Elastic Strain Energy Redistribution and Ground Control through Large Diameter Borehole Pressure Relief in a Coal Roadway under Intense Mining Disturbance

Deep beneath the earth, where coal miners dig tunnels to reach valuable seams, the rock is under immense pressure. This pressure is not static; it is a living force that shifts and surges, especially when heavy machinery cuts through the rock nearby. When a tunnel is dug, the rock that once held the weight of the mountain above is suddenly cut away. The remaining rock tries to find a new balance, often shifting its weight onto the walls and roof of the tunnel. In deep mines, this shift can be violent. The rock stores energy like a coiled spring, and if that energy is released too quickly or unevenly, it can shatter the tunnel walls, crush support beams, and trap workers. For decades, engineers have tried to manage this by reinforcing the tunnel with bolts and steel, but sometimes, adding more strength is not enough. The rock simply pushes back harder. The challenge, then, is not just to hold the rock in place, but to guide how it releases its stored energy, turning a potential explosion into a controlled, manageable shift.

Researchers at the China University of Mining and Technology set out to solve this problem for a specific tunnel, the 50502 haulage roadway at the Nangou Coal Mine. This tunnel was struggling. Despite having a standard support system of steel bolts and cables, the walls were buckling and the floor was heaving upward under the weight of the mining operations above. The team suspected that the existing method of "pressure relief"—drilling large holes into the rock to let it relax—was not quite right. They wanted to understand exactly how the size and depth of these holes changed the way energy moved through the coal. Instead of just guessing, they built a detailed computer model of the tunnel and the rock around it, simulating the drilling of holes of different sizes and depths. They then watched how the energy stored in the rock shifted, looking for the "sweet spot" where the rock would release its stress without collapsing the tunnel.

What they found was a delicate balancing act. When you drill a hole into highly stressed coal, you create a zone of broken rock around the hole. This broken zone acts as a buffer, absorbing some of the stress. However, the rock does not simply disappear; the stress has to go somewhere. The researchers discovered that drilling a hole pushes the area of highest energy concentration deeper into the coal, away from the tunnel wall. This is good, because it keeps the immediate tunnel walls from being crushed. But there is a catch. If the hole is too small, the stress doesn't move far enough away. If the hole is too large, it pushes so much energy deep into the rock that it creates a new, dangerous pocket of high pressure far underground. The team found that the size of the hole matters immensely. As they increased the diameter of the borehole, the area of broken rock grew, but so did the intensity of the energy pocket forming deeper inside. They identified a specific threshold: once the hole diameter exceeded 240 millimeters, the deep energy concentration began to rise sharply, becoming a new risk. Similarly, the depth of the hole was critical. Holes that were too shallow, less than 12 meters deep, failed to move the stress far enough away. It was only when the holes went deeper than 12 meters that the dangerous energy near the tunnel wall began to drop significantly.

The study also looked at the properties of the coal itself. They found that the "stickiness" of the rock, known as cohesion, and the angle at which the rock grains slide against each other, called the internal friction angle, acted as natural brakes. The stronger and more cohesive the coal, the smaller the relief zone became, making it harder to relieve the pressure. Conversely, higher stress from the mountain above or a more horizontal squeeze from the sides made the relief zone larger. The researchers realized that simply drilling a big hole was not a universal solution; the parameters had to be tuned to the specific rock conditions. They determined that for the conditions in the Nangou mine, a hole diameter of 180 millimeters and a depth of 20 meters was the optimal balance. This size was large enough to break the immediate stress near the wall but small enough to prevent a dangerous buildup of energy deep underground.

To test this theory, the team implemented this specific drilling pattern in a 200-meter section of the troubled tunnel. They drilled holes 20 meters deep, spaced one meter apart, and combined this with a reinforced support system that included special truss structures to hold the roof. The results were immediate and measurable. Before the new system, the tunnel floor was rising by 300 millimeters and the walls were squeezing inward by 412 millimeters, causing severe damage. After the new pressure relief and support scheme was installed, the floor heave dropped to 231 millimeters, and the wall convergence plummeted to just 181 millimeters. The roof-to-floor squeezing was reduced from 190 mm to 144 mm, a decrease of nearly a quarter, but the most dramatic improvement was in the side walls, which stabilized by more than half. The researchers concluded that the key to safety in these deep, high-stress tunnels is not just to hold the rock or to drill holes, but to understand how the energy moves. By carefully choosing the size and depth of the relief holes, engineers can guide the rock's energy to a safe location, allowing the tunnel to breathe without collapsing. This approach offers a clear path forward for mines facing similar intense pressure, proving that understanding the hidden flow of energy is just as important as the steel bolts holding the roof up.