Cross-Scale Analysis of Basalt Rock Mass Deformation and Energy Evolution Based on In-Situ Bearing Plate Tests and DEM Simulation

Deep beneath the surface of the earth, where massive concrete dams meet the bedrock, the safety of the entire structure depends on a single, often overlooked truth: rock is never truly solid. To the naked eye, a mountain of basalt looks like a continuous, unyielding block. In reality, it is a complex mosaic of stone fragments held together by invisible cracks, ancient faults, and layers of weathered material. When engineers build a hydroelectric dam, they must know how this hidden mosaic will squish, shift, or settle under the immense weight of the water. If the rock deforms too much, the dam could crack; if it shifts unpredictably, the foundation could fail. For decades, the standard way to measure this behavior has been to press a heavy, rigid metal plate against the rock face in a tunnel and watch how much the ground sinks. This simple test reveals the rock's stiffness, but it only shows the final result, not the hidden mechanics of how the cracks inside the stone are closing, sliding, or breaking apart to allow that movement.

To understand what happens inside the rock during these tests, a team of researchers at the Gushui Hydropower Station in China combined real-world measurements with a powerful computer simulation. They focused on basalt, a common volcanic rock, and conducted a series of rigorous tests where they pressed a 0.52-meter-wide steel plate against six different spots in the tunnel. They applied pressure in five increasing steps, up to a maximum of 10.203 megapascals, and then slowly released it, repeating this cycle five times at each spot. This process allowed them to measure not just how much the rock moved, but how much of that movement was permanent versus how much snapped back like a spring. The researchers then built a digital twin of these tests using a method that treats the rock not as a solid block, but as a collection of thousands of tiny circular particles. By simulating the exact same pressure cycles on these virtual particles, they could watch the invisible dance of forces, cracks, and energy shifts that occur inside the rock, which no camera could ever capture in the real world.

The results of the real-world tests revealed a surprising complexity that simple rock classifications had missed. The team had grouped the test sites by how much the rock had weathered, expecting the "weakly weathered" spots to be softer and the "slightly weathered" spots to be stiffer. However, the data told a different story. One specific test point, located near a small fault line cutting through the rock at a 35-degree angle, behaved very differently from the others. When the researchers included this fault-controlled spot in their average calculations, the "weakly weathered" group appeared much softer than it actually was. When they removed this single point, the average stiffness of the weakly weathered rock jumped significantly, becoming even stiffer than the slightly weathered group. This finding suggests that the presence of a local fault or a specific crack can have a far greater impact on the rock's behavior than the general weathering of the stone itself. The rock's strength is not just a matter of how old or exposed it is, but of the specific, hidden geometry of the fractures running through it.

To see why this happened, the researchers turned to their computer models, which acted like a high-speed microscope for the rock's interior. They created three distinct digital scenarios: one representing the average weakly weathered rock, one for the slightly weathered rock, and a specific model for the fault-controlled spot. As they applied pressure in the simulation, they watched how the tiny particles interacted. In the models representing the more intact rock, the pressure was distributed fairly evenly, and the rock stored energy like a compressed spring, bouncing back well when the load was removed. But in the model with the 35-degree fault, the story changed completely. The fault acted like a weak hinge, causing the force chains—the paths through which pressure travels—to deflect and slide. Instead of the rock compressing uniformly, the movement became concentrated along the fault line. The simulation showed that while the intact rock generated many new tiny cracks as it was squeezed, the faulted rock generated very few new cracks. Instead, its massive deformation came from the existing fault slipping and grinding against itself. This sliding created a large amount of permanent, irreversible movement, explaining why that specific test point sank so much more than the others.

The study also tracked how energy moved through the rock during these tests. In the real world, the researchers measured the total work done by the machine to push the plate down and how much energy was lost as heat or friction during the unloading phase. In the simulation, they could break this down further, seeing exactly how much energy was stored in the contacts between particles and how much was lost to friction as the particles slid past one another. They found a clear pattern: the fault-controlled spot absorbed and dissipated the most energy, confirming that the fault was the primary source of the rock's weakness. The slightly weathered rock, despite having some hidden micro-cracks, held its shape better and stored more energy elastically. The researchers were careful to note that while the computer model could not perfectly replicate the three-dimensional nature of the real rock or the exact time-dependent settling that happens in the field, the trends were undeniable. The model successfully reproduced the specific deformation curves measured in the tunnel, proving that the local structure of the rock, particularly the orientation of faults, is the dominant factor in how the ground will behave under a dam's foundation.

Ultimately, this work provides a clearer picture of what lies beneath the surface of a hydroelectric project. It demonstrates that relying solely on broad categories like "weathering" can be misleading if a specific fault line is present. The rock mass is not a uniform material; it is a collection of distinct zones where the rules of deformation change based on the local cracks and joints. By combining physical tests with detailed simulations, the researchers showed that the most significant movements often occur not because the rock is old or soft, but because a hidden fault is sliding. This insight is crucial for engineers designing massive structures, as it highlights the need to look beyond general rock types and map the specific, hidden fractures that could dictate the stability of the entire foundation. The study confirms that understanding the energy and deformation of rock requires looking at the specific, local story of every crack and fault, rather than just the general age of the stone.