1. Dams are constructed across rivers to store flowing water for uses like hydropower, irrigation, water supply, flood control, and navigation. 2. The key forces acting on a gravity dam include its self-weight, which provides stability, and water pressure from the reservoir, which acts to overturn the dam. Uplift, earthquake loads, silt pressure, and ice pressure are other important forces that must be estimated based on assumptions and available data. 3. The weight of the dam per unit length is calculated based on the cross-sectional area and unit weight of the concrete or masonry used. The total weight acts at the centroid of the cross-section and is the main stabil

1. FORCE ACTING ON GRAVITY
DAM
DIVYA VISHNOI
ASSISTANT PROFESSOR

2. Dams
• Dam is a solid barrier constructed at a suitable
location across a river valley to store flowing water.
Storage of water is utilized for following
objectives:
• Hydropower
• Irrigation
• Water for domestic consumption
• Drought and flood control
• For navigational facilities
• Other additional utilization is to develop fisheries

4. Structure of Dam
Heel
Gallery
Toe
Spillway
(inside dam)
Crest
NWL
Normal
water level
MWL
Max. level
Free board
Sluice way
Upstream Down stream

6. Basic definition
1. 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.
2. 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.
3. 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.
4.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.
5. 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 river bed.

7. Basic definition
6. Heel: contact with the ground on the upstream side
7. Toe: contact on the downstream side
8. Abutment: Sides of the valley on which the structure of
the dam rest
9. Galleries: small rooms like structure left within the dam
for checking operations.
10.Diversion tunnel: Tunnels are constructed for diverting
water before the construction of dam. This helps in
keeping the river bed dry.
11.Spillways: It is the arrangement near the top to release
the excess water of the reservoir to downstream side
12.Sluice way: An opening in the dam near the ground
level, which is used to clear the silt accumulation in the
reservoir side.

9. Forces Acting on a Gravity Dam
• These forces fall into two categories as:
a) Forces, such as weight of the dam and water
pressure, which are directly calculable from
the unit weights of the materials and
properties of fluid pressures; and
b) Forces, such as uplift, earthquake loads, silt
pressure and ice pressure, which can only be
assumed on the basis of assumption of
varying degree of reliability.

10. Forces Acting on a Gravity Dam
• It is in the estimating of the second category
of the forces that special care has to be taken
and reliance placed on available data,
experience, and judgment.
• It is convenient to compute all the forces per
unit length of the dam.

11. 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.
• For convenience, the cross-section of the dam is divided into
simple geometrical shapes, such as rectangles and triangles, for the
computation of weights. The areas and centroids of these shapes
can be easily determined. Thus the weight components
• W1, W2, W3 etc. can be found along with their lines of action.
• The total weight W of the dam acts at the C.G. of its section.

12. Self weight of Dam

47. RTU QUESTIONS
• Describe the various forces acting on gravity
dam with suitable sketch.
• What are the modes of failure of gravity dam?
Explain.

48. References
• Irrigation Engineering & Water Power Engineering
– By Prof B.C.PUNMIA
– Laxmi Publication
• Irrigation Engineering & Hydraulic Structures
– By Prof. Santosh Kumar Garg
– Khanna Publishers
• Irrigation, Water Power Engineering & Hydraulic Structures
– By Prof K.R. Arora
– Standard Publishers Distributions
• Internet Websites
• http://www.aboutcivil.org/
• http://nptel.ac.in/courses/105105110/

49. Thanks