Experimental study on stress field evolution of coal body by drilling and reaming
This paper presents an experimental study using a physical simulation system and advanced pressure sensors to analyze how drilling and reaming with different diameters influence stress field evolution and overlying strata pressure in coal bodies, providing a basis for optimizing pressure relief schemes in deep, high-stress mines.
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
Imagine the Earth's crust as a giant, heavy blanket of rock and coal, piled so deep underground that the weight above is crushing everything beneath it. In this deep, dark world, coal seams are like compressed sponges, storing massive amounts of invisible energy. When miners try to dig into this high-pressure zone, the coal can suddenly snap, releasing that stored energy in a violent explosion of rock and gas called an "outburst." It's a bit like stepping on a balloon that's been inflated until it's about to pop; the deeper you go, the tighter the squeeze, and the more dangerous the pop becomes. To stop this, engineers use a technique called "drilling and reaming." Think of it as poking a hole in that over-inflated balloon. By drilling a hole, you give the compressed coal a place to expand and breathe, releasing the pressure safely before it can cause an accident. But here's the tricky part: how big should the hole be? And how does the pressure actually move around as you drill? If you don't get the size right, you might not release enough pressure, or you might make the rock unstable in a new way. This is the puzzle scientists are trying to solve to keep mines safe and gas extraction efficient.
In this study, a team of researchers from Henan Polytechnic University decided to play "miniature miner" to figure out the perfect drilling strategy. Instead of digging into a real, dangerous mountain, they built a tiny, safe version of the problem in a lab. They created a block of "similar material"—a special mixture of crushed coal, cement, and water—that acted just like real coal but was small enough to fit on a table (300×300×300 mm). They squeezed this block with heavy hydraulic cylinders to mimic the crushing weight of the Earth, then used a robotic drill to bore holes of different sizes, ranging from 30 mm up to 63 mm.
The real magic, however, was in how they watched the pressure move. Usually, scientists have to guess where the stress goes or use computer models that might miss the details. This team used something like a high-tech, super-sensitive "pressure skin." They placed a giant, thin film sensor under the block and smaller sensors on top. When the drill bit cut through the material, these sensors didn't just give a single number; they created a live, colorful map of the stress, showing exactly where the pressure was high and where it dropped. It was like watching a heat map of a crowded room, but instead of body heat, they were tracking the invisible squeeze of the rock.
What did they find? First, they discovered that bigger holes do a better job of releasing pressure. When they drilled a small 30 mm hole, the pressure dropped right around the hole, but the rest of the block stayed tight. As they reamed the hole larger (up to 63 mm), the "pressure relief zone" grew, spreading out like a bubble. However, the shape of this bubble wasn't a perfect cylinder. Instead, it was weirdly shaped: narrow at the top and bottom, but wide and bulging in the middle. Imagine squeezing a tube of toothpaste; the middle bulges out while the ends stay thin. This means that if you drill a hole all the way through a coal seam with the same size drill bit, the middle of the hole gets a great pressure release, but the shallow and deep ends might still be dangerously tight.
The researchers also watched how the weight above the hole (the "overlying pressure") reacted. When they used a small drill bit, the pressure right above the hole jumped up and down wildly, like a shaky hand. But when they used the larger 63 mm bit, the pressure changes were much smoother and gentler. This suggests that bigger holes are less likely to cause the hole to twist or collapse because they don't create such violent swings in pressure.
The team also compared their "pressure skin" method to older ways of testing. They argued that sticking individual sensors inside the rock is like trying to understand a crowd by asking just one person what everyone else is doing—it gives you a single point of data but misses the big picture. Their new method, using the thin film, gave them a full, visual map of the stress field, which is much clearer and less likely to mess up the rock's natural structure.
So, what's the takeaway? The paper suggests that using a single, uniform drill size for the entire length of a hole might not be the best idea. Because the pressure relief zone is "fat" in the middle and "thin" at the ends, the researchers propose that miners might need to be smarter about their tools. They suggest using "adaptive" drilling, where the size of the hole is adjusted depending on how deep you are, to make sure the pressure is released evenly from top to bottom. While this study was done on a small, simulated block of material, the visual evidence they captured provides a strong experimental basis for designing safer, more effective drilling plans in the real, deep mines of the future.
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