Xuesheng Liu

It is difficult to control the mine pressure in the coal face under the close coal seam group. On the basis of the mining background of 4‐1#, 4‐2#, and 4‐3# coal seams in the I01 mining area of the Ningdong mining area, the fracture characteristics and evolution law of the overburden structure under the close coal group are studied. The results show that after the mining of the 4‐2# coal seam, the subsidence of the overlying rock of the 4‐1# coal seam roof increased by about 0.98–1.34 cm. After the mining of the 4‐3# coal seam, it increased by about 0.87–0.97 cm. The subsidence of the roof is reduced by about 11.22% to 27.61%. When the 4‐1# and 4‐2# coal seams are mined, the overburden rock morphology of the stope presents a “trapezium” structure. When the 4‐3# coal seam is mined, the overburden rock morphology of the stope presents a “double trapezoidal” shape form. If the coal seam spacing is small, the roof overburden structure is simplified into a cantilever beam structure. If the spacing is large, the roof overburden structure will form the transfer rock beam structure. Through theoretical analysis, the calculation formula for the overburden failure height and the roof support strength is obtained. The error between the theoretical calculation results and the similar simulation results and the field measurement results is small, which is 4.1%–4.5% and 2.7%, respectively. The reliability of the theoretical calculation results is proven.

Under the typical condition of deep buried weakly cemented overburden, the mining roadway is affected by strong mining stress and high ground stress, its stress environment is complicated, and the disaster behaviors are severe. In this study, the mining disaster behaviors and stress distribution characteristics under this condition are obtained by taking No. 130608 return air roadway of a deep buried weakly cemented overburden mine as the engineering background. Then, the mining stress transfer mechanical model is established, and the mining-induced disaster mechanism is revealed. The engineering criterion M, which can directly reflect the degree of mining-induced disaster, is constructed, and the mining-induced disaster’s degree is divided into four levels. The mining disaster stress adjustment technology’s idea based on “improving load environment + enhancing bearing capacity” is formed, and engineering verification is carried out. The results show that after the application of stress adjustment technology, the mining influence range is shortened from 120 to 80 m advanced working face, the stress of bolt cable is decreased by more than 8%, the surrounding rock’s deformation is decreased by more than 40%, which effectively prevents and controls the mining disaster behaviors, and the disaster reduction effect is remarkable.

Accurately locating the weak links in chamber group is crucial for achieving targeted and directional control for its surrounding rock. In this paper, the topological structure of the chamber group was first established, which divided the chambers into important chambers and general chambers. The chain instability inertia I was defined as the ratio of important chambers m to all chambers n, which can reflect the difficulty of chain instability for chamber group. On this basis, two-chamber, three-chamber, and four-chamber group were analyzed respectively, the topological structure evolution of chamber group were revealed. The concept of structural importance was introduced, which was used to quantitatively characterize the influence of micro-state of each chamber on macro-state of chamber group. Finally, a method of key chambers identification was proposed. When I = 0, there are no key chambers; when I = 1/n, the important chamber is the key chamber; and when I > m/n, chamber with the maximum key coefficient is the key chamber. Moreover, key chambers identification should follow four principles of structure, diversity, dynamic and priority. This research established a quantitative and effective framework for the stability control of chamber group.

How to identify the stress generally in coal seam during borehole drilling process is a hot topic. In engineering, to obtain the stress state by drilling torque is very necessary. This study intends to obtain the stress state and machine drilling torque by numerical simulations. Finite element method (FEM) is employed to simulate the dynamical drilling process, considering variations in coal mechanical properties and drilling torque. Results show that the stress in coal seam is positively correlated with drilling torque and coal modulus of elasticity, with a notable inflection point in stress variation at a drilling depth of approximately 13 m. Two fitting relationship formula between stress and torque, elastic modulus, and drilling depth, in the range from 0 to 13 m and exceeding 13 m, were obtained. The study provides an inversion identification approach by monitoring drilling torque, and it is hopeful to be used for early warning rock burst during pressure relief by borehole drilling.

Small coal pillar instability induced by lateral overlying strata in gob‐side working faces is an engineering problem, which is one of the main bottlenecks restricting safety production. This study investigated the stability and control technologies for small coal pillars through theoretical analysis, numerical simulation, and field testing. First, mechanical parameters of coal and rock mass were determined via numerical simulation. Based on field monitoring, the deformation and failure characteristics in small coal pillars caused by movement of weakly cemented lateral overlying strata were analyzed. The results showed that an “inverted trapezoidal” bearing zone formed above the coal pillar along the working face advancing direction. The advance abutment pressure reaches 39.8 MPa, representing a 62.31% increase compared to the original rock stress, with a maximum stress concentration coefficient of approximately 2.45. A structural mechanics model of the “coal pillar overlying weakly cemented strata” system was established by elastoplastic theory. Combined with field conditions, key technologies for small coal pillars were proposed. A “pressure relief‐grouting” coordinated control strategy was developed and verified by field tests.

The impact risk evaluation for the strip filling of working faces has always been a research hotspot and a difficulty in the field of rock bursts. In this paper, the concept of the critical filling rate is first proposed, and the criterion for identifying the impact risk of the strip filling of a working face is established. Then, the membership function of coal body stress and the coal seam elastic energy index to impact risk was established, and the classification index of the impact risk grade was formed. On this basis, the overall and local evaluation method of the rock burst hazard for the strip filling of working faces was proposed. Finally, the C8301 working face of the Yunhe coal mine was taken as the engineering background, and the impact risk evaluation was carried out. It was found that the overall risk of the C8301 working face was determined as a strong impact risk, and there were six local risk areas, which included two weak impact risk areas, two medium impact risk areas, and two strong impact risk areas. This study can provide guidance and a reference for the impact risk evaluation of strip-filling mining under the same or similar conditions.

Rheological mechanical properties of the soft rock are affected significantly by its main physical characteristics-clay mineral. In this study, taking the mudstone on the roof and floor in four typical mining regions as the research object, firstly, the clay mineral characteristic was analyzed by the X-ray diffraction test. Subsequently, rheological mechanical properties of mudstone samples under different confining pressures are studied through triaxial compression and creep tests. The results show that the clay mineral content of mudstone in different regions is different, which leads to significant differences in its rheological properties, and these differences have a good correlation with the content of montmorillonite and illite-montmorillonite mixed layer. Taking the montmorillonite content as an example, compared with the sample with 3.56% under the lower stress level, the initial creep deformation of the sample with 11.19% increased by 3.25 times, the viscosity coefficient and long-term strength decreased by 80.59% and 53.94%, respectively. Furthermore, based on the test results, the damage variation is constructed considering the montmorillonite content and stress level, and the M–S creep damage constitutive model of soft rock is established. Finally, the test results can be fitted with determination coefficients ranging from 0.9020 to 0.9741, which proves that the constitutive relation can reflect the influence of the clay mineral content in the samples preferably. This study has an important reference for revealing the long-term stability control mechanism of soft rock roadway rich in clay minerals.