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The aim of the study described in this article is to evaluate the suitability of a finite-discrete element method for predicting rock movement during underground mining of mineral deposits by caving systems. A comparison is made of the measured and modelled characteristics of deformation processes, which took place during mining at various deposits. The developed algorithms for simulating sub-level and block caving are described.
Predicting rock and ground surface movements during mining by caving methods is usually based on empirical methods, which cannot take into account all geological conditions. For this, it is necessary to simulate the processes of breaking fractured rock mass and the subsequent behaviour of disintegrated material: movement of rock blocks in the mined-out space and the caving zone.
Discrete elements methods have considerable potential for solving problems with disintegration. For their practical application, it is necessary to understand their specific features and the limits of applicability. The finite-discrete element method is one of the most advanced numerical methods in mechanics of a discrete medium (Munjiza, 2004). To perform geomechanical calculations by this method, Teretau develops Prorock software package. Software processor implements the algorithm of forced stabilization with Coulomb strength criterion. In the calculations, an extreme deformation by dilatancy is simulated, along with variability of strength characteristics, tectonic discontinuities and anisotropy of strength properties, plastic deformation of finite elements. To ensure the high speed, computations are performed on general-purpose processors (Ilyasov, 2016).
Prorock software was used to simulate movement of rock mass, ground surface and host rocks at mines using caving systems: Degtyarsky (Russia), Ridder-Sokolny (Kazakhstan) and Palabora (South Africa).
Caving simulation begins only after the model achieves a state of quiescence with the help of the forced stabilization algorithm (Ilyasov, 2016). This ensures a significant reduction of the impact of inertial oscillations in the system that occur after the simulation is started. After simulation is initiated, elastic modulus, Poisson's ratio and cohesion parameters of rocks in the mining area are gradually reduced to the values corresponding to the disintegrated rock mass (rubble). This is necessary to reduce the number of elements caused by the exclusion from calculations of unrealistic dynamic effects, consisting of high-amplitude oscillations of the system, which cause extensive disintegration around the area with excluded elements. Another consequence of excluding elements from the calculations is significant acceleration of nearby non-excluded finite elements and, consequently, an unnatural increase in the level of disintegration in the mining area. To reduce such effects, the algorithm of plastic deformation of finite elements has been developed and introduced into the program code (Ilyasov, 2016), and a dissipative impact model has been added (Mahabadi, 2012).
Rasskazov, I. Ju. (Mining Institute) | Saksin, B. G. (Mining Institute) | Anikin, P. A. (Mining Institute) | Gladyr, A. V. (Mining Institute) | Potapchuk, M. I. (Mining Institute) | Usikov, V. l. (Mining Institute) | Tereshkin, A. A. (Mining Institute) | Sidljar, A. V. (Mining Institute)
The problem of decrease in risk of technogenic catastrophes at large-scale mining operations is very actual problem for the Far East region where a number of the deposits are burst-hazard. Methods and technical means for the research of a geomechanical condition of rock massif of and results of their application on the burst-hazard underground mines of the East of Russia are considered in the article. Application of a complex of the numerical and instrumental methods is effective for the assessment of burst hazard and geodynamic risk. Measuring complex includes seismo-acoustic and straining devices. Multi-channel geoacoustic ("Prognoz ADS") and microseismic ("Prognoz S") systems of monitoring, a portable device "Prognoz L", a laser deformograph and other measuring tools are included in the measuring complex. Regularities of change of geophysical fields reflecting processes of redistribution of tension in the massif and preparation of the hazard geodynamic phenomena are determined.
Large-scale and intensive development of mineral deposits has the considerable impact on a natural intense strained state of an upper of crust. It leads to rock and tectonic bursts, mine seismicity and other hazard geodynamic phenomena (Henryk M., Mutke G.Z., 2013; Rasskazov, 2008). In the East of Russia there are some areas with high intensity of mining operations (Dalnegorsk, Streltsovsky and other ore areas). It has happened several tens of the strong geodynamic phenomena for the last few years. In some cases they were accompanied by destruction of underground mine excavations and seismic fluctuations of the surface complex. On the Nikolaev polymetallic field located in East Primorye of Russia on March 24, 2016 around an ore deposit the East-1 (block 40) the technogenic earthquake which was followed by strong sound, shaking of all mine field and numerous destructions of mine excavations on −375, −390, −406, −420 m horizons are registered. The volume of the destroyed rock exceeded 400 m3. The geodynamic phenomena led to formation of the open gaps and tens meters long cracks in the rock massif. Soil raisings (10–12 cm) were observed on the borders of gaps. Gaps spatially coincide with elements of tectonic structure of the deposit (Rasskazov et al., 2016a).
The key aim of this study was to compare the slope stability estimation results by the limit equilibrium and numerical modelling methods on the examples of large open pit mines. This study compares factors of safety resulting from standard estimation methods for dry slopes and with account of water table, including impacts of large scale blasting and earthquakes, based on Mohr-Coulomb and Hoek-Brown criteria.
With the increase of the open pit mining depths, the slope stability issues in complex mining and geological conditions are becoming more important.
In the clear majority of cases, open pit slope stability is estimated by limit equilibrium methods and, mainly, in two dimensions. These estimation methods have proven to be sufficiently reliable, however they have certain assumptions that can become very significant in deep pits, where even small changes in the slope angles can have a significant economic impact.
Numerical modelling, and primarily the finite element modelling method, is an increasingly common approach to slope stability estimation. Its main advantage is not only in estimating the slope stability, but also in estimating stress and strain distributions at each point of the rock mass.
The methods of limit equilibrium have been widely tested throughout the world, and specialists have learned to use them for various geomechanical conditions. The finite element method, despite the active development and implementation of newer software, requires sufficiently high qualification to obtain reliable results. However, as our experience shows, the methods of limit equilibrium and numerical modelling can complement each other.
To enable practical application of the finite elements method, it is necessary to test it and its algorithm of safety factor search, named SRF (Strength Reduction Factor), in comparison with the traditional Factor of Safety (FoS). For this purpose, the comparison analysis has been carried out. The results of two-dimensional analysis of different open pits by both methods have been considered, which allowed to estimate the differences and similarities of the finite element method compared to the classical limit equilibrium method (Figure 1). The analysis was carried out in Rocscience software: the method of limit equilibrium was tested in Slide v 6.0, and the finite element method was tested in Phase v 8.0.
The method of reconstruction of stress state in rock mass surrounding underground excavations based on solution of boundary inverse problems of finding values and orientation of external stress field components using the data of full-scale experiments is substantiated. The experiment data are taken from the hydraulic fracturing stress measurement carried out in mines of the Upper Kama Potassium Salt Deposit (mines SKRU-1, 2 and 3). The geomechanical model of the object under investigation is developed and the inverse problem is solved. Using the results and the original code for finite-element modeling of structurally inhomogeneous media with discontinuities, the contour lines of the stress tensor components are plotted. 1 INTRODUCTION Construction and operation of a mine is unimaginable without the integrated geomechanical research, an important element of which is the study into in-situ stresses of rocks (Zang & Stephansson, 2010). This knowledge is the critical condition of the appropriate design choice. The extensive experimental research proves that a full-scale test is a unique possibility to have a quantitative estimate of actual stresses in rock mass. In the course of prospecting and operating a mineral deposit, a huge bulk of direct and indirect real data on stress state of rocks is accumulated but remains scarcely utilized in theoretical modeling. This paper proposes an approach to taking into account such data in geomechanical modeling through solving inverse problems. To that end, a theoretical geomechanical model of stress state will be refined using data of field testing. A test object is selected to be the Upper Kama Potassium Salt Deposit (Russia). The source information for the verification on of the geomechanical model of the object are the data of hydraulic fracturing stress measurement implemented in mines SKRU-1, SKRU-2 and SKRU-3 of Open Joint-Stock Company "Uralkali". 2 FULL-SCALE EXPERIMENTS The experimental assessment of stress state of salt rocks in mines of the Upper Kama Potassium Salt Deposit was carried out at the depth of 330–350 m below daylight surface in 2015–2016. The real data were obtained during hydraulic fracturing stress measurement using the procedure described in (Rubtsova & Skulkin, 2013). According to the experiment data, the rock mass stress state is non-equicomponent: the vertical stresses σv are nearly equal to the lithostatic pressure 6.8 MPa calculated by the overlying formation weight, and the while horizontal stresses exceed σv by 2–3 times.
The problem of assessing the state of stress in mine workings is vital for safe underground mining. The main source of information about a local stress field is usually obtained from measurements on the walls of underground galleries or in boreholes. An alternative to these methods are non-destructive short-term geophysical methods calibrated with a small amount of information from drilled wells and used to monitor stress conditions. Of this category, the greatest attention was paid to the method of acoustic emission and the method of electromagnetic radiation. Both of these phenomena are caused by the rock fracturing. This paper considers the physical basis for the application of acoustic emission in underground conditions based on a modem understanding of the phenomenon and its features.
The release of high concentration of stress in the form of collapsed rocks towards the underground galleries is the main cause of accidents during mining (Zhang et al. 2017a), since the natural distribution of underground stress is significantly altered by the additional stress superimposed by the mining operations, and substantial decrease in strength of rocks (He et al. 20017). Hence, the problem of assessing the state of stress in mine workings is vital for safe mining. The manifold methods have been utilized to monitor the potential for the stress concentration (Zhang et al. 2017b). Hereafter we merely concern the local short-term methods. The main source of information about a local stressed field is usually obtained from measurements the walls of underground galleries or in drilled wells. An alternative to these methods are non-destructive short-term geophysical methods, i.e. acoustic emission (AE) or/ and electromagnetic radiation (EMR) caused by rock fracturing. Acoustic emission (micro-seismic) has been used since the 1920s to assess the underground stress conditions in Poland, South Africa, Canada, the U.S., Australia, China, etc. (He et al. 2017 and references therein). Numerous studies of the AE phenomenon make it possible to understand that the signals AE are a small-scale phenomenon whose properties are analogous to the large-scale radiation of an elastic wave caused by rock-bursts and earthquakes (Kuksenko et al. 1985; Lockner and Rehez 1994; Guha 2001 and references therein; Lei et al. 2003; Thompson et al. 2009; John-son et al. 2013; Goebel et al. 2014; McLaskey and Lockner 2016 and references therein). The formation of cracks and/or macroscopic fractures in rocks is the main cause of the AE exciation, whose energy and spectrum depend on the mechanisms of their generation and rock properties (Michlmayr et al. 2012). Monitoring of AE excitation is applicable for the detection of crack sources, hazard assessment in mines, tunnels, etc (Kim et al. 2015; Kuksenko and Makhmudov 2017 and references therein). It was shown that the AE time-sequence parameters correspond well with the evolution of the rupture events of the rock material at different stages and hence can provide the precursor for the rock damage (Kuksenko et al. 1985; Lockner and Rehez 1994; Rehes 1999; Liang et al. 2017). For example, Cai et al. (2001) characterized rock damage near excavation using AE monitoring. It was shown that the combined AE and EMR methods it possible to accurately assess the risk of high stress accumulation (Frid and Vozoff 2005; Lu et al. 2015; Jiang et al. 2016). The experimental results obtained at the San Pietro gypsum mine, Prato Nuovo, underscore the close correlation between AE and EMR activity, while it was noted that both types of emissions preceded a failure event for approximately one day and 3—4 days, respectively (Carpinteri and Borla 2017). Henceforth, we consider several aspects of the phenomenon of AE, which are important for its application for local short-term stress assessment while features of EMR phenomenon were examined in detail by Frid and Mulev (2018 and references therein).