Numerical simulation of flexible rockfall protection systems

Numerical simulation of flexible rockfall protection systems 

Today's flexible rockfall protection barriers have reached a development stage above which a much greater effort is required to extend their rockfall  retaining capacity. The numerical simulation of these protection barriers enables a more efficient  development of new types due to a reduced number of expensive field tests using  prototypes. The barriers analysed within the research project, this thesis is part of,  consist of steel posts to which supporting and restraining cables are connected. Special brake elements capable to absorb the energy of the falling rock are  integrated into the cables. The rock is caught by steel ring nets spanned by the  supporting cables. These flexible protection systems have gained importance because of their ability to stop a rock gently within a longer braking distance of about 3~-~10~m. This results in a considerable peak-load reduction in all components of the protection system and its foundations.

The specially developed software application {\sc Faro} simulates the dynamic behaviour of a spherical rock stopped by such a protection barrier in many short time-steps by the central differences method.  This enables a detailed view of the dynamics of the modelled barrier and also provides information on its loading and degree of utilisation. The results of the simulations are compared to the field tests carried out within the research project.

The single barrier components are modelled by discrete elements with non-linear material properties. Due to the large displacements to be modelled, geometrical non-linearities are also considered. The elements simulating the net rings and
the cables have been newly developed so as to handle different long distance glides. At first, the net rings can arrange themselves according to their loading. Then, so-called curtain effects take place as several  net rings glide along the supporting cable. Finally, when the brake elements are lengthened, the cables also glide over the heads of the posts or over the ground plates. All cable and rock related gliding effects can also consider the friction.

The falling rock is modelled as a spherical rigid body considering large three-dimensional displacements and rotations, the latter handled by Euler-parameters. A special contact algorithm is used for modelling the interaction between the rock and the barriers, respectively the nodes of the net elements. Frictional effects have also been taken into account. A thin elastic layer around the rock reduces unwanted spurious peaks in the deceleration curve of the rock.

The use of the software provides the possibility to develop new barrier types or to optimise existing ones. Additionally, special load cases that cannot be reproduced in field tests can be checked, as well as special geometrical boundary conditions of a barrier projected to be built at a certain location. For the projecting engineer, the steady comparison of computational results with performed field tests or real rockfall events helps to improve his built finite-element-models and the assessment of the computer simulation.