Mining pressure law and strata movement of thick coal seam mining in isolated panel with multiple adjacent goafs

Deep beneath the earth's surface, where the air is thin and the rock is heavy, miners face a constant battle against the weight of the planet. When a tunnel is dug to extract coal, the layers of rock above it lose their support and begin to shift, crack, and settle. This movement creates immense pressure on the remaining walls and pillars of coal, a phenomenon known as mining pressure. If this pressure is not understood and managed, it can cause the ground to collapse suddenly or trigger violent rock bursts that endanger lives. For decades, engineers have learned to predict these forces in simple, straight tunnels. However, as easily accessible coal runs out, mines are forced to dig into more complicated areas, often leaving behind small, isolated pockets of coal surrounded by empty spaces where other tunnels used to be. These "island panels" are like tiny islands in a sea of empty caverns, and the rules of how the rock behaves around them are far more unpredictable and dangerous than in standard tunnels.

Researchers at the China University of Mining & Technology set out to understand exactly how these complex, isolated pockets behave when they are finally mined. They focused on a specific site in the Yangchangwan Coal Mine in northwestern China, where a thick layer of coal was sandwiched between several previously mined areas. The situation was intricate: the new tunnel was flanked on three sides by old, empty tunnels of different sizes and ages, and it sat directly above another tunnel that had been mined years earlier. To make sense of this chaotic environment, the team built two types of models. First, they used powerful computer simulations to visualize how stress travels through the rock, treating the earth as a collection of blocks that push and pull against one another. Second, they constructed a physical, scaled-down replica of the mine in a laboratory, using layers of sand, gypsum, and other materials to mimic the real rock, and then carefully dug out the coal to watch how the model ground above it reacted.

The results revealed a landscape of stress that was far from uniform. In a typical mine, pressure might build up evenly, but here, the forces were lopsided and intense. As the mining face moved forward, the pressure did not rise smoothly; instead, it surged dramatically in specific spots. When the mining face approached a particular empty tunnel to the north, the energy in the rock piled up, causing the pressure to spike to 28 megapascals. This is a massive amount of force, concentrated in a way that made the rock feel as though it was being squeezed from multiple directions at once. The researchers found that the pressure was not the same on both sides of the tunnel. The upper part of the mining area, which was closer to the older, larger tunnels, bore the brunt of the weight, while the lower part, sitting above the older tunnel that had been mined underneath, actually experienced a release of pressure. It was as if the lower tunnel had acted as a cushion, absorbing some of the weight that would otherwise have crushed the new tunnel.

One of the most critical discoveries involved the thickness of the coal seam itself. The tunnel passed through a section where the coal was eight meters thick, and then transitioned into a section where it was only five meters thick. This change in height acted like a bottleneck for the stress. As the miners moved from the thick section into the thin one, the pressure in the rock ahead of them intensified sharply. The transition zone became a hotspot for stress concentration, where the rock was most likely to fail. The team also identified a specific layer of rock about seventeen meters above the coal that acted as a "key" to the entire system. This hard layer of sandstone controlled how the rest of the rock above it moved. When this layer finally broke, it triggered a cascade of movement that defined the mining pressure. The physical model showed that this layer did not break evenly; instead, it fractured in an uneven, asymmetric pattern, with cracks developing more aggressively on one side than the other.

The study also highlighted the danger of the coal pillars left behind to support the roof. In the area where the new tunnel was being dug, a narrow pillar of coal, only fourteen meters wide, separated the new work from an old, empty tunnel. The simulations showed that this pillar was under extreme stress, reaching peaks of 34 megapascals in some spots. This was nearly double the pressure found in the surrounding rock. The researchers observed that the stress in this pillar was not static; it shifted and grew more intense as the mining face got closer to the empty space. The combination of the uneven rock layers, the different sizes of the neighboring empty tunnels, and the change in coal thickness created a perfect storm of instability. The lower tunnel provided some relief, but the upper tunnels and the changing coal thickness created a complex, uneven pressure field that made the mining process significantly more hazardous than in a standard tunnel.

Ultimately, the work provides a clear map of how these difficult mining conditions behave. The researchers confirmed that the pressure in these isolated islands is not just a matter of depth, but a result of the specific arrangement of old tunnels, the thickness of the coal, and the strength of the rock layers above. They found that the danger is highest where the coal thickness changes and where the tunnel is squeezed between old empty spaces. The findings suggest that to mine these areas safely, engineers must reinforce the narrow pillars and prepare for sudden, intense bursts of pressure, particularly in the transition zones. By understanding that the rock behaves asymmetrically and that the pressure can be relieved in some areas while building up dangerously in others, mine operators can better plan their support systems. This knowledge is vital for extracting the remaining coal resources safely, ensuring that the complex, hidden geometry of the underground world does not catch the miners off guard.