No abstract

One of the most reliable methods for assessing the physical and mechanical properties of rocks as a result of their destruction are laboratory tests using hard or servo-driven test presses. They allow to obtain reliable information about changes in these properties beyond the limit of compressive strength. The results of laboratory tests of rich sulfide ore samples are presented, which made it possible to obtain graphs of their extreme deformation. Both monolithic samples and samples with stress concentrators in the form of circular holes with a diameter of 3, 5 and 10 mm were tested. It was revealed that during the destruction of the samples, the modules of elasticity and deformation decrease by 1.5-2 times, and in the zone of residual strength – by 5-7 times.

Talnakh and Oktyabrsky ore fields are estimated as rockburst-hazardous starting from the depth of 700 m downward according to safety rules. This means that mining is only permitted within certain protected zones. At the present times, such protected zones are generated in underground mines by means of the large-diameter destressing drilling. Despite proved efficiency, the high cost and large amount of the destressing drilling are the grave faults of this approach. Aiming to save drilling cost, it is proposed to make rock mass rockburst-unhazardous using destress blasting. This article gives a brief description of the destress blasting mechanism. This method has been effectively used in relaxation of pillars from stresses before extraction of the reserves from the pillars both in Russia and abroad. In Oktyabrsky Mine stress relaxation of pillars was implemented by slotting, and the drilling and blasting data were available. This study proposes to destress rock masses by means of directional destress fracturing in horizontal plane through blasting of decked charges of special design. The initial parameters for destress blasting using holes with diameters of 76 and 130 mm are determined. The charge design aimed to ensure a zone of fractures in the horizontal plane, at minimized vertical fracturing is described. This information can be used in planning of full-scale tests to refine parameters and application ranges of the method. For the full-scale tests, it is suggested to undertake destress blasting at different blast patterns on different test sites, and to compare the results with the current destressing method (destressing drilling). Efficiency can be proved using geomechanical and geophysical methods. The authors appreciate participation of V. P. Marysyuk and T. P. Darbinyan from NorNickel’s Polar Division in this study.

Before a mineral deposit can be developed deeper, a geomechanical survey should be carried out to determine the stress tensor of the rock mass. The most common in situ stress measurement techniques include the methods of hydraulic fracturing and overcoring. Each of them has its advantages and its drawbacks. The authors of this paper propose to conduct two stages of numerical modelling in order to enhance the efficiency of measurements performed by overcoring method. The first stage involves building a model of drilling to determine the optimum drilling depth. At the second stage, a core drilling model is built. Absolute transverse strains have been determined in reference points of a pilot borehole at each stage of core drilling. The created simulation models help choose the drilling depth of coaxial boreholes, estimate the potential growth of plastic strains that cause core destruction, define the amount of overdrilling to be done to coaxial boreholes and determine the optimum location for the measurement tools. The developed models can also be used to determine stresses by inverse problem solving.

In underground mining with backfilling of mined-out areas, it is necessary to analyze the filling mass quality. Since coring and testing would make a complex process, the method of express analysis comes in handy. As the mechanical methods of express quality analysis are not very accurate, the best option would be to use an acoustic method. But before the latter can be used, it is necessary to conduct a series of laboratory tests in order to under stand the empirical correlation between strength and acoustic pro perties. This paper describes the results of laboratory tests that were carried out for samples of backfill concrete after 7, 28, 45, 70, 90 days of curing to test their strength and acoustic properties. A correlation has been found between the uniaxial compressive strength of the fill and the ultrasonic wave propagation rates. Some field measurements have been taken to determine the propagation rates of longitudinal and transverse waves in the filling mass. Calibration curves have been built for both longitudinal and transverse waves. It was established that for the filling mass that hasn’t cured for 90 days it is the propagation rate of transverse waves that correlates most accurately with the strength; whereas for the filling mass that is older than 90 days it is the propagation rate of longitudinal waves.