Dynamic Response and Energy Absorption Characteristics of Coal- Rock Material Systems under True Triaxial Impact Loading
Through true triaxial split Hopkinson pressure bar tests, this study reveals that the coal-rock interface critically governs stress-wave redistribution and energy absorption under impact loading, driving a transition from cooperative bearing to deformation incompatibility and secondary instability in deep mining environments.
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
Deep underground, where the earth's crust presses down with immense weight and heat, coal mines face a constant, hidden danger. The rock and coal that hold up the tunnels are not static; they are under tremendous pressure from all sides, waiting for a disturbance to trigger a sudden, violent release of energy. This is the world of rockbursts and coal outbursts, where the ground can shatter without warning, endangering lives and equipment. To understand these disasters, scientists must look at how rock and coal behave when they are squeezed by the earth and then hit by a sudden shock, like a falling roof or an explosion. For decades, researchers have studied how single blocks of rock or coal react to such forces, but the real world is rarely so simple. In a mine, coal is almost always sandwiched between layers of harder rock, creating a composite system where the two materials must work together, or fail together. The question that has long puzzled engineers is how this boundary between the soft coal and the hard rock changes the way energy moves and how the system breaks when it is struck.
A team of researchers from China set out to solve this puzzle by recreating the deep underground environment in a laboratory. They built a specialized machine capable of squeezing a sample from three different directions at once, mimicking the true triaxial stress found miles beneath the surface. Into this machine, they placed three types of samples: pure coal, pure rock, and a custom-made block where a piece of coal was glued directly to a piece of rock, representing the natural layers found in a mine. They then subjected these samples to high-speed impacts, striking them with a heavy projectile at speeds ranging from 2.5 meters per second up to 8 meters per second. This setup allowed them to watch, in real-time, how the materials responded to the combination of heavy squeezing and sudden hitting, measuring exactly how much energy they absorbed, how they deformed, and how they finally failed.
The results revealed a clear pattern of how speed changes strength. As the impact speed increased, every sample became stronger and could absorb more energy before breaking. The pure rock was the toughest, able to withstand the highest pressure, while the pure coal was the weakest. The composite block, made of both materials, fell somewhere in between, but its behavior was far more complex than a simple average of its parts. When the composite sample was hit, it did not just break like a single material would. Instead, it showed a unique sequence of events: it reached a peak strength, dropped quickly to a lower but stable level of holding power, and then eventually collapsed completely. This "residual" phase, where the material held on for a while after its main strength was gone, was a distinct feature of the coal-rock combination that neither the pure coal nor the pure rock displayed on its own.
The way energy moved through these samples provided the most critical insight into why the composite behaved so differently. When a shockwave hits a material, some of that energy bounces back, some passes through, and some is soaked up by the material itself as it breaks. The researchers found that the pure rock let a significant amount of energy pass through it, while the pure coal reflected most of it back. The composite, however, acted like a massive energy trap at the boundary. Because the coal and rock are so different in their stiffness and density, the shockwave hitting the interface between them bounced back with incredible force. About two-thirds of the incoming energy was reflected back at the boundary, while only a tiny fraction passed through to the other side. This intense reflection created a zone of high stress right where the coal met the rock, causing damage to start there and spread rapidly into the weaker coal.
This behavior suggests that the danger in deep mines is not just about how strong the coal is, but about how it interacts with the rock surrounding it. The study showed that the hard rock can actually help the coal hold up more weight initially, but this cooperation is fragile. Once the impact is strong enough to damage the coal or the glue-like boundary between them, the load shifts suddenly. The coal, which was being held in check by the rock, begins to crush and slide, forcing the rock to take on more weight than it was designed to carry in that moment. This leads to a secondary, delayed failure that can be just as dangerous as the initial break. The researchers concluded that the interface between the coal and the rock is the most critical weak point in the system. It is where the energy concentrates, where the damage begins, and where the stability of the entire mine roof or floor can suddenly collapse.
For engineers working to keep mines safe, these findings offer a new way to think about risk. It is not enough to look at the strength of the coal or the rock in isolation. The connection between them is a dynamic link that can store and release energy in unpredictable ways. The study suggests that monitoring the boundary zones between hard rock and soft coal is essential, as these are the places where the system is most likely to lose its balance. By understanding that the composite system fails in stages—first by losing its grip at the interface, then by redistributing the load, and finally by collapsing entirely—engineers can design better support systems and warning mechanisms. The goal is to manage the energy before it builds up to the point where the interface can no longer hold, preventing the sudden, violent failures that have plagued deep mining for so long.
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