Three inputs
| Input | Definition | Source |
|---|---|---|
| Mass, m | The largest block expected to detach from the slope — the design block | Block-size inventory on site; the design block is taken from the upper tail of the measured distribution |
| Speed, v | Speed of the block when it reaches the barrier line | Trajectory analysis on the measured slope profile |
| Safety factor | Between 1.1 and 1.5 according to the consequences of failure | Client and consultant; ONR 24810 gives 1.00 / 1.05 / 1.15 by consequence class |
The formula is the kinetic energy of the block: E = ½ · m · v². A 2,000 kg block at 20 m/s carries 400 kJ; the same block at 28 m/s carries 784 kJ. Speed enters squared, which is why a trajectory analysis on a measured profile is worth more than any estimate.
From energy to class
The design energy, multiplied by the safety factor, is compared with the certified MEL of candidate systems: MEL must be at least the design energy. Where blocks arrive frequently — active cuttings, quarry walls — the SEL is checked as well, because the barrier must stay in service between maintenance visits. The class table below gives the certified MEL levels and the SEL that follows from the rule MEL ≥ 3 × SEL; the real SEL is always read from the product certificate.
| MEL | SEL (≥ MEL/3) | Typical use |
|---|---|---|
| 100 kJ | 33 kJ | small stones, gently inclined slopes |
| 500 kJ | 170 kJ | standard road cuttings |
| 1,000 kJ | 330 kJ | highways and railways |
| 3,000 kJ | 1,000 kJ | steep cliffs above settlements |
| 5,000 kJ | 1,700 kJ | large valley slopes |
| 8,000 kJ | 2,700 kJ | mining and quarry walls |
| 12,500 kJ | 4,170 kJ | highest certified class today |
Two more numbers decide the barrier
Energy fixes the class, but not the barrier. The bounce height — the 95th-percentile height at which blocks pass the barrier line — fixes the nominal height. The maximum elongation of the chosen system fixes the clearance to the road or structure behind it; where the space is narrow, a low-elongation type is needed even if the class is the same. Both numbers are in the certificate, not in the catalogue: see Reading a rockfall barrier specification: ETAG 027 to EAD 340059.
What we measure before we choose
- Block-size distribution and the design block, from the source zone.
- Slope profile from drone photogrammetry or laser scanning — see Slope survey: drone photogrammetry and laser scanning.
- Surface type along the fall line: rock, scree, soil, vegetation — the restitution coefficients of the trajectory model.
- Position of the protected asset and the space available for the barrier and its elongation.
- Anchor ground at the barrier line.
Questions on this topic
How is rockfall energy calculated?
E = ½ · m · v², with m the design block mass and v the impact speed from trajectory analysis; the result is multiplied by a safety factor of 1.1 to 1.5 before it is compared with the certified MEL.
Does the energy class also fix the barrier height?
No. The nominal height is chosen from the bounce height at the barrier line (95th percentile from trajectory analysis), and the clearance behind the barrier from the certified maximum elongation.
Sources
- EOTA, EAD 340059-00-0106 / ETAG 027 (2013), Table 2.
- ONR 24810, 2021 (consequence classes and partial factors).