GRAVITY DAM

BVRIT 03-04-2020 16CE310 IE / UNIT 5 / N K & M I

N.Karthikeyan, AP/CE

M.Indumathi, AP/CEP

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Index

1. Types of Gravity Dam

2. Forces Acting on a Gravity Dam

3. Causes of failure of Gravity Dam

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Types of Gravity Dam

A Gravity dam is a structure so proportioned that its own weight

resists the forces exerted upon it.

Types:

1. Masonry Dam

2. Concrete Dam

Suitable across gorges with very steep slopes where earth

dams might slip. 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 3

• A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by primarily using the weight of the material alone to resist the horizontal pressure of water pushing against it.

• Gravity dams are designed so that each section of the dam is stable and independent of any other dam section

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Purpose In winter and spring, gravity dams help control the flow of melting snow in

a river, and in summer they store water to provide it year-round to the

neighboring land.

Without gravity dams, snow and ice would build up in winter and

eventually melt, flowing downhill to a lake or river or the ocean, leaving a

trail of destruction.

By the time all the snow has melted and reached flowing water, the towns

and cities in its path are left with flooded and destroyed properties.

Then, in summer, these cities will run short on water.

A gravity dam helps in capturing the melted snow and rain in a large

area, containing the water for use through the entire year as it slowly drains

over the dam or is guided away through streams, ditches, or pipes, allowing

controlled flow without destruction. 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 5

Bhakra Dam is a concrete gravity dam on the Sutlej River in Bilaspur, Himachal

Pradesh in northern India. The dam forms the Gobind Sagar reservoir. The dam, located

at a gorge near the (now submerged) upstream Bhakra village in Bilaspur district of

Himachal Pradesh of height 226 m. 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 6

Advantages of Gravity Dam

Advantages:

Strong, Stable and Durable

Suitable for moderately wide valleys having steep slopes

Can be constructed to very great heights

Suitable for an overflow spillway section

Maintenance cost is very low

Does not fail suddenly 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 7

Disadvantages of Gravity Dam

Disadvantages:

Gravity dams of great height can be constructed only on sound

rock foundations.

Initial cost is more than earth dam

Takes longer time in construction

Require more skilled labor than earth dam

Subsequent raise is not possible in a gravity dam 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 8

Types The most common classification of gravity dams is by the materials composing the structure:

Concrete dams include

mass concrete dams, made of:

conventional concrete

Roller-Compacted Concrete (RCC)

masonry

hollow gravity dams, made of reinforced concrete

Composite dams are a combination of concrete and embankment dams.

Construction materials of composite dams are the same used for concrete and embankment dams.

Gravity dams can be classified by plan (shape):

Most gravity dams are straight .

Some masonry and concrete gravity dams have the dam axis curved to add stability through arch action.

Gravity dams can be classified with respect to their structural height:

Low, up to 100 feet.

Medium high, between 100 and 300 feet.

High, over 300 feet.

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1. Water Pr.

2. Weight of Dam

3. Uplift Pr.

4. Pr. Due to EQ

5. Ice Pr.

6. Wave Pr.

7. Silt Pr.

8. Wind Pr.

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1

2

3

4

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Forces Acting on a Gravity Dam

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• It is the pressure of water that acts perpendicular on the upstream

face of the dam. For this, there are two cases:

• A. Upstream face of the dam is vertical and there is no water on the

downstream side of the dam

• The total pressure is in horizontal direction and acts on the upstream

face at a height H/3 from the bottom.

•

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1. Water Pressure This is the major external force acting

on dam

Pressure Components on both

upstream and downstream are:

1. Vertical Component

2. Horizontal Component

Unit weight of water, γw=1000 kg/m3

w: specific weight of water. Usually it is taken as unity.

H: height up to which water is stored in m.

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2. Weight of Dam

This is the major resisting force

Generally unit length of dam is considered

The cross section of dam may be divided into

several triangles and rectangles and weights

W1, W2, W3 etc., may be computed

The total weight W of the dam acts at the

C.G. of its section.

Weight = Volume per unit length x Density of

material 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 14

3. Uplift Pressure

The uplift pressure is defined as the

upward pressure of water as it flows or

seeps through the body of the dam or its

foundation. Uplift Pressure

(No Gallery)

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When the water is stored on the upstream side of a dam there exists a head of

water equal to the height up to which the water is stored.

This water enters the pores, fissures, and cracks of the foundation material

under pressure. It also enters the joint between the dam and the foundation at

the base and the pores of the dam itself.

This water then seeps through and tries to emerge out on the downstream end.

The seeping water creates hydraulic gradient between the upstream and

downstream side of the dam.

This hydraulic gradient causes vertical upward pressure. The upward pressure

is known as uplift which is the second largest external pressure. Uplift reduces

the effective weight of the structure and consequently the restoring force is

reduced.

Therefore, it is essential to study the nature of uplift and also some methods

will have to be devised to reduce the uplift pressure value.

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Earthquake waves imparts accelerations to the foundations under

the dam and causes its movement

This earthquake wave may travel in any direction

For design purpose, Horizontal and Vertical directions are

considered.

Seismic Force = Mass x Earthquake Acceleration

According to IS 1893-2002, India was divided into Four zones:

zone II, III, VI, and V.

4. Pressure due to Earthquake

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Earthquake Acceleration

Earthquake Acceleration is usually designated as fraction of

the acceleration due to gravity

It is expressed as α.g

where α is known as Seismic Coefficient

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Seismic Coefficient

Seismic coefficient is divided into

1. Horizontal Seismic coefficient, αh

2. Vertical Seismic Coefficient, αv = 0.75 αh

αh can be determined by one of the two methods

1. Seismic Coefficient Method < 100m height of the dam

2. Response Spectrum Method > 100m height of the dam

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Seismic Coefficient Method

As per IS: 1893-1984,

Horizontal Seismic coefficient, αh = 2 α0

Where α0 =Basic Seismic Coefficient

Basic Seismic Coefficient as per IS 1893:1984

Seismic Zone II III IV V

Basic Seismic Coefficient 0.02 0.04 0.05 0.08 03-04-2020 16CE310 IE / UNIT 5 / N K & M I 21

Effect of Earthquake Acceleration

Effect of Horizontal Earthquake Acceleration, αh g

1. Inertia Force in the body of the dam Inertia Force

= Force x Earthquake Acceleration

= (W/g)(αh g) = W. αh

2. Hydrodynamic Pressure of water

Pey = Cy αh w h

Equation 8.16 (Page 370) where Cy is a dimensionless pressure

coefficient

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• The Inertia Force is the product of mass and acceleration and this force acts in the direction opposite to that of the ground motion.

• If Reservoir is Full - Inertia Force acts in downstream direction

• If Reservoir is Empty - Inertia Force acts in upstream direction

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5. Ice Pressure The ice formed on water surface of the reservoir

is subjected to expansion and contraction due to

temperature variations

Coefficient of thermal expansion of ice is 5 times

more than concrete

The dam face has to resist the force due to

expansion of ice

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This force acts linearly along the length of the dam,

at reservoir level

IS: 6512-1984 recommends 250 kN/m2 applied to the

face of dam over the anticipated area of contact of

ice with the face of the dam.

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6. Wave Pressure Waves are generated on the reservoir surface

because of wind blowing over it.

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7. Silt Pressure The river brings silt and debris along with it.

The dam is, therefore, subjected to silt pressure, Ps, in addition

to water pressure

Where γ’ = submerged unit weight of silt

h = height of silt deposit

Φ = Angle of internal friction

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According to IS : 6512-1972, the silt pressure and

water pressure exist together in submerged silt.

The following are recommended for calculating

forces:

Psh = 1360 Kg/m3

Psv = 1925 Kg/m3

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8. Wind Pressure

It is a minor force acting on dam

Acts on Superstructure of the dam

Normally, wind pressure is taken as 1 to 1.5 kN/m2

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S.

No

Name

Type

1 Anaimaduvu Reservoir,

Belur, Salem Earthen

2

Adavinainarkovil Dam

Mekkarai, Panpoli ,

Tenkasi district

Gravity & Masonry

3 Aliyar Dam

Coimbatore District

Earthen / Gravity &

Masonry

4

Amaravathi Dam

Udumalpet

Tirupur district

Earthen / Gravity &

Masonry

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