Fig。 1 New solid backfilling hydraulic support [5]

As shown in Fig。 1, solid filling materials fall into goaf through dropping holes on solid backfilling conveyor, and then they are compacted by the push-tamp equipment which is set at the rear of hydraulic support, finally the density and ratio of backfilling body can be increased。

2。2 Subsidence reduction mechanism

From the description of solid backfilling mining technology, the goaf is occupied by compacted solid material。 The common solid backfilling materials (gangue, coal ashes, loess) are discrete medium。 While the ground pressure is added, backfilling materials will be compacted, broken and the particle gradation can be improved,    which    can    enhance    the     anti-compact

capability significantly。 Figure 2 shows the stress—strain

curve of the gangue compression in the cylinder compression test。

When the subsidence increases, the reaction of overburden strata suffered from backfilling body raises until it is balanced with the overburden strata’s pressure, and then strata movement stops。 A lot of similar materials and numerical simulation result show that [6−7] the overburden strata  usually  develops  fracturing  zones and bending zones when the solid backfilling technology

Fig。 2 Stress—strain curve of gangue cylinder compression test [6]

is used, and the overlying strata subsides heterogeneously。 Therefore, in the critical mining conditions, the surface subsidence value of solid backfilling mining depends mainly on the subsidence of roof which is determined by mining height and final compression value of the backfilling body。 Compared with the caving method, the roof subsidence space, which is the difference between the mining height and the final compression height of backfilling material, decreases significantly。 This is the main reason why solid backfilling mining can reduce subsidence。

2。3 Slow down subsidence velocity mechanism

In fact, strata movement of solid backfilling mining is a process that overburden strata moves towards goaf and compacts the filling body, then the filling body is consolidated and the anti-compression ability  of  it increases until the reaction is balanced with the press of overburden strata。 After coal exploitation, the ground pressure at the top of coal seam perts to the wings, and at this time, the filling body is unloaded。 While the working face is advancing, the stress on the filling body  will recover to the original rock stress, with the filling body compressed to “release” subsidence space correspondently。 The subsidence space of roof decreases and it derives from the “release” space  of  backfilling  body’s compaction which is the big difference between solid  backfilling mining and caving method。 The subsidence velocity depends on the advancing speed of working  face,  the depth of mining, the maximum subsidence value, the inclined length of mine area, etc。 The results show that subsidence velocity has a positive correlation with the advancing speed of working face, the mining height and the maximum subsidence value, while it has a negative correlation with the depth of mining [8−9]。

The  subsidence  of  solid  backfilling  mining    can

reduce greatly。 Meanwhile, the advancing speed of solid backfilling mining is slower than that by the caving method for its technological characteristics。 Moreover, the subsidence space is caused by backfilling body compaction。 These are the main causes that solid backfilling mining velocity decreases significantly。

3 Subsidence prediction method