A Study On Safe Mining Of Transition From Open Pit To Underground Mining For "Three Under" Resources In Xingqiao Pyrite Mine

Abstract:The transition from open pit to underground mining for "three under" resources in Xingqiao Pyrite Mine has complicated mining conditions and many restricting factors. In this paper, the external environment influencing factors, security boundary pillar thickness, mining method and mining induced surface subsidence were studied, in order to achieve a smooth transition, a safety of surface structures and water, a maximize recovery of underground mineral resources, and a sustainable development of mine. The main conclusions of this paper are summarized as follows:(1) The boundary pillar considers two scheme, a composite boundary pillar with pouring reinforced concrete at bottom of the open pit and a original rock (orebody) boundary pillar without pouring reinforced concrete at bottom of the open pit. Through theoretical analysis and numerical simulation method,24m original rock boundary pillar (orebody above-180m) was recommended for "three under" resources mining.(2) Based on the principle gradual and orderly, from easy to difficult, recommending "three under" resources mining constructed in two phases in the direction of the orebody tendency, fist mining the orebody between-156m and-380m, second the orebody below-380m. The level height is40m,or50m,or60m, fisrt stage mining set230m,-270m,-330m and-380m four levels, second stage mining set-430m,-480m and-530m three levels, and the first mining level is-380m.(3)Through detailed technical and economic comparison, upward horizontal slice stoping and backfilling method was recommended for "three under" resources mining. Stope structural parameters for the pillar is10m, ore room is14m, two steps of extraction. This mining method’s designed recovery rate is84%, loss rate was5%, stope production capacity is330t/d～380t/d. (4) The "three under" resources mining induced surface subsidence was probability integral method and numerical simulation method, and the maximum tilt (i), curvature (K) and level deformation (ε) are1.440mm/m,±0.013mm/m2,±0.503mm/m, which less than the allowed deformation (i=2.5mm/m, K=0.05mm/m2, ε=1.5mm/m) of structures’highest protection level I, the surface structures in a safe state.(5) Considering the complicated hydrogeological conditions within "three under" resources mining area, and to ensure the safety of mining surface structures, the technical measures such as strengthen prevention and control of water, optimizing mining technology, improve the quality of joint top filling, the establishment of rock movement detection system were put forwarded to control the surface rock movement and deformation.