A flame retardant rigid polyurethane foam (RPUF) system containing functionalized graphene oxide (fGO), expandable graphite (EG), and dimethyl methyl phosphonate (DMMP) was prepared and investigated. The results show that the limiting oxygen index (LOI) of the flame-retardant-polyurethane-fGO (FRPU/fGO) composites reached 28.1% and UL-94 V-0 rating by adding only 0.25 g fGO. The thermal degradation of FRPU samples was studied using thermogravimetric analysis (TG) and the Fourier transform infrared (FT-IR) analysis. The activation energies (E_a) for the main stage of thermal degradation were obtained using the Kissinger equation. It was found that the fGO can considerably increase the thermal stability and decrease the flammability of RPUF. Additionally, the E_a of FRPU/fGO reached 191 kJ·mol⁻¹, which was 61 kJ·mol⁻¹ higher than that of the pure RPUF (130 kJ·mol⁻¹). Moreover, scanning electron microscopy (SEM) results showed that fGO strengthened the compactness and the strength of the “vermicular” intumescent char layer improved the insulation capability of the char layer to gas and heat.

The soft-rock roadways in kilometer-deep coal mines are often damaged by large deformation and have to be periodically expanded and repaired, which seriously restricts the safe and efficient production of coal mines. A typical soft-rock roadway in a kilometer-deep coal mine is selected as the engineering, and the main reasons for roadway deformation are analyzed, and the ground stress and mechanical characteristics are obtained. The Flac^3D numerical model, which can accurately reflect the deformation characteristics of surrounding rock in kilometer-deep soft-rock roadway, has been constructed, and the evolution laws of stress field and its damage mechanism have been analyzed with the vertical stress, vertical displacement and plastic zone. The damage range of roadway surrounding rock is elucidated by endoscopic observation, and the synergistic control technology of support and modification is proposed. The on-site engineering application is carried out in Qianyingzi Coal Mine, and the results show that the collaborative control technology of support and modification ensures the safety and stability of deep soft-rock roadway. The results provide important and preliminary technical references for realizing safe and intelligent mining in coal mines.

Coal seam water injection technology enables seam permeability enhancement and facilitates outburst risk reduction. This study investigated the microscale effects of water infiltration on coal and the evolution mechanisms of its mechanical properties. To this end, we systematically analyzed dynamic changes (such as mineral composition, pore structure, and mechanical performance) in coal soaked for various durations using X-ray diffraction, low-field nuclear magnetic resonance (NMR), and uniaxial compression testing. The results indicate: (1) the coal NMR T₂ spectrum displays three characteristic peaks, corresponding to rapid water absorption, uniform transition, and stabilization stages of soaking traditionally divided according to peak area variation trends. (2) The coal strength decreases with progressive soaking, influenced by water content, pore volume, mineral composition, etc. Its compressive strength and elastic modulus drop by 22.4% and 19.5%, respectively, compared to the initial values. (3) The expansion of clay minerals during immersion reduces average pore size. In contrast, quartz particle displacement, pore water movement, and soluble mineral dissolution increase pore volume, reducing the overall structure strength. (4) The dominant factors driving the degradation of mechanical properties vary across immersion stages, including water content and specific mineral concentration. This work offers new insights into how hydraulic technology alters coal seams, providing theoretical support for optimizing water injection strategies in the seam.

In the process of protected seam mining, the reduction in stress and the enhancement of the gas mobility that affects the protected seam are crucial manifestations of the protection effect. Taking the working face of E₈-32010 and the upper D_5-6 coal seam of the Six Mines of Ping Coal Company Limited as the study object, the research method combining theoretical analysis, numerical simulation, and a field test was adopted. In combination with the actual production, we adopted the stress distribution law pertaining to the coal body of the protected seam under the condition of 2.2 m mining height. When the length of the tendency of the working face mining is under the condition of 2.2 m mining height and when the working face mining inclination lengths are 120 m, 160 m, 200 m, 220 m, 240 m, and 280 m, the stress distribution law that regulates the coal body of the protected seam is analyzed, and, based on the stress distribution law, the unpressurized extraction drilling holes are designed, and the effect of the coal body stress at the final hole position of the unpressurized extraction drilling holes on the efficiency of unpressurized extraction is determined through on−site extracting data. The research results indicate the following: (1) as the tendency length of the working face increases, the degree and range of pressure unloading that affects the protected layer are increasing, the stress increase in the deep D_5-6 seam is larger than that of the shallow D_5-6 seam, and the tendency direction can be divided into the stress elevation area, pressure unloading area, and stress elevation area from the shallow zone to the deep zone. Moreover, the minimum stresses in the pressure unloading area are 7.80 MPa, 6.42 MPa, 5.59 MPa, 5.59 MPa, 5.42 MPa, 5.30 MPa, and 5.21 MPa, and the minimum stress is less than 60% of the original stress; (2) the vertical stresses at the final locations of the No. 1, No. 2, and No. 3 drill holes after the protective layer is mined are 16.42 MPa, 10.74 MPa, and 6.72 MPa, respectively, and the pure amount of gas extracted from the unpressurized extracting drill holes has increased immensely; the higher the rate of unloading, the greater the increase, and, the more the unpressurized extracting drill holes are extracted, the greater the increase. The higher the unloading rate, the greater the increase: 19.77–21.31 times, 41.62–41.68 times, and 68.68–74.66 times the pure amount of gas extracted from the corresponding pre−pumping holes; (3) the No. 3 depressurized extraction borehole is 261.02–281.04 times, 191.77–205.55 times, and 138.43–148.18 times higher than the No. 1, No. 2, and No. 3 pre−pressurized extraction boreholes, respectively, and 6.09–7.14 times and 2.28–2.49 times higher than the No. 1 and No. 2 depressurized extraction boreholes, respectively. The research results can not only provide a theoretical basis for verifying the protection effect of the protected layer but also a scientific rationale for the layout of the unpressurized extraction drill holes.

The present research focuses on the mechanical properties and stress evolution of gas-bearing soft coal seams during drilling, which are affected by a multitude of complex factors such as high ground stress, gas pressure, and pre-existing fractures. In this study, a combination of PFC2D (Particle Flow Code in 2 Dimensions) numerical simulation and theoretical analysis is employed to investigate the borehole mechanics and fracture evolution characteristics under diverse complex conditions and to determine the factors influencing different forms of borehole failure in soft coal seams. The principal outcomes are as follows: (1) At a horizontal displacement of 0.1 m from the borehole orifice of the soft coal seam, a stress peak value of 13.9 MPa is attained; the peak value of the coal body contact force is 15.8 MPa; the peak value of the displacement is 0.008 m; and the porosity of the coal body around the borehole ranges from 0.14 to 0.35. (2) With an increase in the number of pre-existing fractures, the inclination progressively aligns with that of the pre-existing fractures. Maximum values of contact force (5.13–51.9 MPa), stress (3.19–37.2 MPa), shape dimension, and fracture angle (140–150°) are achieved under the highest lateral pressure coefficient and gas pressure (1.5 MPa). (3) The borehole energy is directly proportional to the number of pre-existing fractures, the lateral pressure coefficient, and gas pressure. The number of pre-existing fractures has the most significant impact on the damage degree, followed by the lateral pressure coefficient and then the gas pressure. (4) Two types of failure are identified: fracture-dominated failure, which is controlled by the geometric distribution of pre-existing fractures, and stress-dominated failure, wherein the failure zone gradually extends both upward and downward with an increasing lateral pressure coefficient.