During boring of the well hole by any method it should be ensured that bored hole remains straight and vertical.
If well hole is located in consolidate formation, water enters the well hole directly and no casing is required as all the surroundings are quite stable.
But if formations are unconsolidated a casing is necessary to support the outside material and also to help in freely admitting the water into the well.
The casing should either contain perforations or its lower part may be replaced by a screen or strainer so that water may enter the well.
Sometimes in order to increase the effective diameter of the well, the screen or strainer is surrounded by a layer of gravel.
Gravel layer keeps fine material out of the well.
Such a gravel packed well will have a greater specific capacity than one of the same diameter not surrounded by gravel.
The thickness of the gravel may vary with type of formation and method of drilling.
However a minimum thickness of 15 cm is generally used.
While drilling a new tube well it should be ensured that it would not affect the discharge of other nearby existing tube wells.
The distance of the proposed well from existing wells should not normally be less than 1.5 km.
The diameter of the well pipe may be reduced theoretically, as we go deep down the well; but it is not recommended.
A single sized or at the most two sized tube may be used in actual practice.
The velocity of flow through the tube is generally limited between the range of 1.5 m/sec to 4.5 m/sec.
Once the design discharge of the tube well and the velocity of flow are known, the diameters of the pie can be easily found out.
