Mechanical Deterioration and Micro-macro Damage Evolution Mechanisms of Underground Reservoir Coal-Pillar Dams Subjected to Drying-Wetting Cycles

The Hidden Battle Beneath Our Feet

Imagine the Earth's crust not as a solid, unchanging floor, but as a giant, porous sponge made of rock and coal. Deep underground, miners have left behind vast, empty caverns called "goafs" after they've finished digging out the coal. Scientists are now looking at these abandoned spaces as potential goldmines for a different kind of treasure: energy storage. The idea is to flood these old mines with water to create massive underground reservoirs. When the sun shines or the wind blows, we pump water up; when we need electricity, we let it rush back down to spin turbines. It's a brilliant plan to store renewable energy, but there's a catch.

The walls holding back this giant underground lake are made of the leftover coal pillars. These pillars are constantly getting soaked and then dried out as the water levels rise and fall, a process scientists call "drying-wetting cycles." Think of it like a sponge that you soak in water, wring out, let dry in the sun, and then soak again. Over time, this repeated soaking and drying can make a sponge crumble. In the world of geology, this "sponge effect" is a major worry because if the coal pillars get too weak, the whole underground dam could fail. This paper dives deep into exactly how that crumbling happens, using high-tech listening devices and magnetic scanners to see what's going on inside the coal at a microscopic level.

The Story of the Crumbling Pillar

So, what did the researchers actually do? They took chunks of coal from a real mine in China (the Daliuta Coal Mine) and put them through a rigorous "spa treatment" that was actually a torture test. They soaked the coal in water, dried it in an oven at 60°C, and repeated this cycle up to four times. Then, they squeezed these wet-and-dried coal blocks in a giant machine that mimicked the heavy pressure of the earth above them (using pressures of 2, 4, 6, and 8 MPa). While squeezing, they listened to the coal with super-sensitive microphones (Acoustic Emission sensors) and took "MRI scans" of the inside (using Nuclear Magnetic Resonance) to see how the tiny holes inside the coal changed.

Here is what they found, and it's a bit of a shocker: The first time you soak and dry the coal, it takes the biggest hit.

The coal didn't just get a little weaker; it got significantly weaker right away. After just one cycle, the coal's ability to hold together (its compressive strength) dropped by about 17% under lower pressure conditions. By the time they finished four cycles, the strength was down by roughly 33%. It's like a cookie that gets soggy the first time you dip it in milk; the second and third dips make it softer, but the first dip is where it loses its crunch. The researchers also noticed that the coal's "glue" (called cohesion) was the first thing to go, dropping by nearly 36% after four cycles, while its "grip" (internal friction angle) barely changed at all, dropping only about 3%. This tells us that water dissolves the sticky stuff holding the coal particles together much faster than it changes how the particles rub against each other.

But the real magic happened when they listened to the coal "scream" while they squeezed it.

The Sound of Breaking:
The researchers used the Acoustic Emission (AE) sensors to listen for the tiny cracks forming inside the coal. They found that as the coal went through more wet-dry cycles, the "screams" (high-amplitude sounds) started happening much earlier in the squeezing process. In simple terms, the coal gave up and started breaking sooner. They also tracked a number called the "b-value," which is like a report card on the size of the cracks. A high b-value means lots of tiny, harmless cracks; a low b-value means fewer, but much bigger, dangerous cracks. The paper found that as the coal got wetter and drier, the b-value dropped. This suggests the coal was shifting from making a million tiny, manageable cracks to forming a few massive, catastrophic ones that could bring the whole structure down.

The Great Shift: From Tearing to Sliding:
Perhaps the most interesting discovery was how the coal actually broke. At first, the coal tended to break by tearing apart (tensile cracks), like ripping a piece of paper. But after repeated wet-dry cycles, the failure mode changed completely. The coal started to break by sliding past itself (shear cracks), like a deck of cards sliding off a table. The researchers used a special graph (RA vs. AF) to prove this. They found that the proportion of "sliding" breaks jumped from about 64% in dry coal to over 80% in coal that had been through four cycles. The water essentially turned the coal's internal structure into a slippery slide, making it much more likely to shear off and fail.

The Microscopic Sponge Effect:
Finally, they looked inside the coal using NMR, which acts like an X-ray for water and pores. They found that the coal is mostly made of tiny holes (micropores). However, the wet-dry cycles acted like a microscopic chisel. The water dissolved the tiny mineral particles that were holding these small holes together. As these minerals dissolved and fell away, the tiny holes merged and expanded into medium and large holes. The percentage of tiny pores dropped from 95.6% to 88.5%, while the big, dangerous pores grew from a tiny 0.6% to 4.4%. It's as if the water ate away the walls between small rooms, turning them into one giant, unstable hall.

The Bottom Line

This paper suggests that the biggest danger to these underground reservoirs isn't just that the coal gets wet, but that the cycle of getting wet and dry fundamentally changes how the coal breaks. It doesn't just get weaker; it changes its personality. It goes from a material that tears apart to one that slides apart, and it starts failing much sooner than we might expect. The first few cycles do the most damage, dissolving the internal "glue" and turning tiny pores into big, weak spots. While the heavy pressure of the earth (confining pressure) helps hold the coal together a bit, it can't fully stop this transformation.

For engineers building these underground power stations, the takeaway is clear: you can't just look at the coal once. You have to account for the fact that every time the water level rises and falls, the coal is getting a little more slippery and a little more prone to sliding. The first few times this happens are the most critical, and if you don't design for that rapid initial weakening, the whole system could be in trouble. The study doesn't say the project is impossible, but it does say we need to be very careful about how wide we make those coal pillars to keep the water—and the power—safe.