If there was loose and friable material encountered at the depths mentioned where one struck small quantity of water then one should install the inner casing all the way down to the depth at which water was finally struck. It will prevent the fine sand etc. from entering into the well, filling up the borewell and jamming the pump. If there was no soft or friable material encountered, then there is no need to install this casing.
This Inner casing can be installed at a later date but before the pump is lowered in the borewell. If the material is to collapse and fill up the borewell it will happen when the pump is installed and the borewell is dewatered and water along with fines material starts entering the well from the surrounding area. Actually, a casing with proper screen opening and a gravel pack around this PVC casing should be done.
Contributor: Anil Lalwani (https://www.indiawaterportal.org/users/wellwaterworks)
