Recently a borewell was drilled at my site in Bangalore, depth 252 feet. It was dug by Truck mounted pneumatic drilling m/c size 61/2”.  Output of water was good but during the last leg of drilling the driller informed that sand is coming out and it would be good if they stop drilling as sand would collapse and close the already drilled bore. 

I would like to know :
 1) If sand was present at the last leg is their any chance of borewell collapsing? Any preventive methods that can be implemented?
2) How would is sand present at that point so deep nearly 250 feet?
3)Is the presence of sand a natural phenomenon ?
4) How would the water table conditions be down under the sand layer?

N.L.N.Prasad
hintexbgl@hotmail.com