Borewell filled by mud and stones

Hi all

I need help, recently we have drilled 1 borewell of 650 feet, its initial casing pipe was 30feet but after that we got loose soil at 50 and 80 feet. Then we got water at 120feet with boulders and we got good amount of water around 450 feet.

My problem is now my submercible pump is not moving beyond 50feet any solution it will be great help for me?

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