Multi-method monitoring for coastal cliff stability analysis: Integrating geotechnical surveys and terrestrial LiDAR at the Citadel of Bonifacio
This study demonstrates that integrating continuous multi-parameter sensor networks with periodic Terrestrial Laser Scanning surveys effectively quantifies volumetric changes and captures precursor micro-deformations in the soft limestone cliffs of the Bonifacio Citadel, thereby providing critical decision-support data for managing rockfall hazards at vulnerable coastal heritage sites.
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
On the edge of the Mediterranean, where the land meets the sea, rocky cliffs are not just static walls of stone; they are living, breathing landscapes that slowly change shape. These steep faces are constantly battered by the sun, the wind, and the waves, a process that can eventually cause massive sections of rock to break away. For scientists who study these dangers, the challenge is to tell the difference between the normal, daily movements of the earth and the dangerous shifts that signal a collapse is coming. Imagine trying to hear a single whisper in a room full of people shouting; the "shouting" in this case is the natural expansion and contraction of rock caused by the heat of the day and the cold of the night, while the "whisper" is the tiny, irreversible crack that might lead to a disaster. Understanding this difference is vital for protecting the people and history that live on the edge of these cliffs.
At the southern tip of Corsica, France, sits the Citadel of Bonifacio, a historic fortress perched atop a dramatic limestone cliff that towers over the sea. This site is a major cultural treasure and a busy tourist destination, but it is built on geology that is inherently unstable. The cliff is made of soft, sandy rock that is easily worn away by the ocean, creating deep overhangs that threaten to topple the structures above. For years, researchers have worried about the potential for a "massive collapse," a sudden failure of a large block of rock that could endanger the market square and the thousands of visitors who walk there. To solve this puzzle, a team of scientists has spent five years combining two very different ways of watching the cliff: taking incredibly detailed 3D photographs of the entire face and installing a network of sensitive sensors directly into the rock to feel its every movement.
The researchers began by mapping the cliff with a high-tech laser scanner, a device that bounces light off the rock to create a precise 3D model. They did this six times between 2018 and 2023, using scanners placed both at the base of the cliff and far away on the opposite side of the water to capture every angle, even the tricky spots hidden under deep overhangs. This method allowed them to measure the cliff's shape with centimeter-level accuracy. They compared these maps over time to see if the rock had moved, shifted, or crumbled. At the same time, they drilled deep holes into the cliff and installed instruments that could measure the opening and closing of cracks, the tilting of rock blocks, and the temperature changes inside the stone. These sensors sent data back to a computer every hour, creating a continuous stream of information about how the rock was reacting to the weather.
The results of this long-term watch were reassuring but nuanced. When the scientists compared the 3D maps from 2018 to those from 2023, they found no evidence of large-scale movement. The massive blocks of rock that support the Citadel had not shifted in any significant way. The tiny changes they did see in the 3D models were not signs of a coming collapse; instead, they were caused by things like the growth of plants, the shifting of sand at the bottom of the cliff, or the presence of temporary construction scaffolding. The cliff, in terms of its overall structure, remained stable over the five-year period.
However, the story was different when looking at the data from the sensors buried inside the rock. These instruments revealed that the cliff is indeed moving, but it is moving in a predictable, rhythmic way. The rock expands when it gets hot in the summer and contracts when it cools in the winter, much like a metal bridge does on a hot day. This daily and yearly breathing causes the cracks in the rock to open and close by tiny amounts, sometimes less than a millimeter. The sensors showed that this thermal movement is the dominant force acting on the cliff. By using advanced mathematical techniques to filter out this predictable seasonal "noise," the researchers were able to isolate the true, permanent movement of the rock. After removing the effects of the weather, the remaining data showed that the rock was not creeping toward a failure. The tiny, irreversible shifts that do exist are so small that they fall within the margin of error for the instruments, suggesting that the cliff is not currently on the brink of a catastrophic event.
This study highlights a crucial lesson for managing risks in places like Bonifacio: the difference between a temporary reaction and a permanent failure. The cliff is constantly reacting to the environment, but these reactions are reversible. The real danger would be a steady, one-way movement that never stops, which would indicate that the rock is losing its strength. The monitoring system proved that the current movements are just the rock responding to the sun and the sea, not a sign that it is about to fall. By combining the wide-angle view of the laser scans with the deep, sensitive hearing of the sensors, the team created a powerful tool for safety. They can now tell local authorities exactly what is happening on the cliff face, distinguishing between the normal rhythm of nature and the rare, dangerous signals that require immediate action.
The work also points to what might happen in the future. As the climate changes, the sea is rising and storms are becoming more intense, which could eventually wear away the soft rock at the base of the cliff faster than before. This could change the balance of forces, potentially turning the current reversible movements into a permanent slide. The data collected over these five years serves as a baseline, a starting point against which future changes can be measured. For now, the Citadel stands secure, its stability confirmed by a marriage of high-tech scanning and deep-sensing technology that listens to the quiet, steady heartbeat of the stone.
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