The ore body is developed for the flat, and is applied by the sub-column sublevel caving method. The stage height is 50m, the section height is 10m, and the approach is vertically oriented, with a spacing of 10m. The segmented contact roadway is arranged in the north plate, and the direction of the tunnel exit is from south to north. The mining sequence of the ore block is advanced from the center of the ore body to the two wings.

At present, the mining has been reduced to 760m, and the surface has formed a trough that is approximately the same as the ore body's direction and width, and the depth is about 50m. A rock-sinking sinking step was observed on the surface of the north plate, with a moving angle of 47°. The south plate surface forms a steep rock wall that is consistent with the dip angle (Fig. 1).

Fig.1 Sectional view of the four ore body of Fushan Iron Mine

Since the mining of the ore body began at 860m in 1969, a total of 10 sub-sections have been taken, and four ground pressure activities have occurred underground. The main manifestation is that the north plate section is connected to the roadway, and its damage is listed in Table 1. The pressure activity period is 2 to 4 years. Generally, when the recovery approach is close to the north plate contact roadway, the roadway will be unstable.

1. The ground pressure appears to be large along the ore body, the two wings are small, the ore body is large in the north, and the south is small (Fig. 2).

Figure 2 Schematic diagram of the 770m level of the four ore bodies

F9, F2, F8-fault; 1~11,14,18~31-slip number

2. The stress rise zone is within 15~25m from the working face of the roadway, and the stress drop zone is within 15m (Fig. 3).

Figure 3 Stress distribution state above the approach

I-stress reduction zone; II-stress rise zone

3. In the range of 1.5 to 2.5 m on both sides of the working face, stress rise also occurs.

4. There is an unmined body in the upper section, causing stress concentration at the corresponding point in the lower section, causing cracking, gangming and roofing of the incoming spray layer.

5, the way of lagging recovery, the ground pressure is obvious.

In view of the above-mentioned ground pressure activities and their understanding of the law of occurrence, the following comprehensive control technologies for ground pressure are adopted:

1. Adopting the layout scheme of the inclined working face with inclined line (Fig. 4), the oblique line retreating is beneficial to control the scale and sliding time of the north plate sliding body, and the range of the approaching to the connecting roadway is small, which can be improved. The stress state of the roadway reduces the degree of stress concentration.

Figure 4 Schematic diagram of the oblique line layout of the access working surface

2. Adopting the sequence of two wings from the central part of the ore body with large stress to the ground pressure.

3. Implement centralized operations and strengthen mining. The mine production practice proves that a section of mining time is no more than one year, and the roadway and communication roadway will not be damaged by ground pressure.

4. Use appropriate support forms to improve the support effect. The anchor spray is used for the access road and the contact roadway is not damaged by the structure; the anchor spray net is used for the contact zone of the rock; the anchor spray net long anchor cable is mainly used for the fault fracture zone and the individual sections with large ground pressure.

5. If the ore entity is not left behind, if the local pressure large-scale approach fails to recover normally, it should try to loosen the blast to eliminate the adverse effects of the residual entity on the lower section.