Tephra cushioning of ballistic impacts: Quantifying building vulnerability through pneumatic cannon experiments and multiple fragility curve fitting approaches

Ballistic projectiles are the most frequently lethal volcanic hazard close to the vent. Recent eruptions of Ontake in 2014 and Kusatsu-Shirane in 2018 showed that un-reinforced, timber-framed buildings - those typically considered highly vulnerable to the dangerous penetration of ballistics - provided life-saving shelter from ballistic impact. Modelled kinetic energies of some non-penetrating impacts were an order of magnitude above expected penetration thresholds. It has been hypothesised that a pre-existing layer of tephra on the roofs cushioned impacts. To quantitatively test this, and improve our understanding of how buildings respond to projectile impacts, we used a pneumatic cannon to simulate block impacts to clay tiles and reinforced concrete roof slabs covered with tephra layers 0–20 cm thick. Substantially higher impact energies were resisted when tephra was present with 5 cm of tephra approximately tripling the penetration threshold of both building materials. Fragility curves, which relate ballistic hazard intensity with the probability of building damage, were developed from our experimental data following three curve fitting approaches: generalised link models, cumulative link models and data binning. A key benefit of these approaches is that confidence in these curves can be robustly quantified from the data – the first time that this has been attempted for volcanic fragility curves. This study shows how the extent of building damage can be strongly influenced by the sequence of volcanic hazards and provides an example of proactive risk management through testing of physical mitigation strategies in a laboratory environment.