Longwall top coal caving (LTCC) is one of the methods used in thick coal seams. In coal mines where this method is used, some coal is left in place to be retrieved later in order to prevent damage to the main galleries. Stresses and strains created by roof layer caving of the panels advancing towards each other may put pressure on the main galleries. The amount of this pressure increases with time-dependent consolidation of roof layers. Sizes of the safety pillars gain importance in order to prevent this pressure and thus avoid the damages on the supports of the main galleries. In this research, great deformations creating damages on the supports of the main galleries of an underground coal mines in Manisa, Soma-Eynez were examined. Time-dependent deformation amounts were investigated both in situ and with time-dependent numerical modelling analysis. The results obtained by this research provide the sizes of the safety pillars in the panels to be produced.