RESEARCH ON DEFORMATION AND FAILURE CHARACTER AND THE THICKNESS-EFFECT OF THE OVERLYING STRATA TRANSITION FROM OPEN PIT TO UNDERGROUND MINING

With the help of digital photographic measurement technology, the deformation and failure character and dynamical evolution process of the overlying rock strata of NO.2 open-pit of Jingning phosphorite mine of Yunnan Phosphate Chemical Group Co., LTD., was studied during transformation from open-pit to underground mining of sublevel caving method. And we carried similar material simulating experiment to do target researching on these issues. In the meantime, the thickness effects of the deformation and failure character of the overmation and failure character of the overlying rock strata was also investigated. The results showed that:(1)As the stope spatial range is increased, the range and extent of tlying rock strata increases. And the evolution process of mining deformation of the overlying rock strata could be divided into three stages, which contained the stage of roof caving in originally small and local scale, linear continuous increase stage and the whole nonlinear violent collapse stage. (2)After the ore body thickness increasing from 6.80meters to 11.40meters, the mining influence area of overlying rock strata, the extent of deformation and failure and caving angle all extend, and the geometric shape of settlement curve changed from approximate ladle-shaped type to asymmetric half-bow shape. (3)After the full recovery stage, the evolution process of mining deformation and failure of the overlying rock strata appear obviously paroxysmal and timing effects. The conclusions provide a theoretical reference for ground pressure management in mine transition from open pit to underground mining.