Information needed on boulders issue for borewell

We recently drilled a borewell (Marathalli, Bangalore). The total depth of the bore-hole is 900 ft. Though we have found minute sources @200, 500ft, the major source is @800 ft. The yield is ~1.5".

The borewell driller mentioned that there is some loose-soil @300 ft. To analyze this further, we checked with the camera and found that there are boulders b/w 300 to 320 ft. Installing a slotted-pipe casing will be a huge cost (~125/- per ft).

I've read in one of the blogs about a probability of having inner-casing only at the specific depth where boulders occur. Though there are lot of technology advancements, I'm really not sure if this is possible. Can any of you suggest/provide-inputs if this is possible. If so, any contacts ?

Appreciate your help in advance,