A novel steel damping system for rockfall protection galleries

A new steel damping device is proposed to achieve an effective solution for rockfall protection galleries. The system consists of an assembly of cold-formed steel crash boxes, namely Collapsible Tubular Element (CTE), with a transversal section and a length designed to assure, under axial loads, a compressive plastic deformation (folding) without the need of any guide means. Assembled in groups to create a modular cushion, the system reduces the deceleration and the derived impact force due to an impacting block mass during a rockfall event. The influence of both the system module configuration (number of CTEs, presence of restraining cables) and the boulder parameters (mass, angle, and velocity at the impact) is investigated, and a Dynamic Finite Element analyses, along with a full-scale experimental test, have been carried out to evaluate the system performances. The obtained results indicate how the proposed system mitigates the dynamic amplification of the forces transmitted from the impact point to the roof slab, with a mean value of the impact force increased by only 2% within the damping cushion, showing, if compared with the mainly adopted damping cushion systems, a drastic reduction of both impact force and dead load on the gallery's roof. Finally, due to its dimensional stability, the proposed damping device is not affected by compaction over time and by the consequent increase of the impact force.