Listening to the coefficient restitution of rocks

The title of this paper makes reference to an article published by Bernstein (1977) in the American Journal of Physics in which the author presents an acoustic method in order to determine the restitution coefficient of solids. This parameter is of prime relevance on rock fall studies. In rock slopes instability problems the coefficient of restitution is related to the kinetic energy wasted after repeated impacts of a rocky body against the ground. It is probably the most important and most difficult to acquire or to estimate in real situations. In general, there is a necessity of in situ tests where blocks of rocks are thrown down on the slopes. However these tests cannot be performed in places where people or urban equipments could be in risk.
In this paper the acoustic method presented by Bernstein is used for the measurements of normal restitution of rocks. As written before, the method is very well known by physicists but it has not been applied in the geosciences and engineering areas. It consists in the sound recording of successive impacts of a rock sphere against a smooth rocky surface. In order to verify the validity and the precision of the method the gravity acceleration was back calculated. The gravity acceleration value demonstrated the applicability of the technique. A parametric sensibility analysis of restitution coefficient was already carried out simulating a real rock fall problem using the RocFall software. It could be demonstrated that the length of rock path was greater using the experimental data than that obtained from data available in literature. The probable reason for this difference should be related to the poor rock description and classification in the previous work. So the method can be understood as a potential one for a better knowledge of rock restitution coefficient in the future.