ARCH DAMS

Dear Friends Greetings ! Today let us have a look over Arch dams.

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.

Many early arch dams were built of rubble, ashlar masonry, etc.

However, practically all arch dams constructed during recent years have been built of concrete.

The famous Hoover Dam of U.S.A., constructed in 1936, is an 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).

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.

Types of Arch Dams.

The arch dams may he divided into the following  types:

1. Constant radius dam.

2. Variable radius dam, and

3. Constant angle dam.

4.Double Curvature dam.

Classification of an arch dam depends on its angle, radius and curvature. There are four types of arch dams i.e. constant radius arch dam, variable radius arch dam, constant angle arch dam, double curvature arch dam.

An arch dam is curve shaped solid wall which is generally built with cement concrete. It is preferred where river valleys are formed.

The working nature of arch dam is like partly cantilever retaining wall and partly arch action. The whole curved wall stands with its large base and transfers the loads to the two ends of the dam using horizontal thrust.

Another important point is that the load carrying capacity of arch dam also depends on its curvature, i.e. greater the curvature larger will be the load resisted. Different types of arch dams constructed and used in the world and their characteristics are discussed below.

Types of Arch Dam Construction

Arch dams are generally classified into four main types:

Constant radius arch dam

Variable radius arch dam

Constant angle arch dam

Double curvature arch dam

Constant Radius Arch Dam

Constant Radius Arch dam, in which outer side curve of arch is built with constant radii at all elevations. At the same time the inner curve of arch is constructed with decreasing radius from top to bottom of dam.

The outer side curve of an arch is called as extrodos while the inner curve is called as introdos. Extrados always faces the upstream side and intrados faces the downstream side.

The thickness of dam is more at the base to withstand against hydrostatic pressure from water and minimum thickness is provided at the top. The cross section of constant radius arch dam look like right angled triangle in which vertical line represents extrados and hypotenuse represents intrados.

In this type of arch dam, all the centers of circular curves of intrados lie on same line and on the same line the centers of extrados also lie. So, this dam can also be called as constant center arch dam.

The constant radius arch dams require large amount of concrete for its construction. These are the most common types of arch dams and they are generally preferred for U-Shaped valleys.

The central angles of constant radius arch dam are not constant. The angle made by every arch ring at its center vary at different elevations. The central angle is maximum at the top of the dam and minimum at the bottom of the dam.

But there are some recommendations in which the safest angle for an arch dam is derived as 133o 34’. But due to its shape it is not possible to provide constant angle throughout the dam.

Hence, this safest angle is provided for the middle arch ring. From the middle ring the angle will increase towards top and decrease towards base of dam. But at the top of the dam the angle should not be more than 150o hence, during its design the top angle is managed to below 150o.

Variable Radius Arch Dam.

Variable radius arch dam, as in the name itself the radii of both extrados (outer side) and intrados inner side) curves are varied with respect to elevation. The radius is maximum at the top of the dam and minimum at its base. In this case also central angles of arch rings are varied at different heights hence it is also called as variable center arch dam.

The center line or crown line on downstream face is looks like vertical line and both sides of this line look like battered from the top view. At abutments also, it looks like vertical line similarly upstream side also looks vertical.

Hence, we can provide over hangings at the crown as well as at the abutments which can reduce the designed thickness of the dam to reduce the amount of concrete and make it economical.

In general, variable radius arch dam is economical than constant radius arch dam. It uses only 58% of the amount of concrete used for constant radius arch dam. This type of arch dam is more suitable for V-shaped valleys.

Constant Angle Arch Dam.

Constant angle arch dam, is a dam in which central angle of every arch ring is constant throughout the height of an arch dam. It is obtained by the modification of variable radius arch dam. Safe and best central angle 1330 34’ is used as central angle in this case.

For this type of arch dam, it is necessary to provide over hangings at the crown and at abutments. The provision of over hangings also be done when the foundation is stronger and is not suitable for weaker foundations.

This type of arch dam is more economical than the above two types and requires just 43% of the amount of concrete used to build constant radius arch dam. Constant angle arch dam is more suitable for V-shaped valleys.

Double Curvature Arch Dam

Double curvature arch dam, in which curvature is provided not only in horizontal direction but also in vertical direction. It means the cross section of double curvature also looks like curve.

The whole dam is looks like shell type so, it is also called as shell arch dam.

Because of double curvature, the thickness of arch wall is reduced. But this non-vertical type dams are difficult to construct compared to other three types. They also require stronger foundations. Shell arch dams are more suitable for very narrow valleys.

Why no shutter has been provided in the arch portion of Idukki( Double curvature) dam?

Idukki dam is a double curvature thin arch dam.it means dam is curve both in X and Y direction.look like half broken coconut.so if we provide the shutter will be in immersed condition. So shutters are provided in cheruthoni (5 shutters).

In Tehri dam a funnel shaped arrangement is provided at F.R.L. the surplus water will go into funnel and reach a shaft from there through a tunnel water will be let out. This arrangement is called "Morning glory".but Idukki is double curvature and "thin" dam so this arrangement is not possible.

colorodo dam(Hoover dam) is single arch dam,so shutter is there.

Few important and Wonderful arch dams Enjoy the Technology.

Hoover dam's beauty at its Height.

Jingpin-I Dam i

Currently, the tallest arch dam in the world is the 305 metres (1,001 ft) Jingpin-I Dam in China, which was completed in 2013. 
Daniel-Johnson Dam

The longest multiple arch with buttress dam in the world is the Daniel-Johnson Damin Quebec, Canada. It is 214 meters (702 ft) high and 1,314 meters (4,311 ft) long across its crest.

With warm regards.

Leela.G