Abstract: It formed a subsidence area and expanded the offset in surface because of underground mining. The mine waste reservoir was limited to storage much more tailings and expropriation was restricted by limited land. In order to solve these two problems, it was intended to backfill the disease area with semi-dry cemented blocks about the actualities in Jinshandian Iron Mine. When the overlying and surrounding strata collapse again, the filling block will be broken and pulverized easily, and which would induce debris flow in mined-out area. Therefore, there need to study the tailings' pulverization rate when the surface collapses again and the possibility of inducing debris flow. Accordingly, the cemented tailings blocks with strength of 1 MPa and 3 MPa were compressed level-to-level respectively. The mixture with the cemented tailings blocks with strength of 6 MPa was compressed level-to-level too. While it was compressed, the vertical stress and strain were recorded. When the cemented blocks were crushed and pulverized in some load level, some samples were taken to do screening test and study the curve of broken blocks’ particle size distribution. According to the results of experiments as follows, when the collapse appeared in mined-out area and the cemented blocks may be compressed, the failure fillings’ pulverulent ratio was 8%-15%. The failure fillings’ pulverulent ratio can be reduced by filling the mixture which contains cemented blocks and waste rocks when surrounding rocks collapse. Combined the results of debris flow test, the possibility of debris flow was little; even if it has, it can also ensure the normal production in underground mining area

Key words: backfill, tailings, compression, gradation, pulverization rate, debris flow