It is the normal practice to drill 6½-inch bore wells in the hard rock areas of Bangalore with blank casing installed deep into the hard rock and no casing pipe at all beyond that depth so that only clear water free of turbidity enters the well from one or more water-bearing fractures within hard rock. Care is taken to disallow any muddy water entering the well from surface or shallow depths by sealing the annular spacing between the casing pipes and between the casing pipe and the adjoining geological formation.
Construction of a tube well drawing filtered water from sand and pebbles admixed with mud through slotted pipe requires great care as there is danger of finer material in the aquifer entering directly into the well through screen slots. To avoid this, select proper screen size having bearing on the grain size of the aquifer, or develop the well by removing fine particles in the aquifer through excessive pumping, or install a gravel pack around the slotted pipe.
Contributor: R Jagadiswara Rao (https://www.indiawaterportal.org/users/rjagadiswara)