We often find that suitable rockfall protection designs are overlooked as the mitigation comes as an emergency when time and money is limited. However, even with limited time and money, the designer must ensure that the rockfall protection system is fit for the purpose. But what goes into a comprehensive design? It first starts with the rockfall risk mitigation process and involves characterising the rockfall hazard and evaluating the risk posed by the hazard. This is done through a site assessment to estimate rockfall frequency and size of rocks/blocks. Then analyse the consequences rockfall will have on structures, persons, and services, and evaluate the risks against a tolerance criteria and value. This is where good experience comes into play.
When the risk is then estimated, the correct mitigation option must be selected. This can include measures such as avoidance (moving the people or infrastructure), source treatment (nailing down rocks) or installation of rockfall protection systems. Once the correct option is selected, the specific design will commence. It must also be noted that non-engineered measures also exist and includes rockfall monitoring programmes, warning signs and warning systems.
The design starts off with data from the site assessment, rock characteristics, slope morphology, rockfall frequency and trajectory, which is then used in 2D or 3D modelling to determine the correct structure or combination of structures and the location thereof. These can be active or passive mitigation structures. The structure/s should then be designed, and could consist of rockfall barriers, rockfall embankments, or drapery systems. A rockfall barrier design must determine the position, maximum energy levels versus the service energy levels, downslope deflection, elongation, anchors, post foundations, and type of corrosion protection coating on the steel components. An embankment design must determine the type, position, size, and the global and internal stability of the embankment. Designs for drapery systems, simple or secured, should determine the type of drapery mesh, its strength and durability, configuration, and type of anchors and cables required. The anchor design should include more specific the size, depth, and spacing required.
Furthermore, other design factors like frequency of rockfall, access to the slope, constructability, inspection and maintenance requirements, aesthetic and environmental issues, property ownership, and construction specifications must all be taken into consideration.
Prepared by
Christian Mynhardt (BTech. CIV)