Gravity Dam- Y910026

GRAVITY DAM FORCES ACTING ON GRAVITY DAM THEORETICAL & PRACTICAL PROFILE OF GRAVITY DAM Y910026 ASHISH SANGHAVI

GRAVITY DAM WHAT IS A GRAVITY DAM ? A gravity dam is a structure so proportioned that its own weight resists the forces exerted upon it. These dams are most permanent ones, requires little maintenance and is most commonly used. WHAT MATERIALS ARE USED FOR ITS CONSTRUCTION ? • Concrete • Masonry

FORCES ACTING ON GRAVITY DAM Following Are The Forces Acting On A Gravity Dam 1. Water pressure 2. Weight of dam 3. Uplift pressure 4. Earthquake Forces 5. Ice pressure 6. Wave pressure

7. Silt pressure 8. Wind pressure

1. WATER PRESSURE • • •

This is the major external force acting on dam. The intensity of pressure varies triangularly – zero intensity at the top of water surface – to a value wh at any depth h below water surface. In both the cases whether dam face vertical or inclined water pressure will form a vertical face and water pressure will be as shown in diagram.

P = wh2 2

Dark blue Triangle’s Area denote water pressure magnitude

2. WEIGHT OF DAM • •

This is the major resisting force. It is generally calculated as weight per unit length and act along centroid.

Weight/m= Area X Density of material •

In case when incline face, Weight= C.S. Area of dam x Density of material + C.S. Area of water X Density of water

Cross sectional area shown in blue outline

Vertical face

Inclined face

3. UPLIFT PRESSURE • Water seeps through the – pores and fissures of the foundation & dam material – joints between the body of the dam and its foundation This exerts upward 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

D A M

UPLIT PRESSURE

4. EARTHQUAKE FORCE • Due to earthquake primary and secondary waves are set up in the earth‘s crust. • The waves impart acceleration to the foundation under the dam which produces an inertia force in the body of the dam and sets up stresses initially in the lower layers and gradually in the whole body of the dam. • These may cause vertical and horizontal acceleration waves • Vertical acceleration waves cause no serious damage. • Horizontal acceleration waves cause – Inertia forces in dam body – Hydrodynamic Pressure of Water

5. ICE PRESSURE • In extreme cold climate, the top surface of reservoir freezes into ice. • Due to variation in temperature , such ice expands during day time. • This expansion exerts pressure on dam called ice pressure • Ice pressure acts along length of dam at reservoir level. • Its magnitude varies from 25 to 150 t/m2

D Expanding Ice creating pressure on dam

U/S

A M

D/S

6. WAVE PRESSURE • Waves are generated on reservoir surface due to wind blowing over it. • Wave pressure depends on height of waves developed. • Height of wave depend on velocity of wind. Wind

D

A M

7. STILT PRESSURE • The river brings silt & debris along with it which gets deposited to an appreciable extent when dam is constructed. • The dam is therefore subjected to silt pressure in addition to water pressure. • If Ys is submerged unit weight of silt & Ф is angle of internal friction & hs is height to which silt is deposited , silt pressure is

Ps= 1 Ys hs2 1- sinФ 2

1+ sinФ

• If the upstream face is inclined , the vertical weight of silt supported on the slope also acts as vertical force

8. WIND PRESSURE • It is a minor force & is hardly accounted while designing dam • Wind pressure is required to be considered only on that portion of superstructure which is exposed to action of wind. • Normally wind pressure is taken as 1 to 1.5 kN/m2 for the area exposed to wind pressure. Wind

D A M

ELEMENTARY OR THEORETICAL PROFILE OF GRAVITY DAM • The elementary profile of a gravity dam is a triangular section having zero width at the water level where hydrostatic pressure is zero, and maximum base width B where maximum hydrostatic pressure head acts.

PRACTICAL PROFILE OF GRAVITY DAM • The elementary profile is not possible from practical point of view as dam requires – Road at the top – Additional Load due to Roadway Top – Free Board

Extra Portion to Balance Resultant in Middle Third Portion

PRACTICAL PROFILE OF GRAVITY DAM • Hence the practical profile will be different from the theoretical profile. • Due to the provision of the above three, the resultant will act outside the outer middle third, and tension at the heel and compression at the toe will occur. To eliminate these tension and compression, some extra concrete will be essential in the upstream face.

THEORETICAL PROFILE PRACTICAL PROFILE Free Board

Roadway Loads Roadway

Water Pressure Diagram Extra Portion to Balance Resultant in Middle Third Portion & thus avoid tension condition

P

W

R

Middle Third Portion

PRACTICAL PROFILE OF GRAVITY DAM

PROFILE DIFFERENCE THEORETICAL PROFILE

PRACTICAL PROFILE

Has a shape of right angle triangle

Modified shape to make suitable for practical requirement

Has no top width as water pressure at top is zero

Has top width for construction of road

Free board is not provided

Free board provided

When reservoir empty more stable as shape is right angle triangle

When reservoir empty less stable because of modified shape