,
• d) Horizontal filter,
• e) Transition zones/transition filters,
• f) Intermediate filters,
• g) Rock toe, and
• h) Toe drain.
Dams 1
Inclined/Vertical Filter
• Inclined or vertical filter abutting
downstream face of either impervious core
or downstream transition zone is provided
to collect seepage emerging out of
core/transition zone and thereby keeping
the downstream shell relatively dry. In the
eventuality of hydraulic fracturing of the
impervious core, it prevents the failure of
dam by piping.
Horizontal Filter
• It collects the seepage from the inclined/vertical
filter or from the body of the dam, in the absence
of inclined/vertical filter, and carries it to toe
drain. It also collects seepage from the
foundation and minimizes possibility of piping
along the dam seat.
Inner Longitudinal and Inner
Cross Drains
• When the filter material is not available in the
required quantity at reasonable cost, a network
of inner longitudinal and inner cross drains is
preferred to inclined/vertical filters and horizontal
filters. This type of drainage feature is
generally adopted for small dams, where the
quantity of seepage to be drained away is
comparatively small.
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Rock Toe and toe drain
• The principal function of the rock toe is to provide
drainage.
• It also protects the lower part of the downstream slope
of an earth dam from tail water erosion.
• Rock available from compulsory excavation may be
used in construction of the rock toe.
• Where this is not possible and transportation of rock is
prohibitively costly, conventional pitching should be used
for protecting the downstream toe of the dam.
• The top level of the rock toe/pitching should be kept
above the maximum tail water level (TWL).
Concrete Diaphragm
• A single diaphragm or a double diaphragm
may also be used for seepage control
(Figure 46). Concrete cutoff walls placed
in slurry trench are not subject to visual
inspection during construction, therefore
require special knowledge, equipment and
skilled workmen to achieve a satisfactory
construction.
Relief Wells
• Relief wells are an important adjunct to most of the preceding basic
schemes for seepage control.
• Prevent excess hydrostatic pressures in the downstream
portion of the dam, which could lead to piping.
• They also reduce the quantity of uncontrolled seepage flowing
downstream of the dam
• . Relief wells should be extended deep enough into the foundation
so that the effects of minor geological details on performance are
minimized.
Dams 1
failure
• The various modes of failures of earth
dams may be grouped under three
categories:
• 1. Hydraulic failures
• 2. Seepage failures, and
• 3. Structural failures
• This type of failure occurs by the surface
erosion of the dam by water. This may
happen due to the following reasons:
Hydraulic failures
• 1. Overtopping of the dam which might have been
caused by a flood that exceeded the design flood for the
spillway. Sometimes faulty operation of the spillway
gates may also lead to overtopping since the flood could
not be let out in time through the
• spillway. Overtopping may also be caused insufficient
freeboard (the difference between the maximum
reservoir level and the minimum crest level of the dam)
has been provided. Since earth dams cannot withstand
the erosive action of water spilling over the embankment
and flowing over the dam’s downstream face, either
complete or partial failure is inevitable (Figure 22)
Dams 1
• 2. Erosion of upstream face and shoulder
by the action of continuous wave action
may cause erosion of the surface unless it
is adequately protected by stone riprap
and filter beneath (Figure 23).
Dams 1
• 3. Erosion of downstream slope by rain wash. Though
the downstream face of an embankment is not affected
by the reservoir water, it may get eroded by heavy rain
water flowing down the face, which may lead to the
formation of gullies and finally collapse of the whole dam
(Figure 24).
Dams 1
• 4. Erosion of downstream toe of dam by
tail water. This may happen if the river
water on the downstream side of the dam
(which may have come from the releases
of a power house during normal operation
or out of a spillway or sluice during flood
flows) causes severe erosion of the dam
base. (Figure 25).
Dams 1
Seepage failures
• The water on the reservoir side
continuously seeps through an
embankment dam and its foundation to the
downstream side. Unless a proper design
is made to prevent excessive seepage, it
may drive down fine particles along with its
flow causing gaps to form within the dam
body leading to its collapse. Seepage
failures may be caused in the following
ways:
Piping through dam and its foundation:
• This is the progressive backward erosion
which may be caused through the dam or
within its foundation by the water seeping
from upstream to the downstream (Figure
26)
Dams 1
Sloughing of downstream face:
• This phenomena take place due to the
dam becoming saturated either due to the
presence of highly pervious layer in the
body of the dam. This causes the soil
mass to get softened and a slide of the
downstream face takes place (Figure 28)
Dams 1
Structural failures
• These failures are related to the instability
of the dam and its foundation, caused by
reasons other than surface flow (hydraulic
failures) or sub-surface flow (seepage-
failures). These failures can be grouped in
the following categories:
• 1. Sliding due to weak foundation: Due to the presence
of faults and seams of weathered rocks, shales, soft clay
strata, the foundation may not be able to withstand the
pressure of the embankment dam. The lower slope
moves outwards along with a part of the foundation and
the top of the embankment subsides (Figure 29) causing
large mud waves to form beyond the toe.
Dams 1
• 2. Sliding of upstream face due to sudden
drawdown: An embankment dam, under filled up
condition develops pore water pressure within
the body of the dam. If the reservoir water is
suddenly depleted, say due to the need of
emptying the reservoir in expectation of an
incoming flood, then the pore pressure cannot
get released, which causes the upstream face of
the dam to slump (Figure 30).
Dams 1
• 3. Sliding of the downstream face due to
slopes being too steep: Instability may be
caused to the downstream slope of an
embankment dam due to the slope being
too high and / or too steep in relation to
the shear strength of the shoulder
material. This causes a sliding failure of
the downstream face of the dam (Figure
31).
Dams 1
• Damage caused by burrowing animals or water soluble
materials: some embankment dams get damaged by the
burrows of animals which causes the seepage water to
flow out more quickly, carrying fine material along with.
This phenomena consequently leads to piping failure
within the body of the dam, finally leading to a complete
collapse. Similarly, water soluble materials within the
body of the dam gets leached out along with the
seepage flow causing piping and consequent failure.
Dams 1
Dams 1
Dams 1
Dams 1
Embankment Dam
Slip Failure of Earth Dam
Buttress Dam
Buttress Dam
Buttress Dam
: is a gravity dam reinforced by structural supports.
Buttress
:a support that transmits a force from a roof or wall to another
supporting structure.
This type of structure can be considered even if the foundation
rocks are little weaker.
Buttress Dam
Typical Sections of
Buttress Dams
Shapes of Buttress Dam
Buttress Dam
Multiple-Arch Dam
(Buttress Dam)
Types of Dam
Earthfill 58%
Timber Crib 2%
Other 16%
Rockfill 3%
Concrete 11%
Stone Masonry 10%
Dam Failure
June 5, 1976: the failure in the Teton Dam led to flooding in the
cities of Sugar City and Reburg in Idaho. The dam failure killed 14
people and caused over $1 billion in property damages.
The dam failed because the bedrock was not strong enough to
support the structure. Currently the dam is once again used for
hydroelectric power.
Teton Dam, Idaho
Dam Failure
July 17, 1995 : a spillway gate of Folsom Dam failed, increasing
flows into the American River significantly. The spillway was
repaired and the USBR carried out an investigation of the water
flow patterns around the spillway using numerical modelling.
No flooding occured as a result of the partial failure, but flooding
is still a major concern for this area. It seems that the Folsom Dam
may be due for a height increase as an answer to this concern
Folsom Dam, USA
Arch Dam
• Curved in plan
• Carries major part of water load
horizontally to abutment by arch action
• Remaining load by cantilever action as in
case of gravity dam
• V and even U shaped valley suitable
Arch Dam
Section
Constant radius
• Radius of extrodos are equal at all elevations
from top to bottom
• Radius of introdos are decreasing from top to
bottom
• Center of all extrodos, introdos and & centerline
of horizontal arch rings all lie on same vertical
line (at one point) that passes through
centerline of horizontal arch ring at the crest
• Increase thickness towards the base
• Minimum thickness at the top
Constant radius
• Most economical central angle for an arch
dam is 133-134 degree
• Such angle can be adopted at one place
about mid height
Arch Dam
Variable Radius
• Radius of extrodus and introdus vary at
various elevations being max. at top and
min at bottom
• Centers of various rings at different
elevations
• Donot lie on a same vertical line
• More economical as concrete uesd is 82
% of constant radius
Constant angle
• Centeral angle of horizontal arch rings are
same at all elevations but radii do vary
• Thus it can be designed at best value of
centtral angle 133-134 degree
• So most economical
• Such dame cannot be used when
foundation are weak
Arch Dam
Thin cylinder theory
Stresses are assumed to be
same as in thin cylinder of
equal outside radius
Cantilever action is absent
allow
hr
t



T=thickness of arch at any
elevation wrt radius(r at that
elevation) ;
This thickness increased linearly
with depth
Central angle for minimum
concrete
Arch Dam
Example Profiles of Existing Dam

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Dams 1

  • 1. Dams Definition of Dams Advantages and Disadvantages of Dams Classification of Dams Types of Dams
  • 3. What is a Dam? A dam is a structure built across a stream, river or estuary to retain water. Dams are made from a variety of materials such as rock, steel and wood.
  • 7. Definitions Heel: contact with the ground on the upstream side Toe: contact on the downstream side Abutment: Sides of the valley on which the structure of the dam rest Galleries: small rooms like structure left within the dam for checking operations. Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry. Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.
  • 8. Advantages of Dam Irrigation Water Supply Flood Control Hydroelectric Recreation Dams gather drinking water for people. Dams help farmers bring water to their farms. Dams help create power and electricity from water. Dams keep areas from flooding. Dams create lakes for people to swim in and sail on.
  • 9. Disadvantages of Dam Dams detract from natural settings, ruin nature's work Dams have inhibited the seasonal migration of fish Dams have endangered some species of fish Reservoirs can foster diseases if not properly maintained Reservoir water can evaporate significantly Some researchers believe that reservoirs can cause earthquakes
  • 12. Three Gorges Dam Type: Concrete Gravity Dam Cost: Official cost $25bn - actual cost believed to be much higher Work began: 1993 Due for completion: 2009 Power generation: 26 turbines on left and right sides of dam. Six underground turbines planned for 2010 Power capacity: 18,000 megawatts Reservoir: 660km long, submerging 632 sq km of land. When fully flooded, water will be 175m above sea level Navigation: Two-way lock system became operational in 2004. One-step ship elevator due to open in 2009.
  • 14. Three Gorges Dam Shipping Locks Shipping Locks
  • 16. Hoover Dam Location: Arizona and Nevada, USA Completion Date: 1936 Cost: $165 million Reservoir Capacity:1.24 trillion cubic feet Type: Arch/ Gravity Purpose: Hydroelectric power/flood control Reservoir: Lake Mead Materials: Concrete Engineers: Bureau of Reclamation The Hoover Dam is a curved gravity dam. Lake Mead pushes against the dam, creating compressive forces that travel along the great curved wall. The canyon walls push back, counteracting these forces. This action squeezes the concrete in the arch together, making the dam very rigid. This way, Lake Mead can't push it over. Today, the Hoover Dam is the second highest dam in the country and the 18th highest in the world. It generates more than four billion kilowatt-hours a year, that's enough to serve 1.3 million people!
  • 17. Classification of Dams Storage Dam Detention Dam Diversion Dam Coffer Dam Debris Dam Classification based on function
  • 18. Classification of Dams Storage Dam: 1. To impound water to its upstream side During periods of excess and deficient supply 2. Reservoir or lake is formed 3. Irrigation , water power generation etc Classification based on function
  • 19. Classification of Dams Detention Dam: 1. Water is stored during floods and release gradually @ safe rate 2. Ist type: water is stored & then released 3. 2nd type: water is not released, water seeps in pervious banks Water level in well rises Lift irrigation is possible 4. Seeping may be sufficient that surface water Classification based on function
  • 20. Classification of Dams Diversion Dam: 1. Raise water level in river & thus provides head for carrying or diverting water into canals e.g. weir or barriage Classification based on function
  • 21. Classification of Dams Coffer Dam Classification based on function
  • 22. Classification of Dams Debris Dam: 1.detention dams are constructed across tributary carrying large silt and sediments 2. Debris dams traps the sediments and thus to exclude the sediments to flow to the main reservoir formed on main river. Classification based on function
  • 23. Classification of Dams Classification based on hydraulic design Classification based on material of construction Overflow Dam/Overfall Dam Non-Overflow Dam Rigid Dam Non Rigid Dam
  • 24. Classification of Dams Classification based on structural behavior Gravity Dam Arch Dam Buttress Dam Embankment Dam
  • 25. Gravity Dam Gravity dams are dams which resist the horizontal thrust of the water entirely by their own weight. Concrete gravity dams are typically used to block streams through narrow gorges.
  • 26. Gravity Dam Cross Section Plain View Material of Construction: Concrete, Rubber Masonry
  • 27. Gravity Dam ADVANTAGES • External forces are resisted by weight of dam • More strong and stable • Can be used as overflow dams also with spillway feature • Highest dams can be made as gravity dams cuz of its high stability • Specially suited for heavy downpour; slopes of earthen dams might get washed away • Less maintenance required • Gravity dam does not fail suddenly but earthen dams
  • 28. Gravity Dam DISADVANTAGES • Can be made only on sound rock foundation • Initial cost is high • Takes more time to construct if materials are not available • Requires skilled labour
  • 29. Arch Dam An arch dam is a curved dam which is dependent upon arch action for its strength. Arch dams are thinner and therefore require less material than any other type of dam. Arch dams are good for sites that are narrow and have strong abutments.
  • 30. Arch Dam Cross Section Plain View Material of Construction: Concrete
  • 31. Arch Dam Curved in plan Carried load horizontally to its by arch action Balance of water load is transferred to the foundation by cantilever action Advantages adapted in gorges where length is small in proportion to height dam require less material can be made in moderate foundation cuz of load distribution as compared to gravity dams
  • 32. Arch Dam Curved in plan Carried load horizontally to its by arch action Balance of water load is transferred to the foundation by cantilever action Disadvantages require skilled labor speed of construction is slow require strong abutments of solid rock of resisting arch thrust
  • 33. Buttress Dam Buttress dams are dams in which the face is held up by a series of supports. Buttress dams can take many forms - the face may be flat or curved.
  • 34. Buttress Dam Cross Section Plain View Material of Construction: Concrete, Timber, Steel
  • 35. Buttress Dam • Retain water between buttress • Less massive than gravity Dams • Ice pressure: ice tends to slide over the inclined U/S so this factor is unimportant • when future increase in reservoir; Future extension is possible by extending buttress and slab • Power house can be made B/W buttress; thus reducing cost • Can be designed to accommodate moderate movement of foundation without any serious damage
  • 36. Buttress Dam Disadvantages skilled labor requirements deterioration of u/s as very thin concrete face
  • 37. Embankment Dam Embankment dams are massive dams made of earth or rock. They rely on their weight to resist the flow of water.
  • 38. Embankment Dam Cross Section Plain View Material of Construction: Earth, Rock
  • 39. Earth and rockfill Dam • Made of locally available soil & gravels • Can be made on any type of available foundation • Can be constructed rapidly • Cheaper • Future consideration can be made (raising height) Disadvantages • Vulnerable to damage by floods • Cannot be used as overflow dams not suitable where heavy downpour is more common • High maintenance cost
  • 40. Types of Dam Factors governing selection of types of dam A Narrow V-Shaped Valley : Arch Dam (top width of valley less than ¼ th of height) A Narrow or Moderately with U-Shaped Valley : Gravity/Buttress Dam A Wide Valley : Embankment Dam Rolling plain: earth dam Topography-Valley Shape
  • 41. Types of Dam Factors governing selection of types of dam Solid Rock Foundation : All types Gravel and Coarse Sand Foundation : Embankment/Concrete Gravity Dam Silt and Fine Sand Foundation : (earth)Embankment/ low concrete Gravity Dam but not rockfill Clay foundation: earth dams Geology and Foundation Condition
  • 42. Types of Dam Factors governing selection of types of dam • if large spillway capacity is required; overflow concrete dam • if small spillway capacity is required; earth dam with separate site for spillway • In case of earth dam, where no other site of spillway is available, Earth dam with central concreting of spillway may be preferred Spillway size and location
  • 43. Types of Dam Factors governing selection of types of dam Climate conditions Availability of construction materials Environmental considerations Overall cost General considerations Communication road link, rail roads Locality: free from mosquitoes as labor and staff colonies are constructed
  • 45. Gravity Dam Forces on Gravity Dam 1. Weight of the dam 2. Water pressure 3. Uplift pressure 4. Wave pressure 5. Earth and Silt pressure 6. Earthquake forces 7. Ice pressure 8. Wind pressure 9. Thermal loads.
  • 47. Gravity Dam • Dead load = weight of concrete or masonry or both + weight of such appurtenances as piers, gates and bridges. • Unit weight of concrete (24 kN/m3) • For convenience, the cross-section of the dam is divided into simple geometrical shapes. • Thus the weight components W1, W2, W3 etc. can be found along with their lines of action.
  • 48. Gravity Dam Forces on Gravity Dam Gravity or weight of dam W When W = Weight of dam  = Specific weight of material  = Volume of dam Weight of Dam
  • 49. Gravity Dam Forces on Gravity Dam Water Pressure • Water pressure on the upstream face is the main • destabilizing (or overturning) force acting on a gravity dam. • Tail water pressure helps in the stability. • The water pressure always acts normal to the face of dam.
  • 50. Gravity Dam Forces on Gravity Dam Free-body diagram of cross section of a gravity dam
  • 51. Gravity Dam Forces on Gravity Dam Water Pressure
  • 52. Gravity Dam Forces on Gravity Dam Water Pressure
  • 53. Gravity Dam Forces on Gravity Dam Water Pressure the weight of water is found in two parts PV1 and PV2 by dividing the trapezium ABCD into a rectangle BCDE and a triangle ABE. Thus the vertical component PV = PV1 + PV2 = weight of water in BCDE + weight of water in ABE. The lines of action of PV1 and PV2 will pass through the respective centroids of the rectangle and triangle.
  • 54. Gravity Dam Forces on Gravity Dam Uplift Pressure Water has a tendency to seep through the pores and fissures of the material in the body of the dam and foundation material, and through the joints between the body of the dam and its foundation at the base. The seeping water exerts pressure. The uplift pressure is defined as the upward pressure of water as it flows or seeps through the body of dam or its foundation.
  • 55. Gravity Dam Forces on Gravity Dam Uplift Pressure A portion of the weight of the dam will be supported on the upward pressure of water; hence net foundation reaction due to vertical force will reduce. The area over which the uplift pressure acts has been a question of investigation from the early part of this century. One school of thought recommends that a value one-third to two-thirds of the area should be considered as effective over which the uplift acts. The second school of thought, recommend that the effective area may be taken approximately equal to the total area.
  • 56. Gravity Dam Forces on Gravity Dam uplift Pressure
  • 57. Gravity Dam Forces on Gravity Dam uplift Pressure in case of drainage gallery
  • 58. Gravity Dam Forces on Gravity Dam Silt Pressure IS code recommends that a) Horizontal silt and water pressure is assumed to be equivalent to that of a fluid with a mass of 1360 kg/m3, and b) b) Vertical silt and water pressure is determined as if silt and water together have a density of 1925 kg/m3. The gradual accumulation of significant deposits of fine sediment, notably silt, against the face of the dam generates a resultant horizontal force, Ps. a) Submerged unit weight of silt b) Angle of internal friction c) Height to which silt is deposit
  • 59. Gravity Dam Forces on Gravity Dam Ice Pressure Ice is subjected to expansion and contraction due to temperature variations 􀀹magnitude of forces varies b/w 250 kN/m2- 1500kn/m2 applied to the face of dam over the anticipated area of contact of ice with the face of dam. 􀀹The problem of ice pressure in the design of dam is not encountered in India except, perhaps, in a few localities.
  • 60. Gravity Dam Forces on Gravity Dam Wind Pressure Wind pressure does exist but is seldom a significant factor in the design of a dam. Wind loads may, therefore, be ignored.
  • 61. Gravity Dam Forces on Gravity Dam thermal Pressure 􀀹Even the deflection of the dam is maximum in the morning and it goes on reducing to a minimum value in the evening. 􀀹Measures for temperature control of concrete in solid gravity dams are adopted during construction. 􀀹 Thermal are not significant in gravity dams and may be ignored.
  • 62. Gravity Dam Forces on Gravity Dam Wave Pressure The upper portions of dams are subject to the impact of waves. 􀀹Wave pressure against massive dams of appreciable height is usually of little consequence. 􀀹The force and dimensions of waves depend mainly on the extent and configuration of the water surface, the velocity of wind and the depth of reservoir water. 􀀹The height of wave is generally more important in the determination of the free board requirements of dams to prevent overtopping by wave splash. 􀀹 An empirical method has been recommended by T. Saville for computation of wave height hw (m), which takes into account the effect of the shape of reservoir and wind velocity over water surface rather than on land by applying necessary correction.
  • 63. Gravity Dam Forces on Gravity Dam Wave Pressure Wind velocity of 120 km/h over water in case of normal pool condition and 80 km/h over water in case of maximum reservoir condition should generally be assumed for calculation of wave height if meteorological data is not available. 􀀹Sometimes the following Molitor’s empirical formulae are used to estimate wave height H= height of waves in meters V= wind velocity in km/h F=straight line of water expanse in km
  • 64. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 65. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 66. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 67. Gravity Dam Forces on Gravity Dam Earthquake forces Seismic coefficient represents max. earthquake acceleration values
  • 68. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 69. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 70. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 71. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 72. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 73. Gravity Dam Forces on Gravity Dam Earthquake forces Horizontal· acceleration causes two forces: (1) Inertia force in the body of the dam, and (2) Hydrodynamic pressure of water. • Inertia forces(energy required to move or acc. The object): The inertia force acts in a direction opposite to the acceleration imparted by, earthquake forces and is equal to the product of the mass of the dam and the acceleration. • For dams up to 100 m height , • at the top of the dam • horizontal seismic coefficient =1.5 times seismic coefficient αh then reducing linearly to zero at the base • It causes an overturning moment about the horizontal section adding to that caused by hydrodynamic force.
  • 74. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 75. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 76. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 77. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 78. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 79. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 80. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 83. Gravity Dam Forces on Gravity Dam Earthquake forces
  • 89. Compression or crushing • • ∑V= total vertical force • b= base width • e= eccentricity of the resultant force from the centre of the base • The positive sign will be used for calculating normal stress at the toe, since the bending stress will be compressive there, and • negative sign will be used for calculating normal stress at the heel.
  • 90. Allowable compressive stresses of dam material= 3000kN/m2 If pmin exceeds this , dam may fail by crushing If pmin comes out –ve, or e>b/6, tension will produce pmin pmax
  • 91. Cc- normal stress Cbc- bending stress
  • 94. Principal stress at u/s or at heel
  • 96. A dam will fail in sliding at its base, or at any other level, if the horizontal forces causing sliding are more than the resistance available to it at that level. The resistance against sliding may be due to friction alone, or due to friction and shear strength of the joint. Externat horizontal forces < shear resistance Or μΣV/Σ H > 1 sliding This represents Factor of safety and is always > 1
  • 98. Reservoir is full, three forces act on dam i.e P, W and U and the resultant of these forces pass through the outer most middle third point.
  • 99. If C=0 i.e no uplift considered
  • 100. If C=0 i.e no uplift considered
  • 106. Multiple step method of design of gravity dam • For economical design, dam is divided into various zones
  • 108. Zone 1: • ice sheet exist • Controlled by free board • Width is determined by economical and practical consideration
  • 110. Zone 2 • u/s and d/s remain vertical • For full reservoir case, resultant force passes thru outer third point of plane CC1 • Empty reservoir case, resultant force lies within middle third
  • 112. Zone 3 • u/s face vertical and d/s face inclined
  • 114. Zone 4 • Upstream face begins to batter such that line of resultant lie along corresponding extremities of middle third • Plane ee1 is governed by criteria such that maximum inclines pressure @ d/s toe for reservoir full condition equal to allowable limit • Design of zone 4 ; by dividing zone into number of blocks till bottom zone 4 is reached
  • 116. Zone 5 • d/s slope is flattened so that maximum inclined pressure @ d/s toe under reservoir full condition remains within working stress i.e. resultant of forces lies within middle third
  • 118. Zone 6 • Conditions of designed are determined by maximum pressure@ both u/s and d/s faces under reservoir empty and full conditions. • Line of resultant under both conditions lie well within middle third
  • 120. Zone 7 • Max compression at d/s toe exceeds working limit • Zone is usually eliminated • If height of Dam is so large that it exceeds zone 6, various changes are made in upper zones so that height lies till zone 6. e.g. using superior materials or height of the dam is reduced
  • 121. Embankment Dam Earth-Fill Embankment Dam A earth-fill dam in Australia.
  • 123. Embankment Dam Earth Dams: • most simple and economic (oldest dams) • built of natural materials. • constructed with low-permeability soils to a nominally homogeneous profile (single material) • 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. 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
  • 125. Embankment Dam simple zoning finer more cohesive soils placed adjacent to the impervious core element coarser fill material towards either face. Central core checks the seepage Transition zone prevent piping through cracks (that may develop in the core) Outer zone gives stability to the central impervious fill Clay with fine sand as material of impervious core Coarse sand gravel as outer shell Transition filters are provided in between these 2 zones when there is abrupt change in zones
  • 127. Embankment Dam Diaphragm Earth Dam Thin impervious core(diaphragm) surrounded by earth or rockfill Core made up of impervious soil, concrete, steel timber etc Water barrier to prevent seepage through the dam Core rest on impervious foundation material to avoid excessive underseepage
  • 129. Embankment Dam Types: 1.Homogeneous embankment type 2.Zoned embankment type 3.Diaphragm type
  • 130. Embankment Dam Seepage calculations in embankment dams Location of phreatic line Phreatic line, also variously also called as saturation line, top flow line, seepage line, etc. is defined as the line within a dam in a vertical plane section below which the soil is saturated and there is positive hydraulic pressure. On the line itself, the hydrostatic pressure is equal to atmospheric pressure, that is, zero gauge pressure. Above the phreatic line, there will be a capillary zone in which the hydrostatic pressure is negative Since the flow through the capillary zone is insignificant, it is usually neglected and hence the seepage line is taken as the deciding line between the saturated soil below and dry or moist soil above in a dam section.
  • 131. Embankment Dam Seepage calculations in embankment dams The flow of the seepage water below the phreatic line can be approximated by the Laplace Equation ∂2 φ/ ∂x 2 + ∂ 2 φ/ ∂y 2 = 0 where φ=k*h is the velocity potential, k= permeability of soil; h= head causing flow the streamlines are perpendicular to the equipotential lines
  • 132. Embankment Dam Homogeneous dam with horizontal drainage filter
  • 133. Embankment Dam It is assumed that the phreatic line which emanates at P, meets the horizontal drainage blanket at B and is, for most of its downstream part, a parabola (first proposed by A. Casagrande). This curve is termed as the Base Parabola and is assumed to have its focus at A, the upstream edge of the horizontal drainage blanket. The Base Parabola, on its upstream part is assumed to meet the reservoir water surface at a point P0 that is 0.3L upstream of P, as shown in Figure 49. In order to obtain the Base Parabola, one has to consider P0 as the centre, and draw an arc A-R, with the radius equal to P0-A. The point R is on a horizontal line at the same elevation of the reservoir surface. From point R, a perpendicular is dropped on to the top surface of the horizontal drainage blanket to meet it at a point C. Knowing the focus, the directrix and the point P0, a parabola can be drawn, which gives the Base Parabola shape. It may be recalled that point B is mid way of points A and C. At its upstream point, however, the parabola has to be modified such that it takes a curve upwards and meets the point P with the gradient of the phreatic line being perpendicular to the dams upstream face.
  • 137. components • Core • Casing or shell • Cutoff • Slope protection measure • Internal drainage system • Surface drainage
  • 141. Drainage system • The conventional types of seepage control and drainage features generally adopted for the embankment dam are: • a) Impervious core, • b) Inclined/vertical filter with horizontal filter, • c) Network of inner longitudinal drain and cross drains, • d) Horizontal filter, • e) Transition zones/transition filters, • f) Intermediate filters, • g) Rock toe, and • h) Toe drain.
  • 143. Inclined/Vertical Filter • Inclined or vertical filter abutting downstream face of either impervious core or downstream transition zone is provided to collect seepage emerging out of core/transition zone and thereby keeping the downstream shell relatively dry. In the eventuality of hydraulic fracturing of the impervious core, it prevents the failure of dam by piping.
  • 144. Horizontal Filter • It collects the seepage from the inclined/vertical filter or from the body of the dam, in the absence of inclined/vertical filter, and carries it to toe drain. It also collects seepage from the foundation and minimizes possibility of piping along the dam seat.
  • 145. Inner Longitudinal and Inner Cross Drains • When the filter material is not available in the required quantity at reasonable cost, a network of inner longitudinal and inner cross drains is preferred to inclined/vertical filters and horizontal filters. This type of drainage feature is generally adopted for small dams, where the quantity of seepage to be drained away is comparatively small.
  • 148. Rock Toe and toe drain • The principal function of the rock toe is to provide drainage. • It also protects the lower part of the downstream slope of an earth dam from tail water erosion. • Rock available from compulsory excavation may be used in construction of the rock toe. • Where this is not possible and transportation of rock is prohibitively costly, conventional pitching should be used for protecting the downstream toe of the dam. • The top level of the rock toe/pitching should be kept above the maximum tail water level (TWL).
  • 149. Concrete Diaphragm • A single diaphragm or a double diaphragm may also be used for seepage control (Figure 46). Concrete cutoff walls placed in slurry trench are not subject to visual inspection during construction, therefore require special knowledge, equipment and skilled workmen to achieve a satisfactory construction.
  • 150. Relief Wells • Relief wells are an important adjunct to most of the preceding basic schemes for seepage control. • Prevent excess hydrostatic pressures in the downstream portion of the dam, which could lead to piping. • They also reduce the quantity of uncontrolled seepage flowing downstream of the dam • . Relief wells should be extended deep enough into the foundation so that the effects of minor geological details on performance are minimized.
  • 152. failure • The various modes of failures of earth dams may be grouped under three categories: • 1. Hydraulic failures • 2. Seepage failures, and • 3. Structural failures • This type of failure occurs by the surface erosion of the dam by water. This may happen due to the following reasons:
  • 153. Hydraulic failures • 1. Overtopping of the dam which might have been caused by a flood that exceeded the design flood for the spillway. Sometimes faulty operation of the spillway gates may also lead to overtopping since the flood could not be let out in time through the • spillway. Overtopping may also be caused insufficient freeboard (the difference between the maximum reservoir level and the minimum crest level of the dam) has been provided. Since earth dams cannot withstand the erosive action of water spilling over the embankment and flowing over the dam’s downstream face, either complete or partial failure is inevitable (Figure 22)
  • 155. • 2. Erosion of upstream face and shoulder by the action of continuous wave action may cause erosion of the surface unless it is adequately protected by stone riprap and filter beneath (Figure 23).
  • 157. • 3. Erosion of downstream slope by rain wash. Though the downstream face of an embankment is not affected by the reservoir water, it may get eroded by heavy rain water flowing down the face, which may lead to the formation of gullies and finally collapse of the whole dam (Figure 24).
  • 159. • 4. Erosion of downstream toe of dam by tail water. This may happen if the river water on the downstream side of the dam (which may have come from the releases of a power house during normal operation or out of a spillway or sluice during flood flows) causes severe erosion of the dam base. (Figure 25).
  • 161. Seepage failures • The water on the reservoir side continuously seeps through an embankment dam and its foundation to the downstream side. Unless a proper design is made to prevent excessive seepage, it may drive down fine particles along with its flow causing gaps to form within the dam body leading to its collapse. Seepage failures may be caused in the following ways:
  • 162. Piping through dam and its foundation: • This is the progressive backward erosion which may be caused through the dam or within its foundation by the water seeping from upstream to the downstream (Figure 26)
  • 164. Sloughing of downstream face: • This phenomena take place due to the dam becoming saturated either due to the presence of highly pervious layer in the body of the dam. This causes the soil mass to get softened and a slide of the downstream face takes place (Figure 28)
  • 166. Structural failures • These failures are related to the instability of the dam and its foundation, caused by reasons other than surface flow (hydraulic failures) or sub-surface flow (seepage- failures). These failures can be grouped in the following categories:
  • 167. • 1. Sliding due to weak foundation: Due to the presence of faults and seams of weathered rocks, shales, soft clay strata, the foundation may not be able to withstand the pressure of the embankment dam. The lower slope moves outwards along with a part of the foundation and the top of the embankment subsides (Figure 29) causing large mud waves to form beyond the toe.
  • 169. • 2. Sliding of upstream face due to sudden drawdown: An embankment dam, under filled up condition develops pore water pressure within the body of the dam. If the reservoir water is suddenly depleted, say due to the need of emptying the reservoir in expectation of an incoming flood, then the pore pressure cannot get released, which causes the upstream face of the dam to slump (Figure 30).
  • 171. • 3. Sliding of the downstream face due to slopes being too steep: Instability may be caused to the downstream slope of an embankment dam due to the slope being too high and / or too steep in relation to the shear strength of the shoulder material. This causes a sliding failure of the downstream face of the dam (Figure 31).
  • 173. • Damage caused by burrowing animals or water soluble materials: some embankment dams get damaged by the burrows of animals which causes the seepage water to flow out more quickly, carrying fine material along with. This phenomena consequently leads to piping failure within the body of the dam, finally leading to a complete collapse. Similarly, water soluble materials within the body of the dam gets leached out along with the seepage flow causing piping and consequent failure.
  • 180. Buttress Dam Buttress Dam : is a gravity dam reinforced by structural supports. Buttress :a support that transmits a force from a roof or wall to another supporting structure. This type of structure can be considered even if the foundation rocks are little weaker.
  • 181. Buttress Dam Typical Sections of Buttress Dams Shapes of Buttress Dam
  • 183. Types of Dam Earthfill 58% Timber Crib 2% Other 16% Rockfill 3% Concrete 11% Stone Masonry 10%
  • 184. Dam Failure June 5, 1976: the failure in the Teton Dam led to flooding in the cities of Sugar City and Reburg in Idaho. The dam failure killed 14 people and caused over $1 billion in property damages. The dam failed because the bedrock was not strong enough to support the structure. Currently the dam is once again used for hydroelectric power. Teton Dam, Idaho
  • 185. Dam Failure July 17, 1995 : a spillway gate of Folsom Dam failed, increasing flows into the American River significantly. The spillway was repaired and the USBR carried out an investigation of the water flow patterns around the spillway using numerical modelling. No flooding occured as a result of the partial failure, but flooding is still a major concern for this area. It seems that the Folsom Dam may be due for a height increase as an answer to this concern Folsom Dam, USA
  • 187. • Curved in plan • Carries major part of water load horizontally to abutment by arch action • Remaining load by cantilever action as in case of gravity dam • V and even U shaped valley suitable
  • 189. Constant radius • Radius of extrodos are equal at all elevations from top to bottom • Radius of introdos are decreasing from top to bottom • Center of all extrodos, introdos and & centerline of horizontal arch rings all lie on same vertical line (at one point) that passes through centerline of horizontal arch ring at the crest • Increase thickness towards the base • Minimum thickness at the top
  • 190. Constant radius • Most economical central angle for an arch dam is 133-134 degree • Such angle can be adopted at one place about mid height
  • 192. • Radius of extrodus and introdus vary at various elevations being max. at top and min at bottom • Centers of various rings at different elevations • Donot lie on a same vertical line • More economical as concrete uesd is 82 % of constant radius
  • 193. Constant angle • Centeral angle of horizontal arch rings are same at all elevations but radii do vary • Thus it can be designed at best value of centtral angle 133-134 degree • So most economical • Such dame cannot be used when foundation are weak
  • 194. Arch Dam Thin cylinder theory Stresses are assumed to be same as in thin cylinder of equal outside radius Cantilever action is absent allow hr t    T=thickness of arch at any elevation wrt radius(r at that elevation) ; This thickness increased linearly with depth
  • 195. Central angle for minimum concrete
  • 196. Arch Dam Example Profiles of Existing Dam