| Nonpillar sublevel caving is a major mining method used in China's underground iron mines. Due to the restriction of traditional ore drawing theory, for a long time, small structural parameter of about 10m× 10m has been used in most mines of this type. Its backward mining mode, scattered operation points, low efficiency of equipment and high product cost have restricted the development of China's underground mines. In recent years, nonpillar sublevel caving method in foreign countries has been developing towards the orientation of "Two largeness, one height and one reduction", that is to use large structural parameter, high efficiency large equipment and high intensity mining so as to reduce the preparatory engineering amount and further to reduce the mining cost.Large spacing nonpillar sublevel caving means that its access space is greater than its sublevel height and it has solved the problem of the restriction of sublevel height by drilling equipment capacity. The enlargement of between-access space will slightly increase the depth of centering hole, which will not make difficult the drilling, explosive loading and blasting operation. The existing mining equipment can meet the requirement of large spacing mining. It makes no great change in the technology and is easy to operate. The increase in one-time caved ore amount is beneficial for the large mining equipment to display the efficiency. The reduction in stripping ratio can reduce the production cost so as to achieve enormous economic benefit.Due to the increase in access space, the stress concentration state of roadway is improved and the stability of extraction opening is increased. In addition, the enlargement of access space can reduce the access number and thus the exposed ore rock area, which is favorable to the improvement of the ground pressure of stope. China's most ore bodies in metallic mines are characterized by deep lying, great ground pressure. Soft and fractured structure and unstable surrounding rock, with some having developed joints, fissures and faults and some having large powder content and very poor ore stability. Some ore bodies are hard but very fractured. The ground pressure is a major factor affecting the mining production and safety. The adoption of large spacing is no doubt very beneficial to the improvement of the ground pressure of this type of mines and the promotion of the mine development.Meishan Iron Mine was the first in China to use large spacing nonpillar structural parameter. It increased the access space to 20m from the original 15m, as a result, the access number was reduced by 25% and great economic benefit achieved. At present the large spacing nonpillar theory has been popularized in many domestic mines, such as Beiminhe Iron Mine, Dahongshan Iron Mine, etc.Based on the basis of the collection and investigation of the data of geological prospecting and mining district investigation and related data, the monitoring on the stress and displacement of Meishan Iron Mine was made. Three-dimension continuous medium finite element calculation program was used in the 3D finite element analysis of rockmass of Meishan Iron Mine. The ground pressure conditions at different structural parameters (15x15m, 15m><18m, 15mx20m) were analyzed. Through the comparison of the monitoring data with data analog, the ground pressure change law in large spacing nonpillar sublevel caving was studied.The study shows that with the increase of access space from 15m to20m, the stress state between intermittent pillars can be improved. That is, the access spaceenlargement will benefit the space stability. From the displacement (deformation) and the change in least safety rate, it can be seen that the increase in access space can be beneficial to the stability of intermittent pillar or stope. The increase in space will improve the ground pressure condition and therefore, is of great importance to the unstable ore body mining district or other deposits with unstable ore rock.
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