Study of the Influence of Block Caving Underground Mining on the Stability of the Overlying Open Pit Mine

Block caving mining method is one of the most viable and preferred mass underground mining methods for low grade orebody. However, in combined surface and underground mining, the breakthrough of an underground caving under an open pit may result in subsidence at the bottom of the pit. This can pose a serious threat for the stability of the pit and the safety of the mining. In order to ensure the stability of the open pit and guarantee the safety of the mine infrastructures and equipment as well as the continuation of underground mining operations in all serenity, it is extremely important to control pit bottom subsidence. In this paper, the numerical simulation software Phase 2 was used to study the influence of the backfill materials on the stability of an open pit during underground block caving mining. Four undercut levels were considered in the model. The undercuts were located at 250, 500, 750 and 1000 m from the bottom of the pit respectively. Backfilling of subsiding zones of the pit bottom with waste rock was realized each time before moving from a mining level to another. The result obtained show that the pit bottom subsidence increased when moving from the mining level located at 250 m from the pit bottom to the mining level located at 500 m. After that, it begun to decrease progressively with the increasing depth of mining. This is due to the accumulation of waste rock in the stope column. In addition, the tensile stress above the goaf decreased with the increasing mining depth.