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With the production activities continue progressing into deeper underground spaces, the rising ground stress poses new challenges in the fracturing of hard rock. Previous research mostly focused on the outer actions of blasting on conventional rock mass, while research on the inner actions of blasting via discrete element method (DEM) is relatively scarce, especially for deep-buried hard rocks under high ground stress. Relying on the pre-cracking project of hard rock protective layer in the thousand-meter deep well of Pingdingshan coal mine, this paper aims to numerically investigate the fracture mechanism of deep-buried hard rock under blasting loads via DEM. To this end, the algorithm of simulating explosion load is improved. The improved algorithm ensures a more reliable correspondence between numerical results and engineering practice, significantly enhancing the accuracy and credibility of calculations. After the calibration of mesoscopic parameters on the basis of laboratory tests, a series of parametric study, including confining pressure, peak blast stress and lateral stress coefficient, have been performed to understand the effects of in-situ stress on the behaviors of rock blasting. The obtained numerical results exhibit that confining pressure inhibits the fracture growth: under low confining pressure, confining pressure mainly inhibits the development of fractures in sparsely fractured zone while the crack growth in densely fractured zone and crushed zone is also inhibited under high confining pressure. According to the stress state, hoop peak stress is more sensitive to confining pressure than radial peak stress. Rock breakage in the vicinity of blasthole is essentially controlled by the radial peak stress, while crack propagation in the far-field is mainly induced by the hoop peak stress. With different lateral stress coefficients, the failure characteristics of rock mass are principally related to the hoop stresses in the vertical direction. The obtained numerical results and mesoscopic analysis are capable of providing new insights into the fracturing mechanism of deep-buried hard rock.

A microseismic (MS) monitoring system in a mine can monitor the MS signals generated by coal rock rupture and blasting waves and can distinguish the two types of waves more clearly to monitor and analyze the rupture and evolution process of coal rock. According to the nonlinearity characteristics of the waveform, the fractal characteristics of a mine’s MS and blasting waves are analyzed by simple fractal and multifractal theory, and the simple fractal dimension $D$ and multifractal parameters are obtained, respectively. Results show that the simple fractal dimension $D$ reflects the complexity and frequency structure of the wave. The simple fractal dimension $D$ of a blasting wave is larger than that of a mine MS wave, which indicates that the blasting wave is relatively complex with higher frequency, while the mine MS wave is relatively simple with lower frequency. However, the simple fractal dimension $D$ can only describe the wave integrity features, not the local features. The multifractal parameters can describe the local characteristics of the wave more finely, and the multifractal spectrum describes the probability information of the singularity exponent $a$. The singularity exponential range and multifractal spectral width $\mathrm{\Delta }\alpha$ of the blasting wave are smaller than those of the mine MS wave, which indicates that the probability measure of distribution unevenness and the degree of partial parameter fluctuation of the blasting wave are more severe than those of the mine MS wave. Wave signal analysis based on simple fractal and multifractal methods can not only obtain the characteristics of the wave strength and spectral structure but also other important information, such as local singularity. Therefore, it is possible to more clearly and conspicuously identify mine MS and blasting waves, so that coal rock rupture can be monitored more accurately.

A layered rock mass is a special type of geological body. The existence of a bedding surface may lead to a poor cutting effect (over/under-excavation of the surrounding rock), falling of blocks, or collapse, thereby affecting most constructions in areas with such rocks. Given the lack of a proper quantitative analysis method for surrounding rock damages, the construction process of layered surrounding rock tunnels becomes difficult. To address these problems, three types of cut blasting models with single, double, and four holes are studied in this paper. With this, the LS-DYNA program is used to analyze the behaviors of stress wave propagation, crack propagation, and fracture modes, as well as fracture mechanisms of mudstone, sandstone, and layered rock. Using the image processing technology and fractal theory, the fractal dimension change trend and progressive damage evolution behavior of the three types of rocks under different cut blasting conditions are determined. Also determined is the corresponding relationship between the fractal dimension and the rock damage degree. The results indicate that crack initiation, propagation, bifurcation, and fractal dimension evolution are more closely related to the phenomenon where the compression wave is ahead of the tension wave, and the$\setminus$incompatible deformation of the bedding under single-hole blasting. Under double-hole and four-hole blasting, the phenomena, such as spalling, bedding crack penetration, and fracture connection between the explosive holes are caused mainly by the effects of stress concentration, reflected tension waves, and stress wave superposition. Moreover, under different blasting conditions, the rocks exhibit a similar progressive damage process, i.e. a rapid increase at first, then a slow rise, and finally a stabilization phase. The dynamic damage degree of the rock exhibits a linear increasing trend under different blasting holes. The study results provide a useful reference for blasting scheme design and optimization of underground engineering projects.