Kinematical Back-Analysis of Block Propagations for Evaluating the Rockfall Hazard

Rockfall hazard is due to sudden rock scarps failure triggering dangerous block propagation which can cause severe damage to built-up areas and to communication lines. To establish adequate mitigation measures against rockfall phenomena it is necessary to evaluate realistic possible block trajectories and also the block energy has to be estimated. Field data acquired by detailed survey of recently occurred block propagation on actual slopes is precious information for determining the main kinematical and geomechanical parameters (restitution coefficient E at the impact, rolling friction, etc.). This paper reports the field evidence and the kinematical study of the motion of two blocks (A and B) mobilised by a rockfall in Lavone (Valtrompia, northern Italy) on 14 February 1987. The two sequences of impact marks left by the blocks on the ground surface were measured and the lithostratigraphical features of the debris slope were surveyed. On the basis of the field-collected input data, several computer simulations were carried out to calculate the coefficients of restitution (E) satisfying the trajectory conditions. Calculated output values, obtained by running a specific automatic program for rockfall modelling, show that rebound trajectories require high coefficients of restitution (0.8 £ E £ 0.9). Back-calculated impact velocities range from 9.2 to 19.8 m/s. Trajectory heights vary from 0 to 2.4 metres above the slope surface. Block trajectories differ considerably according to the circumstances of initial air projection i.e. to initial rebound angle (a_r). The calculated values of a_r denote a considerable range (36°), emphasizing the high variability and the random nature of this parameter. The described case history shows that rockfall computer analyses can be an effective tool to describe the bouncing propagation of single blocks but care must be taken in choosing the restitution coefficient E and the geometrical parameters of initial air projections.