Study on Prevention and Control of Instability Disasters at Mining Faces with Hard Roof and High-gas Soft Coal Seams
This study addresses instability disasters in hard-roof, high-gas soft coal seams by revealing that the coupling of periodic roof weighting and residual gas pressure drives the risk, and validates that a combined prevention strategy of deep-hole pre-splitting blasting and cross-measure hydraulic reaming effectively mitigates these hazards.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep within the earth, where coal seams lie hidden beneath layers of rock, miners face a constant, silent struggle against the forces of geology. Two specific conditions make this underground work particularly treacherous: a roof made of exceptionally hard rock that refuses to break, and coal that is so soft it crumbles easily while holding high-pressure gas. When the roof is hard, it acts like a stiff, unyielding beam that spans wide distances before finally snapping. When it does snap, it releases a massive, sudden shockwave that squeezes the coal wall below. Simultaneously, the gas trapped inside the soft coal acts like an invisible pressure cooker, weakening the coal's internal structure and making it far more likely to fail under stress. If these two forces—the heavy, sudden impact of the roof and the weakening pressure of the gas—happen at the same time, the result can be a violent collapse of the mine wall, known as rib spalling, which endangers workers and halts production.
Researchers at the Chongqing Research Institute in China set out to solve this specific problem at a coal mine in Shaanxi Province. They focused on a mining face where the coal was extremely soft and the roof was a thick layer of hard sandstone. By combining field observations, computer simulations, and laboratory tests, they uncovered exactly how these two dangers work together to cause disasters. Their investigation revealed that the frequency of wall collapses was not random; it spiked dramatically whenever the hard roof broke in its natural cycle, and the danger was even worse in areas where the gas pressure remained high. The study showed that gas pressure alone could reduce the strength of the coal by nearly half, meaning that a roof collapse that might be manageable in dry conditions becomes catastrophic when the coal is saturated with high-pressure gas.
To test their theories, the team first looked at the behavior of the hard roof. Using a computer model that simulates how rock fractures, they confirmed that without intervention, the roof would span over 60 meters before breaking for the first time, and then break again roughly every 26 meters. These wide gaps meant that when the roof finally gave way, it did so with tremendous force, creating a large, unstable zone. The researchers also conducted physical tests on coal samples in a lab, subjecting them to different levels of gas pressure. They found that as the gas pressure increased, the coal became significantly weaker and more brittle, losing almost half of its ability to bear weight. This confirmed that the gas was not just a separate hazard, but a critical factor that degraded the coal's ability to hold up against the roof.
Armed with this understanding, the team developed a two-part strategy to prevent these collapses. First, to deal with the hard roof, they implemented a technique called deep-hole pre-splitting blasting. Instead of waiting for the roof to break naturally over a wide distance, they drilled deep holes into the roof rock and used controlled explosions to create a series of deliberate, smaller fractures. This intervention forced the roof to break much sooner, reducing the distance it could span from over 60 meters down to just 20 meters, and cutting the regular breaking cycle from 26 meters to about 13 meters. By breaking the roof into smaller, more manageable pieces, they prevented the buildup of massive energy that usually leads to violent collapses.
Second, to address the gas and the soft coal, they used a method called hydraulic reaming in the tunnels below the coal seam. This process involved drilling holes and then expanding them with high-pressure water to crush the surrounding soft coal and create a large, open cavity. This action dramatically increased the permeability of the coal, allowing the trapped gas to flow out much more easily and quickly. The result was a significant drop in the gas pressure remaining in the coal, bringing it down to a safe, low level. This not only reduced the risk of gas-related explosions but also restored some of the coal's natural strength, making it better able to withstand the pressure from the roof.
When the team applied both of these methods together at a new mining face, the results were striking. The combination of weakening the roof's impact and strengthening the coal by removing gas pressure drastically reduced the number of wall collapses. In the treated area, the frequency of instability events dropped to nearly zero during normal mining periods and remained very low even when the roof was breaking. The hydraulic supports, which hold up the roof, experienced much lower pressure than before, indicating that the dangerous energy had been successfully dissipated. The study concludes that by tackling the roof and the gas simultaneously, it is possible to turn a highly unstable and dangerous mining environment into a safe and manageable one, offering a reliable solution for other mines facing similar geological challenges.
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