Arch dam.pptx

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 Introduction to Arch Dam Conditions favoring to Arch dam Classification of Arch Dam 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. Arch dams are particularly adapted to the gorges where the length is small in proportion to the height. 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. Because of much less base width, the problems of uplift pressure are minor. 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. It requires skilled labor and sophisticated form work. The design of an arch dams are also quite specialized The speed of construction is normally slow.  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 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. A V- Shaped Valley with strong abutments is more suitable and ideal condition for the construction of Arch dam. 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. 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 eleva tions, 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.

11. Buttress Dam

12. 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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