TYPES OF TUBE WELLS
The tube wells may be further classified into following three types:
- Strainer wells
- Cavity well.
- Slotted wells.
All the three types of tube wells have been discussed one by one.
1. Strainer Type Tube Well.
- It is the most commonly used type of tube well in our country.
- It is so common that whenever we talk of a tube well, it is automatically understood referring to strainer type tube well.
- This tube well consists of blind pipes and strainer pipes or screen pipes.
- This tube well is generally unsuitable for very fine sandy strata, because in that case screen openings are likely to get choked easily.
- The boring of the hole is generally carried out by using casing pipe of about 5 to 10 cm larger than the diameter of the well pipe.
- Thus a 15 cm diameter well, will require a bore hole of 20 to 25 cm diameter.
- After boring the hole, the well pipe assembly consisting of blind and strainer pipes is lowered into the bore hole.
- The lengths of blind and strainer pipes are so adjusted that the blind pipes about against impervious layers and strainer pipes against the water bearing aquifers
- A short blind pipe should be provided at the bottom so as to permit settlement of any sand particles that have passed through the strainer.
- The well is generally plugged at bottom by cement concrete.
2. Cavity Type Tube Well.
- This is such a tube well which does not require any strainer.
- It draws its supplies from a cavity developed in the water bearing layer at the bottom end of the well.
- In strainer type tube wells flow of water through strainers is horizontal and radial.
- But in this well water entres well from bottom of the well without passing through any screen and flow of water is normal to the spherical cavity formed at the bottom.
- The impervious layer is punctured and a cavity is developed by pumping, by a sand pump.
- In the initial stage of pumping, fine sand comes out with water and consequently a cavity is formed.
- As the spherical surface area of the cavity increases outwards, the radial critical velocity decreases and sand particles stop entering.
- The principle behind this well is same as that of a deep well.
- The only difference is that in case of open deep well only first aquifer lying below mota layer is punctured whereas in case of strainer tube well any aquifer lying at any position can be tapped.
3. Slotted Type Tube Well. .
- This well is made at such places where sufficient depth of water bearing stratum is not available even at a depth of 80 m to 100 m and also
- when suitable strong roofing layer is not available for the construction of a cavity well.
- This well consists of a slotted wrought iron tube penetrat a highly pervious confine aquifer.
- The size of the slots may be 25 mm × 3 mm and space 10 to 12 cm centre to centre.
- First of all a casing pipe 40 cm diameter is lowered and aquifer is penetrated for a depth of about 5 m.
- The slotted pipe, sometimes also known as education pipe of 15 cm diameter is then lower.
- Gravel is then pour from top in the annular space left between inside of the casing pipe and outside.
- The gravel is filled for 3 to 4 mhigher than the slotted pipe.
- Now the casing pipe is withdrawn 5 cm at a time and well is developed with the help of compressed air pumped into the education pipe.
- Finally when casing pipe is fully withdrawn, the annular space between the casing pipe and the education pipe suitably plugg.
- By developing the well with the help of compressed air, the sand surrounding the gravel is freed from finer particles.
- filter getting choked are very much reduced.
- The difference between slotted tube well and strainer tube well are the following.
- Slotted well can tap only one water bearing strata whereas strainer well can tap many.