Abstract: Analyzing mechanics of overburden movement between traditional collapse upward mining and backfill upward mining, this paper has found that it would fail to mine in a certain close distance coal seams because upper coal and rock structures are damaged when applying traditional collapse upward mining. But backfill upward mining can reduce the damage level of the upper coal seam with a certain filling ratio. Using composite beam limit rupture and plastic zone development height discriminance to design filling ratio, the condition of backfill upward mining feasibility has been proposed, and then the backfill upward mining critical filling ratio has been obtained. Results have shown that according to geological conditions of Papaya mine , when upper coal seam (9^# coal) does not break, the critical filling ration of coal mining in sub-coal seam (10^# coal) is 82.1%, when plastic zone development height of sub-coal seam (10^# coal ) reaches upper coal seam (9^# coal), the critical filling ratio of the former is 70.4%, which means when implementing backfill mining 9^# coal in Papaya mine, critical filling ratio for backfill mining is 82.1%.

Key words: backfill upward mining, close distance coal seams, numerical simulation, filling ratio