Bolter opr & co-worker were installing 6' bolts. Top was uneven w/sandstone. The oprs reduced spacing of straps from 2' to 20"". They were scaling loose rock & material prior to bolting. Rocks & sandstone would not pry down. EE stated when pushing bolt w/wrench, simultaneously w/other opr, pushing bolt w/plate into strap when rotating bolt a piece of rock fell 5', struck rt hand.