Dynamic Response and Damage Regularity of Water-Saturated Sandstone Under F–T Cycles and Cyclic Impact Loading

In the high, cold reaches of western China, the ground itself is under constant siege. Here, the temperature swings wildly between day and night, and across the seasons, water trapped inside the rocks freezes and thaws in a relentless, rhythmic cycle. When water freezes, it expands, prying open tiny cracks within the stone. When it thaws, the water seeps deeper into those widened gaps, only to freeze again and push further. Over years, this invisible pressure slowly turns solid rock into something fragile and crumbly. This process, known as freeze-thaw weathering, is a major threat to the safety of mines, roads, and tunnels built in these regions. The danger is compounded when the rock is already saturated with water, as the ice has more room to grow and cause damage. Engineers need to understand exactly how this weathered, water-logged rock behaves when it is hit by sudden, heavy forces, such as the blasts used in mining or the vibrations from heavy machinery. If the rock fails unexpectedly, it can trigger landslides or collapse, putting lives and infrastructure at risk.

To solve this puzzle, a team of researchers from mining institutes and universities in China set out to watch how water-saturated sandstone reacts when it is battered by repeated impacts after being subjected to many freeze-thaw cycles. They chose red sandstone, a common rock type in the region, and cut it into perfect cylinders. First, they soaked the rocks in water until they were completely saturated, ensuring no air remained inside. Then, they placed these wet rocks into a machine that could cycle the temperature between freezing and warm conditions, mimicking the harsh environment of the highlands. They created groups of rocks that had endured zero, fifteen, thirty, forty-five, and sixty of these freezing and thawing cycles. After this treatment, the researchers took each group and subjected them to a series of low-stress impacts, similar to a heavy hammer striking the rock five times in a row. They used a specialized device that could measure the exact force the rock could withstand and how much it stretched or deformed with every single blow. To see what was happening inside the stone without breaking it apart, they used industrial computed tomography, a powerful 3D X-ray technology, to scan the rocks after the impacts and reveal the hidden cracks.

The results painted a clear and worrying picture of how the rock's strength fades. The team found that the more freeze-thaw cycles a rock endured, the less able it was to handle even a single impact, let alone a series of them. For the rocks that had never been frozen, the fifth impact still left them standing strong, with a high peak stress. However, for the rocks that had been through sixty cycles of freezing and thawing, the fifth impact caused their strength to drop by nearly two-thirds compared to the fresh rocks. At the same time, the rocks became much more stretchy and prone to deformation. The researchers observed that as the number of impacts increased, the rocks that had suffered more freeze-thaw damage began to fail much faster. While the fresh rocks could survive five hits before showing major signs of breaking, the most damaged rocks began to crumble after just two or three hits. The internal structure of the stone was clearly compromised; the repeated freezing and thawing had already weakened the bonds between the grains, making the rock unable to recover from the shock of the hammer blows.

Looking inside the rocks with the 3D scanners revealed exactly how this failure unfolded. In the rocks that had not been frozen, the damage was slow to appear. It took four or five impacts before small cracks began to form at the edges and corners. But in the rocks that had been through many freeze-thaw cycles, the damage started almost immediately. The cracks did not appear randomly; they consistently began at the edges and corners of the cylinders, where the temperature changes were most extreme and the ice could form fastest. As the impacts continued, these initial cracks grew and spread inward, eventually connecting to form larger fractures that tore through the stone. The researchers used a mathematical concept called fractal dimension to measure the complexity of these cracks. They found that as the rocks suffered more freeze-thaw damage and more impacts, the pattern of cracking became more intricate and chaotic, indicating a more severe state of failure. The most damaged rocks showed a rapid increase in this complexity, signaling that the stone was losing its integrity much faster than the fresh ones.

The study concludes that the combination of water saturation, freeze-thaw weathering, and repeated impact creates a dangerous synergy that accelerates rock failure. The damage is not just a simple sum of the two factors; rather, the freeze-thaw cycles prepare the rock to fail catastrophically under impact loads that it would otherwise withstand. The failure always starts at the surface, where the temperature changes are most violent, and then spreads inward. This means that in real-world engineering, such as open-pit mines in cold regions, the outer layers of rock slopes are the most vulnerable points. If these surfaces are already weakened by seasonal freezing and thawing, they are far more likely to collapse under the vibration of mining blasts or the weight of heavy equipment. The research provides a clear warning that the long-term safety of structures in these cold, wet environments depends on accounting for this cumulative damage, as the rock's ability to resist sudden forces diminishes rapidly once the cycle of freezing and thawing has taken its toll.