The Innonet project: understanding the capacity of flexible protection systems against rockfall in natural terrain

In the last 30 years, rockfall barriers made of steel wire nets have become established worldwide as a protective solution, are meanwhile CE certified and the question inevitably arises as to the effect of natural impacts, i.e. impacts from boulders that strike the net at any point, possibly also rotating as they do so. In 2019 an Innosuisse-sponsored research project was granted to the WSL Institute for Snow and Avalanche Research SLF together with the industry partner Geobrugg, for testing fully instrumented rockfall barriers, in natural terrain in the Swiss Alps, aiming at finding improvements to the capacity of a rockfall barrier outside of the certification standards. The awareness that the capacity of a rockfall barrier is different depending on the impact location, and how to deal with the so-called remaining capacity of rockfall barriers, in load cases outside the approval tests, differ worldwide. In some countries, specialized designers are aware of this fact and solve the problem by over-dimensioning the rockfall barriers to ensure the availability of residual capacity outside of the middle field. In other countries however, authorities and/or designers assume that a 1000kJ rockfall system absorbs this energy even in marginal areas or in case of an eccentric hit. Protective solutions are consequently not necessarily designed properly. This research project tries to assess the performance and the residual capacity of rockfall barriers, after being impacted by various load cases, to improve the current knowledge. Several field campaigns were conducted, in which rocks of different shapes and sizes are projected into the netting of the rockfall barrier and its structure (cables and posts). The barrier is equipped with sensors to measure the loading on different elements of the protection system. In addition, the test blocks (up to 3’200 kg) are also equipped with sensors that measure the rotation and the acceleration during the fall and on impact with the barrier. In combination with high-resolution drone recordings and video recordings from different viewing angles, the trajectories and velocities of the individual blocks can be reconstructed in detail, enabling further insights into the interaction of all parameters. The barrier was left in place since construction and is enduring its third winter without maintenance. A field survey (snow depth and density, loads on cables, posts, etc) was undertaken in the winters 19/20 and 20/21, and further surveys will take place this current winter. This contribution will present the evaluation of the rockfall test data. It allows an understanding of the remaining capacity of a barrier, the influence of rockfall rotation onto the protection system itself as well as the importance of the impact location. Forces measured in the system show a variation of up to 40% when compared to the standard testing results. The goal is then to assess if additional tests can be carried out to the standardized tests, to better prepare a rockfall barrier for the field.