This is depends on your Area and quality of mud, Hard rock or soft, this subject is not related to bore well digger, you should masseur the safety of your SUBMERGE motor, if stuck inside... means loose of bore and motor, in this case bore digger people will decide how many pipe you have to put inside.i am interested in digging a borewell in my land but the engineer says that because it is not solid rock but loose rock his machine will get damaged.any suggestions?thanks