Microscopic fatigue damage behaviour due to dry and wet cycling on the deterioration law of mechanical properties of coal bodies with different water saturation levels

Deep beneath the earth's surface, where coal miners work, the environment is a harsh combination of crushing weight, intense heat, and high pressure. In these deep layers, the rock surrounding the coal seams is constantly stressed, and when water is present, it changes the game entirely. Water is not just a passive liquid in these tunnels; it seeps into the tiny holes and cracks within the rock, acting like a wedge that can pry things apart or dissolve the glue holding them together. When the water level rises and falls repeatedly—drenching the rock and then letting it dry out—the material undergoes a slow, invisible transformation. This cycle of wetting and drying is a common occurrence in nature and in mining operations, yet its long-term effect on the stability of the rock has remained a complex puzzle. Understanding how this repeated soaking and drying weakens the coal is critical for preventing sudden, dangerous collapses and ensuring the safety of those working underground.

Researchers from China University of Mining and Technology and Guizhou University set out to solve this puzzle by looking at coal samples in a way that had never been done before. They took blocks of coal from a deep mine and subjected them to a rigorous series of tests, mimicking the natural cycles of rain and drought that occur over years, but compressed into a laboratory setting. The team created four groups of coal samples and subjected them to zero, three, six, or nine cycles of soaking in water for two days and then drying in an oven for one day. To see what was happening inside the rock without breaking it, they used a powerful industrial scanner, similar to a medical CT scan but with much higher resolution, to build a three-dimensional map of the tiny pores and cracks within each sample. They also used a technique that measures how much nitrogen gas sticks to the surface of the coal to count the microscopic holes, and they examined the samples under a microscope to see the physical changes at a grain level. Finally, they tested how strong the coal was by blasting it with high-speed impacts while squeezing it from all sides, simulating the violent conditions of a deep mine.

What they discovered was a story of initial expansion followed by a sudden collapse. As the number of wet-dry cycles increased, the coal did not simply get weaker in a straight line; its internal structure went through a dramatic evolution. In the early stages, the repeated swelling and shrinking of the coal caused new cracks to open up and existing ones to connect, making the rock more porous and its internal network more complex. However, after a certain point, this process backfired. The constant stress of swelling and shrinking caused the tiny bridges between the coal particles to break, leading to a collapse of the pore structure. The researchers found that the total volume of the holes and the surface area of the cracks first grew larger as the cycles increased, but then began to shrink as the internal structure crumbled and some of the smaller holes were blocked by minerals that had dissolved and then re-deposited. This meant that while the coal became more damaged, the nature of that damage changed from a network of open, connected cracks to a more chaotic, collapsed mess.

The impact of this structural change on the strength of the coal was severe and predictable. The team found that the more cycles the coal endured, the less force it could withstand before breaking. This weakening was not linear; it followed a specific curve where the strength dropped sharply at first and then continued to decline more gradually. The effect was even more pronounced when the coal was fully soaked with water. Water-saturated coal samples were significantly weaker than dry ones, and the presence of water inside the pores acted like a hydraulic press, pushing the cracks open from the inside and making the rock fail much faster under pressure. The researchers observed that the energy required to break the coal decreased as the cycles increased, meaning the rock became easier to destroy. When the coal was saturated, it absorbed more energy during the impact because the water inside forced the cracks to spread further, but this came at the cost of overall stability.

To understand why this happened, the team looked closely at the chemical and physical processes at work. They found that water does more than just fill holes; it chemically attacks the minerals that hold the coal together. Certain minerals, like calcite, dissolve when they come into contact with water and carbon dioxide, weakening the bonds between the coal particles. As these minerals dissolve, the structure loses its integrity. Furthermore, the water promotes chemical reactions that create new, oxygen-rich groups on the surface of the coal molecules, making the material more attracted to water and less stable. This chemical loosening, combined with the physical stress of the water pressure inside the cracks, creates a perfect storm for failure. The researchers used computer simulations to model these processes, confirming that the combination of swelling, chemical dissolution, and pore collapse accurately predicted the weakening they saw in the lab.

The implications of these findings are direct and practical for the mining industry. The study suggests that in areas where coal seams are exposed to fluctuating water levels, the rock is not just getting wet; it is being systematically dismantled from the inside out. The repeated cycles of wetting and drying create a fatigue that eventually leads to a loss of strength that cannot be ignored. For mine engineers, this means that simply draining water is not enough; they must also consider how the rock has been weakened by past cycles. The researchers recommend using stronger support systems, such as combining bolts with a layer of sprayed concrete to seal the rock surface, effectively isolating the coal from the wet-dry environment. By understanding that the rock's strength degrades in a specific, predictable pattern, mining operations can better anticipate where and when the ground might give way, turning a potential disaster into a manageable engineering challenge.