Here is a term paper on ‘Dams’ for class 6, 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Dams’ especially written for school and college students.
Term Paper on Dams
Term Paper Contents:
- Term Paper on the Definition of Dam
- Term Paper on the Classification of Dams
- Term Paper on the Stability of Dams
- Term Paper on the High and Low Dam Profiles
- Term Paper on the Control of Temperature in Dams
- Term Paper on the Construction and Contraction Joints in Dams
- Term Paper on the Galleries in Dams
- Term Paper on the Forces Acting on a Dam
- Term Paper on the Selection of Site and Type for Dams
Term Paper # 1. Definition of Dam:
Dam is a solid barrier generally impervious in nature constructed at the narrow outlet of a catchment area or in a valley. It holds up the flow of water to raise the water level to a fixed height to form a reservoir on the upstream side. Dam serves two purposes.
Firstly it retains water to create an impounding reservoir and secondly it passes the water over or through it when required or when the water is surplus.
The difference between a dam and a weir is that former impounds water on its upstream side permanently while the latter only raises the level of flowing water at the site.
Term Paper # 2. Classification of Dams:
Dams could be classified in different ways as follows:
(a) According to the Materials used in Construction:
(b) According to the Purpose Served by Them:
(i) Storage dam.
(ii) Diversion dam.
(iii) Detention dam.
(i) Storage Dams:
Dams constructed for the purpose of storing water during high floods and then for supplying the stored water when necessity arises are called storage dams. Bhakra dam and Talwara dam are storage dams.
(ii) Diversion Dams:
A small dam constructed only to raise the water level on the upstream side for diverting it into canals is called a diversion dam. It is actually called weir or a barrage. Nangal dam is an example of diversion dam.
(iii) Detention Dams:
Small dams constructed to delay and to detain the flow of flood water are called detention dams. They are called check dams also. It is one of the popular methods of controlling sedimentation of reservoirs.
(c) According to the Hydraulic Design:
(i) Over-flow dam.
(ii) Non-over flow dam.
(i) Over-Flow Dams:
When water flows over the crest of a dam it is known as over-flow dam. Solid gravity dam is overflow type dam.
(ii) Non-Overflow Dams:
Dams which do not allow water to flow over the crest of a dam are called non-overflow type dams. Earth and rockfill dams are non-overflow type dams.
(d) According to the Principles Adopted for Obtaining Their Stability:
(i) Arch dam.
(ii) Buttress dam.
(iii) Gravity dam.
(i) Arch Dams:
In arch dams the load is transferred to the abutments mainly by arching action.
(ii) Buttress Dams:
In buttress dams the load is mainly transferred to the supporting buttresses.
(iii) Gravity Dams:
In gravity dams stability against external load is achieved by the weight of the dam itself.
(e) According to the Design Features of Gravity Dams:
(i) Low dam.
(ii) High dam.
(i) Low Dam:
It is designed on the basis of a theoretical profile of solid gravity dam.
(ii) High Dam:
It is not a simple structure as low dam. Here allowable stresses in the masonry are exceeded. Therefore dam section is modified to achieve stability.
It may be seen that main classification is according to the material used for construction of dam. Under other classifications, dams from this list reappear owing to different criteria adopted in design or different purpose served by them.
Term Paper # 3. Stability of Dams:
Following design principles ensure stability of dam:
Principle 1:
To avoid tension in any part of dam, resultant of all the forces acting on the dam above any joint should remain within the middle-third of the joint width. This condition should be achieved when the reservoir is full and also when empty.
Principle 2:
Sliding should always be resisted fully.
If shear is neglected this condition is achieved when:
Tangent of θ is less than the allowable coefficient of friction at the place under consideration. θ is the angle between the vertical force and the resultant of all the forces, Fig. 13.7.
where HP is total horizontal forces and SW is total vertical forces
The condition for no sliding is Tan θ < f.
Where, f is the coefficient of friction of the material. The value of f varies from 0.6 to 075.
If shearing is also included this condition is achieved when sum of total frictional resistance at the joint under consideration and ultimate shearing strength of the joint exceeds the total horizontal forces above the joint for all conditions.
Mathematically condition may be expressed as:
where Sa is the unit shearing strength of the material.
A is the area of the joint.
Sf is the shear friction factor of safety. The value of Sf is generally taken between 4 and 5.
Principle 3:
Compressive stresses should never exceed the safe prescribed limit.
Principle 4:
So far as possible there should be no tension, internally, in the dam section.
Term Paper # 4. High and Low Dam Profiles:
A low dam is designed on the basis of theoretical profile. The resultant of the two main forces (water pressure and self-weight of dam) always passes through the middle third of each horizontal joint in the dam section. In this type allowable stresses on the masonry are not exceeded. Design of the dam is not altered or affected because of the allowable stress limitations.
A high dam is not as a simple structure as low dams. Here allowable stresses on the masonry are often exceeded. As a result resultant of two main forces (water pressure and self-weight of dam) passes out of the middle third of some horizontal joint in the dam section.
The dam section is given an extra slope or batter on the upstream side or the downstream side as per requirements. Thus the section is modified for achieving stability in high dams. Fig. 13.11 makes the distinction clear between the two types.
Foundation Grouting:
In actual practice ideal foundation is not commonly available. So, generally weak and faulty foundation is made strong before constructing a dam. The method adopted to strengthen the foundation is called grouting. Grouting means injecting a slurry of water and cement (ratio 3: 1 generally) with certain admixtures. If there is water flowing in the cavities of the foundation asphalt may be used for grouting.
Thus foundation grouting is adopted firstly to prevent leakage and to reduce the uplift by tightening the foundation. Secondly grouting helps in consolidating broken seamy foundation.
General procedure of grouting is as follows:
Under the heel of a dam series of holes is drilled. The diameter of a hole is usually 7.5 cm. The distance between two holes and the depth of holes depend upon the nature of foundation.
The holes are then washed with water under pressure. The pressure of the water is generally kept equal to the head of water against the dam.
In the hole threaded pipe is fitted and anchored properly. The pipe is connected to grouting machinery. Through the pipe grout mixture is injected under pressure. Finally grout hole is sealed at the top. Generally grouting of one hole is done completely before starting drilling of the adjacent holes.
According to the functions served methods of grouting may be:
(a) Curtain or cut-off grouting, and
(b) Consolidation grouting.
According to the procedure adopted while grouting the holes the methods may be subdivided as:
(1) Inclined grouting
(2) Cris-cross grouting
(3) Split-spacing grouting
(4) Stage Grouting.
Term Paper # 5. Control of Temperature in Dams:
When hydrolysis of cement takes place lot of heat of hydration is generated in first few days in the mass concrete placed in the dam. The surface of concrete block open to atmosphere cools quickly than the interior of the blocks. Thus steep temperature gradient is created. This temperature gradient is harmful. It produces internal temperature stresses in the concrete mass. To prevent production of abnormal stresses it is necessary to flatten the temperature gradient of the interior and exterior of the concrete block.
Following methods can be used successfully to avoid production of abnormal temperature stresses:
(i) Low heat cement may be used in the concrete.
(ii) The height of each block may be reduced. Generally it is restricted to 1.5 m. Thus more surface area will be exposed to the atmosphere. At least after 5 days interval second layer of concrete block should be laid over the first so as to give enough time for cooling of the first layer.
(iii) Ice or refrigerated water may be used for preparing-concrete.
(iv) Refrigerated water may be circulated through the pipes embedded in the concrete blocks permanently.
Term Paper # 6. Construction and Contraction Joints in Dams:
It is clear that whole dam cannot be constructed at a stretch as one monolithic mass. Naturally construction will be done in stages. Thus to facilitate work, joints are provided.
According to function served the joints in a dam can be divided as:
(1) Construction joints; and
(2) Contraction joints.
Construction joints facilitate construction of a dam to proceed in small lifts. Thus all the horizontal joints are called construction joints. The lift or the distance between two construction joints is kept 1.5 m. It is in accordance with the procedure of cooling of concrete.
Fig. 13.12 shows construction and contraction joints.
Temperature stresses may produce unsightly cracks in a dam. Contraction joints avoid these cracks. Of course all contraction joints are construction joints too. Though contraction joints avoid haphazard and dangerous cracks, regular joints or openings are introduced.
Construction joints are of two types, namely:
(i) Transverse joints, and
(ii) Longitudinal joints.
As it is shown in Fig. 13.12 transverse joints are continuous from the upstream face to the downstream face. Longitudinal joints are staggered or broken. Spacing of these joints is generally kept 15 metres.
To provide more strength at joints and to minimise water leakage through the joints various types of keys are provided as shown in Fig. 13.13. Sometimes water seals of various types are also provided at joints to reduce leakage.
Term Paper # 7. Galleries in Dams:
A gallery is an opening or a passage left in the dam which runs longitudinally. This opening is left in the completed structure of the dam and it is generally of rectangular shape with flat or semi-circular roof. The gallery has an access either through elevators or lift towers or through cross galleries at the ends.
The location, size, name and shape of a gallery depend on its use or purpose.
A gallery may serve single or various purposes mentioned below:
(a) Drainage:
To provide space, for drainage of water percolating through the upstream face of a dam or seeping through the foundation.
(b) Drilling:
To provide, space for drilling holes and for grouting the foundation.
(c) Headers:
To provide space for equipment used in artificial cooling of the concrete blocks and grouting of the construction joints.
(d) Inspection:
To, provide access to the interior of the dam for inspection purposes.
(e) Equipment:
To provide access to and to make room for mechanical equipment used in the operation of gates in spillway and sluices.
Fig. 13.14 shows a dam section and various types of galleries.
Term Paper # 8. Forces Acting on a Dam:
i. External Water Pressure:
It is the pressure of water on the upstream face of the dam mainly.
Total horizontal pressure acts on the upstream face at a height H/3 from the bottom.
The pressure diagram is triangular and the total pressure is given by:
where w is density of the water. Usually it is taken as unity. Units are gm/cc (or 1000 kg/m3)
H is the height upto which water is stored in cm (or in m).
When the u/s face has batter, in addition to the horizontal water pressure P, there is a vertical pressure of the water. It is due to the water column resting on the upstream sloping face (Fig. 13.4).
The vertical pressure P2 acts, on the (length b) portion of the base.
The vertical pressure is given by:
Pressure P2 acts through the centre of gravity of the water column resting on, the sloping upstream face.
When there is water on the downstream side of the dam pressure may be calculated similarly. The water pressure on the downstream face actually stabilizes the dam. Hence as an additional factor of safety it may be neglected.
ii. Uplift Pressure:
Seepage takes place through all types of dams, concrete as well as earth, under the pressure of stored water. The water seeps through the pores of the foundation and the dam material if the material is permeable. If not, the seepage water finds its way through imperfectly bonded foundation and construction joints.
The seeping water tries to emerge out on the downstream side. The seeping water creates a hydraulic gradient between upstream and downstream ends of the dam. The hydraulic gradient causes vertically upward pressure. This upward pressure is known as uplift. The uplift reduces effective weight of the structure. Thus the stabilising force is reduced by the uplift (Fig. 13.5).
Uplift is given by:
U is uplift pressure; B is base width of dam, and H is the height upto which water is stored.
The total uplift acts at B/3 from the heel or upstream end of the dam. In solid gravity dams which are founded on impervious foundations it is made sure that the uplift does not reduce the stabilising force so much that the dam becomes unstable. Uplift is generally reduced by providing drainage pipes or holes in the dam section.
In case of earth dams constructed on permeable foundations if the seepage water while coming out at d/s face retains sufficient pressure and velocity it erodes and carries away the soil particles. Thus the piping starts from the downstream face of the dam. It leads to complete wash out of the foundation material.
As a result continuous cavities are formed below the dam and ultimately the dam fails. For safety of earth dams path of percolation of the seepage water is increased by providing either a cut-off or a drain or a filter. It lowers the pressure and velocity of seepage water at exit.
The theory of flownets helps in determing the residual pressure at any point in the dam or in the foundation analytically. It helps in selecting suitable measures to reduce piping. The flownet is a diagram which represents two sets of curves. They intersect each other orthogonally and obey Laplace’s equation.
One set of curves are called flow lines. They indicate paths of percolation followed by seepage water. Other set of curves are equipotential lines. They are the lines which join points having equal residual head of water. The flownet can be constructed graphically by trial and error method.
Fig. 13.6 shows a typical flownet for a homogeneous isotropic foundation without a cut-off or a drain. AB is the length of impervious portion of the dam.
The dotted lines are flow lines and the full lines are equipotential lines. The flownet may be constructed with innumerable curves. In practice, however, the flownet is drawn with limited number of curves in such a way that the flownet gives elementary squares between successive set of lines. It may be noted that longer the path required by flow lines further apart will be the equipotential lines. Then loss per unit length will be smaller.
Also velocity will be less and seepage per unit area will be less. By introducing a cut-off or a drain the shape of flownet materially changes. It shows that the pressure distribution also changes. This method helps in finding out analytical solution to the problem of stability of a dam structure.
iii. Self-Weight of Dam:
It is the only force which stabilizes the structure. Total weight of the dam is supposed to act through the centre of gravity of the dam section in vertically downward direction. Density of the material used in construction is about 2.4 gm/cc.
iv. Silt and Earth Pressure:
Earth material deposited in the reservoir against the face of dam also exerts pressure on the dam. Like external water pressure earth also exerts pressure in vertical and horizontal directions.
v. Ice Pressure:
In very cold regions, upper layers of water in the reservoir freeze to form ice. Also seepage water gets frozen. When ice is formed it exerts pressure on the sides of contact due to increase in volume.
vi. Earthquake Pressure:
An earthquake can be defined as a vibration of surface of earth caused by a disturbance of the rocks beneath the surface. Damage to the dam during earthquake is mostly due to vibrations in earth in horizontal directions. When the ground beneath the dam is suddenly moved to one side the dam structure tends to remain in its original position due to its inertia.
The horizontal thrust is due to:
(a) Inertia of water resting against the dam, and
(b) Inertia of the dam itself relative to the foundation on which it rests.
The effect of inertia of the water depends on the slope of upstream face. A flat slope reduces horizontal component of the force due to the shock wave. For this reason the earth and rock fill dams resist earthquake shocks much better than the solid gravity dams.
The effect of inertia of the dam itself depends upon the density of material used in the dam and the position of centre of gravity in relation to the foundation.
The usual method of considering the inertia effect is to assume an additional external horizontal force equal to the mass of the dam multiplied by some arbitrary percentage of the gravitational acceleration “g”. In case of solid gravity, dam’s earthquake acceleration ranges from 0.1 to 0.2 g. The determination of increase in water pressure due to earthquake is rather difficult. Various formulae have been evolved to calculate it which are beyond the scope and hence are not dealt with here.
vii. Wind Pressure:
Dam structure is exposed to high velocity wind blowing over the reservoir. It is however minor in design of dams.
viii. Wave Pressure:
Similarly, dam structure is exposed to impact of waves produced on the reservoir surface due to high velocity winds. Wave pressure is given by formula p = 2 w hw2
where hw is height of waves. Height of wave likely to be generated is obtained from standard wave height formula which gives wave height in terms of length of reservoir and the velocity of wind.
Term Paper # 9
. Selection of Site and Type for Dams:
i. Site for Dams:
While selecting a site for a dam following points should be taken into consideration:
(1) The water storage should be largest for the minimum possible height and length. Naturally site should be located in a narrow valley.
(2) Good foundation should be available at moderate depth.
(3) Good and suitable basin should be available. Cup shaped basin with fairly water-tight bed and sides should be available.
(4) Materials for construction should be available at a dam site or near it.
(5) For discharging surplus water a spillway is provided. There should be good and suitable site available for spillway construction. It may be in the dam itself or near the dam on the periphery of the basin.
(6) Value of the property and land likely to be submerged by the proposed dam should be sufficiently low in comparison with the benefits expected from the project.
(7) Dam site should be easily accessible in all seasons.
(8) The catchment on the upstream should contribute good and sufficient water to the basin. The catchment should not be easily erodible otherwise excessive silt will come in the reservoir.
(9) There should be suitable site available for providing living accommodation to the labourers and engineering staff.
(10) Overall cost of construction and maintenance of the dam should be taken into consideration.
ii. Types for Dams:
Following topographical and geological factors affect selection of a type of dam:
(a) Nature of Foundation:
If there is sound rock formation present in the foundation, any type of dam can be adopted. Of course selection of particular type is subject to the consideration of economic height suitable for each type.
An earth dam is suitable for poor rock and earth foundations particularly when the height of the proposed dam is intermediate.
Presence of large depths of over-burden on rock layer usually necessitates construction of an earth dam in preference to solid gravity dam. Sometimes buttress dam can be economically constructed in such conditions.
(b) Nature of Valley:
An arch dam is very suitable for narrow valley provided good rock abutments are available.
If gorge with rocky bed is available solid gravity dam is best suited.
If valley is wide and foundation is also weak buttress dam can be adopted.
For any width of valley with good foundations steel dam is most suited.
For any width of valley with any foundation and low height of water to be stored, timber dam is suited.
For wide valley with gentle side slopes an earth dam or rock-fill dam is preferable.
(c) Permeability of Foundation Material:
When uplift pressure exerted on the base of a dam is excessive an arch dam can be selected. Obvious reason for its selection is uplift is not an important design factor in arch dams.
When foundations are pervious an earth dam is suitable. The reason is that an earth dam has a longer base in comparison with the height. The water which seeps below the foundation and emerges out at downstream face of the dam practically loses its entire head in the path of percolation.
In addition following points should also be considered:
(d) Suitable site for locating a spillway.
(e) Availability of construction material may sometimes dictate the choice.
It is very essential to study existing merits and demerits of the site while selecting type of dam.
First consideration should be given to safety.
Secondly the choice is generally limited by the funds available. The cost of construction is governed by the prices of materials available.
That in general the most permanent and safe dam will be found to be most economical one.