The study of the behaviour of reinforced embankments for rockfall protection, resulted not to be adequately explored.
Particularly, the topic of those structures’ behaviour, subjected to the impact of single elements having high impact energyand generally with velocity from “very rapid” to “extremely rapid”, that is from 0.3 m/min to over 30 m/sec, shouldbe faced.
The modern modelling systems of the velocity of the boulders downhill allow to estimate with remarkable precision the height of the protection structure and most of all the location in term of distance of the detachment point along the main unloading directions.
As far as wire netting rock protection barriers are concerned, the state-of-the-art, allowed a resistance capacity up to a total energy over 3000 kJ, indicatively the energy of a boulder of 3 m3 which moves at the speed of about 30 m/sec (100 km/h).
Most of the experts consider the reinforced embankments as a structure which can be set in a range of higher absorption energy compared to the wire netting rock barriers currently on the market. Thus the 2500-3000 kJ threshold seemed to be widely exceeded and, in 1999, in collaboration with Politecnico di Torino, TENAX carried out some full-scale tests.
The full-scale tests allowed a quantitative evaluation of the above mentioned mechanisms as well as the development of mathematical models to estimate the behaviour of a given reinforced embankment subject to a fixed impact, providing a realistic design aid for such structures.
Checking the behaviour of the reinforced embankments for rockfall protection when high energy impacts occur, provides to government Boards and designers a valid solution in all those cases that cannot be dealt with traditional methods.
Description and test procedures
The testing facility where the reinforced embankment for rockfall protection has been tested was located in a valley where a cableway system enabled cubic boulders (l=1.55 m. ca. weighting W=5.000 Kg, and l=1.50 m, weighting W=8.800 Kg) to run in semi-free falling for about 50m.
A release device allowed the boulder to hit the embankment wall at a velocity of 30 m/s with an impact energy of 2500 kJ and 4500 kJ respectively.
The impact phases were recorded through a system of three video cameras with a time code display and a 25 photograms/s taking speed.
The choice of constructing a steep isosceles earth wall leads to a reduction of the soil mass that will oppose the impact. The consequent reduction of the energy absorption capacity is already compensated by the reinforcement provided by geogrids.
The filling soil, found on the site, is made up of weathered limestone material with grain size distribution of gravel and of sand with the presence of silts and clays.
The embankment was not pierced by the boulder which fell at the bottom of the ground wall.
The scheme shows the position of the impact which occurred only in correspondence of the fifth and sixth portion of the wall producing the soil detachment practically only in these portions.