The driller probably reached this sandy layer after pinching out a hard clay layer and sand was coming out below the clay roof. Then its better not to put the strainer and keep it open at the bottom of the well so that it would behave as a Cavity type Well. Cavity would be formed in the water bearing stratum at the bottom of clay roof during drawing of water from the bottom of the well and water from the aquifer will enter the well pipe through this cavity in a spherical pattern of flow. After continuous pumping, the spherical area of the cavity will increase outwards with the result that the velocity of flow will reduce and consequently, the sand particles will stop entering the well pipe. In the beginning sandy water will be obtained from this cavity type tube well but with the passage of time clear water will come out.
Development of cavity well
A centrifugal pump is usually better suited for developing a cavity well than an air compressor. The cavity should be developed slowly and with great care keeping low discharge. Otherwise, sand is likely to rush into the casing and choke it. When discharging water becomea clear, the depression may be increase slightly, which may result in further sand being drawn out. This process will be repeated till the sand-free discharge is obtained. The pumping may then be stopped for an hour and then resumed. The discharge after restarting may again contain sand. The pumping may be continued till the clear water is obtained. The procedure may be repeated till the well is developed fully.
When the pink colored granite is fissured and highly crushed it changes into a formation similar to that of a river sand with pinkish and yellowish tinches, that are due to pyroxenite, feldspars and quartz minerals.
There is a need to drill the bore well and blow out the silica rich sand like particles. This will come out with moisture and sometimes with flow of water. Blow out a maximum of about a truck load of the particles or more and keep on blowing air until there is no more sand coming out. Keep 5 mm diameter or 10 mm diameter pebbles ready near the bore well.
When the nice sand like crushed particle have been pumped out from the bore, stop the drilling and blowing of air and pour the pebbles inside it while slowly the rig is rotating but the air is not coming out.
This can be done by many drillers who have drilled in Gujarat, U.P and Rajasthan. Replace double the amount or just near to the volume of the sand or silt you removed from the borewell.
The collapsing of bore will not take place and due to hydrostatic pressure the ground water will come to the static water level and the yield will be more than double the yield of the normal bore wells. If the surrounding bore wells are yielding for 5 H.P and your bore well will yield for 10 to 15 H.P.
Please do not do this if you get clay like silt or if very nice black or white colored powder is coming out from the bore well as water yield would be very low. If coarse grained sand gravel like material is coming out only then will this method work.
A. Rajamohamed Ambalam (http://www.indiawaterportal.org/directory/rajamohamed-ambalam)
Bangalore is a hard rock terain and there is no possibility of sand as such at depths of 250 ft. There is a possibility that the bore hole encountered soft formation at that depth and has a tendency to back fill the bore hole. It is quite natural to encounter such formations at these depths. The chances of bore hole collapsing depends on the rock formations above 250 ft. If the soft formation was met only at 250 ft and drilling stopped immediately then it is not likely to cave in.
However if it gets caved/ filled up to the extent of choking the water source, flush the well with air compressors, clean out the soft material, and lower a smaller diameter pipe with perforations into it (3" or 4" diameter appropriate for the bore well) and the well can be used again. Drilling rig crews are usually are aware of this method. But make sure that there is actually a case of soft material as has been reported.
M R Ramesh
Highly fractured granite or brecciated granite yields sand like material even at greater depths. Measure the depth to the bottom of the bore well. Let us assume that the borewell got filled upto 200 feet and the water level is at a depth of, say, 100 feet below ground level. Fit a 7.5 HP submersible pump at around 175 feet and test the well for quantifying the yield or the driller may pump the well using the compressor and a V-notch. If the yield is sufficient for the given purpose, then start using the well. There is no use in drilling further after a collapsible formation has been encountered.
C Udaya Shankar