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Here is an essay on ‘Dams’ for class 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Dams’ especially written for school and college students.
Essay on Dams
Essay # 1. Introduction to Dams:
A dam may be defined as a solid barrier constructed at a suitable location across a river valley with a view of impounding water flowing through that river.
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Dams are constructed for achieving any one or more of the following objectives:
(i) Generation of hydropower energy;
(ii) Providing water for irrigation facilities;
(iii) Providing water supply for domestic consumption and industrial uses;
(iv) Fighting droughts and controlling of floods;
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(v) Providing navigational facilities;
Additional benefits coming from dams are development of fisheries and recreation facilities in the reservoirs created by them and also the overall greenery effect all along the reservoirs.
In a country like India where rainfall is erratic and depends considerably on the vagaries of seasonal winds – the monsoons, importance of dams can hardly be overemphasized. Hundreds of thousands kilometer long irrigational canals being fed by reservoirs created by around four thousand minor and major dams spread throughout the country have been responsible, to a great extent, for making India self-sufficient in food production.
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Essay # 2. Types of Dams:
Although no two dams are exact copy of each other, it has been a practice to classify these structures on the basis of:
(a) Design of construction, whether the load of the body of the dam is transmitted on the foundations or to the abutment rocks; such as gravity dams, arch dams, buttress dams etc.
(b) Material of construction, such as concrete, rockfill or earthfill dams;
(c) Size of the construction, such as small dams and large dams;
The well-known main types of dams are the gravity dams, the arch dams and the embankment dams.
International Congress on large dams defines a large dam as one which has a height of more than 15 m from the lowest portion of the general foundation area to the crest.
Dams with heights falling between 10-15 m but satisfying the following conditions are also classed among the large dams:
(i) Length of the crest of the dam is greater than 500 m;
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(ii) Capacity of the reservoir is not less than 1 million cubic meters;
(iii) Maximum flood discharge not to be less than 2000 cubic meters/second.
In India, there were about 4000 large dams meeting the above definitions at the end of twentieth century. The greatest number of large dams are in Maharashtra (1508) followed by Madhya Pradesh (752), Gujarat (365), Karnataka (213), Orissa (149) Rajasthan (128) and Uttar Pradesh (109). Punjab had only two dams whereas Haryana, Mizoram, Nagaland and Sikkim did not have a single large dam till then.
The tallest dam (the Bhakra Dam) with a height of 226 m and length at crest of 518 m is located in Himachal Pradesh and the longest dam (the Hirakud dam) having a height of 59 m and length of about 5 km (4800 m) is built across Mahanadi in Orissa. The Bhakra Dam is a gravity dam and the Hirakud Dam an embankment dam. The only arch dam, the ldduki Dam is located in Kerala.
The salient features of principle types of dams are as follows:
a. Gravity Dams:
A gravity dam is a solid masonry or concrete structure, generally of a triangular profile, which is so designed that it can safely stand against a pre-calculated volume of water by virtue of its weight. All the forces arising in such a dam – as due to the thrust of the impounded water and the massive weight of the dam material – are assumed to be directly transmitted on to the foundation rocks. Hence the strength of the foundation rocks is the most critical factor in their design.
A gravity dam, when properly designed and carefully constructed, is considered among the safest types.
Many derived types of gravity dams have also been constructed with advantage. The Buttress Dam is such a type in which a thin concrete slab is supported from the downstream side by buttresses thereby saving considerable construction material. The upstream face in a gravity dam may be vertical or inclined.
Similarly, the axis of the dam may be straight or a curvature may be induced in the design of the dam. The buttresses in such dams are narrow, heavily loaded structures which take most of the load from the dam and transmit the same to isolated foundations under them. Hence rocks must be exceptionally strong under the buttresses.
b. Arch Dams:
An Arch Dam, as the name implies, is an arch-shaped solid structure mostly of concrete, which is designed in such a way that a major part of the thrust forces acting on the dam are transmitted mainly by the arch action, (and also cantilever action at the base) on to the abutment rocks, that is, rocks forming the left and right sides of the stream valley. Hence, such dams can be built even on those sites where the foundation rocks may not be sufficiently strong.
Two main types of Arch Dams are:
i. The constant radius Arch Dam, in which the radius of curvature throughout the structure is constant and upstream face, is vertical.
ii. The variable radius dams, in which curvatures are different on the upstream and downstream sides. An arch dam having a curvature both in horizontal and vertical alignment is often called a cupola dam.
Arch dams are better suited for narrow valleys with strong and uniformly sloping walls or abutments. In ideal situation, they offer many advantages over the other types of dams. Arch dams are quite thin walled compared to gravity dams and hence lighter in weight.
Sometimes the designers mix the better points of both the gravity and arch dams and prefer to design a mixed arched-gravity-dam. A combination of series of arch dams called the Multiple Arch Dams is sometimes applied with advantage when the valley is too wide for a single arch or gravity dam.
The ldduki dam in Kerala (India) is an important Arch Dam of our country.
c. Embankment Dams:
These include a variety of non-rigid structures which are built over wide valleys with varying foundation characteristics from easily available materials such as earth and rock fragments. These are generally of trapezoidal shape. In design they may be made up of a single type of material (such as earth fill or rock fill) or a combination of more than one material.
Their main advantage over other types of dams is that they can be constructed even on weak foundations such as unconsolidated Weak River or glacial deposits. An embankment dam is constructed as a homogeneous construction but very commonly with a properly compacted core of an impervious material such as clay. Concrete cores with proper cover are also provided in many embankment dams.
Depending upon the type of material used, the embankment dams may be an earth fill dam or rock fill dam or mixed-type embankment. The clay core wall is made up of simple dug up and cleared, thoroughly compacted, puddled clay or rolled clay. It is followed by two or more layers of proactive transitional layers before the actual “fill” starts. The Hirakud dam in Orissa is one of the longest embankment dams of our country.
Essay # 3. Geotechnical Considerations for Designing and Constructing Dams:
Whereas a decision regarding placing a dam across a particular river and creating a reservoir or basin is always based on socio-economic considerations, its design and construction are essentially civil engineering activities involving important geotechnical parameters.
Detailed answers to following main questions have to be obtained:
(i) The exact location where the dam should be placed against the river along its longitudinal profile;
(ii) The type of dam that will be most suitable for that particular site;
(iii) The availability, cost and quality of the materials required for the construction of the dam.
Obviously, answers to above questions would involve very systematic and thorough geological investigations along the river valley in general and in some preliminarily selected areas in particular. The problem may be divided, for discussion purpose, into two categories – geotechnical considerations for dam sites and for reservoir sites. The two are, however, intimately interlinked and both the dam site and the reservoir area must be geologically suitable for a safe, stable and economical project.
Essay # 4. Selection of Sites for Dams:
The Objectives:
The main object of placing a dam across a river is to impound its water behind the dam.
Naturally, this would require that:
(a) Topographically, a place which is most suitable for the purpose is selected. Ideally, it would be a narrow gorge or a small valley with enough catchment areas available behind so that when a dam is placed there it would easily store a calculated volume of water in the reservoir created upstream.
This should be possible without involving significant uprooting of population, loss of cultivable land due to submergence or loss of existing construction. Also, strategically the location of a dam, especially a major project, is so decided as to cause minimum damage to the public in case of its destruction or failure.
(b) Technically, the site should be as sound as possible – strong, impermeable and stable. Strong rocks at the site make the job of the designer much easy – he can evolve best deigns. Impermeable sites ensure better storage inventories.
Stability with reference to seismic shocks and slope failures around the dam, especially upstream, are a great relief to the public in general and the engineer in particular. The slips, slides, and slope failures around and under the dam and susceptibility to shocks during an earthquake could prove highly hazardous.
(c) Constructionally, the site should not be far off from deposits of materials which would be required for its construction. All types of major dams require millions of cubic meters of natural materials—earth, sand, gravel and rock – for their construction. Their non-availability in the adjoining areas would make the project cost too high, may be even unfeasible.
(d) Economically, the benefits arising out of a dam placed at a particular site should be realistic and justified in terms of land irrigated or power generated or floods averted or water stored. Dams are invariably costly structures and cannot be placed anywhere and everywhere without proper analysis of cost-benefit aspects.
(e) Environmentally, the site where a dam is proposed to be placed and a reservoir created, should not involve ecological disorder, especially in the life cycles of animals and vegetation and man. The fish culture in the stream is the first sector to suffer a major shock due to construction of a dam. Its destruction may cause indirect effects on the population. These effects require as thorough analysis as for other objects. The dam and the associated reservoir should become an acceptable element of the ecological set up of the area.
Geological Characters for Investigation:
For achieving the above objects, thorough and systematic investigations of following geological characters of the areas in general and of the preliminarily selected site in particular would have to be carried out.
(1) Geology of the Area:
Preliminary geological surveys of the entire catchment area followed by detailed geological mapping of the reservoir area have to be conducted.
These should reveal:
(i) Main topographic features,
(ii) Natural drainage patterns,
(iii) General characters and structures of rock formations such as their stratification, folding and faulting and igneous intrusions, and
(iv) The trend and rate of weathering and erosion in the area.
Such a study when interpreted properly would rule out some areas for the dam placement and help in identifying the locations that are most suitable topographically and economically, where further detailed geological investigations could be carried out. For obtaining the above information, conventional geological and geophysical surveys need to be conducted.
(2) Geology of the Site:
(a) Lithology:
The single most important feature that must be known thoroughly at the site and all around and below the valley up to a reasonable depth is the Lithology, i.e. types of the rocks that make the area. Surface and subsurface studies using the conventional and latest techniques of geological and geophysical investigations are carried out.
Such studies would reveal the type, the composition and textures of the rocks exposed along the valley floor, in the walls and up to the required depth at the base. Rocks are inherently anisotropic materials, showing variation in properties in different directions.
Yet, it is of great significance to know what class of rocks make up the area – igneous, sedimentary or metamorphic; and also which type and sub-type is more prevalent; and whether it is only one class of the rock existing there, or more types of the same or different classes of the rocks are found.
It is possible that the entire site may be made up of one type of rock, say, for example, fine textured sandstones; it is also possible that it may have alternate layers of sandstones, shales and clays, all of varying types. Complex lithology definitely poses challenging design problems.
(b) Structures:
Along with lithology, the structural features of rocks of the site are also thoroughly investigated. This involves detailed mapping of planes of weakness like bedding planes, schistosity, foliation, cleavage, joints, shear zones, faults and fault zones, folding and the associated features.
It is because each one of these features modifies the engineering properties of the rocks to a great extent. While mapping these features, special attention is given to recording their attitude, spacing and nature. Joints, for instance, may not be as harmful when sparsely developed and of a closed nature, but these may render the same rock very weak and permeable when profusely developed and of open type.
Shear zones have to be searched, mapped and treated with great caution. In some cases, these may be developed to such an extent that the rock may necessitate extensive and intensive rock treatment (e.g. excavation, backfilling and grouting etc.). In still other cases their development may be to an unmanageable scale. In such cases cost factors may demand the abandoning of the site for a better alternative.
Following is a brief account of the influence of more important structural features of rocks on dam foundations:
i. Dip and Strike:
The strength of sound, unfractured stratified rock is always greater when the stresses are acting normal to the bedding planes than if applied in other directions. This being so, horizontal beds should offer best support for the weight of the dam. The resultant force (due to weight of the dam and thrust of the impounded water) is always inclined downstream. As such, gently upstream dipping layers offer best resistance to the resultant forces in a dam. They also serve as a natural obstruction for leakage (Fig. 23.5C).
It is also easy to understand that the easiest direction for slippage in stratified rocks is along the bedding planes. Consequently, the most UNFAVOURABLE strike direction is the one in which the beds strike parallel to the axis of the dam and the dip is downstream (Fig. 23.5.B). It must be avoided as far as possible. Therefore, other conditions being same, beds with upstream dips are quite favourable sites for dam foundations.
ii. Faults:
These structures can be source of danger to the dam in a number of ways. Thus:
(i) The faulted rocks are generally shattered along the rupture surfaces;
(ii) Different types of rocks may be present on either side of a fault plane. Hence, sites with fault planes require great caution in calculating the design strength in various sections of the dam. In case some fault surface or zone gets ignored or overlooked, the stability of dam gets endangered.
(iii) Dams founded on beds traversed by fault zones and on major fault planes are more liable to shocks during an earthquake compared to dams on non-faulted rocks. This single factor in itself is of great importance, especially when the area in which dam is proposed happens to be seismically active. It is, therefore, always desirable to avoid risk by rejecting sites traversed by faults, fault zones and shear zones for dam foundations.
But when topographic, lithological and/or economic factors do not leave a choice for an alternative site, then the nature, extent and age of the fault should be thoroughly investigated. Generally small scale fault zones and shear zones can be treated effectively by grouting. But in the case of major shear zones, weak material would have to be excavated and the space backfilled with hard material like concrete up to the required depth.
iii. Folds:
The most notable effects of folds on rocks are – shattering and jointing along the axial planes and stressing of limbs. Consequently, dams aligned along axial regions of folds would be resting on most unsound rocks in terms of strength. Similarly, in synclinal bends dams placed on the upstream limbs would run the risk of leakage from beneath the dam. Further, the balance of forces in the stressed limbs would be disturbed if these are opened up during construction of diversion tunnels and galleries.
iv. Joints:
No sites are totally free from jointing. Hence, sites cannot be abandoned, even if profusely jointed. However, the detailed mapping of all the aspects and characters of jointing as developing in the rocks of proposed site has to be taken up with greatest caution. The geometry of joints, their intensity, nature and continuity with depth, all must be thoroughly established and their effects on the site rocks evaluated and remedial measures taken in advance.
Occurrence of micro-joints has to be established with still greater care as such joint systems, if left untreated, could be source of many risks. In the limestone rocks that formed foundation and abutments at Salal Dam in Jammu (Kashmir), the micro-joints presented considerable difficulties in detection and treatment.
Engineering Properties of Rocks:
Only lithological and structural studies are not fully sufficient for the selection of a site for a dam. A thorough testing – both in laboratory and in situ of the site rocks for their most important engineering properties has to be carried out. Such properties include – compressive strength, shear strength, modulus of elasticity, porosity, permeability and resistance to disintegration on repeated wetting and drying.
(a) Strength Parameters:
The determination of strength of the foundation rocks in the case of gravity dam and also of abutment rocks in the case of arch dams are considered starting points in working out designs of these dams. In either case construction of a dam involves transfer of a tremendous load due to the weight of the dam and weight and thrust of stored water on to a very small area of foundation or abutment rocks.
These loads are both of compressive and shearing nature. The capacity of the rocks vis-ά-vis these loads has to be clearly established. This is achieved by testing the strength properties in the laboratory on the core samples obtained from different and critical locations at the site using conventional laboratory techniques.
These are complimented with in-situ studies of the same properties using static and dynamic techniques. The static methods involve excavating trenches, tunnels or bore-holes within the rock mass at site and loading a section by pressure exerting machines such as hydraulic jacks. Settlements and strains are recorded at different load increments from which parameters like bearing strength, modulus of elasticity and Poisson’s ratio are calculated.
The dynamic methods involve creating seismic shocks artificially at selected locations and recording the velocity of shock waves so generated through the rocks of the site and the abutments. The shock wave velocity is related to the density, rigidity, porosity and permeability and structural constitution of the rocks.
Results obtained from all the three investigations, i.e. laboratory, in-situ static and dynamic testing are compiled and correlated to obtain a fair assessment of the strength parameters of the rocks at the site.
(b) Porosity and Permeability:
A dam is essentially a water impounding structure and as such water impounded by it must not find easy avenues of escape other than those provided in the design i.e. sluices and spillways. However, perfectly impervious rock throughout the site may not be available in all cases. Most rocks are permeable to some extent. It is, therefore, essential that investigations are carried out to establish fully the magnitude of permeability of the rocks at the site.
Porosity and permeability are also investigated, like strength related properties, both in the laboratory and in-situ. Programmes are then chalked out to make the rocks water tight, especially in critical zones, by artificial treatment, such as grouting.
Slaking Test:
This is an important test where rocks are weakly cemented or poorly compacted as in shales, argillaceous sandstones, conglomerates, limestones and pyroclastic rocks. Reservoir rocks, especially in the immediate upstream vicinity of the dam, are liable to be exposed a number of times in a year when water is drawn out from the reservoir for the proposed use.
They would undergo drying during the exposure. Similarly, when the water level rises during high precipitation time, these rocks are submerged. Such alternate drying and wetting may cause disintegration by crumbling and removal of cementing material and other mass wasting processes.
Essay # 5. Forces Acting on a Dam:
Any major dam is a highly complex engineering structure where more than one type of forces comes into play. A proper estimation and analysis of all these forces is essential to ensure stability of the dam.
In any dam, following type of forces are always involved:
(a) Weight of the Dam:
Each dam is a massive construction where thousands of tonnes of material are placed on limited space to form a huge barrier. In gravity dams and embankment dams, it is the weight of the dam which is primarily responsible for holding the water back on the upstream side. For calculating the total force due to the weight of the dam, it is divided into convenient units or sections and weight of each section acting on its centre of gravity is determined.
The resultant from all the sections is summed up and is taken as expression for the total weight acting at the C.G. OF THE DAM. Evidently, the forces arising due to the weight of the dam are compressive in nature. If by any chance, they happen to be greater than the allowable stresses of the material within or below the dam, the latter is likely to fail by crushing.
(b) Water Pressure:
Since the dam is supposed to impound water, it is required to resist horizontal forces acting due to weight of the water impounded on its upstream side. This water pressure can be calculated by using rules pertaining to hydrostatic pressure distribution (Fig. 23.8).
Thus, in a gravity dam with a vertical upstream face, the water pressure would be equal to W*H at the base of the dam and zero at the surface level, where W and H stand for the unit weight of water (1000 kg/m) and height of water in meters, respectively.
The resultant force due to the weight of the water on the dam would be given by the expression:
p = 1/2 WH2
acting at H/3 from the base (i.e. at C.G. of the dam). When the upstream face is not vertical, (Fig. 23.8B) the weight due to water is resolved into horizontal component and a vertical component.
(c) Uplift Pressure:
Although it may be desirable to make a dam an absolutely impervious structure, it may be practically impossible at economic costs. Many pores and minute cracks and joints are left in the dam body and also in the foundation rocks. Water is likely to find its way into these minute openings through seepage and gradually fill them up.
It has been now fairly established that the trapped water exerts an upward pressure on the body of the dam which is, in no case, unimportant. This pressure, called the Uplift Pressure must also be assessed, analysed and accounted for in the dam design for ensuring stability of the structure.
In a gravity dam without a drainage gallery, the uplift pressure is broadly taken as of trapezoidal shape, being maximum at the upstream toe (equal to maximum reservoir head) and minimum at the downstream toe (equal to the tailwater head). In order to provide some built-in relief from the uplift pressure, drainage galleries are often provided in the dams. In other cases cut-off channels are constructed under the upstream face. A third method is intensive pressure grouting of the foundations to minimize chances of development of uplift pressure.
(d) Earthquake Forces:
Dams get disturbed during earthquakes like all the other structures standing on the ground. The disturbance in the dams would be highly dangerous because they store huge volumes of water under great hydrostatic head, which when let loose due to failure of dam, could create havoc.
Hence, dams that are constructed in areas known to be seismically active from the past records must necessarily be designed to withstand additional forces that are likely to arise during a future shock or shocks. The seismic forces, therefore, are the fourth major class that are supposed to be acting on the dams, besides those due to weight of the dam, weight of the water and uplift pressure.
An analysis of seismic forces liable to act on a dam during a shock is a highly complex problem, which needs solution from at least three aspects:
(a) Forces developing due to vertical acceleration of the ground during a shock both in an upward and in a downward direction.
(b) Forces developing due to horizontal acceleration when the reservoir behind the dam is empty (horizontal inertia force).
(c) Forces developing due to horizontal acceleration when the reservoir is full (the hydrodynamic forces).
It attempts analysis of such forces, except to mention some basic factors. Thus:
(i) While considering the effect of vertical acceleration, it is the acceleration in downward direction which is considered a greater threat to the stability of the dam;
(ii) While considering the effect of horizontal acceleration in reservoir full condition, the hydrodynamic force calculations must take into consideration the shape of the upstream face and also water pressure distribution in static conditions;
(iii) While considering the effects of horizontal inertia force, the direction in which the seismic shock is most likely to act should be chosen with great caution.
The dam safety designs would then demand incorporation of such additional strength in the dam so that it is capable of withstanding sum total of all the disturbing forces during a future shock without collapsing, overturning or bursting.
In most countries of the world including India, the land surface has been classified into zones of seismic activity. In areas of ‘high’ seismic activity, sub-zoning has also been attempted or could be attempted. A factor of safety is then taken into consideration while designing a dam in such an area. This is called ‘seismic factor’ and is expressed in terms of acceleration as a fraction due to gravity, e.g. 0.1 g, 0.15 g or 0.05 g and so on.
(e) Other Forces:
In addition to the above major forces acting on a gravity dam, forces caused due to silting in immediate vicinity of the upstream face (silt pressure) and due to the waves caused by strong winds blowing over the reservoir water may also be considered. Generally these types of forces are of theoretical significance only and easily ignored. Similarly in areas of cold climate, the top of surface in reservoirs may actually freeze and expand causing lateral forces between 25 to 150 t/m2. Their influence on the top line of dam structure may have to be considered carefully.
Essay # 6. Relative Suitability of Different Rocks:
The number of major dams constructed in different parts of the world runs in thousands; in India alone there are more than 4000 major dams constructed till the end of 20th century. But, no two dams might be called duplicate of each other.
This is so because no two sites are exactly similar in all geological details and as such design of each dam has to be in accordance with the site conditions for that dam. Viewed from this angle a generalization regarding suitability of different rocks as dam sites may be erroneous. It may be, however, of great assistance and guidance value while choosing the sites for detailed investigations when choice is available.
It is from this angle that the following brief account of relative suitability has to be viewed:
(a) Igneous Rocks:
The massive igneous rocks like granites, syenites, diorites, gabbros, dolerites and the like may be classed among the excellent category as they possess compressive strength, shear strength and modulus of elasticity, far in excess than required for very high dams.
When undisturbed by secondary processes such as folding and faulting and unaffected by weathering and erosion, these are invariably almost impermeable. But, difficulties may arise even with these rocks when they are frequently traversed by planes of weakness such as fault zones, shear zones, joints, disconformities and secondary intrusions, or, when they are highly weathered in localized zones.
Similarly, rocks of igneous origin but volcanic character have to be viewed with great caution.
Three main types of difficulties may arise with such rocks:
(i) When congealed in layered form, their contact planes may also be planes of weakness;
(ii) In between the volcanic rock layers, thick or thin sedimentary layers be intervening (the intertrappean beds) causing heterogeneity and variation in reaction to the applied loads;
(iii) Sometimes these may be rich in interconnected cavities left due to escape of gases from lava during cooling, rendering them quite permeable.
However, other things being the same, sites made up of igneous rocks are generally safe and stable and more suitable.
(b) Sedimentary Rocks:
Massive, well-cemented, thoroughly compacted and fine textured sedimentary rocks generally form sound, stable and durable sites for dams. But when rocks of this group occur in a deformed and profusely jointed or layered fashion, great care and caution are necessary while placing dams on them.
Sandstones with siliceous cements (quartzites) are generally reliable. Those varieties of sandstones which have clayey or ferruginous cements, and those rich in mica and other weak minerals may require thorough treatment for improving their qualities. Joints occur in almost all sedimentary rocks and must be thoroughly studied and treated. Sandstones form the major rocks at the site of Bhakra Dam in India.
Limestones, which are carbonate type sedimentary rocks, are always to be viewed with caution. These are more often richly traversed by solution cavities and channels. They are also open to gradual solvent action of water, which of course cannot be overlooked.
In spite of these negative points, massive, thoroughly compact and fine-textured limestones form quite sound foundations when these defects are properly taken care of. The well-known Salal Dam in Jammu is located in a limestone region and site rocks are fine textured dolomitic limestones often traversed by micro-joints.
Shales are perhaps the most troublesome sedimentary rocks as far as their suitability for dam sites is concerned. Thoroughly compacted and well cemented and hardened shales may prove to be quite suitable. However, the common varieties – soft, friable and too clayey shales – can hardly be trusted as foundation rocks.
In fact, even the apparently suitable shales often present a variety of problems chief among which are:
(i) Gradual consolidation under load that may lead to too much settlement of the dam (and hence create risk of failure);
(ii) Liability to shear failure when these happen to occur in thin layers inclined downstream;
(iii) Rapid deterioration under conditions of alternate wetting and drying when these happen to form abutment rocks;
(iv) Unpredictable elastic constants.
In view of the above negative qualities shales and clays are always considered among unreliable sites for dams.
(c) Metamorphic Rocks:
This group of rocks exhibits greatest variation in terms of suitability for dam sites. Some varieties are hard, compact, thoroughly crystalline, well-knit and massive and the ideal rocks for dam foundations such as granitic gneisses, quartzites and marbles. However, some other varieties like schists, slates and phyllites may pose considerable troubles, and require very thorough investigations with regard to their filiation, mica content, fracturization, cleavage and so on.
Still, many dams have been located even on these rocks after proper investigations and treatment. The Dul Hasti Project in Kistwar (J & K – India) and the Uri Project, also in Kashmir, India, have gneissic and phyllitic rocks as their sites. Schists and mylonites occur in parts at the Hirakud dam site in Orissa.
Essay # 7. Geological Problems after Dam Construction:
Erosion below Spillways:
The Problem:
Reservoir water discharged over the spillway of dam generally acquires such velocities that are capable of causing deep erosion in any type of soil or rock below the spillway. Silt, sand, gravels and boulders are easily removed by such action whereas rocks with open joints or bedding planes are virtually plucked out of their places gradually but surely.
Control:
The aim of all methods of control of erosion below the spillways should be to dissipate the extra energy that reservoir water gains due to increased velocity during fall below spillway. The best method for dissipating extra energy within a limited space is to make the falling water strike against the tail water in a properly designed manner.
This would result in dissipation of extra energy in creation of turbulence in the tailwater and not in too much affecting the base materials of the river bed below. A simple method for producing such turbulence is by creating a hydraulic jump below the spillway. For this jump to occur, the most essential condition is a requisite depth below the jump.
So, the main problem with the dam design engineer is to obtain the required depth:
(i) Either, by constructing a small auxiliary dam below the apron or by excavating the river bed when the depth of the tail water is insufficient to create the jump;
(ii) Or, in case the water depth is more than sufficient for the birth of the jump, by providing a sloping apron;
(iii) Or, by devising any other economic methods.