Abstract
Where a mined-out area underlies a slope, it is a direct threat to slope safety and stability. This is of particular concern where a mined-out area underlies the slope of an open-pit mine, and it has a serious impact on the design and safety measures used for the mine. If a mined-out area underlying the final slope of an open-pit mine is not treated adequately and at the appropriate time, it may cause the slip failure of the final slope during the service life of the mine, posing a serious threat to the safety of personnel and equipment during the stripping phase. In light of the potential for such problems, this paper analyzes the instability mode and failure characteristics of an open-pit slope near a mined-out area in China using geological field survey and the polar stereographic projection method. The scale span method, in combination with engineering analogy and consideration of open-pit mining technology, is then used to determine the critical safety thickness at which pretreatment of mined-out areas should be carried out. A pretreatment process to infill the mined-out area during construction of open-pit mine steps is put forward, and its effects on slope stability and reliability are comprehensively evaluated. The results show that circular sliding is the most appropriate instability mode for a slope near a mined-out area. The failure initiates through breakage in the roof of the mined-out area, which induces subduction sliding of the free face of the slope at the left boundary of the mined-out area and subsequent failure of the entire regional slope. Comprehensive analysis methods are used to determine that the critical safety thickness at which a mined-out area under the final open-pit slope should be pretreated is 24 m. The recommended treatment countermeasure is to transfer filling slurry into the mined-out area through drilling holes in benches. This can satisfy the stability and reliability requirements for the slope under different working conditions.
Highlights
A large proportion of mining in China uses the open-pit method
Illegal mining, which has been on the rise since the 1980s, has resulted in numerous undocumented underground goafs around such mines [1] and these present a clear threat to open-pit mine safety, especially where a mined-out area underlies the open-pit slope and seriously affects slope stability [2,3,4]
Mining and blasting generate large disturbing shock forces, and the resulting stress redistribution and consequent changes to the engineering geology conditions will affect the internal mechanics of slope failure [5, 6]
Summary
A large proportion of mining in China uses the open-pit method. Slope stability is a core safety issue at such mines. Mining and blasting generate large disturbing shock forces, and the resulting stress redistribution and consequent changes to the engineering geology conditions will affect the internal mechanics of slope failure [5, 6] This can cause mined-out areas to collapse instantaneously, directly threatening overall slope stability and the lives of the miners. Based on uncertainty measurement evaluation and an analytic hierarchy, Dong et al carried out a comprehensive risk evaluation of underground goafs using multiple indices and found that this method can reduce the risk of accident and improve the mining environment in practice [21] Another manner by which a mined-out area under an open-pit can be effectively treated is by using the downward deep hole cutting mining caving method of strip mining, as this method does not influence the stability of the final slope. Pretreatment measures for mined-out areas are proposed, and a comprehensive evaluation of the effects of the treatment on slope stability and reliability is carried out, providing a reference for engineering practice at mines where similar conditions pertain
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