Challenges of efficient underground mining of thick flat coal seams are studied. An effective method to secure stability of site development workings in overworked layers is suggested. A conclusion is made on advisability of stress-relief workings construction in preparation of overworked layers. Parameters of slicing mining system and stress-relief workings are justified. Assessment of useful mineral loss enhancement and production cost reduction is made due to introduction of the proposed method. Possible areas are determined for application of this method.