Designs and Construction of Gravity dams
Classification of Dams
Dams can be classified in the number of ways. But most usual ways of classification i.e. types of dams are mentioned below:
Based on the functions of dams, it can be classified as follows:
- Storage dams: They are constructed to store water during the rainy season when there is a large flow in the river. Many small dams impound the spring runoff for later use in dry summers. Storage dams may also provide a water supply or improved habitat for fish and wildlife. They may store water for hydroelectric power generation, irrigation or for a flood control project. Storage dams are the most common type of dams and in general, the dam means a storage dam unless qualified otherwise.
- Diversion dams: A diversion dam is constructed for the purpose of diverting water of the river into an off-taking canal (or a conduit). They provide sufficient pressure for pushing water into ditches, canals, or other conveyance systems. Such shorter dams are used for irrigation, and for diversion from a stream to a distant storage reservoir. It is usually of low height and has a small storage reservoir on its upstream. The diversion dam is a sort of storage weir which also diverts water and has a small storage. Sometimes, the terms weirs and diversion dams are used synonymously.
- Detention dams: Detention dams are constructed for flood control. A detention dam retards the flow in the river on its downstream during floods by storing some flood water. Thus the effect of sudden floods is reduced to some extent. The water retained in the reservoir is later released gradually at a controlled rate according to the carrying capacity of the channel downstream of the detention dam. Thus the area downstream of the dam is protected against flood.
- Debris dams: A debris dam is constructed to retain debris such as sand, gravel, and drift wood flowing in the river with water. The water after passing over a debris dam is relatively clear.
- Coffer dams: It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry. A coffer dam is thus a temporary dam constructed for facilitating construction. These structures are usually constructed on the upstream of the main dam to divert water into a diversion tunnel (or channel) during the construction of the dam. When the flow in the river during construction of hydraulic structures is not much, the site is usually enclosed by the coffer dam and pumped dry. Sometimes a coffer dam on the downstream of the dam is also required.
STRUCTURE (PARTS) OF A GRAVITY DAM
- Toe: Portion of structure in contact with ground or river-bed at downstream side.
- Abutment: Sides of the valley on which the structure of the dam rest.
- Galleries: Small rooms like structure left within the dam for checking operations.
- Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side.
- Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.
- Crest: The top of the dam structure. These may in some cases be used for providing a roadway or walkway over the dam.
- Parapet walls: Low Protective walls on either side of the roadway or walkway on the crest.
- Heel: Portion of structure in contact with ground or river-bed at upstream side.
- Spillway: It is the arrangement made (kind of passage) near the top of structure for the passage of surplus/ excessive water from the reservoir.
- Abutments: The valley slopes on either side of the dam wall to which the left & right end of dam are fixed to.
- Gallery: Level or gently sloping tunnel like passage (small room like space) at transverse or longitudinal within the dam with drain on floor for seepage water. These are generally provided for having space for drilling grout holes and drainage holes. These may also be used to accommodate the instrumentation for studying the performance of dam.
- Free board: The space between the highest level of water in the reservoir and the top of the structure.
- Dead Storage level: Level of permanent storage below which the water will not be withdrawn.
- Diversion Tunnel: Tunnel constructed to divert or change the direction of water to bypass the dam construction site. The hydraulic structures are built while the river flows through the diversion tunnel.
Force acting on a Dam structure
In the design of a dam, the first step is the determination of various forces which acts on the structure and study their nature. Depending upon the situation, the dam is subjected to the following forces:
1. Water pressure
2. Earthquake forces
3. Silt pressure
4. Wave pressure
5. Ice pressure
6. Self weight of the dam.
The forces are considered to act per unit length of the dam. For perfect and most accurate design, the effect of all the forces should be investigated. Out of these forces, most common and important forces are water pressure and self weight of the dam.
1. Water Pressure
Water pressure may be subdivided into the following two categories:
I) External water pressure:
It is the pressure of water on the upstream face of the dam. In this, there are two cases:
(i) Upstream face of the dam is vertical and there is no water on the downstream side of the dam. The total pressure is in horizontal direction and 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
P1 = wh2/2
Where w is the specific weight of water. Usually, it is taken as unity.
H is the height up to which water is stored in m.
(ii) Upstream face with batter and there is no water on the downstream side
Here in addition to the horizontal water pressure P1 as in the previous case, there is vertical pressure of the water. It is due to the water column resting on the upstream sloping side. The vertical pressure P2 acts on the length ‘b’ portion of the base. This vertical pressure is given by
P2 = (b x h2 x w) + (0.5b x h1 x w )
Pressure P2 acts through the centre of gravity of the water column resting on the sloping upstream face.
If there is water standing 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) Water pressure below the base of the dam or Uplift pressure
When the water is stored on the upstream side of a dam there exists a head of water equal to the height upto which the water is stored. This water enters the pores and fissures of the foundation material under pressure. It also enters the joint between the dam and the foundation at the base and the pores of the dam itself. This water then seeps through and tries to emerge out on the downstream end. The seeping water creates the hydraulic gradient between the upstream and downstream side of the dam. This hydraulic gradient causes vertical upward pressure. The upward pressure is known as uplift. Uplift reduces the effective weight of the structure and consequently, the restoring force is reduced. It is essential to study the nature of uplift and also some methods will have to be devised to reduce the uplift pressure value.
With reference to figure, uplift pressure is given by
Pu = (wH x B)/2
Where Pu is the uplift pressure, B is the base width of the dam and H is the height upto which water is stored.
This total uplift acts at B/3 from the heel or upstream end of the dam. Uplift is generally reduced by providing drainage pipes or holes in the dam section. Self weight of the dam is the only largest force which stabilizes the structure. The total weight of the dam is supposed to act through the centre of gravity of the dam section in vertically downward direction. Naturally when specific weight of the material of construction is high, restoring force will be more. Construction material is so chosen that the density of the material is about 2.045 gram per cubic meter.
2. Earthquake Forces
The effect of earthquake is equivalent to an acceleration to the foundation of the dam in the direction in which the wave is travelling at the moment. Earthquake wave may move in any direction and for design purposes, it is resolved into the vertical and horizontal directions. On an average, a value of 0.1 to 0.15g (where g = acceleration due to gravity) is generally sufficient for high dams in seismic zones. In extremely seismic regions and in conservative designs, even a value of 0.3g may sometimes by adopted.
Vertical acceleration reduces the unit weight of the dam material and that of water is to (1- kv) times the original unit weight, where kv is the value of g accounted against earthquake forces, i.e. 0.1 when 0.1g is accounted for earthquake forces. The horizontal acceleration acting towards the reservoir causes a momentary increase in water pressure and the foundation and dam accelerate towards the reservoir and the water resists the movement owing to its inertia. The extra pressure exerted by this process is known as hydrodynamic pressure.
3. Silt Pressure
If h is the height of silt deposited, then the forces exerted by this silt in addition to the external water pressure, can be represented by Rankine formula
acting at h/3 from the base.
ka = Coefficient of active earth pressure of silt =
= angle of internal friction of soil, cohesion neglected.
= submerged unit weight of silt material.
h = height of silt deposited.
4. Wave Pressure
Waves are generated on the surface of the reservoir by the blowing winds, which exert a pressure on the downstream side. Wave pressure depends upon wave height which is given by the equation
for F < 32 km, and
for F > 32 km
Where hw is the height of water from the top of crest to bottom of trough in meters.
V – wind velocity in km/hour
F – fetch or straight length of water expanse in km.
The maximum pressure intensity due to wave action may be given by Pw = 2.4γwhw and acts at hw/2 meters above the still water surface.
The pressure distribution may be assumed to be triangular of height 5hw/3 as shown in the figure.
Hence total force due to wave action acting at 3hw/8 above the reservoir surface and given by
5. Ice Pressure
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 and exerted by the expanding ice. This force acts linearly along the length of the dam and at the reservoir level. The magnitude of this force varies from 250 to 1500 kN/sq.m depending upon the temperature variations. On an average, a value of 500 kN/sq.m may be taken under ordinary circumstances.
6. Weight of dam
The weight of dam and its foundation is a major resisting force. In two dimensional analysis of dam, unit length is considered.
The stability of a dam can be analysed in the following steps:
i. Consider unit length of the dam.
ii. Work out the magnitude and dimensions of all the vertical forces acting on the dam and their algebraic sum, i.e. ?V.
iii. Similarly work out all the horizontal forces and their algebraic sum i.e. ?H.
iv. Determine the lever arm of all these forces above the toe.
v. Determine the moments of these forces about the toe and find the algebraic sum of all those moments, i.e. ?M.
vi. Find out the location of the resulting force by determining its distance from the toe.
vii. Find out the eccentricity (e) of the resultant (R) using
It must be less than b/6 in order to ensure that no tension is developed anywhere in the dam.
viii. Determine the vertical stresses at the toe and heel using
ix. Determine the maximum normal stresses.
x. Determine the factor of safety against overturning as equal to
xi. Determine the factor of safety against sliding using
Sliding factor =
Shear friction factor (S.F.F)
Causes of failure of a Gravity Dam
Failure of gravity dam occurs due to overturning, sliding, tension and compression. A gravity dam is designed in such a way that it resists all external forces acting on the dam like water pressure, wind pressure, wave pressure, ice pressure, uplift pressure by its own self-weight. Gravity dams are constructed from masonry or concrete. However, concrete gravity dams are preferred these days and mostly constructed. The advantage of gravity dam is that its structure is most durable and solid and requires very less maintenance.
A gravity dam may fail in following modes:
- Overturning of dam about the toe
- Sliding – shear failure of gravity dam
- Compression – by crushing of the gravity dam
- Tension – by development of tensile forces which results in the crack in gravity dam.
Overturning Failure of Gravity Dam
The horizontal forces such as water pressure, wave pressure, silt pressure which act against the gravity dam causes overturning moments. To resist this, resisting moments are generated by the self-weight of the dam.
If the resultant of all the forces acting on a dam at any of its sections, passes through toe, the dam will rotate and overturn about the toe. This is called overturning failure of gravity dam. But, practically, such a condition does not arise and dam will fail much earlier by compression.
The ratio of the resisting moments about toe to the overturning moments about toe is called the factor of safety against overturning. Its value generally varies between 2 and 3.
Factor of safety against overturning is given by
FOS = sum of overturning moments/ sum of resisting moments
Fig : sum of external horizontal forces greater than vertical self-weight of dam (overacting, sliding occurs)
Sliding Failure of Gravity Dam
When the net horizontal forces acting on gravity dam at the base exceeds the frictional resistance (produced between body of the dam and foundation), The failure occurs is known as sliding failure of gravity dam.
In low dams, the safety against sliding should be checked only for friction, but in high dams, for economical precise design, the shear strength of the joint is also considered.
Factor of safety against sliding can be given based on
- Frictional resistance
- Frictional resistance and shear strength of the dam
Factor of safety based on frictional resistance:
Gravity Dam Failure due to Tension Cracks
Masonry and concrete are weak in tension. Thus masonry and concrete gravity dams are usually designed in such a way that no tension is developed anywhere. If these dams are subjected to tensile stresses, materials may develop tension cracks. Thus the dam loses contact with the bottom foundation due to this crack and becomes ineffective and fails. Hence, the effective width B of the dam base will be reduced. This will increase pmax at the toe. Hence, a tension crack by itself does not fail the structure, but it leads to the failure of the structure by producing excessive compressive stresses.
For high gravity dams, certain amount of tension is permitted under severest loading conditions in order to achieve economy in design. This is permitted because the worst condition of loads may occur only momentarily and may not occur frequently.
Gravity Dam Failure due to Compression
A gravity dam may fail by the failure of its material, i.e. the compressive stresses produced may exceed the allowable stresses, and the dam material may get crushed.
For detailed notes on "Design and Construction of Concrete Gravity Dams" by NPTEL, Click here
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