In this article we will discuss about:- 1. Meaning of Open Wells or Dug Wells 2. Classification of Open Wells 3. Cavity Formation 4. Construction 5. Well Development 6. Well Troubles.
Contents:
- Meaning of Open Wells or Dug Wells
- Classification of Open Wells
- Cavity Formation in Open Wells
- Construction of Opens Wells
- Well Development
- Well Troubles
1. Meaning of 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 increased, 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.
2. Classification of Open Wells:
Open wells may further be classified into two types:
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.
3. Cavity Formation in Open Wells:
When water is pumped out of the well there is fall in the level of water. This fall in the level of water from the static water table conditions causes water held in the surrounding soil to flow towards the well. As more and more water is drawn from the well the level of water goes on depressing causing increase in depression head.
Increased depression head leads to increase in the velocity of percolating water. Velocity of percolating water is maximum at the face of the well but goes on decreasing as distance from the well increases. This is because depression head goes on decreasing as we move away from the well. At certain rate of withdrawal of water from the well, the velocity increases to such an extent that it starts lifting up of the soil particles.
As more and more soil particles are lifted, a hollow is formed in the bottom of the well, resulting in increased effective area so that ultimately the velocity falls below the critical value and then no further sand goes out of the well. This forms a cavity below the bottom of the well. The formation of such a cavity below shallow well is dangerous as it may lead to the subsidence of the well lining. The maximum rate of taking out of water from such wells is therefore limited.
In case of deep wells resting on a mota layer the cavity formation below the bored hole is not dangerous. A large cavity may deliberately be formed so as to increase the area of percolation and thus higher yields can be obtained.
4. Construction of Opens Wells:
Open wells may be constructed in following three ways:
1. Kacha Wells:
These wells are of small depths and can be constructed in boulder or hard soils. They can be constructed at places where water table is very near to the surface of the ground. They can be only temporary source of irrigation water. They may collapse easily and thus sometimes prove very dangerous.
2. Wells having Pervious Lining:
In this, the lining of the well is done in dry bricks or stones without using any mortar. The seeping water can enter the well through the open joints of dry masonry. Thus inflow in this case is radial and not axial. Such wells are generally plugged at the bottom by means of concrete.
If bottom is not plugged, then water will be entering the well both, from bottom as axial flow and sides as radial flow. When such wells are constructed in fine sandy soil, the pervious lining is generally surrounded by gravel to act as vertical filter. This vertical filter prevents ingress of sand along with seeping water.
3. Lined Wells:
In this case wells are lined by stone or brick masonry or by cement concrete. For constructing such a well first of all a pit is excavated with the help of hand tools. The diameter of the pit is kept slightly larger in diameter than the finished diameter of the well. After the pit has been dug a kerb is laid at its bottom and masonry is raised over it.
The masonry is raised above ground. Now earth is excavated and bailed out of well and masonry is simultaneously raised over lining wall projecting above the ground level. As the earth is excavated from below the kerb, the kerb along with masonry above, sinks in the hollow thus formed.
This process of sinking and raising masonry above the projected lining wall is continued till well reaches the water table. After this excavation is done with the help of Jhams which is nothing but a self-closing bucket which is tied to a rope and worked up and down over a pulley.
When Jham is thrown into the well, it dislodges some soil and as the Jham is pulled up the soil cuttings get retained but the water oozes out. Sinking is continued till mota layer is reached. After this a small diameter hole is bored at the centre of the well to puncture the below lying confined aquifer.
If mota layer is not available then bottom of the well is filled with gravel, boulders etc. Gravel filled at the bottom acts as a filter and prevents sand particles from being lifted along with seeping water. Flow in this well is from the bottom only.
Sometimes, to utilize the water available in top most aquifer, small weep holes are left in the pucca lining of the well. In that case, along with water from bottom of the well, water from the sides can be admitted in the well.
Well Shrouding:
It is a process by which fine particles of soil coming along the seeping water into the well, are prevented from entering the well. It is accomplished by interposing a coarse grained material such as gravel between strainer pipe and the aquifer soil. This process is essential in sandy and unconsolidated formations of aquifer and also for slotted tube wells.
5. Well Development:
It is a process by which the fine particles are removed from aquifer formation surrounding the strainer pipe of the well.
Following favourable effects are obtained from well development:
(i) Specific capacity of the well increases.
(ii) Flow of sand into the well is prevented.
(iii) Life of the well is increased.
The well can be developed by following methods:
(i) Pumping:
In this method, pumping of water is done non-continuous, irregularly and at variable rate. By this fine material surrounding the well gets agitated and carried out of the well.
(ii) Surging:
A bailer or hollow surge block is moved up and down in the well briskly. By this finer particles are agitated and enter the well from where they are pumped out. Sometimes calgon is also added to well water so as to act as dispersing agent for fine particles.
(iii) Compressed Air:
In this method compressed air is continuously injected which develops an air lift pump system in the well which carries sand particles with water and are pumped out. After developing with compressed air, surging is also done to fully develop the well.
(iv) Back Washing:
In this method water is forced in the reverse direction by means of compressed air pressure. All the sand and clay material which is truck around the strainer is agitated and removed.
(v) Dry Ice Method:
It is a chemical method in which hydrochloric acid and solid sodium dioxide, known as dry ice is used. First of all HCI is poured into the well and well is capped. Now compressed air is forced into the well which forces the HCI solution into the soil formation.
Now cap is removed and blocks of solid sodium dioxide (dry ice) are put into the well. Due to sublimation CO2 gas is released and this gas develops a very high pressure in the well. When the pressure is released muddy water is automatically thrown out of the well. This method is not used in India.
6. Well Troubles:
Failure of the well or reduction in the yield is not uncommon.
Following may be the reason and remedies:
1. Reduction of discharge may be due to a draft greater than the percolation of water. Lowering the pumps may increase the yield for the time being, but permanent remedy is to reduce pumpage.
2. The casing pipe or screen pipe may have collapsed, partially or completely, blocking the flow of water. This trouble necessitates replacement of the casing or screen pipes. If the material in which the well is drilled is unconsolidated it may have to be abandoned.
3. Casing pipe may have corroded or leaked. Due to this, water may escape out into the ground or contaminated ground water may enter the well. The remedy lies in withdrawing and replacing the casing pipe. The life of the casing pipe depends upon the character of water handled by it. If water being handled is highly corrosive, C.I. or concrete casing may be used.
4. Screens cause trouble by corrosion or incrustation. There is little that can be done about corrosion, but some remedies are available against incrustation. All forms of incrustation are remedied by acid treatment.