Gravity dam

Brief idea about Gravity dam and forces acting on gravity dam.

1. PRESENTATION ON GRAVITY DAM

2. INTRODUCTION

3. Gravity Dam It is a masonry or concrete dam which resists the forces acting on it by its own weight. These dams are heavy and massive wall-like structures of concrete in which the whole weight acts vertically downwards. Its c/s is approximately triangular in shape. As the entire load is transmitted on the small area of foundation, such dams are constructed where rocks are competent and stable.

4. Gravity dams are rigid concrete dams which ensure stability against all loads by virtue of their weight alone. They transfer all the loads to the foundation and hence are built when the foundation is strong rock. A typical section of a gravity dam is shown. Gravity Dams

5. Famous Gravity Dams Around The World

6. BHAKRA DAM HIMACHAL, INDIA HEIGHT 741 feet (226  m)

7. FONTANA DAM NORTH CAROLINA, USA HEIGHT 700 feet (210 m)

8. LAKE VYRNWY DAM LIVERPOOL, UK HEIGHT 146 feet (45  m)

9. NUREK DAM TAJIKISTAN HEIGHT 984 feet (300  m)

10. SHASTA DAM NORTH CALIFORNIA,USA HEIGHT 602 feet (183 m)

11. Structure of gravity dam Heel Gallery Toe Spillway (inside dam) Crest NWL (Normal water level) MWL (Max. water level) Free board Sluice way Upstream Down stream

12. Purpose of Gravity dam

13. ADVANTAGES OF GRAVITY DAM Strong , Stable and Durable Suitable for moderately wide valleys having steep slope Loss of water by seepage in gravity dams is less . Can be constructed to very great heights Suitable for an overflow spillway section Maintenance cost is very low Does not fail suddenly

14. 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 Design of gravity dams is very complicated DISADVANTAGES OF GRAVITY DAM

15. Forces Acting On Gravity Dam Water Pressure Weight of the Dam Uplift Pressure Silt Pressure Wave Pressure Wind pressure Ice pressure Earthquake Pressure

16.  Water Pressure is the most major external force acting on a gravity dam .  On upstream face pressure exerted by water is stored upto the full reservoir level. The upstream face may either be vertical or inclined .  On downstream face the pressure is exerted by tail water. The downstream face is always inclined Water Pressure

17. Weight of the Dam The self weight of the gravity dam is the main stabilising force which counter balances all the external forces acting on it.

18. Uplift Pressure Uplift pressure is the pressure exerted by water as it seeps through the body of the dam or its foundation . Seeping water exerts pressure on the base of the dam and it depends upon water head.

19. Silt Pressure Sediment deposition in the reservoir results in a force acting horizontally on the upstream face. This force is assumed to have a hydrostatic distribution.

20. Wave Pressure Waves are generated on the surface of the reservoir by the blowing winds, which exert a pressure on the upstream side. Wave pressure depends upon wave height

21. Wind pressure The top exposed portion on the dam is small & hence the wind pressure on this portion of dam is negligible .

22. The ice which may be formed on the water surface of the reservoir in cold countries may sometimes melt and expand. The dam face is subjected to the thrust exerted by the expanding ice . Ice pressure

23. Dams are subjected to vibration during earthquakes . Vibration affects both the body of the dam as well as the water in the reservoir behind the dam . The most danger effect occurs when the vibration is perpendicular to the face of the dam. Earthquake Pressure

24. Sliding – It may take place on a horizontal joint above formation, on the foundation. Sliding takes place when total horizontal forces are greater than the combined shearing resistance of the joint and the static friction induced by total vertical forces . Causes of gravity dams Failure

25. Overturning – A dam fails in overturning when total horizontal forces acting on the dam section are quite great in comparison with total vertical forces. In such cases the resultant of two passes outside the limits of the dam. Causes of gravity dams Failure

26. Dam may fail when tension is produced in the concrete. Dam may fail in crushing due to compression. Causes of gravity dams Failure

27. To prevent overturning, the resultant of all forces acting on the dam should remain within the middle-third of the base width of the dam . In the dam, the sliding should be fully resisted when the condition for no sliding exists in the dam section . In the dam section, the compressive stresses of concrete or masonry should not exceed the permissible working stress to avoid failure due to crushing . There should be no tension in the dam section to avoid the formation of cracks . The factor of safety should be maintained between 4 to 5 Precaution against dams Failure