Boulders in bore well in Kengeri, Bangalore, Karnataka - Is a PVC casing needed?

I got drilled borewell @ bangalore, near Kengeri. At 74 feet got the water But i have drilled up to 350 feet..

But i have some problem ( as per the drilling service provider, ) that  got some Boulders between at 76-80 feet  & at 140 feet, (very little).  However after that there is no sign of boulders. finally got 4&1/2 inch water.

Now drilling service provider is advising to have a PVC casing with STILT which will cost Rs.200/ per feet.

Please advice me whether it is necessary to insert the same.

 

Thanks in advance,

 

N V SIMHA

 

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