CLASSIFICATION OF WELLS
The wells may be classified under two heads.
1. Open wells or dug well.
2. Tube wells.
1. Open Wells or Dug Wells.
- The ancient wells were all open wells.
- They are constructed by digging large diameter holes in the earth.
- Although there have been some improvement in methods of digging the wells, in some areas old conventional methods are still prevalent.
- These wells are initially used to get water for drinking purposes, but later on their use for irrigation purpose was also appreciated.
- Open wells are generally open masonry wells having diameters varying from 2 m to as much as 7 m.
- Their depth seldom exceeds 20 m. Their discharge is about 18 m3/hr or about 0.005 cumecs depending upon the depth of water table and nature of soil comprising the aquifer.
- The walls of the open well may be built of precast concrete, blocks, brick, or stone masonry.
- The wall is known as staining wall or lining wall of the well.
- The thickness of lining or staining wall varies between 38 cm and 75 cm depending upon the depth of the well.
- The yield of an open well is limited because such wells can be dug only to a limited depth.
- The ground water storage is also limited in the top most aquifer.
- Moreover open wells cannot be worked at velocity of percolation more than the critical velocity.
- The velocity more than the critical, causes dislocation of soil grains and consequently develops hollows behind the well lining.
- Hence, limited velocity of percolation, limited depth of well and limited ground water storage in the top most aquifers, put limit on the yield of the open well.
- However yield of the open well can be increase, if open well is constructed in top aquifer but in addition 8 to 10 cm diameter hole is bored to puncture the lower layers of water bearing strata.
- Open wells may further be classifie into two types.
1. Shallow wells and
2. Deep wells.
1. Shallow Well.
- It is such an open well which gets its supplies from upper most aquifer.
- If such wells penetrate for small depth under water table, they may even go dry in summer.
- This is because water from the ground reservoir will soon be drawn out and as soon as water table goes below the bottom of the well it will go dry.
- Hence in order to get water from such wells for more time or for the year round they should be carried deep into the ground water.
- Quality of water obtained from shallow wells may not be good for drinking but it is quite good for irrigation purposes.
- In fact shallow wells can hardly provide substantial irrigation as availability of water is very limited.
2. Deep Well.
- The deep wells get their supplies from water bearing straws lying below an impervious stratum.
- The theory of deep wells is based on the percolation of water into the under lying aquifers from out-crops.
- The out-crop is the exposed area of the pervious layer or aquifer, lying below the impervious layer.
- It is this area from where rain water enters the pervious layers lying below the impervious layer.
- After getting entry into the layer from out-crop water starts seeping along the slope of the aquifer and reaches the well site.
- During travel from out crop to well site, water gets thoroughly purified.
- But during this process certain salts may get dissolved in water and make it hard.
- Deep wells may get supplies from more than one confined aquifer.
- Deep wells are not necessarily deeper than shallow wells. It is not the depth of the well but the criteria of getting their supplies.
- Shallow wells get their water from top most layer but deep wells get their supplies from confined aquifers lying below the impervious layers.
- A ‘shallow well’ might have more depth than a ‘deep well’.
- The impervious layer on which deep wells generally rest and draw their supplies from pervious formations lying below the impervious layer, through a hole is known as Mota layer.
- The term Mota layer is also sometimes known as ‘Matbarwa’ or ‘Magasan’ layer.
- This layer refers to a formation of clay, cemented sand, Kankar or other hard materials which are often found lying a few metres below the water table in the sub soil.
- These names are not used for hard material layers lying above the water table.
- The main advantage of ‘mota’ layer is that it gives structural support to the open well resting on its surface.
- It is useful for unlined and partly lined wells and is indispensible for a heavy masonry well which would not remain stable without Mota layer support under steady use.
- The Mota layer is found throughout the Indo-Gangetic plain.
- The Mota layer may be present as continuous layer or may be in form of local formation.
- It has different thicknesses at different places.