The dam is an important civil engineering structure that is multifunctional and used throughout the world. From simple water supply works, 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 earthen dam. The earthen dam is the type of embankment dam that is composed of the earth which is compacted into subsequent layers of suitable thickness. This dam is also called as earth dam.
In other words, 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 transported and dumped to site. It is then spread in layers at suitable depths and duly compacted utilizing tamping rollers, sheep foot rollers, vibratory rollers or other earth rolling equipment.
The earthen dam is the earliest type of dam known to have been constructed from the beginning of human civilization.
Some of the notable earthen dam built across the world can be listed as follows:
1. Tarbela Dam, Pakistan
2. Mica Dam, Canada
3. Tataragi Dam, Japan
Components of Earthen Dam
A typical earthen dam consists of the following components:
The foundation is the supporting component that withstands both horizontal as well as vertical load. The foundation is made up of soil.
The casing is the component of an earthen dam that protects the inner core. The upstream and downstream slopes of a casing have to be decided based on the type of dam, height, availability of material and the condition of the foundation. A flatter slope is built in case of low permeability earth. 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 earthen 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.
Materials Used in Earthen Dam
The following materials are commonly used in the earthen dams:
1. Clayey Material
2. Sandy Soil
3. Murum, Sandy silt ( For casting)
4. Rock masonry ( For pitching and Riprap)
5. Cement, steel, line and other materials in a small amount for the construction of spillway and outlets.
Types of Earthen Dam
The earthen dam can be classified into the following types:
1. Based on Mode of Construction
a. Homogeneous Type
In this type of earthen dam; Homogeneous sections are constructed with one type of soil. Soil should have sufficient frictional resistance, low permeability and should be available in adequate quantity near the site. They are rarely used.
b. Zone Type
This type of earthen dam is commonly used. It consists of mainly two parts i.e. Core and Casing. The core is generally constructed with clayey soil like black cotton soil to improve the water tightness against seepage. The casing is made up of soft rocks, gravel or murum which provides stability to the dam.
2. Based on the Method of Construction
a. Rolled Fill Type
i. It is the common method of constructing the earthen dam.
ii. Earthmoving types of machinery are used for excavating and placing of soil.
iii. Soils are placed in layers with a thickness of 20cm.
iv. Compaction is done at optimum moisture content condition.
b. Hydraulic Fill Type
i. Construction, excavation, Transporting and placing of soil is carried by a hydraulic method.
ii. No needed of compaction as the soil gets consolidated during the operation of hydraulic.
Conditions of Stability of Earthen Dam
a. The dam height should be greater than the water overflow height.
b. The seepage line should be maintained and repaired in time after noticing in downstream (d/s) face of the dam.
c. Both downstream(d/s) and upstream (u/s) of the dam should be enough stable to resist many worst conditions.
d. The d/s and u/s should be constructed in such a way that water overflow can’t resist there.
e. It’s necessary to find pore pressure in the dam’s foot.
f. The foundation of the dam should be within safe shear stress limit.
g. The problem of water waves and burrowing of animas is common in dam sites. Thus, the u/s slope should be conserved.
Causes of Failure of Earthen Dam
The earthen dam has less rigidity that prone to cause failure. There are mainly four causes of failure of the earthen dam. They are explained below:
a. Hydraulic Failure
This is the most possible failure occurred in an earthen dam. And this failure occurs due to the following reasons:
Due to improper design, spillway may have insufficient capacity to handle the overtopping flow of water in the dam and this leads to failure.
The water waves at u/s slope may cause erosion and that may lead to the failure.
Seepage, heaving and cracking may occur at u/s face of the dam due to frost action.
Gullies (erosion water concentrated drainage) on the d/s face may cause due to heavy rainfall resulting failure.
b. Seepage Failure
Seepage in the foundation may lead to piping or sloughing(soil removal process from wet d/s face) that may cause the failure of the dam.
Seepage through the embankment & abutment leads to internal erosion resulting failure.
c. Structural Failure
The dam with extreme steeper slopes leads to slip of d/s and u/s slopes.
The unstable water level in the reservoir cause slips of u/s.
d. Earthquake Failure
Earthquake magnitude determines the amount of failure. The weak earthen dam may be collapsed due to the combined force of water and seismic waves.
Advantages of Earthen Dam
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 the ground underneath.
4. The equipment and plant required are simple and small.
Disadvantages of Earthen Dam
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.
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