This paper presents an innovative method for using foam concrete as a typical building material for soft structures in underground coal mines subjected to dynamic loading. To understand the behaviour of foam concrete under impact loading, a total of 30 specimens with a diameter of 50 mm and a height of 50 mm were experimentally tested using a 75 mm diameter split Hopkinson pressure bar (SHPB) device. The key parameters investigated in the present study included the type of foam concrete (fly ash and sand), the density of foam concrete (1000, 1200 and 1400 kg/m3), and the impact velocity (3.0, 4.0, 5.0, 6.0, and 7.0 m/s). Six specimens were also tested under static loading for comparison. The stress-strain curve of foam concrete under impact loading showed three stages, started with a linear elastic stage, followed by a yield stage and ended with a pore wall destruction stage. The test results also indicated that the dynamic increase factor, ultimate compressive strength, tenacity, and specific energy absorption increase with the strain rate under the same density. In particular, both the failure model and the behaviour of foam concrete were affected by the impact velocity. The findings of this research provide a reference for further research on the application of foam concrete in underground coal mines.
A novel method for fracturing coal is presented in this paper. A chemical solution is injected into coal under high pressure, whereby the coal is fractured and subsequently weakened by chemical erosion over time to produce an anti-impact soft structure. In this study, the mechanical properties of coal under chemical erosion were investigated, and the fracturing design parameters were optimized. The uniaxial compression test and the split Hopkinson pressure bar (SHPB) test were used to determine the dynamic and static mechanical properties of coal after 20 days of immersion in different chemical solutions. After chemical solution erosion, the dynamic and static compressive strengths and elastic modulus of the coal decreased according to an exponential power law in the damage variable. The chemical treatment increased the duration of the pore compaction stage and decreased that of the elastic deformation stage, while decreasing the brittleness and increasing the ductility of coal. The acoustic emission (AE) curve of the immersed coal samples consisted of four stages corresponding to those of the stress-strain curve: pore compaction-closure, a slowly rising linear elastic regime, steady-state prepeak crack propagation, and unsteady crack propagation at the peak strain. The increase in the damage variable of the coal sample from chemical erosion led to a lower dissipated energy, a higher fractal dimension, and a more fragmented coal sample. The effect of the investigated chemical solutions on weakening the coal mechanical properties decreased in the following order: alkaline solution > acidic solution > NaCl solution > distilled water. The experimental results provide a reference for weakening fractured coal seams.
This paper studies the evolution and control of surrounding rock under different pressure relief support conditions in mine roadways in which rockburst events have occurred. The evolution of fractures in the surrounding rock was determined from borehole images obtained with a digital panoramic borehole camera, and the surface displacement due to the rockburst events in the mine roadway was measured. According to the existing problems of the original support system of the roadway, a new coupled support system to prevent rockburst events in mine roadways was proposed, resolving both the pressure relief and support of the roadway. Field measurements indicate that the effect on the roadway under the coupled method of pressure relief and support was more satisfactory than that under the original support system. With the coupled support method, the surface displacement of the roadway was approximately 0.6 m, fractures were distributed only in the soft structures and bolt anchorage areas, and the maximum depth of the fractures was 2.95 m. By contrast, under the original support system, fractures were distributed throughout the roadway surrounding rock, and the maximum depth of fractures was 6.75 m. This coupled roadway support technology of pressure relief and support effectively maintains the stability of the rock surrounding the roadway and ensures the safety of the working face. The research results can provide a reference for damage prevention and support of mine roadways prone to rockburst events.
A rock burst usually causes a roadway collapse or even an instant blockage. When the deformation energy accumulated in the surrounding rock exceeds the minimum energy required for the dynamic destruction of the surrounding rock of a roadway, a rock burst accident will occur. According to statistics, 85% of rock burst accidents occur in roadways. This paper establishes a strong-soft-strong structural model for the rock burst stability control of the surrounding rock of a roadway, and the anti-impact and antiseismic mechanisms of the mechanical model are analysed. The strength, stress transfer, deformation, and energy dissipation characteristics of the strong-soft-strong structure are studied. The stress criterion and energy criterion of rock burst failure in small internal structures are derived for roadway support design. The support scheme of “anchor cable active support + hydraulic lifting support + soft structure energy absorption” is proposed. A steel pipe can be inserted into a borehole drilled into the small internal structure to realize the proposed innovative protection technology for small internal structures by creating a soft structure that can release, absorb, and transfer the pressure by repeatedly cracking the coal and rock mass. The innovation of cracking technology for soft roadway structures has been realized. The roadway tested with this strong-soft-strong enhanced surrounding rock control technology met the production requirements during the mining period. The field test was successful, and the expected support effect was achieved. This work provides a reference for roadway support under similar conditions and can be popularized and applied.
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