Presence of loose rocks in borewell

Can we install slotted PVC pipes for the loose portion only to avoid blocking of borewell?

The borewell was dug in May 2016. Total depth is 900 ft. Pump lowered to 780 ft. Yield 1.5 in. It was fine till June 2017. After the heavy rains in Bangalore, the water availability reduced. We suspected problem with the pump and tried to remove the pump. It was not possible to take it out. We suspected accumulation of silt and decided to scan the borewell. The scanning revealed the presence of loose rocks at the depth of 60 to 70 ft just below the casing(60ft). The loose rocks have fell and blocked the pump from removing. After 70 ft the well is clear without loose rocks.

My question is 1. How to remove the pump? 2. What should be done to avoid this in future?