Here is a term paper on ‘Gravity Dams’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Gravity Dams’ especially written for school and college students.
Term Paper on Gravity Dams
Term Paper Contents:
- Term Paper on the Definition of Gravity Dam
- Term Paper on the Classification of Gravity Dams
- Term Paper on the Profile of a Gravity Dam
- Term Paper on the Construction of Gravity Dams
- Term Paper on the Causes of Failure of Gravity Dam
- Term Paper on the Advantages and Disadvantages of Gravity Dams
Term Paper # 1. Definition of Gravity Dam:
A gravity dam is that dam in which stability of dam against external forces (e.g., water pressure, earthquake etc.) is achieved by the weight of the dam itself.
This type of dam is the most permanent one and hence it is very commonly used. It requires minimum maintenance. It may be constructed at all localities, only consideration is strength of the foundation. Gravity dam may be constructed with concrete or masonry.
Term Paper # 2. Classification of Gravity Dams:
Gravity dams may be classified according to the material used for constructing them:
(i) Masonry Dams:
This type of dam is constructed with masonry. Exterior faces of the dams are constructed with coursed masonry. It is made watertight by pointing the joints with cement. A plaster of cement is also applied. The interior could be either in coursed masonry or rubble masonry.
(ii) Concrete Dams:
They are built block by block by pouring good quality concrete. Between the blocks proper joints and seals are provided to take care of stresses developed during curing of concrete.
(iii) Reinforced Cement Concrete Dams:
In order to provide extra strength and to economise on the mass of concrete reinforcement is used. In such dams steel bars are embedded in the concrete as per design requirement. With this technique it is now possible to keep hollow spaces inside the body of the dam very conveniently. Such hollows provide space for inspection, instrumentation and machinery.
Term Paper # 3. Profile of a Gravity Dam:
Stored water exerts pressure perpendicularly on the face of a barrier. As the depth of water increases the pressure also increases. Naturally the barrier or the dam should have minimum thickness at the top and the thickness should be increased with the depth of water. A figure with an apex at the top and some thickness at the bottom with a straight vertical face is nothing but a right angled triangle.
The most important condition for stability is that the resultant should remain in the middle third portion of a base.
Taking two extreme conditions, namely reservoir full and reservoir empty the dimensions of a triangular dam section can be fixed as follows:
The water is stored to a height of H metres. Naturally the height of the triangle is H metres. The base width of this triangle is, say B metres.
When the reservoir is empty the only force acting on the dam is self-weight. It passes through the centre of gravity of the triangle. Obviously it will pass through point A as shown in Fig. 13.8. Point A is at 5/3 distances from the heel of the dam or point O. Thus point A is one extremity of the middle third.
When the reservoir is full resultant R, of self-weight W and water pressure P will meet the base at some other point. The base width should be selected in such a way that it passes through other extremity of the middle third. That is, it should pass through point D.
Base B in terms of H is found out as mentioned below:
Therefore elementary profile of a gravity dam is a right angled triangle with a base width equal to H/√ρ. Where H is height of the water stored and p is specific gravity of the material used in the dam. This section will be safe against overturning and failure due to tension.
A dam with the top as an apex of a triangle is not practicable. It is essential to give some thickness to the top of the dam. Mr. Bligh has given an empirical formula for finding out thickness of dam at the top.
The formula is:
a = 0.552 √H
where ‘a’ is the top width of the dam in metres.
In elementary profile, water is supposed to be stored right upto to the top. Any slight addition of water may cause overtopping of the dam which is very undesirable. The level of existing water may rise due to wind, waves and unanticipated floods. Sufficient free-board should be provided. Generally 2 to 3 metres of net free-board is provided. The amount of free-board depends on peak flood, height of waves, intensity of wind etc.
Thus the theoretical profile of Fig. 13.8 can be modified as Fig. 13.9. It is a practical profile of a dam.
Whenever resultant force acts eccentrically, that is not in the centre of the base; stress intensity at two ends is given by the equation:
where e is an eccentricity of the resultant force. It is the distance between the centre of the base and the line of action of the resultant.
when e = B/6, that is when the resultant passes through the either extremities A and D of Fig. 13.8.
That is the stress on one side is compressive, of value 2W/B whereas on the other side it is 0 as shown in Fig. 13.10 (a).
When e is less than B/6 both values of S are positive as shown in Fig. 13.10 (b). That is there is compression on both sides.
When e is greater than B/6 the stress diagram will be as shown in Fig. 13.10 (c) In this case there is tension on one side.
From the above discussion it is clear that if e is more than B/6 the structure will not be safe as concrete is weak in tension. Obviously the value of e should be kept less than B/6 that is the resultant should remain in the middle third of the base.
It is also essential to see that the compressive stress in not exceeding the limits of safety.
Term Paper # 4. Construction of Gravity Dams:
Whole dam cannot be built as a monolithic mass at one stretch. Technique of construction of various types of gravity dams is similar in principle to any construction activity by that particular material.
For example in case of concrete dams various common steps involved are proper batching and mixing of aggregate, pouring and compaction of concrete, curing of concrete, finishing and provision of construction joints. However, there are certain constructional features which are special in case of dams.
Term Paper # 5. Causes of Failure of Gravity Dam:
A solid gravity dam may fail because of the following reasons:
i. Sliding:
Sliding may take place on a horizontal joint above the foundation, on the foundation or on a horizontal seam in 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.
ii. 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 the two passes outside the limits of the dam.
iii. Dam may fail when tension is produced in the concrete.
iv. Dam may fail in crushing.
Term Paper # 6. Advantages and Disadvantages of Gravity Dams Versus Earth Dams:
Advantages:
(i) Gravity dams are more suitable in narrow valleys (V gorges) than earth dams.
(ii) Maintenance cost of gravity dams-is less than that of earth dams.
(iii) Failure of dams is not very sudden as that of earth dams.
(iv) Gravity dams may be built to any height but height of earthen dam is limited.
(v) Outlets or sluices are provided in the body of the gravity dams, but in case of earth dams the spillways are provided separately.
(vi) Loss of water by seepage in masonry dams is less as compared to earth dams.
Disadvantages:
(i) Gravity dams are costlier than earth dams.
(ii) In construction technical experts are required.
(iii) Gravity dams require hard foundation.
(iv) Design of gravity dams is complicated and takes lot of time and labour.