Specifics of permafrost rock mass’ physical-mechanical properties study in evaluation of ore deposit geotechnical conditions

The aim has been to study the geotechnical conditions of certain ore deposits occurring in a permafrost zone, with the focus on the stress-strain properties of the rocks mass. The study was conducted in 2000-2018 at the Laboratory of Geomechanics and Rock Physics, INRTU. The main physical-mechanical properties of the rocks and ores have been defined in accordance with the requirements of the Russian Federation standards. The characteristics were selected based on the most common methods of estimating the geotechnical conditions of ore deposits, and on the stress-strain calculation for the structural components of the filed development systems. The analysis of the rock stress-strain characteristics has been carried out with the account of the main structural and cryogenic features of the ore deposit rock mass, and of rock outcrop in the mine workings. The study has distinguished three types of geotechnical conditions: permanent-frozen rock zone, frozen-to-thaw rock transition zone, and permanent-thaw rock zone. When evaluating the ore deposit geotechnical conditions, a differentiated approach to determining the rock stress-strain properties is suggested: natural dry state for the permanent-frozen rock zone; water-saturated state after 25 freezing-thawing cycles for the transition zone; natural dry and water-saturated state (depending on the hydrogeological factors) for the permanent-thaw rock zone. It is suggested that the conversion of the sample rock strength properties into those of the rock mass is done using the differentiated coefficient of structural slackening with the account of the permafrost, rock jointing, and the mining’s’ geometrics. When evaluating the burst risk and the brittle fracture susceptibility, the rock cryogenic state should be taken into account. It is recommended that the brittle fracture susceptibility is defined by either of the following methods: by the Kuznetsov criterion (the ratio of the rock’s compressive strength to its tensile strength), by out-of-limit deformation (the ratio of the rock’s modulus of elasticity to its modulus of decrease), or a by combination of the two. Based on the research results, a series of methodological and normative documents have been developed for regulating the rock pressure and defining the geotechnical parameters for the conditions of cryolythozone ore deposits underground mining.

permafrost zone, physical-mechanical properties of rocks, geotechnical conditions, rock burst risk, stability of mine workings

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