This document discusses the key forces acting on a gravity dam, including its weight, water pressure, uplift pressure, silt pressure, wave pressure, and earthquake forces. It defines key terms like structural height, maximum base width, and hydraulic height. It also provides details on how to calculate or estimate the various forces, for example explaining that water pressure acts normal to the face of the dam and can be calculated based on horizontal and vertical components. Uplift pressure is defined as the upward pressure of water seeping through the dam or its foundation. Earthquake forces cause random vibrations that impart accelerations to the dam's foundation.
Topics:
1. Types of Gravity Dam
2. Forces Acting on a Gravity Dam
3. Causes of failure of Gravity Dam
4. Elementary Profile of Gravity Dam
5. Practical Profile of Gravity Dam
6. Limiting height of Gravity Dam
7. Drainage and Inspection Galleries
Types- selection of the suitable site for the diversion headwork components
of diversion headwork- Causes of failure of structure on pervious foundation- Khosla’s theory- Design of concrete sloping
glacis weir.
Topics:
1. Types of Gravity Dam
2. Forces Acting on a Gravity Dam
3. Causes of failure of Gravity Dam
4. Elementary Profile of Gravity Dam
5. Practical Profile of Gravity Dam
6. Limiting height of Gravity Dam
7. Drainage and Inspection Galleries
Types- selection of the suitable site for the diversion headwork components
of diversion headwork- Causes of failure of structure on pervious foundation- Khosla’s theory- Design of concrete sloping
glacis weir.
WEIRS VERSUS BERRAGE
TYPES OF WEIRS
COMPONENT PARTS OF A WEIR
CAUSES OF FAILURE OF WEIRS & THEIR REMEDIES
DESIGN CONSIDERATIONS
DESIGN FOR SURFACE FLOW
DESIGN OF BARRAGE OR WEIR
Canal fall- necessity and location- types of falls- Cross regulator and
distributory head regulator- their functions, Silt control devices, Canal
escapes- types of escapes.
It contains detailed information about a Gravity Dam........it also conataims the information in brief & pictures giving a clear view of the Gravity Dams...........It also contains formulas with details of their terms.........
Topics:
1. Causes of Failures of Weirs on Permeable Foundations
2. Bligh’s Creep Theory
3. Lane’s Weighted Creep Theory
4. Khosla’s Theory
5. Application of Correction Factors
6. Launching Apron
WEIRS VERSUS BERRAGE
TYPES OF WEIRS
COMPONENT PARTS OF A WEIR
CAUSES OF FAILURE OF WEIRS & THEIR REMEDIES
DESIGN CONSIDERATIONS
DESIGN FOR SURFACE FLOW
DESIGN OF BARRAGE OR WEIR
Canal fall- necessity and location- types of falls- Cross regulator and
distributory head regulator- their functions, Silt control devices, Canal
escapes- types of escapes.
It contains detailed information about a Gravity Dam........it also conataims the information in brief & pictures giving a clear view of the Gravity Dams...........It also contains formulas with details of their terms.........
Topics:
1. Causes of Failures of Weirs on Permeable Foundations
2. Bligh’s Creep Theory
3. Lane’s Weighted Creep Theory
4. Khosla’s Theory
5. Application of Correction Factors
6. Launching Apron
DAMS
Types of dams
Selection of dam sites
Geological characters for investigation
Selection of the dam type
Gravity dams
butress dams
embankment dams
arch dams
cupola dams
composite dams
Bhakra Dam
Mir Alam multi-arch dam
Idukki Dam
Tehri Dam
Ujani Dam or bhima dam
FORCES ACTING ON GRAVITY DAM
The Bureau of Indian Standards code IS 6512-1984 “Criteria for design of solid gravity dams” recommends that a gravity dam should be designed for the most adverse load condition of the seven given type using the safety factors prescribed.
1. Load combination A (construction condition): Dam completed but no water in reservoir or tail water
2. Load combination B (normal operating conditions): Full reservoir elevation, normal dry weather tail water, normal uplift, ice and silt (if applicable)
3. Load combination C: (Flood discharge condition) - Reservoir at maximum flood pool elevation ,all gates open, tail water at flood elevation, normal uplift, and silt (if applicable)
4. Load combination D: Combination of A and earthquake
5. Load combination E: Combination B, with earthquake but no ice
6. Load combination F: Combination C, but with extreme uplift, assuming the drainage holes to be Inoperative
7. Load combination G: Combination E but with extreme uplift (drains inoperative)
Water Pressure (P) is the major external force exerted by the water stored in the Reservoir on the upstream face of the dam. It can be calculated by the law of hydrostatic pressure distribution; which is triangular in shape as shown in Fig. 3.3.
(a) When u/s face is vertical :
When the upstream face is vertical, the intensity of pressure is zero at the water surface and equal to γw • H at the base.
Earth quake pressure, Horizontal Component(PH) , (ii) Vertical Component(PV) = Weight of water in ABCD portion ,
2. Weight of the Dam :
The weight of the dam per unit length is given by the product of the area of crosssection of the dam and the specific weight of the Construction material, i.e. concrete, and masonary it acts vertically downwards at the centre of gravity of the section.
dam may be divided into smaller sections of simple geometrical shapes such as triangles,rectangles, etc.
weight of each of these acting at its centre of gravity may be considered.
Weight of any part of dam = cross-sectional area of that part x specific weight of material
3. Uplift Pressure :
Uplift pressure is defined as the force exerted by water penetrating through the pores, cracks, fissures within the body of the dam, at the contact between the dam and its
foundation, and within the foundation.
acts vertically upwards
it causes a reduction in the effective weight
Ice Pressure :
Ice pressure is exerted on a dam by a sheet of ice formed on the entire water surface of the reservoir, when it is subjected to expansion and contraction with changes in temperature.
The coefficient of thermal expansion of ice being five times more than that of concrete, the dam face has to resist the force due to expansion of ice. This force acts linearly along the length of the dam, at the reservoir level.
As per IS : 6512 - 1984, ice pressure may be taken equal to 250 kN/m2 applied to the face of the dam over the anticipated area of contact of i
Types of Gravity Dam
Forces Acting on a Gravity Dam
Causes of failure of Gravity Dam
Elementary Profile of Gravity Dam
Practical Profile of Gravity Dam
Limiting height of Gravity Dam
Drainage and Inspection Galleries
A gravity dam is a solid structure, made of concrete or masonry, constructed across a river to create a reservoir on its
upstream. The section of the gravity dam is approximately triangular in shape, with its apex at its top and maximum width at bottom.
The section is so proportioned that it resists the various forces acting on it by its own weight. Most of the gravity dams are solid, so that
no bending stress is introduced at any point and hence, they are sometimes known as solid gravity dams to distinguish them from hollow
gravity dams in those hollow spaces are kept to reduce the weight. Early gravity dams were built of masonry, but now-a-days with
improved methods of construction, quality control and curing, concrete is most commonly used for the construction of modern gravity
dams.
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2. Gravity Dam
Forces acting on gravity dam
Computation of forces
Content
Weight
Water pressure
Uplift pressure
Silt pressure
Wave pressure
Earthquake forces
3. Gravity Dam
A gravity dam is a solid structure, made of concrete or masonry,
constructed across a river to create a reservoir on its Upstream.
The section of the gravity dam is approximately triangular in shape,
with its apex at its top and maximum width at bottom.
The section is so proportioned that it resists the various forces acting
on it by its own weight.
Gravity dams are usually consist of two sections; namely, the non-
overflow section and the overflow section or spillway section.
4. Basic definitions
Axis of the dam: is the line of the upstream edge of the top (or crown)
of the dam. The axis of the dam in plan is also called the base line of the
dam. The axis of the dam in plan is usually straight.
Length of the dam: is the distance from one abutment to the other,
measured along the axis of the dam at the level of the top of the dam.
Structural height of the dam: is the difference in elevations of the top
of the dam and the lowest point in the excavated foundation. It,
however, does not include the depth of special geological features of
foundations such as narrow fault zones below the foundation. In
general, the height of the dam means its structural height.
5. Basic definitions
Toe and Heel: The toe of the dam is the downstream edge of the base,
and the heel is the upstream edge of the base.
Maximum base width of the dam: is the maximum horizontal distance
between the heel and the toe of the maximum section of the dam in
the middle of the valley.
Hydraulic height of the dam: is equal to the difference in elevations of
the highest controlled water surface on the upstream of the dam (i. e.
FRL) and the lowest point in the riverbed.
7. Forces Acting on a
Gravity Dam
1. Weight of the dam
2. Water pressure
3. Uplift pressure
4. Wave pressure
5. Earth and Silt pressure
6. Earthquake forces
7. Ice pressure
8. Wind pressure
9. Thermal loads.
8. Weight of Dam
Main stabilizing force in a gravity dam.
Dead load = weight of concrete or masonry or both + weight of such
appurtenances as piers, gates and bridges.
Weight of the dam per unit length is equal to the product of the area
of cross-section of the dam and the specific weight (or unit weight) of
the material.
Unit weight of concrete (24 kN/m3) and masonry (23 kN/m3) varies
considerably depending upon the various materials that go to make
them.
9. Water pressure
Water pressure on the upstream face is the main destabilizing (or
overturning) force acting on a gravity dam.
Tail water pressure helps in the stability.
The water pressure always acts normal to the face of dam.
It is convenient to determine the components of the forces in the
horizontal and vertical directions instead of the total force on the
inclined surface directly.
12. Uplift pressure
Water has a tendency to seep through the pores and fissures of the
material in the body of the dam and foundation material, and through
the joints between the body of the dam and its foundation at the base.
The seeping water exerts pressure.
The uplift pressure is defined as the upward pressure of water as it
flows or seeps through the body of dam or its foundation.
A portion of the weight of the dam will be supported on the upward
pressure of water; hence net foundation reaction due to vertical force
will reduce.
13. Earth pressure
Gravity dams are subjected to earth pressures on the downstream and
upstream faces where the foundation trench is to be backfilled. Except
in the abutment sections in specific cases, earth pressures have usually
a minor effect on the stability of the structure and may be ignored.
Silt is treated as a saturated cohesionless soil having full uplift and
whose value of internal friction is not materially changed on account of
submergence.
15. Ice pressure
Ice expands and contracts with changes in temperature.
In a reservoir completely frozen over, a drop in the air temperature or
in the level of the reservoir water may cause the opening up of cracks
which subsequently fill with water and freezed solid. When the next
rise in temperature occurs, the ice expands and, if restrained, it exerts
pressure on the dam.
Good analytical procedures exist for computing ice pressures, but the
accuracy of results is dependent upon certain physical data which have
not been adequately determined.
Ice pressure may be provided for at the rate of 250 kPa applied to the
face of dam over the anticipated area of contact of ice with the face of
dam.
16. Wind Pressure
Wind pressure does exist but is seldom a significant factor in the design
of a dam.
Wind loads may, therefore, be ignored.
17. Thermal Load
The cyclic variation of air temperature and the solar radiation on the
downstream side and the reservoir temperature on the upstream side
affect the stresses in the dam.
Even the deflection of the dam is maximum in the morning and it goes
on reducing to a minimum value in the evening.
Measures for temperature control of concrete in solid gravity dams
are adopted during construction.
Thermal are not significant in gravity dams and may be ignored.
18. Wave pressure
The upper portions of dams are subject to the impact of waves.
Wave pressure against massive dams of appreciable height is usually
of little consequence.
The force and dimensions of waves depend mainly on the extent and
configuration of the water surface, the velocity of wind and the depth
of reservoir water.
The height of wave is generally more important in the determination
of the free board requirements of dams to prevent overtopping by
wave splash.
19. Wave pressure
Wind velocity of 120 km/h over water in case of normal pool condition
and of 80 km/h over water in case of maximum reservoir condition
should generally be assumed for calculation of wave height if
meteorological data is not available.
Sometimes the following Molitor’s empirical formulae are used to
estimate wave height.
where Vw = wind velocity in km/hr and F = fetch length of reservoir in
km.
20. Earthquake forces
An earthquake sets random vibrations (waves) in the earth's crust,
which can be resolved in any three mutually perpendicular directions.
This motion causes the structure to vibrate.
The waves impart accelerations to the foundations under the dam
and causes its movement.
Acceleration introduces an inertia force in the body of dam and sets
up stresses initially in lower layers and gradually in the whole body of
the dam.
The vibration intensity of ground expected at any location depends
upon the magnitude of earthquake, the depth of focus, distance from
the epicenter and the strata on which the structure stands.
21. Earthquake forces
The earthquake force experienced by a structure depends on its own
dynamic characteristics in addition to those of the ground motion.
Response spectrum method takes into account these characteristics
and is recommended for use in case where it is desired to take such
effects into account.
IS:1893 - 1984 code specifies design criteria under earthquake
condition.
As per IS Code, for dams up to 100 m height, the seismic coefficient
method shall be used for the design of the dams; while for dams over
100 m height the response spectrum method shall be used.