The dam is an important civil engineering structure that is multi-functional and used throughout the world. From simple water supply and irrigation works to huge hydropower generation plants and disaster control works; all require the construction of the dam. One such important type of dam is the embankment dam.
In a very simple sense, an embankment dam can be understood as a large water impounding structure. Embankment dam essentially is made up of earth and rock fragments. Embankment dams are flexible structures that can deform in accordance with the deflection of the foundation without any significant damage. Such type of dam has been used for various purposes from the beginning of civilization. Some of the embankment dams around the world are about 2000 years old.
The notable examples of embankment dam can be listed as follows:
1. Nurek Dam, Tajikistan ( 300m high)
2. Tehri Dam, India ( 260.5m high)
3. Alberto Lleras Dam, Colombia ( 247m high)
4. Mica Dam, Canada ( 240m high)
5.Oroville Dam, USA ( 235m high)
6. Keban Dam, Turkey ( 210m high)
A typical embankment dam consists of the following components:
The foundation of an embankment is the supporting component that withstands both horizontal as well as vertical load. The foundation may be made up of rock or soil. The foundations in which rock is used as the chief material has high strength and free from faults and defects. Sand and gravels may also be used for the construction of the foundation as they provide good support. However, the seepage through them must be taken care of properly.
The casing is the component of an embankment dam that protects the inner core. The upstream and downstream slopes of a casing have to be decided on the basis of the type of dam, height, availability of material and the condition of the foundation. A flatter slope is built in case of low permeability material. The desirable range for downstream slope varies from 2:1 to 2.5:1 while the range for upstream slope varies from 2:1 to 4:1.
The core is the component of an embankment dam that checks the seepage of water through the body of the dam. Due to this reason, it is commonly referred to as an impermeable barrier. The material used for the construction of core should be selected depending upon the topography, availability of material, diversion considerations and suitability. The position of the core may be either central or inclined upstream. The core must be constructed such that its top-level lies at least 1m above the maximum water level. The minimum width of the core should preferably not less than 3m.
TYPES OF EMBANKMENT DAM
An embankment dam can be basically classified on the basis of the fill material used into the following types:
I. Earthen Dam
The earthen dam is the type of embankment dam that essentially consists of suitable soil compacted into layers by some mechanical means. The soil used must have acceptable engineering properties. It is usually obtained from burrow pits or excavations. Most commonly, the soil from the excavation or pits is transported and dumped to site. It is then spread in layers at suitable depths and duly compacted by means of tamping rollers, sheep foot rollers, vibratory rollers or other earth rolling equipment.
Some of the advantages of the earthen dam can be listed as follows:
1. Utilization of locally available material.
2. The design and construction are relatively simple and easy.
3. Such dams have high resistance to settlements and movement of ground underneath.
4. The equipment and plant required are simple and small.
Some of the disadvantages of the earthen dam can be listed as follows:
1. An earthen dam has a high risk of being damaged by the heavy flow of water. Hence, additional protective structures may have to be constructed.
2. If the compaction is not adequate or improper, the foundation becomes weak and prone to failure.
3. Such dams require high maintenance to check for tree growths, erosion, seepage etc.
II. Rock-Fill Embankment Dam
Rock-fill embankment dam is the type of embankment dam that essentially consists of a fragment of rocks with an impervious core. Usually, the core and the rock fragments are separated by a series of transition zones made up of suitable graded materials. The rockfill materials must be duly compacted by the use of rubber-tired, steel-wheel or vibratory rollers.
Some of the advantages of the rockfill dam can be listed as follows:
1. The initial cost of dam construction is relatively low.
2. Such dams are suitable for any type of foundation.
3. The construction process can be carried out rapidly. It is a fast process.
Some of the disadvantages of the rockfill dam can be listed as follows:
1. The life span of the rockfill dam is comparatively less.
2. Such dams are not suitable to be constructed as high dams.
3. Rockfill dams require frequent maintenance. The cost of maintenance is also high.
4. There exists the possibility of seepage through the dam.
DESIGN OF EMBANKMENT DAM
The design of a typical embankment dam consists of the following steps:
1. Preliminary investigation is first carried out. This is done in order to collect the necessary topographical and hydrological data of the proposed site. Aerial photographs and large-scale maps are studied. If required, the field visit should be done.
2. The next step includes the determination of catchment yield. Catchment yield is an important factor for assessing the feasibility of the dam. It is calculated using the following formula:
Y= RF * A * 1000
Y = Catchment Yield in a cubic meter(cu.m)
RF= Annual runoff for a catchment in mm
A = Catchment Area in square kilometre (sq. km)
3. The storage capacity is then computed as,
Q = LTH/6
Q = storage capacity in a cubic meter (cu. m)
L= Length of the dam wall at full supply level in meter
T= Throwback in meter
H= maximum height of embankment in meter
4. The height of the embankment dam is computed using depth-capacity curve and reservoir area capacity curve.
5. Then, freeboard must be added to the height of embankment so as to maintain safety. Generally, 10-15% of the freeboard is provided to the highest flood level.