Mechanism and control of strong ground pressure in shallow buried ultra-large mining height working face with directly overlying double key strata
This study investigates the mechanism of strong ground pressure in shallow buried ultra-large mining height faces with double key strata through field observation and simulation, establishing a theoretical support resistance model and demonstrating that hard roof pre-splitting effectively reduces fracture intervals and stress concentration to ensure stable mining conditions.
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 digging a giant hole in the ground, but instead of a shovel, you're using a massive machine that eats through a thick layer of coal. This is the world of deep-earth mining, specifically looking at "ultra-large mining height," where the machine is so tall it can swallow a ten-meter chunk of rock in one go. The big challenge here isn't just getting the coal; it's keeping the roof from crashing down. In mining, the layers of rock above the coal are like a heavy, stacked Jenga tower. Usually, when you pull a block out, the blocks above shift and lock together in a stable arch, holding the weight. But when you dig a hole that's ten meters high, the blocks above have to rotate too far to lock in place. They slip, slide, and lose their grip, sending a massive shockwave of pressure straight down onto the miners and their machines. This is called "strong ground pressure," and it's the reason why some mining faces are dangerous or even impossible to work. Scientists have long known that certain hard layers of rock, called "key strata," act like the main beams holding up this roof. But what happens when you have two of these heavy beams right on top of each other in a shallow, super-tall mine? That's the puzzle this paper tries to solve.
The researchers, led by Zekun Wang and Hualei Zhang from Anhui University of Science and Technology, decided to investigate this specific nightmare scenario: a shallow mine with a ten-meter high working face and two hard "key strata" layers directly above it. They wanted to understand exactly how these two layers interact, how they break, and how much force they dump onto the hydraulic supports (the giant metal legs holding up the roof). To do this, they didn't just guess; they built a miniature version of the mine in a lab using sand, gypsum, and whiting, scaled down 100 times. They also created computer models and looked at real data from the Caojiatan coal mine in Shaanxi, China, where this exact situation was happening.
Here is what they discovered. When the mine starts, the lower of the two hard layers breaks first. Because the hole is so tall, this broken layer doesn't form a nice, stable arch. Instead, it acts like a long, wobbly diving board that snaps off and slides down. This causes a "small-period weighting," which is like a rhythmic, moderate thumping on the supports. It's annoying and puts stress on the equipment, but it's manageable. However, the real trouble starts with the second, upper hard layer. This layer is huge and heavy. When it finally breaks, it doesn't just fall on its own; it crashes down so hard that it forces the lower layer to break again, even if it wasn't ready. This creates a "large-period weighting" event. It's like a domino effect where the top domino falls, knocking over the middle one, which then smashes into the bottom one, sending a massive, synchronized shockwave of weight onto the mine floor. The study found that during these big events, the pressure is so intense that the existing supports in the mine (rated for 29,000 kN) simply aren't strong enough to hold the roof up; the math showed the load could hit 30,000 kN or even 40,000 kN in simulations.
So, how do you stop a ten-meter high roof from crushing you? The team proposed a clever trick: "roof pre-splitting." Imagine the hard roof layers are like a giant, unbreakable chocolate bar. Instead of waiting for it to snap naturally under the weight of the earth (which causes the big, dangerous crash), the miners use high-pressure water to shoot cracks into the rock before the mining machine even gets there. This weakens the "chocolate bar," making it break into smaller, more manageable pieces earlier. The simulations showed that this technique was a game-changer. By pre-splitting the hard roof, they reduced the distance between breaks (the "periodic weighting interval") by about 57% for the lower layer and 56% for the upper layer. More importantly, it dropped the maximum stress concentration on the working face by nearly 20%.
When they tested this idea in the real world at the Caojiatan mine, the results were promising. After implementing the pre-splitting, the violent, synchronized crushing events became much less frequent and less intense. The "weighting interval" (how far the mine had to advance before the roof gave a big thump) became more predictable, averaging around 16.7 meters, and the ground pressure behavior stabilized. The paper concludes that while you can't just build a stronger support to solve this problem (because the forces are too high), you can change the game by breaking the roof yourself, on your own terms, before it decides to break you. It's a reminder that sometimes, the best way to handle a heavy load is to break it into smaller pieces before it gets too heavy to carry.
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