Classification of wells

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 classified into two types.

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.

• 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.