In view of the increasingly prominent risk of mining-induced seismicity during the westward shift of coal mining development in China, this paper systematically analyzes the distribution patterns, typological characteristics, and occurrence mechanisms of mining-induced seismicity in Chinese coal mines. Besides, the formation mechanism of mining-induced seismicity in long-wall working face dominated by hard-thick roof is examined, and the elastic wave attenuation coefficients of mining-induced seismicity for typical mines in the Ordos Basin is inverted. Finally, a key integrated technology for coordinated prevention and control via surface fracturing, which combines vertical well layered fracturing with horizontal well precision fracturing, is proposed and then validated through engineering practice. The results indicate that mining-induced seismicity in China exhibits significant spatial clustering, and the frequency of mining-induced seismicity is generally positively correlated with regional coal production. Furthermore, high-incidence areas correspond to regions of intensive coal mining activities, confirming that mining operations are the primary trigger. Additionally, based on mining methods and techniques, mining-induced seismicity can be categorized into two types: that induced by shortwall mining and that induced by longwall mining. The former is primarily triggered by the instability of coal pillars in goafs, while the latter mainly results from the delayed fracture of hard-thick roofs. In the deep long-wall working faces of the Ordos region, mining-induced seismicity is predominantly of the roof movement type. A roof load-bearing structure model was constructed, and a formula for calculating the energy released during roof fracture was derived. Consequently, the relationships among hard-thick roof fracture and rock tensile strength, roof fracture and stratum thickness, as well as stratum thickness and energy release were systematically elaborated. Notably, the average elastic wave attenuation coefficient (α) for typical Ordos mines was determined to be 0.001, elucidating the mechanism behind mining-induced seismicity in long-wall working face dominated by hard-thick roof. A technical framework encompassing well-type optimization, layout design, and horizon identification was established. Engineering practice demonstrated that surface fracturing can achieve artificial pre-fracturing to weaken the roof, markedly reducing both the frequency and energy release of mining-induced seismicity and thereby enhancing the safe advance efficiency of working faces. Overall, this study provides theoretical support and an engineering paradigm for the safe and efficient prevention and control of mining-induced seismicity and rockburst in hard-thick roof mining areas of Western China.
In order to enhance the effect of rockburst prevention and control in gob-side working faces under thick-hard key strata, an integrated approach encompassing theoretical analysis, numerical simulation, and engineering application was adopted to investigate the main influencing factors and quantitative determination methods for the rockburst prevention width. Results indicate that both static and dynamic loads acting on the gob-side working face during mining are significantly influenced by the adjacent goaf. The static load is primarily governed by the pre-mining spatial structure of the overburden and its load transfer mechanism, whereas the dynamic load is predominantly induced by the fracturing of the thick-hard key stratum and the subsequent instability of the spatial structure of the overburden during active mining. By considering the overburden characteristics of the gob-side working face under a thick-hard key stratum, the quantitative relationship among the transferred load of the overburden (Pc), the goaf width (L), and the fracture height (h) can be established. Taking into account both the pre-mining overburden spatial structure and the strata movement state during mining, gob-side working faces were classified into three typical categories Type X1+X2, Type X1+\begin{document}$ \overline{{\mathrm{X}}2} $\end{document}, and Type \begin{document}$ \overline{{\mathrm{X}}1} $\end{document}+\begin{document}$ \overline{{\mathrm{X}}2} $\end{document}. The bearing capacity of gob-side working faces and the external loads they sustain were identified as the principal factors controlling the working face width. By constructing a ratio function between the bearing capacity and loading force, a comprehensive rockburst mechanical criterion and a quantitative method for determining the optimal rockburst prevention width of gob-side working faces were established. The ranges of rockburst prevention widths were calculated based on field cases of gob-side working faces under thick-hard key strata in different mines, and the rationality of the theoretical research was verified by numerical simulation results and the actual rockburst manifestation during mining. The research findings are expected to provide practical guidance and engineering references for rockburst prevention and control in gob-side working faces under thick-hard key strata.
Aiming at the increasingly severe rockburst hazards in deep mining roadways with multiple thick-hard roofs in northwestern China, this study established an engineering mechanical analysis model for rockburst in such roadways, based on analyses on coal-bearing stratigraphic structures and in-situ rockburst characteristics of six rockburst-prone mines affiliated with two typical deep mining areas in Ordos. The analysis identified two key structural elements affecting the surrounding rock stress distribution and structural stability of mining roadways, i.e., the "inverted right trapezoid zone", which bears the weight of overlying strata and transfers strata stress, and the "inclined block", which restricts the expansion of the "inverted right trapezoid zone" and protects section coal pillars. Centered on the core concepts of energy absorption and structure stabilization, decoupling and energy dissipation enhancement, stress transfer and release, and yielding support and resistance, we proposed a force-structure cooperative prevention and control mechanism for rockburst in deep mining roadways with multiple thick-hard roofs. Furthermore, we clarified the targeted rockburst prevention technical measures, including roadway roof structure optimization and surrounding rock stress control. Field practices at typical rockburst-prone working faces verified the effectiveness of the proposed force-structure cooperative prevention and control technique, providing theoretical guidance for rockburst prevention and control in deep mining roadways with thick-hard roofs in northwestern China.
To address the unclear rock burst prevention mechanism of underground directional hydraulic fracturing for a near-field hard roof, this study took the 302 working face of a coal mine in Inner Mongolia as the engineering background. Using a combination of theoretical analysis, numerical simulation, and field tests, it investigated the overburden movement and structural fracture, stress evolution, and energy release characteristics of the fractured stope, and revealed the rock burst prevention mechanism. Theoretical analysis results indicate that the peak mining-induced stress in the working face with a near-field thick-hard roof declines as the roof fracture span decreases. Hydraulic fracturing achieves mining-induced stress regulation by altering the rock strata fracture angle and the periodic fracture mode. Numerical simulation results reveal that compared with the unfractured model, after fracturing of the near-field key stratum, the average fracture angle increases by 14.4°, and the peak contact forces during initial and periodic fractures decrease by 32.8% and 19.9%, respectively. The overburden displacement and microseismic activity in the stope are remarkably enhanced. To be specific, the maximum displacement in the overburden coupled zone grows by approximately 3.7 m; the total frequency of microseismic events rises 3.6-fold, while the frequency of events with energy greater than 1×105 J falls by 82.6%, suggesting a transformation of microseismic activity from "low-frequency, high-energy" to "high-frequency, low-energy". Field tests show that after fracturing, the roof weighting range of the working face is noticeably reduced, the continuous weighting length stabilizes at 6–8 m, and the proportion of microseismic events with energy greater than 5×103 J drops to 1.2%. By increasing the fracture angle of the near-field key stratum, altering its periodic fracture mode, and changing the microseismic energy-frequency characteristics of the stope, underground hydraulic fracturing achieves mining-induced stress regulation and rock burst prevention. The research results can provide a reference for rock burst prevention under similar conditions.
Thick-hard roofs, characterized by high strength and good integrity, are widely distributed in the coal measure strata in the northwest energy base. Conventional hydraulic fracturing has the problems of high initiation pressure, limited fracture propagation, and difficult control of weakening range. Considering these problems, this paper proposed a synergistic weakening method that integrates liquid nitrogen jet pre-cooling with subsequent pressurized fracturing. Through physical simulation tests and thermal-fluid-solid-damage multi-field coupling numerical simulations, the mechanical response, damage evolution, and weakening zone expansion law at the end of pre-cooling were analyzed under different pre-cooling pressure and time conditions. The results show that liquid nitrogen jet pre-cooling can weaken the pressure-bearing capacity of the specimen in the subsequent pressurized fracturing stage. With the extension of pre-cooling time and the increase of pre-cooling pressure, the peak pressure is reduced by 19.1% and the peak time is shortened by 31.0%. The acoustic emission (AE) response characteristics show that the micro-crack activity and energy release have occurred in the pre-cooling stage, and the AE count and energy ratio move forward to the pre-cooling stage. Besides, the overall b-value decreases with the extension of the pre-cooling time, indicating that the crack activity gradually shifts from scattered micro-crack initiation to large-scale and localized propagation. The numerical simulation results show that the low-temperature affected zone and the damage zone are first formed near the injection hole and then expand radially. Raising the pre-cooling pressure will promote the advancement of the distal dominant crack front. At 1500 s, compared with the 1 MPa case, the damage area ratio and Req under 2 MPa increase by 5.8% and 2.8%, whereas R95 and Rmax increase by 17.5% and 8.5%, respectively, which suggests that higher pre-cooling pressure mainly enhances the outward extension of cracks. Liquid nitrogen jet pre-cooling changes the subsequent pressurized fracturing process from crack initiation in an intact specimen to propagation and coalescence along the pre-damaged zone, thereby reducing the threshold for subsequent pressurized fracturing. This study provides a theoretical reference for efficient permeability enhancement and fine control of thick-hard roofs in coal measure strata.
