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The manifestation of strainbursts is related to the strain energy stored in the rock and how this energy is released and transitioned during unstable dynamic failure. It is of significance to investigate the role of energy evolution and transition during strainbursts and how it is influenced by surrounding rocks. For this purpose, experimental and numerical studies were performed in this study. Composite rock–coal specimens were loaded under the uniaxial compressive condition to produce strainbursts in the laboratory. Numerical simulation was then performed to reproduce the strainburst process observed in the laboratory and investigate the energy-absorbing-transition process associated with the spontaneous instability of the coal in the composite coal–rock specimens. The result has provided direct evidence of the conversion of elastic energy stored within a rock into the forms of kinetic energy to be released during the strainbursts process. For composite rock specimens under unconfined compressive conditions, a softer rock with lower Young’s modulus tends to store more elastic energy than a stronger rock and thus contribute more to the strainbursts that occurred at a surrounding rock. This loading condition is analog to the pillar loading condition in which the pillar is loaded by the roof and floor of the pillar due to the stress distribution resulting from excavation. The kinetic energy of ejected rock in strainbursts is not solely from the elastic energy stored in the zone where the bursts occur but also from the surrounding rocks. Most of the kinetic energy comes from the elastic energy stored within the busting zone and the contribution of the elastic energy stored in surrounding rocks is not significant.

Brittle failure is a fundamental failure pattern in many different materials, from small nanoscale materials with single crystals to the large earth crust. Many efforts have been dedicated to understanding the brittle failure mechanisms of individual brittle and semi-brittle materials. Limited studies have been conducted on the brittle failure of composite materials with interaction and energy feedback between different materials. Here we investigated the brittle failure pattern of coal–rock composite materials under uniaxial compression by laboratory tests and numerical simulations. We used a high-speed camera to capture the failure of coal–rock specimens. For all three tested coal–rock combined specimens, the rock failed with a splitting pattern that resulted from a single tensile fracture that developed sub-parallel to the loading direction. We regarded this brittle failure as a sliding-induced tensile fracture from frictional drag that was caused by unequal lateral deformation of the rock and coal under identical axial loading. The tensile crack propagated stably at ~ 0.05 times the Rayleigh wave speed c R . We observed an unstable failure pattern of the coal samples that was characterized by the ejection of small pieces from the coal specimen surface. This behavior is attributed to the strain energy that is stored in the rock specimen, which releases when the coal fails. The excessive strain energy transitions into dynamic energy during coal failure. Our findings provide insight into the brittle failure mechanisms of composite materials and have significant implications at scales relevant to seismicity, engineering applications and geohazards.

In underground mining practice, the rock bolt support system is the major support pattern to control the deformation and stability of openings. A rock bolt is generally subjected to complex loads including tension, torsion, bending and shear, which result from the deformation of excavations and exposure to dynamic loads that are generated by rockbursts. An understanding of the response of rock bolt under complex conditions is of great importance for rock bolt support design and practice. New sophisticated equipment has been developed for this purpose. This work involved a comprehensive experimental study on the mechanical behavior of rock bolts under complex loads. The results show that rock bolt pre-tensioning by torque application to the nut can result in decreases in tensile strength and elongation because the rock bolt is subjected to a combination of tension and distortion. When a pre-tensioned rock bolt is subjected to a shear load, the maximum shear force can reach up to 80% of the tensile capacity of the rock bolt. Higher impact energy results in a longer period of dynamic loading and a larger irreversible plastic deformation on the rock bolt, in contrast to a rock bolt that is subjected to low impact energy. The capacity and especially the deformation capacity of a rock bolt may decrease significantly after successive containment of the deformation of the surrounding rock mass from rockbursts.

The grouted bolt, combining rock bolting with grouting techniques, provides an effective solution for controlling the surrounding rock in deep soft rock and fractured roadways. It has been extensively applied in numerous deep mining areas characterized by soft rock roadways, where it has demonstrated remarkable control results. This article systematically explores the evolution of grouted bolting, covering its theoretical foundations, design methods, materials, construction processes, monitoring measures, and methods for assessing its effectiveness. The overview encompassed several key elements, delving into anchoring theory and grouting reinforcement theory. The new principle of high pretensioned high-pressure splitting grouted bolting collaborative active control is introduced. A fresh method for dynamic information design is also highlighted. The discussion touches on both conventional grouting rock bolts and cable bolts, as well as innovative grouted rock bolts and cables characterized by their high pretension, strength, and sealing hole pressure. An examination of the merits and demerits of standard inorganic and organic grouting materials versus the new inorganic–organic composite materials, including their specific application conditions, was conducted. Additionally, the article presents various methods and instruments to assess the support effect of grouting rock bolts, cable bolts, and grouting reinforcement. Furthermore, it provides a foundation for understanding the factors influencing decisions on grouted bolting timing, the sequence of grouting, the pressure applied, the volume of grout used, and the strategic arrangement of grouted rock bolts and cable bolts. The application of the high pretensioned high-pressure splitting grouted bolting collaborative control technology in a typical kilometer-deep soft rock mine in China—the soft coal seam and soft rock roadway in the Kouzidong coal mine, Huainan coal mining area, was introduced. Finally, the existing problems in grouted bolting control technology for deep soft rock roadways are analyzed, and the future development trend of grouted bolting control technology is anticipated.

Rockbursts cause damage to underground excavations in a sudden and violent manner and are associated with mining-induced seismic events. We propose a simple experimental method to study strainburst process in the laboratory, that simply involves a common compression testing apparatus and rock-coal-rock specimens. Strainbursts of coal samples are successfully produced and the burst process is monitored using high-speed camera and acoustic emission sensors. The strainburst mechanism is characterized by an initial ejection of small coal fragments followed by the ejection of large coal blocks. We found that the strainbursts are caused by the elastic strain energy stored in the rock samples during the uniaxial compression. The amount of the transferred energy is significantly less than the elastic energy stored in the coal sample but plays an important role in triggering strainbursts. The greater the transferred energy, the greater the damage severity of strainbursts occurred in the coal sample. Tensile cracking subparallel to the vertical loading direction and tangential compressive stress appears to play a dominant role in the strainburst failure mechanism.

Interbedded rock layers are a typical geological structure in coal measures. In underground mining practices, the disturbance resulting from the extraction of long-wall panels changes the loading rate of roof–coal structures. To investigate the mechanical behavior and failure characteristics of roof–coal structures under different loading rates, composite coal–rock specimens with a height ratio of 1:1 was prepared and loaded under uniaxial compressive conditions with different loading rates. Acoustic emission signals and the deformation process of sandstone and coal were monitored. The fracture morphology was analyzed by scanning electron microscope (SEM), and the mean diameter of debris (da) was calculated to quantitatively evaluate the fragmentation. It is found that the sensitivity of the mechanical behavior to heterogeneity tends to decrease with the loading rate. The higher the loading rate, the less likely localized failure occurs in the pre-peak stage, the smoother the axial stress–axial strain curves before peak stress, and the greater the uniaxial compressive strength and Young’s modulus. The higher the loading rate, the more brittle the post-peak behavior and the severer the damage of the coal sample at the final loading stage. The coal sample tends to fail through intragranular cracking under a high load rate and intergranular cracking under a low loading rate. The sandstone sample exhibits deformation rebounding in the post-peak stage, providing direct evidence of energy transition from strain energy stored in the sandstone to cracking energy of the coal sample during coal failure. The higher the loading rate, the greater the rebounding speed of the sandstone, leading to severer damage to the coal sample.

Grouting is an effective method to improve the integrity and stability of fractured rocks that surround deep roadways. After years of research and practice, various theories and a complete set of grouting technologies for deep roadways with fractured rocks have been developed and are widely applied in Chinese coal mining production. This paper systematically summarizes and analyzes the research results concerning the theory, design, materials, processes, and equipment for the grouting and reinforcement of fractured rocks surrounding deep roadways. Specifically, in terms of grouting methods, pregrouting, grouting‐while‐excavation, and postgrouting methods are explored; in terms of grouting theory, backfill grouting, compaction grouting, infiltration grouting, and fracture grouting theories are studied. In addition, this paper also studies grouting borehole arrangement, water‐cement ratio, grouting pressure, grouting volume, grout diffusion radius, and other grouting parameters and their determination methods. On this basis, this paper explores the physical and mechanical properties of organic and organic‐inorganic composite grouting materials, and assess grouting reinforcement quality testing methods and instruments. Taken as the field cases, the application of pregrouting in front of heading faces, grouting‐while‐excavation, and postgrouting in the Kouzidong coal mine are then introduced, and the effects of the grouting reinforcements are evaluated. This paper proposes a development direction for grouting technology based on problems existing in the grouting reinforcement of fractured rocks surrounding deep roadways. Grouting theories and technologies for the reinforcement of fractured rocks surrounding deep roadways are reviewed. Field cases of the application of pregrouting in front of heading faces, excavation‐while‐grouting, and postgrouting in the Kouzidong coal mine are introduced. A development direction for grouting technology is proposed based on problems existing in the grouting reinforcement of fractured rocks surrounding deep roadways. Highlights Grouting theories and technologies for the reinforcement of fractured rocks surrounding deep roadways are reviewed. Field cases of the application of pregrouting in front of heading faces, excavation‐while‐grouting, and postgrouting in the Kouzidong coal mine are introduced. A development direction for grouting technology is proposed based on problems existing in the grouting reinforcement of fractured rocks surrounding deep roadways.

The post-peak characteristics of coal serve as a direct reflection of its failure process and are essential parameters for evaluating brittleness and bursting liability. Understanding the significant factors that influence post-peak characteristics can offer valuable insights for the prevention of coal bursts. In this study, the Synthetic Rock Mass method is employed to establish a numerical model, and the factors affecting coal post-peak characteristics are analyzed from four perspectives: coal matrix mechanical parameters, structural weak surface properties, height-to-width ratio, and loading rate. The research identifies four significant influencing factors: deformation modulus, density of discrete fracture networks, height-to-width ratio, and loading rate. The response and sensitivity of post-peak characteristics to single-factor and multi-factor interactions are assessed. The result suggested that feasible prevention and control measures for coal bursts can be formulated through four approaches: weakening the mechanical properties of coal pillars, increasing the number of structural weak surfaces in coal pillars, reducing the width of coal pillars, and optimizing mining and excavation speed. The efficacy of measures aimed at weakening the mechanical properties of coal is successfully demonstrated through a case study on coal burst prevention using large-diameter borehole drilling.

Accurate estimation of the triaxial compression behavior of jointed coal is essential for coal mining. Few studies addressed the triaxial compression behavior of large-scale rock mass, especially with real joint geometry. We employed a numerical synthetic rock mass (SRM) method to study the triaxial compression behavior of jointed coal. Jointed-coal specimens were constructed based on in-situ joint measurements and microparameter calibration against laboratory experiments. A series of triaxial compression tests under different loading orientations and confining pressures were numerically performed to obtain joint and confining-pressure effects on the triaxial compression behavior and reveal the failure mechanism of jointed coal. Results suggest that the triaxial compression behavior of the jointed coal has strong joint and confining-pressure effects. Joints weaken the strength and elastic modulus, reduce the lateral deformation, and affect the geometries of the shear-rupture surface. An increase in the confining pressure causes the peak and residual strength increase significantly. With an increase in the confining pressure, the elastic modulus increases sharply at low confining pressure, the mechanical behavior transitions from brittleness to ductility, the failure mode transitions from shear-rupture surface to plastic flow, and the joint effect diminishes and even disappears. The jointed coal fails by means of a shear-rupture surface under triaxial compression loading with a confining pressure (which is not too high), and the geometries of the shear-rupture surface vary with the distribution of joints.

The evolution of mining-induced stress field in longwall panel is closely related to the fracture field and the breaking characteristics of strata. Few laboratory experiments have been conducted to investigate the stress field. This study investigated its evolution by constructing a large-scale physical model according to the in situ conditions of the longwall panel. Theoretical analysis was used to reveal the mechanism of stress distribution in the overburden. The modelling results showed that: (1) The major principal stress field is arch-shaped, and the strata overlying both the solid zones and gob constitute a series of coordinated load-bearing structures. The stress increasing zone is like a macro stress arch. High stress is especially concentrated on both shoulders of the arch-shaped structure. The stress concentration of the solid zone in front of the gob is higher than the rear solid zone. (2) The characteristics of the vertical stress field in different regions are significantly different. Stress decreases in the zone above the gob and increases in solid zones on both sides of it. The mechanical analysis show that for a given stratum, the trajectories of principal stress are arch-shaped or inversely-arched, referred to as the “principal stress arch”, irrespective of its initial breaking or periodic breaking, and determines the fracture morphology. That is, the trajectories of tensile principal stress are inversely arched before the first breaking of the strata, and cause the breaking lines to resemble an inverted funnel. In case of periodic breaking, the breaking line forms an obtuse angle with the advancing direction of the panel. Good agreement was obtained between the results of physical modeling and the theoretical analysis.