Technologies For Coordinated Mining Of Open-pit And Underground Mining At Southeastern Of Gongchangling Iron Mine

The southeast area in Gongchangling has two iron belts,the lower iron belt contains the magnetite,and the underground mining is conducted to the lower iron belt at first, then the open pit mining method is used in the upper iron belt which is the Hematite. When underground mining is+196m～+152m, open-pit mining is+448m-+388m step.and the maximum difference of upper and lower iron belt reaches to about 300 m. The goaf is caving through surface, and the surface is formed the big subsidence near the open-pit, hence,the production safety is threatened by it of the open-pit.Meanwhile,due the work face of lower iron belt is designed in+152m level,it won’t start to mine downwards until the open-pit mining ends. The output of the southeastern area will not link up in 5 to 10 years.In order to solve these problems,this paper adopts a high efficient mining technique based on three rules (granular rock flowing rule,rock mass caving rule and ground pressure activities rule),and the comprehensive analyse technology for coordinated mining of upperand below ore veins.First, the incidence conditions of fall or slip for side wall rock were analyzed, which came to the conclusion that, wall stability of collapse pit is maintained jointly by the strength of lateral rock and lateral supporting force for the mobile granular rocks in pit and that the balance relationship and subsided status of granular media in pit could be attributed to mobile status of granular media.Second, the flow continuity of granular media in collapse pit was studied by experimental tests. According to the granular rocks at the collapse pit bottom and in the mine mouth at underground, comprehensive assessments on granular rocks and block distribution in the collapse pit were made. Besides, on the basis of block distribution, arching experiments were conducted and the critical span ratio of granular arching was analyzed during the period of ore drawing, therefore, under the actual width of the collapse pit, there was little posibility of arching phenomenon occuring when the granular rock in pit is undergoing the process of falling down.Third, the relationship between surface subsidence and mining depth of central region was analyzed statistically. Specifically, because the upper iron belt in the central area has a long service life, a large mining depth and fully developed of collapse pit, the relationship between the range of surface subsidence, mining depth and the upper angle was statistical, which indicated that as for the granular rocks making a large impact on the range of the collapse pit, the height of granular column is generally less than 1/3 entire column height, however, as for the granular rocks taking on a small impact on the range of the collapse pit, the maximum height of deep granular column accounted for 82%of the height of the entire granular column. In view of that, the new concept of supporting rock stability of the critical granular columns was come up with, and the relationship between critical granular columns and orebody angle was analyzed as well.Fourth, the supporting effect of granular rocks in collapse pit on the side wall was analyzed, which draw the conclusion that the combined effect of active side pressure and passive lateral pressure of scattered column in collapse pit hindered a large area chipping from the side wall rock, which enabled the critical granular column to form a more stable support structure to adapt to the changes of side pressure. Apart from that, the relationship module between the mining depth, the angle of hanging wall, conventional displacement angle, critical depth and the actual displacement angle model was summarized. Moreoever, this disterssion proposed that reducing the critical depth was an effective way to reduce the range of subsidence.Fifth, the rationality of the critical granular columns theory was verified by the numerical simulation method. In this chapter, Numerical simulations on the surface subsidence of the upper iron belt in the central area was conducted. By setting PFC discrete element numerical model, a series of numerical simulation on the relationship between surface displacement and velocity & displacement of side wall collapse pits dispersion, stress and strain of wall dispersion, as well as the rock fracture were executed. From the simulation results, we can conclude when the depth of collapse pit reaches a certain value, the deformation on the hanging wall tends to zero, which verifies the exsitence of critical grnular column,sixth, based on physical experiments, it came to the conclusion that the flow of granular racks in collapse pit was continuous. Besides, analysis of the security of the rock collapse pit was made, and a technical solution to the problem of how to deal with dumping rock from Southeastern open-pit was proposed. After the implementation,transport distance of discharging waste rock has been reducd, which has effectively increased the production capacity of the open-pit mining.Seventh, according to the method for determining the critical granular column proposed by this dissertation, the critical granular column height of the lower iron belt in Southeastern was suggested. Furthurmore, in the light of granular height, the reasonable mining depth in Southeastern was calculated, that is, under the existing level of mining, further speed of the lower iron belt with mining could be increased, and open-pit height difference in underground could be up to 475m, which could release the underground mining capacity.Overall, this dissertation presented a new concept of critical granular rock column, and developed a technology of rumping rock to the collapse pit, maintaining the safety of the open-pit mining and improving the efficiency of rock, saving the cost of dumping rock, removing the constraints of the lower iron belt limited mining. This collaborative mining technology of open-pit and underground for steep and parallel ore veins, exactly the method for steep multi-layered orebody first the underground mine the the lower ore belt and then the open-pit mine the upper iron belt. Underground mining offer the place for dumping rocks from open-pit mining, and open-pit mining plays a role in relief for underground deep mining, these two methods complement each other, to provides a new efficient way exploiting anxious inclined and parallel ore veins mining for domestic metal mining.