Dams 1

Dams
Definition of Dams
Advantages and Disadvantages of Dams
Classification of Dams
Types of Dams

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.

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
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
Shipping Locks
Shipping Locks

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!

Storage Dam
Detention Dam
Diversion Dam
Coffer Dam
Debris Dam
Classification based on function

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

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

Diversion Dam:
1. Raise water level in river & thus provides head
for carrying or diverting water into canals e.g.
weir or barriage

Coffer Dam

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 hydraulic design
Classification based on material of construction
Overflow Dam/Overfall Dam
Non-Overflow Dam
Rigid Dam
Non Rigid Dam

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
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
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
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
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
• 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
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
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.

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
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
Free-body diagram of cross
section of a gravity dam

Gravity Dam
Water Pressure

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
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
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
uplift Pressure

Gravity Dam
uplift Pressure
in case of
drainage
gallery

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
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
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
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
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
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
Earthquake forces

Gravity Dam
Earthquake forces
Seismic coefficient represents max. earthquake acceleration values

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.

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

Principal stress at u/s or at heel

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

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

Multiple step method of design
of gravity dam
• For economical design, dam is divided into
various zones

Zone 1:
• ice sheet exist
• Controlled by free board
• Width is determined by economical and
practical consideration

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

Zone 3
• u/s face vertical and d/s face inclined

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

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

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

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.

components
• Core
• Casing or shell
• Cutoff
• Slope protection measure
• Internal drainage system
• Surface drainage

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.

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.

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.

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)

• 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).

• 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).

• 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).

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)

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)

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.

• 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).

• 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).

• 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.

Embankment Dam
Slip Failure of Earth 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

• 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

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

• 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

Dams 1

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