Introduction to Arch Dam and Buttress Dam

1. By
Prof. V. S. Chaudhari (Assistant Professor)
Department of Civil Engineering
Sanjivani College of Engineering, Kopargaon-423603
Subject: Dams and Hydraulic Structures
Introduction to Arch and Buttress Dams
Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon, 423603.
An Autonomous Institute Affiliated to Savitribai Phule Pune University, Pune

2. Contents
1. Introduction to Arch Dam
2. Conditions favoring to
Arch dam
3. Classification of Arch
Dam
4. Introduction to Buttress
Dam

3. Introduction
 Arch dam may be defined as a solid wall curved in plan standing across the entire width of
the river valley in a single span.
 The dam body is usually made up of cement concrete, although rubble and stone masonry has
also been used in the past.
 An arch dam is a dam curved in plan and carries at major part of its water load
horizontally to the abutments by arch action. This part of water load depends primarily
upon the amount of curvature.
 The balance of the water load is transferred to the foundation by cantilever action.
 The thrust developed by the water load carried by arch action essentially requires strong side
walls of the canyon to resist the arch forces.
 The weight of arch dams is not counted on to assist materially in the resistance of external
loads. For this reason, uplift on the base is not an important design factor.
 The famous Hoover dam of U.S.A., constructed in 1936, is an arch dam of constant radius type
having the following dimensions:
Height 222 m (726 ft); Length 380 m (1244 ft); Top width 13.7 m (45 ft) and Base thickness 201 m (660 ft).
 In India the arch dam named as Iddiki Dam is built across the river Periyar in Kerala having a height of
170 m, width of 7.6 m, and base thichness of 19 to 8 m.

4. Advantages of Arch Dam.
1. Arch dams are particularly adapted to the gorges where the length is small in proportion to the
height.
2. For a given height, the section of an arch dam is much lesser than a corresponding gravity dam.
Hence, an arch dam requires less material and is, therefore, cheaper.
3. Because of much less base width, the problems of uplift pressure are minor.
4. Since only a small part of water load is transferred to the foundation by cantilever action, an arch
dam can be constructed in moderate foundations where gravity dam requiring sound foundation
rock may be unsuitable.
Disadvantages of Arch Dam.
1. It requires skilled labor and sophisticated form work. The design of an arch dams are also quite
specialized
2. The speed of construction is normally slow.
3. It requires very strong abutments of solid rock capable of resisting arch thrust.
Hence, it is not suitable in the locations where strong abutments are not available.
Unfortunately, only few sites are suitable for this type of dam.

5. Conditions favoring to Arch Dam
1. In case of Arch dam, amount of curvature of the dam, shape of the valley plays vital role because
arch action is depend on the curvature of the dam, shape of the valley.
2. A V- Shaped Valley with strong abutments is more suitable and ideal condition for the
construction of Arch dam.
3. L/H ratio should be less than 1.5
Where, L= Crest length of V shaped valley
H= Height of Valley
According to current practice the L/H ratio of 2 to 6 is also considered as practicable.
4. U shaped valley also fairly suitable for the construction of arch dam

6. Types of Arch Dams
The arch dams may he divided into the following three types:
1. Constant radius dam.
2. Variable radius dam, and
3. Constant angle dam.

7. 1. Constant radius dam.
 A constant radius arch dam is that in which the radii of the outside curved surface are equal to all
elevations from top to bottom.
 The centers of all such circular arch’s called extrodos wall, therefore evidently lie on one vertical line.
 However the intrados has gradually decreasing radius from top to bottom, so as provided
increased concrete thickness towards the base for accounting the proportionally increasing
hydrostatic water pressure of the reservoir. The dam body will therefore be triangular in C/S with U/S
face vertical and minimum thickness at the top.
U/S Face
(extrados
wall)
D/S Face
(intrados wall)
River Flow
Water

8.  Evidently it is only the radii of the intrados which decreases with depth, while the centers of all
such circular arch’s continue to lie on the same vertical line, on which the center of extrados lies.
 Hence in such dam’s the center of extrados and intrados as well as the center line of horizontal
arch ring at various elevations lie on a straight vertical line that passes through center of
horizontal arch ring at the crest.
 Such dam is therefore sometimes called as constant center arch dam, although strictly speaking,
this center is not at one point but lies at different height and along one vertical line.

9. Variable Radius Arch Dam :
 The dams in which the radius of the extrados and intrados curves diverges at different heights is
called the variable radius arch dams.
 In such a dam, the radius at the top of the dam is maximum while it is minimum at the base,
these types of dams are suitable for both U-shaped and V-shaped basins for their proper
construction.
 Such a dams is generally more economical than a stable radius arch dams.

10. Constant angle arch dam:
 The dam in which the middle angles of the arches are constant and radial at different heights is
called the constant angle arches.
 In such dams, a central angle of the arch rings at all elevations is identical magnitude from the
height of the dams to its base.
 For economical design of constant angle arch dams, the central angle should be from 133°- 134°
based on the thin cylinder principle.
 But in the actual practice of construction, the central angle can vary from 100 ° to 150 °
depending on the size of the valley.
 In such dams the maximum arch action is established in the arch rings at all elevations, the
thickness of a constant-angle arch dam is smaller than that of a constant radius or variable
radius arch dams.
 It has been shown that the volume of concrete is minimum when the central angle is 133° 34′.
 A variable radius arch uses about 58% of the concrete required in constant radius arch dams.
 A constant angle arch dams requires about 42.6% of the concrete required in constant radius
arch dams.
Hence, a constant angle arch dam is the most economical.

12. Buttress Dam

13. What is Buttress Dam
 A buttress dam is a modification or improvisation of gravity dam.
 Buttresses are nothing but solid walls which are constructed parallel to the water
flow with some specified thickness at regular intervals.
 On these buttresses, an arch slab or inclined deck slab is provided to support
upstream water.
 Generally, gravity dam requires more concrete. To minimize the cost, concrete
quantity is minimized by clearing it from the unstressed area of dam. Finally, an
important innovation takes place which is nothing but buttress dam.

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