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Rock shape should be given greater consideration in risk assessments

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Researchers from the WSL Institute for Snow and Avalanche Research SLF and ETH Zurich have spent four years conducting rockfall experiments, enabling them to compile the most comprehensive set of measurement data to date. Among other things, the results show that wheel-like rocks have a wider lateral spread than cube-shaped rocks. The new findings have important implications for hazard assessment and the dimensioning of protective structures. Based on the measurement data, calculation programs can be calibrated and refined to give a more accurate representation of reality.


Rockfall is a very real threat in an Alpine country like Switzerland. In order to assess the hazard at a given location and plan protective measures, engineering firms use computer models to calculate how far falling rocks can roll. However, the models are not yet able to adequately take into account the extent to which the mass, size or shape of a rock influences its movement. This would require real-world measurement data to be fed into the models, but until now such data were only available sporadically, since no systematic rockfall studies had been conducted.



First comprehensive experiments

That has now changed after researchers from the SLF and ETH Zurich spent over four years carrying out rockfall experiments. "This has allowed us to compile the largest set of measurement data to date," says Andrin Caviezel, SLF researcher and lead author of the study. The researchers used artificial rocks in the form of concrete blocks fitted with sensors, which they rolled down a slope near the Flüela Pass in the Swiss canton of Grisons. They compared different shapes and masses, reconstructed the complete trajectories and determined speeds, jump heights and runout zones (see info box). They have just published their results in the prestigious scientific journal Nature Communications.


Lateral spread

The most significant finding is that the direction a rock rolls in depends much more on its shape than on its mass. While cube-shaped boulders plunge straight down the line of greatest slope, wheel-shaped rocks often pull away to one side and so may threaten a much wider area at the base of the slope. "This needs to be taken into consideration when assessing danger zones, but also when determining the location and dimensions of rockfall nets," explains Caviezel. Because wheel-like rocks hit rockfall nets with their narrow side, their energy is concentrated on a much smaller area than is the case with cube-like rocks – so protective nets need to be stronger.


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This aerial photograph of the slope shows in blue the places where the cube-shaped rocks (weight: 2,670 kg) came to rest – most are within a very restricted area. The red cross indicates the release point. Source: SLF
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By contrast, the wheel-shaped rocks (weight: 2,670 kg), represented here in purple, ended up spread over a much wider area. Source: SLF

More realistic models

The data are now being entered into the RAMMS::ROCKFALL simulation program developed at the SLF. As well as factoring in the shape, the aim is to represent more realistically how the rock's speed is affected by the way it impacts and bounces off the ground. "This will enable us to offer an enhanced program that engineering firms can use to make more reliable calculations," says Caviezel. The data set is also available on the EnviDat platform, where it is freely accessible to other research groups. They can use it to calibrate their own algorithms or to develop new, more accurate models providing enhanced protection against rockfall.


The rockfall experiments in numbers

Number of concrete blocks: 183
Weight of the blocks: 45, 200, 800 and 2,670 kg
Number of reconstructed trajectories: 82
Usable impacts: 1,394
Maximum jump height: 11.1 m
Maximum speed: 30.3 m/s = 109 km/h

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Wheel-shaped (left) and cube-shaped (right) concrete blocks, each weighing 2,670 kilograms. Photo: Martin Heggli, SLF
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SLF researcher Andrin Caviezel prepares one of the sensors developed by ETH Zurich, which were fitted inside the blocks. Photo: Martin Heggli, SLF
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A helicopter transports a block to the release point at the top of the slope. Photo: Martin Heggli, SLF
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One of the concrete blocks positioned on the tilting platform that will be used to set it in motion. Photo: Martin Heggli, SLF