State various types of tube wells and explain construction of Slotted Type tube well.
Answers
1. Strainer Type Tube Wells:
In tube wells the metal pipe driven in ground is perforated to allow only clear water to enter the hole. It is obvious that if no other means is adopted the perforations in the metal tube will have to be made very fine. It is very costly process.
As an alternative wire net may be wrapped on the cylindrical frame of small diameter but it is liable to break as it is very delicate. So the best and most commonly adopted practice is to provide a pipe with fairly big perforations and surrounding that is a wire net or a strainer with smaller openings.
The wire net with finer openings excludes the objectionable soil particles from entering the tube well. Total area of the openings in the metal tube and in the strainer is kept the same. This is because if the area of opening is same the velocity of inflow will be the same.
Moreover there is some annular space left between the perforated metal tube and the strainer. If the space is not left the strainer may rest directly over the tube and consequently the open area of perforations will be reduced, Fig. shows the section of a strainer type tube well.(see image)
Normally the mesh size of the wire net or strainer is kept equal to D60 to D70 of the surrounding soil. This type of well derives water from one aquifer of unlimited extent or from a confined aquifer or from number of aquifers.
2. Cavity Type Tube Well:
In this type water contribution to bore hole takes place through the bottom layer only. From Fig. 18.2, it is clear that in principle it is similar to the category of the deep wells under open wells.
2.Cavity Type Tubewell
It derives water from the previous layer underlying the hard impervious layer. The tube well is taken down till it penetrates the impervious or mota layer and reaches the water bearing layer. In the initial stages when the water is pumped out fine sand comes in the tube well with the water and consequently a hollow or cavity is formed at the bottom. The bottom of the cavity for some thickness is thus made free of finer particles.
After the cavity formation only clear water enters the tube well. Since the rate of pumping is more, the velocity of water entering the coarse sand layer is critical but when it comes in the hollow the velocity is reduced. Finally the water enters the tube well at the bottom with a velocity lower than the critical velocity. The main difference between a strainer and cavity tube well is that in the former the inflow is radial whereas in the later it is spherica
3.3. Slotted Type Tube well:
Sometimes the nature of subsoil formation is not anticipated correctly. Obviously bore hole driven for constructing strainer well will be a failure. If a mota formation is present cavity well may be resorted to. But if neither of the conditions are existing the slotted tube well can be rightly constructed. There should be of course an aquifer present at the bottom. In the bore hole (say 36 cm diameter) a 15 cm diameter education pipe is lowered till it reaches the bottom. The bottom of the education pipe is slotted as shown in Fig. (see image 3)
Slotted Type Tubewell
- The size of the slots may be 25 mm x 3 mm with 12 mm spacing. As the slots are quite wide, to avoid sand entry in the pipe a filter of shingle is provided at bottom, surrounding the slotted pipe portion. Finally before withdrawing the 36 cm diameter pipe casing the shingle is poured in the annular space between the education pipe and the casing pipe. The development of this well is done gradually with the compressed air. Thus the slotted tube well, unlike strainer well receives inflow only at bottom through shingle shrouding.
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