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The article analyzes efficiency of safety and support of underground excavations at great depths in potassium salt rock masses as a case-study of the Starobin and Petrikov deposits. Various protection technologies are discussed. It is found that owing to formation of block structures and cohesion loss zones nearby the perimeter of mine excavations, the expansion gaps can have both beneficial and adverse influence on excavation stability. The relaxation excavation technology allows a great reduction in the dimensions of limiting state zones in rock mass surrounding mine openings. The model of block movement is proposed, and it is shown that mutual displacements of blocks by more than 1-2 mm in the vicinity of an excavation perimeter can be a cause of the dynamic failure of blocks into mined-out areas. It is recommended to install rock bolting as soon as cohesion loss is detected on sliding surfaces.

Using the mathematical modeling, the author performs the stress-strain analysis of stratified rock mass in the course of mining of close-spaced coal seams with regard to linear change in temperature field with depth. The problem is considered in an incoherent quasistatic formulation: first the temperature field is determined and then the stress-strain assessment is made using the Duhamel-Neumann law. The problem solving used the finite element method. It was assumed that the rock mass layers occur in complete contact. The initial stress state was found on the basis of Dinnik’s hypothesis. The features of the initial stress state, which influence stress redistribution during coal mining, are determined. The variants of close-spaced coal seam mining are discussed, and the related stress state peculiarities are highlighted as compared with mining with no regard to the temperature gradient.

The methane emission parameters are determined in the face areas of the Boldyrev seam, Kirov Mine, and seams 50 and 52, Yalevsky Mine. It is shown that coal of the same seam can have greatly different gas emission rates determined in a few hours after coal detachment from rock mass. The gas emission rate depends on the regularity of microstructure, determined by calculation of information entropy and statistical complexity using digital images of coal surface from an electron microscope. The lower gas emission rate is a feature of coal with a less uniform (more chaotic) microstructure, which is explained by the higher number of cut-off bonds in the aliphatic component of coals, while these bonds are the adsorption centers and prevent faster diffusion of methane.

The authors compare the strength characteristics of rocks, determined by standards GOST 21153.8-88 and ASTM D7012-04. The research involved 7 rock lithotypes (shale, diabase, metasomatic rock, marble, gneiss, quartzite, garnet) and bulk compression, uniaxial compression and indirect tension testing. Mohr’s circles and failure envelopes are plotted for each rock lithotype. The differences between the standards are found in terms of adhesion factors and internal friction angles: ASTM yields higher adhesion factors; internal friction angles vary depending on the rock type. The calculation formula allows predicting the uniaxial compression strength at an error of 10.5%. The failure envelopes are updated using the calculated data. The results highlight the significance of proper selection of a failure envelope plotting procedure in engineering design.

The increase of an undercut height relative to haulage drifts is one of the effective measures aimed to enhance operating safety of the latter. The FEM-based stress-strain change modeling is performed in rock mass during ore block mining in complex geological conditions at different vertical distances between development drifts and stopes. The research reveals an inversely proportional dependence between the undercutting level height relative the haulage level and the total length of operating excavations in the influence zone of stoping. The determined dependence makes it possible to select an optimal height of the undercut level for the benefit of stress control in rock mass and towards minimization of the adverse influence of stresses on mine infrastructure.

The fluid transport analysis in fractured porous rocks requires investigating flow in large fractures and inside porous blocks. This is possible with the flow model with the hereditary-type sources, which allows adding the flow analysis with the pressure difference of fluid in blocks and fractures, which is called a piezometric wave. Its influence on the change in the stress-strain behavior of structural elements in a seam (matrix) is investigated. The theoretical studies validate the possibility of the seam matrix transition to pre- and post-limiting deformation. It is shown that zones of probable pressure-induced disintegration in rocks adjoin the highly permeable zones of seams. Origination of such zones greatly reduces permeability of confined aquifers and efficiency of drainage well. The necessary conditions for these processes to develop and their prevention activities are set forth.

Physical lab-scale simulation finds out that in polydisperse broken ore draw, the medium undergoes structuring which leads to the formation of a consolidated block shaped as a flaring cone. Along the side surfaces of the cone, slide zones originate as layers of small size particles. The angle of the cone generators is governed by the internal friction angle of the medium, which depends on the degree of fragmentation and on the grain size composition. An empirical formula is proposed for calculating the angle of the cone generators. The obtained results make it possible to develop an algorithm and a method for optimizing parameters of structural elements in the caving systems of mining with self-propelled machinery.

The article analyzes interaction of a hydraulic fracture and a permeable crack nearby a void in a uniform medium under triaxial compression. 2D modeling used the finite element method. Possible paths of the created fracture are set as cohesive layers composed of elements which can deform and fail depending on an applied stress. A fluid flow is modeled by additional points with crossflow of fluid from the created fracture into the permeable crack when they intersect. The numerical studies are presented for the nature of the hydraulic fracture-permeable crack interaction depending on the: distance between their interaction point and the void; stress state; strength and internal friction factor of the medium; fluid viscosity. On the basis of the obtained results, for the creation of an antiseepage screen, it is recommended to perform hydraulic fracturing in the vicinity a void at a distance equal to the void radius.

The authors analyze the main approaches to the assessment of the dynamic fracture tendency in rocks on a laboratory scale and report the data of experimental investigation of rock samples. It is shown that the dynamic fracture tendency of rocks depends on their loading conditions, namely, on the stiffness ratio of the loading system and loaded object, on the energy input rate (loading/deformation rate) and on the type of the stress state. It is found that the change in the loading parameters of rock samples during a lab experiment can influence their behavior in fracture; for this reason, in assessment of the dynamic fracture tendency in rocks, it is necessary to consider not a sample, individually, but the whole press-sample system.

The article describes the experimental investigations of stresses and strains in a tubular sample made of an equivalent geomaterial subjected to static compression, torque and multiple weak impacts. The tests show that the sample pre-loaded by the static compression and then additionally loaded by the torque at the shearing stresses of 5-7% of the compression stresses elongates and its diameter reduces and the compressive stresses grow. This process lasts for 120-200 h. The additional loading of the sample by the multiple weak impacts gradually leads to the stress relaxation and to the increase of the axial and circumferencial strains. The process of relaxation is nonmonotonous, with the stress picks and drops. During loading, the elastic energy accumulates and releases in the sample with a period of 80-120 h.