Attenuators are rockfall protection solution combining rockfall drapes and flexible rockfall barriers. The system is used as a passive protection measure – the barrier breaks the fall and the drape steers the rocks into a designated runout zone.
The top part of the attenuator intercepts rockfall and guides it under the drape. In the middle part of the system, rolling or falling blocks and debris are led down the slope to the safe runout zone. Rock blocks and debris accumulate at the bottom safely and can be cleared with routine periodic maintenance. This is in an area that has easy access. Clearing can be done in a short time thus reducing maintenance costs.
The attenuator systems are made of flexible wire netting designed to reduce the intensity (kinetic energy) of rockfall and guide their trajectory. Debris is collected on the slope and released with less energy before reaching the next attenuator or runout zone. Attenuators are used when the slope height is too great for one barrier. It allows for large quantities of debris to collect and filter through each attenuator before reaching the toe of the slope. This results in a moderately controlled form of slope protection.
Application of rockfall attenuator systems are suited to regions of high frequency rockfall whereby cleaning and maintenance are better managed as opposed to standard rockfall barriers that catch and contain rockfall. Attenuator systems can be applied to dissipate the energy of rockfall to the design value of the existing installations or site specific design conditions. These systems should be considered with the underlying terrain as well as the expected rockfall hazard.
Large scale field tests
Geobrugg is currently busy with 1:1 large scale field tests in Canada. Previous tests in Switzerland and Japan have proven very good. Examples of successful installations done during 2014 in the USA and New Zealand, is given here. The Oregon Department of Transport, approached Geobrugg for a permanent rockfall mitigation strategy after two rockslides caused the closure of two lanes on the Clackamas Highway (OR224), USA, forcing motorists to take a detour of over an hour. The Department of Transport investigated the slopes and evaluated the rockfall hazard for a permanent mitigation strategy. Through the design process, Geobrugg provided technical support on various flexible rockfall migitation systems. The customer chose to use an Attenuator Hybrid Rockfall Barrier System.
The Attenuator System in this particular project utilises a GBE500A barrier post and rope infrastructure supporting TECCO® G80/4 mesh that drapes down to the safe runout zone at the toe of the rock slope. Two attenuator systems were used in this project. The one system is 92 m long with 4m high posts, having an approximately 25 m long TECCO mesh draped tail. The second system is much longer, at 300 m in length. Posts were either 4 m, 6 m or 3 m high. Specialized posts were developed in order to transition between the various system heights in a continuous manner, optimizing post spacing and rope layouts. The draped TECCO mesh tail on these systems are up to 30 m long terminating about 3 m above the toe of the slope.
In New Zealand, Geobrugg was approached for a rockfall protection solution for the section above the State Highway (SH6) near Diana Falls that would actively control the frequent high energy rockfalls. The best solution was the construction of a 5m high 3 000 kJ and two 6 m high 5 000 kJ Geobrugg attenuator barriers. The smaller system (3000kJ) was installed over 250 m above the highway. The first of the 5 000 kJ systems was installed 150 m up and the final, and largest barrier, was installed about 50 m above the road. The bottom rockfall attenuator was a huge structure with a 35 t high tensile steel ROCCO® ringnet drape to control and channel the falling rocks and debris to the roadside for the maintenance crew to clear.
The attenuator system combines the proven strengths of both barrier and drape to provide a low maintenance protection. Attenuators are designed and built with high tensile steel wire. This solution is self-cleaning and virtually maintenance-free and deserves a closer look for situations in South Africa.
