A water well is a hole or shaft, usually vertical, excavated in the ground for bringing groundwater to the surface.
Water wells may be classified as:
1. Open Wells or Dug Wells:
Open wells are the wells which have comparatively large diameters but low yields (or discharges) and are not very deep. The diameters of the open wells usually vary from 1 m to 10 m. The yield of such wells in most of the cases is about 20 m3/hour or less. However, a properly constructed open well penetrating a permeable aquifer can yield 100 to 300 m3 hour.
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The depths of open wells may generally range from 2 m to 20 m. Since these wells are usually constructed by digging, these are also known as dug wells. The walls of an open well may be built of brick or stone masonry or precast concrete rings. The thickness generally varies from 0.5 m to 0.75 m depending on the depth of the well.
Open wells may be further classified as:
(a) Shallow open wells
(b) Deep open wells
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Shallow open wells are those which rest in the top water bearing strata and draw their supplies from the surrounding material. On the other hand deep open wells are those which rest on impervious strata and draw their supplies from the pervious formation lying below the impervious strata through bore holes made in the impervious strata.
The impervious strata is generally known as mota layer and it is a layer of clay, cemented sand, kankar or other hard materials. The term mota layer is however not applied to layers of hard materials laying above the water table.
The main advantage of such a mota layer is that it gives structural support to the open well resting on its surface. Further since the pervious formation below the mota layer generally contain large quantity of water, the yield of deep wells is more than that of shallow wells.
It may, however, be mentioned that the nomenclature of shallow and deep open wells is purely technical and it has nothing to do with the actual depth of the well because sometimes a shallow well may have more depth than a deep well.
2. Tube Wells:
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A tube well is a long pipe sunk into the ground intercepting one or more water bearing strata. As compared to open wells the diameter of tube wells are much less and usually range from 80 mm to 600 mm.
The tube wells may also be further classified as:
(a) Shallow tube wells
(b) Deep tube wells
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Shallow tube wells have their depths limited to about 30 m and may have a maximum yield of about 20 m3/hour. On the other hand deep tube wells may have maximum depth of about 600 m and may yield more than 800 m3 hour. Different methods are used for the construction of shallow and deep tube wells.
The tube wells may also be classified as:
(a) Strainer type tube well
(b) Cavity type tube well
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(c) Slotted type tube well
A brief description of these three types of tube wells is given below:
(a) Strainer Type Tube Wells:
It is the most common and widely used type of tube well such that the term tube well in general refers only to a strainer type tube well. In this type of tube well the pipe introduced into the ground is an assembly of strainer pipes (also called strainers) and ordinary pipes (called blind pipes) which are alternately placed. The lengths of the strainer and the blind pipes are so adjusted that the strainer pipes rest against aquifers (or water bearing strata) and the blind pipes rest against aquicludes (or impervious strata).
The various types of strainers that are commonly used for these tube wells are indicated below. In general the strainer consists of a fine wire mesh (or screen) wrapped round a slotted or perforated pipe with a small annular space between the two. The total area of the openings of the slotted pipe is kept equal to the area of the openings of the wire mesh so that no change in the velocity of flow occurs between the two.
The annular space between the pipe and the wire mesh is required, otherwise the wires of the mesh will cover a large portion of the area of the openings of the pipe. The water enters the well pipe through the wire mesh and hence the sand particles of size larger than the size of the mesh openings are prevented from entering the well pipe. This reduces the danger of sand removal and hence larger velocities of flow can be permitted in this case.
The size of the openings of the wire mesh is generally kept equal to D60 to D70 of the surrounding soil. Some strainers consist of only the slotted pipes without wire mesh wrapped round them, in which case the slots are of relatively very small size so as to prevent the entry of sand particles into the well pipe.
The strainer type tube well is generally unsuitable for very fine sandy strata, because in that case the size of mesh opening will have to be reduced considerably which may result in choking of the strainer, and if the screen openings are kept bigger, the well will start discharging sandy water.
The diameter of the bore hole or casing pipe for a strainer type tube well is generally kept 50 to 100 mm more than the diameter of the well pipe. Thus for a 150 mm diameter well, a bore hole of 200 to 250 mm diameter shall be drilled.
After the casing pipe is removed the annular space between the well pipe and the surrounding strata is backfilled with gravel.
A strainer type tube well may draw water either from an unconfined aquifer of unlimited extent or from one or more confined aquifers lying one over the other. At the bottom a short blind pipe is provided so as to permit settlement of any sand particles if passed through the strainer. The well is generally plugged at bottom by cement concrete.
Abyssinian tube well is a special type of strainer well in which the diameter of the well pipe is kept equal to 38 mm (1.5 in.) and the strainer is provided only for a length of about 1.2 to 1.5 m (4 to 5 ft.).
Types of Strainers:
Some of the common types of strainers used for the tube wells are as follows:
(i) Cook strainer
(ii) Tej strainer
(iii) Brownlie strainer
(iv) Ashford strainer
(v) Leggett strainer
(vi) Phoenix strainer
(vii) Layne and Bowler strainer.
The above noted different types of strainers are briefly described below:
(i) Cook Strainer:
It consists of a solid drawn brass tube having wedge-shaped horizontal slots. The slots are made with a slot cutting machine from inside of the tube. The slots are wider at inside and narrower at outside.
The gage of the slots depends on the coarseness of the surrounding soil and varies from 0.15 to 0.4 mm. This is a relatively costly strainer and is commonly used in U.S.A. and other western countries.
(ii) Tej Strainer:
It is similar to Cook strainer, but consists of brass tube made from a brass sheet bent round to form the tube with its vertical joint being brazed. The slots are cut in the sheet before it is bent. This strainer is generally manufactured from 75 mm diameter and upwards, and in 2.5 m length which may be joined together by means of screwed collars of brass. Tej strainers are manufactured in India and are commonly used in our country.
(iii) Brownlie Strainer:
It consists of a polygonal convoluted tube of steel plate having perforations and a copper wire mesh wrapped around it. The mesh consists of heavy parallel copper wires woven with copper ribbons. This is the best type of strainer as it has the advantage of keeping the wire mesh (or straining material) away from the perforated tube. The convolutions are straight sided and the tube is star-shaped.
(iv) Ashford Strainer:
It consists of a perforated tube with a wire wound round it over which a wire mesh is soldered. The wire keeps the mesh away from the tube. The wire mesh is protected and strengthened by a wire net around it. It is a delicate strainer and needs careful handling.
(v) Leggett Strainer:
In this strainer a cleaning device is provided which is in the form of cutters. The cutters can be operated from the ground surface. When the cutters are turned in they clean the slits.
(vi) Phoenix Strainer:
It consists of a mild steel tube in which slots are cut from inside by a slot cutting machine. The tube is cadmium plated to keep it free from danger of choking from corrosion caused by chemical action.
(vii) Layne and Bowler Strainer:
It consists of a wedge-shaped steel wire wound to a suitable pitch round a slotted or perforated steel or wrought iron pipe. The strainer pipes are joined together by screwed collars. This strainer is of a very robust type and is generally used in oil wells.
(b) Cavity Type Tube Well:
A cavity type tube well consists of a pipe sunk into the ground and resting on the bottom of a strong clay layer. It does not utilise strainers and hence it draws its supplies from the bottom and not from the sides as in the case of a strainer type tube well. In the initial stages of pumping fine sand comes out with water and consequently a cavity is formed at the bottom.
The water from the aquifer enters the well pipe through this cavity. As the pumping is continued the spherical area of the cavity increases outwards with the result that the velocity of flow is reduced and consequently the sand particles stop entering the well pipe. Hence in the beginning sandy water is obtained from a cavity type tube well but with the passage of lime clear water is obtained.
Since water is drawn from the bottom, only one particular aquifer can be tapped by the cavity type tube well. Thus in principle a cavity type tube well is similar to a deep open well, with the difference that whereas a deep open well taps only the first aquifer just below the mota layer, a cavity type tube well may tap any lower strata.
The essential difference in the flow pattern of strainer type tube well and a cavity type tube well is that due to the provision of strainers the flow in a strainer type tube well is radial, whereas in a cavity type tube well the flow is spherical. Further in a strainer type tube well the area of flow section may be increased by increasing the length of the strainer pipe (only in the case of an unconfined aquifer of large extent) while in the case of a cavity type tube well the area of flow section is increased by enlarging the cavity at the bottom of the well. The cavity formed due to certain discharge gets enlarged if an increased discharge is pumped out from the well.
(c) Slotted Type Tube Well:
A slotted type tube well consists of a pipe which is slotted for part of its length at one end and for the rest of the length it is plain pipe. The slotted portion of the pipe is usually about 5 m long and it penetrates the confined aquifer. The size of the slots is usually 25 mm x 3 mm and these are at 10 to 12 mm spacing.
In order to prevent the fine sand particles entering the well pipe, it is surrounded by a mixture of gravel and coarse sand (bajri). The mixture of gravel and coarse sand placed around the well pipe is called shrouding, and it is poured from the top into the annular space between the well pipe and the casing pipe before withdrawing the casing pipe.
The shrouding must extend upto about 3 to 4 m above the top level of the slotted portion of the well pipe. Thus in this case the diameter of the casing pipe is kept 200 to 250 mm more than the diameter of the well pipe. For example for a well pipe of 150 mm diameter casing pipe of about 400 mm diameter is required.
A slotted type tube well differs from a strainer type tube well in the following two respects:
(i) In a strainer type tube well the strainer pipes are surrounded by wire mesh to prevent the fine particles from entering the well pipe, while in a slotted type tube well the gravel and coarse sand shrouding serves this purpose.
(ii) A strainer type tube well can tap one or more confined aquifer lying one over the other, whereas a slotted type tube well can tap only one aquifer.
Under certain circumstances as indicated below it may not be possible to provide either a strainer type tube well or a cavity type tube well. For example if sufficient depth of water bearing stratum is not available even upto a depth of 75 to 100 m then strainer type tube well cannot be provided. Similarly if a suitable strong clay strata is not available then cavity type tube well cannot be provided. In such circumstances a slotted type tube well can be provided if at least one stratum having sufficient quantity of water is available.