Norilsk Nickel’s Polar Division, Norilsk, Russia:

V. P. Marysyuk, Chief Geotechnical Engineer – Director of Center for Geodynamic Safety, Candidate of Engineering Sciences
S. Yu. Shilenko, Deputy Director of Occupational and Industrial Safety

Saint-Petersburg Mining University, Saint-Petersburg, Russia:
A. A. Andreev, Leading Engineer of Scientific Center for Geomechanics and Mining Industry Problems, aa-andlex@yandex.ru

M. A. Vilner, Engineer of Scientific Center for Geomechanics and Mining Industry Problems

The paper considers the issue of assessing the proportion of unfilled voids in ore mining by the cut-and-fill method with cemented paste backfill. The efficiency of such mining systems is directly related to the proportion of backfill in the mined-out voids. The underfill of such voids may lead to negative processes in undermining, such as rock movements, dynamic events induced by rock pressure and growth of water-conducting fractures. The mined-out void fill factor is the coefficient A which is the ratio of the backfill volume to the void volume. According to standards, it should not be less than 0.98. Such value of the coefficient governs an unacceptable underfill sometimes. The paper substantiates the maximum underfill with regard to the recently predominant transition to the cut-and-fill mining method, as well as the causes of the increasing underfill, such as the uncontrolled roof failures. Based on the statistical processing of in-situ data, it is shown that the proportion of the neglected underfill can reach 0.13% under the most  unfavorable conditions, which is lower than the rated value of the allowable underfill and isincapable to lead to a significant change in the nature of rock movements. The calculated values of the mined-out void fill factor are 0.995–0.998 at the mineable ore body thickness of 20–50 m. Thus, in the modern conditions of copper–nickel ore mining, particularly at Talnakh, the fill factors in the cut-and-fill stoping practice need to be adjusted. The fill factors obtained in this study can be used for the more reasonable calculations when solving issues related to rock mass movements, which, among other things, can have a positive influence on the cost of protection of undermined objects.
The authors appreciate participation of members of Norilsk Nickel’s Polar Division Yu. N. Nagovitsin, M. P. Sergunin, A. A. Kisel, A. A. Bazin and A. K. Ustinov in this study.

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