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Tunnel failure caused due to geologic heterogeneity and variability in both mining processes and tunnel arrangement in deep metal mines is very complex. In this paper, a quantitative risk assessment for deep tunnel failure was conducted using clustering and logistic regression. The study was performed by considering various representative evaluation indexes of geological conditions, mining disturbance, and microseismic (MS) data. Comprehensive Seismic Event Clustering (CSEC) methodology was used to analyse the MS data to filter noise data and identify seismic sources. Additionally, peak particle velocity (PPV) was used to quantitatively evaluate the influence of MS activities on the response of tunnel which would aid in determining the PPV distribution characteristics of a large regional tunnel. Furthermore, Excavation Vulnerability Potential (EVP) index was used to quantitatively evaluate the effect of site conditions on response of the tunnel. Thus, based on logistic regression, PPV and EVP were used to establish a risk assessment model to determine the tunnel damage levels and probability of damage occurrence. The successful application of the proposed method at the Ashele copper mine demonstrates that it is capable of conducting a risk assessment for tunnel failure in deep metal mines. [ABSTRACT FROM AUTHOR]

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In the mining process of deep metal mines, different types of rock mass instability failures are caused by strong mining disturbance. It is beneficial to master the fracture mechanism of rock mass in time to effectively prevent and control the ground pressure disasters. Microseismic signals are generated by the propagation and expansion of cracks inside the rock mass that contain plentiful information about the structural changes of rock mass. The ratio of the radiated energy of S and P waves (Es/Ep) of microseismic events can fast and effectively calculate the rock fracture mechanism, which is widely used for ground pressure hazard risk assessment. In this paper, this method was used to analyze the fracture mechanism of rock mass around deep stope in Hongtoushan copper mine and Ashele copper mine. Furthermore, the spatial distribution characteristics and proportion changes of microseismic events with different fracture mechanisms along with the mining process were studied. The results show that tensile cracks play a dominant role, accounting for 62% of the total events, during non-shear fracturing of the rock mass caused by the stoping unloading effect, while shear cracks occupy 68% of the total events during orebody slip failure. When the physical and mechanical properties of the orebody and rock mass are significantly different, slip failure along their contact zone is prone to occur under blasting disturbance. During deep mining, it is necessary to control the exposed area of the roof by each stoping, especially during the earlier mining stage, to avoid tensile stress concentration. The temporal and spatial variation of tension cracks and shear cracks induced by roof damage obtained in this paper can guide the prevention and control of ground pressure disasters in deep mining effectively. [ABSTRACT FROM AUTHOR]

The purpose of this ISRM Suggested Method is to describe a methodology for in situ acoustic emission monitoring of the rock mass fracturing processes occurring as a result of excavations for tunnels, large caverns in the fields of civil, rock slopes and mining engineering, etc. In this Suggested Method, the equipment that is required for an acoustic emission monitoring system is described; the procedures are outlined and illustrated, together with the methods for data acquisition and processing for improving the monitoring results. There is an explanation of the methods for presenting and interpreting the results, and recommendations are supported by several examples. [ABSTRACT FROM AUTHOR]

In deep metal mines, seismic hazards (for example, rock bursts and roof collapses) occur easily due to the influence of complicated geological conditions, high-stress environments and strong blast disturbances. A microseismic (MS) monitoring technique was used to evaluate rock mass stability in the Hongtoushan copper mine in China. The changes in the multiple MS parameters, including the apparent volume, energy index, spatial correlation length, fractal dimension and b value, during the mining process were presented. The results showed that the MS sequences decayed following Omori's power-law and that the rock mass returned to a relatively stable state after approximately 20 days of blasting. After each stoping, the proportion of large-scale fractures increased, the MS events become more concentrated, and the long-range correlations of the rock mass weakened. These changes caused b value, fractal characteristics and spatial correlation length to decrease significantly. Although the risk of a large-scale rock mass failure was reduced, the risk of a local failure increased. As the mining process continued, stress and deformation gradually increased, and the areas with concentrated stress and large deformation were not consistent. Control measures for hazards should be implemented according to the corresponding occurrence mechanisms as determined from the differences in the rock mass physics mechanics. [ABSTRACT FROM AUTHOR]

An acoustic emission (AE) testing of rock cracking was performed under uniaxial loading conditions by precut varisized circular holes in selected brittle granites. Based on AE-source location technique and AE-theory for moment tensor analysis, rules of the temporal-spatial evolution of micro-cracks in different failure mechanisms were explored and types of micro-cracks were analyzed as well. The results revealed that the micro-cracks are uniquely easy to generate in the positions where stress are concentrated. Tensile fractures are easy to form on the roof and floor of a circular hole, while shear fractures are easy to be found on both sides. The locations of initial cracks generated around the holes in the loading process are the direction or vertical direction of maximum principle stress. Macroscopic crack orientation agrees with the direction of maximum principle stress approximately. As the size of circular opening increases and the relative size of pillar decreases, shear cracks are dominant with the percentage more than 45%, tension cracks are fewer, accounted for less than 40% of the total events, and mixed-mode cracks represent a minimum proportion, despite the decrease of percentage of shear cracks. The findings of this work can serve for supporting design of tunnel or roadway to avoid collapse. [ABSTRACT FROM AUTHOR]

The purpose of this ISRM Suggested Method is to describe a methodology for in situ microseismic monitoring of the rock mass fracturing processes occurring as a result of excavations for rock slopes, tunnels, or large caverns in the fields of civil, hydraulic, or mining engineering. In this Suggested Method, the equipment that is required for a microseismic monitoring system is described; the procedures are outlined and illustrated, together with the methods for data acquisition and processing for improving the monitoring results. There is an explanation of the methods for presenting and interpreting the results, and recommendations are supported by several examples. [ABSTRACT FROM AUTHOR]

This work presents an experiment on the acoustic emission (AE) of coarse grain granites with two square-shaped precut holes under uniaxial loading. Studies were carried out on the temporal-spatial evolution behavior of micro-cracks by AE mechanisms with the use of the simplex location method and the moment tensor theory, with further analysis in comparing the numerical simulations using the software RFPA $$^\mathrm{2D}$$ (Rock Failure Process Analysis). The results show that during the loading process, from beginning to rock failure, shear-mode micro-cracks are prominent, constituting more than 60 % of the total events; next most common are tensile-mode micro-cracks a less than 35 % of the total events. Variations of micro-cracks of the three modes during the loading process have the same increase tendency, i.e. fewer were generated in the initial loading stage, with a rapid increase when the stress values are between 40 and 60 % of the peak stress, and a rate dimunition before rock failure. It is observed that the tensile stress concentration is prone to appear at the tops of the two holes in the form of tensile type cracks, while the shear stress concentration usually appears at the bottom in the middle region of the specimen in the form of shear type cracks. The findings of the present work may serve as guidance for the prevention of roof and floor collapse in the stope exploration of mines. [ABSTRACT FROM AUTHOR]

oustic emission tests of the core specimens retrieved from boreholes at the depth over 1000 m in Hongtoushan Copper Mine were carried out under uniaxial compressive loading, and the numerical test was also done by using the rock failure process analysis (RFPA) code, based on the procedure for estimating geo-stresses by the Kaiser effect under uniaxial compression. According to the statistical damage mechanics theory, the Kaiser effect mechanism was analyzed. Based on these analyses, it is indicted that the traditional method of estimating geo-stresses by the Kaiser effect is not appropriate, and the result is usually smaller than the real one. Furthermore, the greater confining compression in the rock mass may result in a larger difference between the Kaiser effect stresses acquired from uniaxial loading in laboratory and the real in-situ stresses. [ABSTRACT FROM AUTHOR]