Rock falls can have a serious impact on infrastructure, but designing systems to protect the public from the effects is not straightforward as Claire Symes finds out.
The only sound breaking the silence of the late autumn sunshine at the Consorzio Triveneto Rocciatori test centre in Fonzaso, Italy, was a series of siren bleeps that provided the waiting audience with warning that a 17.6t block of concrete was about to drop 38m onto a rock fall barrier. But this was just a test. If this was a real rock fall situation then road or rail users in the vicinity would have no warning of the impending impact and would be dependent on authorities having installed sufficient rock fall barriers to protect them.
The audience of geotechnical experts and infrastructure clients from all over the world was gathered last month to watch Maccaferri’s prototype P650 fencing system undergo a European standard ETAG 27 test. But having a compliant system is only part of the equation; having a clear understanding of the problem and risks at each site are key to finding the right solution.
Speaking at a conference organised by Maccaferri ahead of the live testing, Colorado Department of Transportation rockfall programme manager Ty Ortiz highlighted some of the challenges of understanding the fall mechanisms: “Our experience was based on trying to develop rock fall attenuators for the I-70 at Georgetown. There had been two major accidents on the road and we used to frequently have to close the road to clear rock fall debris.”
The site had barriers but they weren’t sufficient to stop rocks from reaching the road level, so Otiz’s team carried out a number of live tests with different barrier systems. The eventual solution involved using some barriers specifically to slow rocks rather than halt them completely.
“You need to understand the dynamic load of the flow, as well as the static load on the barrier”
Not every site has large blocks, though, and some areas experience more problems with debris flow. According to University of Parma professor Andrea Segalini, these events are not as energetic as individual block falls but can continue for long periods and over a wide area. “Characterisation is key to the modelling,” he says. “You need to understand the dynamic load of the flow, as well as the static load on the barrier, and consider the drag force exerted if the flow overtops it.”
Once a barrier is in place, there is also maintenance to consider. “Maintenance needs to be considered at the design stage,” says California Engineering Services senior engineering geologist John Duffy. “To be effective the barriers need to be empty, but clearing them under load is not a simple task - we developed a sacrificial connection to allow for easier dismantling.”
Rock falls rarely occur as isolated incidents, so it is likely that a barrier that has already stopped one rock will have to resist further falls before maintenance can be carried out. This is covered by ETAG 27, so Maccaferri’s P650 barrier was subjected to a second rock fall, successfully withstanding a 2.2t concrete block on top of the 17.6t fall. The firm said more tests would be carried out to definitively prove the energy needed to break the barrier.
Consorzio Triveneto Rocciatori rock fall specialist Giorgio Cavallet says: “Once a block has fallen onto the barrier it is no longer dynamic and the barrier will become stiffer and stiffer with each successive rock fall, so it is important for the maintenance operation to understand how and when the system will break.”
Barrier system in action
Motorists using National Road 47 between Padua and Trento in Italy are at lower risk of having their journey interrupted by rock falls following installation of a new barrier system at Valsugana.
The vertical dolomite rockface above the road often results in 0.5m diameter rock falls, and occasionally blocks of up to 1.5m diameter fall from the face.
The solution installed at the site is a 100m long, 6m high RMC/300A fence near the lower slope with two RMC/500A fences - 70m and 50m long, and 7m high - further up the rock face. According to main contractor SJLES this combination will protect the road even if the main impact is followed by a swarm of other blocks.