The shape of the rock should be taken into account more in risk assessments

Rockfall is a very real threat in an Alpine country like Switzerland. In order to assess the danger at a given location and to plan protective measures, the design offices use computer models to calculate how far falling rocks can roll. However, models are not yet able to adequately take into account the extent to which the mass, size or shape of a rock influences its motion. This would require the introduction of real-world measurement data into the models, but so far this data has only been available sporadically, as no systematic rockfall studies have been conducted.

First full experiences

That has now changed after researchers at the WSL Institute for Snow and Avalanche Research SLF and ETH Zurich spent more than four years conducting rockfall experiments. “This 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 man-made 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 Graubünden. They compared different shapes and masses, reconstructed the complete trajectories and determined the speeds, jump heights and runout zones (see box). They have just published their results in the scientific journal Natural communications.

Lateral diffusion

The most significant finding is that the direction in which a stone rolls depends much more on its shape than on its mass. While cube-shaped boulders plunge directly along the line of greatest slope, wheel-shaped boulders often swerve to one side and thus can threaten a much larger area at the base of the slope. “This should be taken into account when evaluating danger zones, but also when determining the location and dimensions of anti-stone nets,” explains Caviezel. Since wheel-shaped rocks hit rock guard nets with their narrow side, their energy is concentrated in a much smaller area than is the case with cube-shaped rocks – protective nets should therefore be more solid.

More realistic models

The data is now entered into the RAMMS :: ROCKFALL simulation program developed at SLF. In addition to taking shape into account, the goal is to represent more realistically how the speed of rock is affected by the way it impacts and bounces off the ground. “This will allow us to offer an improved program that engineering firms can use to perform more reliable calculations,” says Caviezel. The dataset 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 that provide enhanced rockfall protection.

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