What is Earthen dams and Types of the earthen dam

Earthen dam

• Earthen dams are the most ancient type of embankments as they are generally built of locally available materials in their natural state with minimum amount  of processing , but in early days the cost of carriage and dumping of the material was fairly high however the modern development in the earth moving equipments have considerable  reduce the cost of carriage and dumping of the  materials
• on the other hand the cost of the gravity dam increased because of the increase in the cost of masonary country accept terms are still cheaper as the used locally available materials and less skilled labour is needed for them, gravity and arc dams required sound Rock foundations ,whereas our 
• Earthen dams can be easily built on earth foundation  
• Earthen dams are most susceptible to failure then rigid gravity dam or arch dam
• Embankment dams are cheaper but the maintenance cost is high.
• Earthen dams are more earthquake resistant than that of the gravity dams due to some elsatic nature of the soil.

Types of Earthen Dams

1. Homogenous Dam
2. Zoned Dam
3. Diaphragm Dam

Homogeneous Dam

• These dams are constucted with a single material throughout.
• These Dams are only suitable for low to moderatly high dams i.e 30m-50m .
• These dams are succeptible to seepage and piping failure so large sections are required .
• The internal drainage system controls the pheratic line within the dam body and to reduce the resistace to the flow of water.
• Filters are provided to control the piping actions.

Pheratic line 

It is the top most saturated line.

Above the pheratic line the dam body is dry on the downstream side.

Types of Drainage systems for the homogeneous dam

1. Homogeneous Earthen dam with toe drain
2. Homogenous Earthen dams with horizontal blanket
3. Homogeneous Earthen dam with inclined chimney  with horizontal blanket.
4. Homogenous Earthen dam with strip filter connected to horizontal drain.

Zoned Dams

These Dams are made up of two different materials.

In this dams the inner core is made up of the impermeable materials and the shell of the dams can be made up of the pervious materials.

The impervious material is such that it has permeability of the range of the 10^-7 m/s or less and at the same time it must posses the UCS of atleast  200N/m2.

The main work of the impervious material is to reduce seepage through the dam body.

The pervious materials  which surrounds the core of the dam are provided just for the stability point of view.

The layer between the impervious and pervious medium is known as transition layer , and its main purpose is to stop the mixing of the two different materials with in each other.This layer is made up of geomaterials of the size which is in between the size of materials in core and the shell.

The materials used in the impervious core is not pure clay due to high shrinkage and swelling characterstics ,hence we mix some gravels in the clay i.e GC (clayey gravel) which is most suitable for the construction of the core .

GC prevents swelling and shrinkage of the soil and also impart strength to the soil.

Types of drainage for zoned dams

1. Zoned dam with central vertical core and toe drain
2. Zoned dam with centrral vertical core and chimney filter with the horizontal blanket
3. Inclined clay core zoned dam with chimney filter with the horizontal blanket.
4. Zoned dam with the central vertical core and strip filter with horizontal drain

Diaphragm type dams

These types of the dams are just like the zoned dam but the only difference is in the thickness of the impervious core.

In this type of dam the thin impervious core is present which is surrounded by the earth or rocks of bigger size .

The Diaphragm can be made up of impervious soil, cement,steel,timber.

If the thickness of the diaphragm at any elevation is less than 10m or less than the height of the embankment above the corresponding elevations, the dam is said to be the diaphragm dam.

  

Courtesy - Google

The main works of the different type of sections of the dam are as follows

Shell 

it is outer most part of the dam 

its main purpose is to give the stability to the core of the dam.

The shell can be made up of locally available pervious materials in case of the zoned and diaphgram dams.

Core of the dam

it is the innermost part of the earthen dam which is constructed to reduce the seepage through the body of dams.

It is made up of highly impervious materials like clay.

Since clay has high shrinkage and swelling characterstics it cant be directly used in the core ,in clay some gravel should be mixed to reduce the swelling and shrinkage of the impervious core.

For core of the dam clayey gravel(GC) is most suitated material.

 Cuttoff trench 

it is used when the foundation is on the soil .

Its main work is to reduce the seepage flow through the foundation of the dam.

It is also helpful in to prevent the quick sand conditions , it increases the path of flow which helps into the decreament of the flow velocity of the water.

Transition zones

it used to avoid the mixing of the clay zones in to the permeable zones.

Rip rap 

Rip rap is the process of applying the layer of the stones on the upstream and downstream side but generally it is not used in the downstream side due to economy . 

we can use Sod or turping in down stream side to prevent it from erosions.

it is used to prevent the dam from the erosion.

Berms 

Berms are the simply break in the slope of the downstream side of the dam.

it is used to decrease the velocity of the runoff water during the rainy seasons which will heps in to reduce the erosion of the soil.

it can be used as pavement for repairing vehicles.

it can be used as drainage.

Its width is generally veries between2 to 6m depending on the requirement  and purpose.