Geomechanical Stability Assessment of Underground Chalk Heritage Galleries Based on TLS-Derived 3D Geometry and Numerical Modelling

Beneath the surface of many historic towns lie networks of tunnels and chambers, carved long ago from soft, porous rock to extract building materials. These underground spaces are not merely empty voids; they are complex geological structures where the stability of the ground depends on a delicate balance between the weight of the earth above, the strength of the rock itself, and the geometry of the openings. In places like Chełm, Poland, where centuries-old chalk tunnels run directly beneath a bustling market square, this balance is critical. The rock, known as writing chalk, is naturally strong when dry but becomes fragile and prone to crumbling when it absorbs water. Over time, weathering and moisture can weaken the rock, causing blocks to detach from the ceiling or walls, and in extreme cases, leading to the collapse of the ground above. Understanding how these ancient structures hold together requires more than just looking at them; it demands a way to see the invisible forces of stress and strain that act upon them every day.

To address the safety of these historic spaces, a team of researchers turned to a modern approach that combines high-precision scanning with advanced computer simulation. They focused on a specific section of the Chełm Chalk Tunnels, a unique monument of Polish mining history that was heavily developed in the sixteenth and seventeenth centuries before being abandoned due to safety concerns. The researchers began by mapping the underground galleries in extreme detail using terrestrial laser scanning, a technique that bounces laser beams off surfaces to create a precise three-dimensional digital copy of the tunnels. This process captured the irregular shapes, narrow passages, and rough textures of the chalk walls, which are too complex for traditional measuring tools to record accurately. This digital model served as the foundation for a virtual reconstruction of the underground world, allowing the scientists to study the tunnels without disturbing the fragile environment.

With the geometry of the tunnels locked into a computer model, the team needed to understand the physical properties of the rock itself. They collected samples of the chalk from the site and tested them in a laboratory to see how the material behaved under pressure, both in its natural damp state and when dried out. The tests revealed a stark difference: the chalk was significantly stronger when dry, but its ability to hold weight dropped considerably when moisture was present. This finding was crucial because the tunnels are known to be damp, with condensation often visible on the walls. The researchers used these laboratory results to program the computer model, ensuring that the virtual rock behaved exactly like the real rock would under similar conditions. They then simulated the weight of the buildings and the traffic on the market square above, as well as the natural pressure of the surrounding earth, to see how the underground chambers responded.

The simulations showed that, under current conditions, the selected chambers remain globally stable. The computer calculated a safety margin that exceeded the minimum threshold required to prevent a large-scale collapse, suggesting that the tunnels are not in immediate danger of failing entirely. However, the analysis also highlighted specific areas where the rock is under significant stress. The computer maps revealed that while the overall structure holds, there are localized zones, particularly along the sidewalls and in the roofs of some chambers, where the rock is being pushed to its limits. These areas correspond to the thin layers of chalk that have been observed detaching from the walls in the real tunnels. The study found that the distribution of this stress changes depending on the pressure from the side; when horizontal pressure increases, the load shifts from the walls to the ceiling and floor, altering which parts of the tunnel are most at risk.

One of the most important findings was that the tunnels appear to have been naturally shaped over time to maximize their stability. The chambers often have an elliptical shape with a vertical orientation, which happens to be an efficient form for resisting the downward pressure of the earth. The researchers noted that the miners likely adjusted the shape of the tunnels as they worked, carving away weak spots and leaving behind a structure that could support itself. Despite this inherent stability, the study emphasizes that the current safety is not a guarantee for the future. The simulations did not account for the slow, long-term effects of weathering or the gradual weakening of the rock as it absorbs more water over decades. While the tunnels are safe today, the continuous process of moisture-induced degradation means that the weak spots identified in the computer model could eventually grow, leading to local collapses or damage to the surface above.

The value of this research lies in its ability to pinpoint exactly where the risks are, rather than just offering a general assessment of safety. By combining the precise 3D geometry of the laser scans with the real-world behavior of the chalk, the researchers created a tool that can guide future monitoring and reinforcement efforts. They identified that the deepest parts of the tunnels are particularly sensitive to changes in the stress field, and that the presence of water is the single most important factor threatening the rock's strength. This approach allows engineers to focus their attention on the specific sections of the tunnels that need support, rather than treating the entire network as a uniform problem. For the historic chalk galleries of Chełm, this means that the path forward involves careful observation of the damp zones and targeted reinforcement of the areas where the rock is already showing signs of strain, ensuring that these underground monuments can be preserved for future generations without risking the safety of the city above.