Assessing roof vulnerability to collapse from tephra fall loading, with relevance to Ascension Island

Tephra fall can significantly increase loading on roofs and by better understanding the factors that influence roof vulnerability to collapse, mitigations can be more appropriately targeted. My research assessed tephra sliding behaviour, modelled tephra loads and developed a GIS tool to estimate how vulnerable individual buildings are to roof collapse.

In the laboratory I confirmed the geomechanical properties of synthetic tephra that influence deposit sliding behaviour (bulk density, grain size distribution, internal angle of friction) match well to natural samples. I then used synthetic samples to characterise how tephra slides on roofs and validated my results with field observations during the 2021 Cumbre Vieja eruption (La Palma, Canary Islands). Grain size, roof material and pitch were the key factors influencing sliding. I compiled initial sliding coefficients for three roof materials on simply pitched roofs, based on the Eurocode approach for snow loads. These coefficients estimate the fraction of tephra load on the ground that is expected to remain on a roof.

I conducted a probabilistic tephra fall hazard analysis for Ascension Island, using knowledge from the geological record and data from analogous eruptions on São Miguel, Azores. I modelled possible future eruption scenarios and provided probabilities of tephra fall loads exceeding threshold values for roof failure. There was a 50 % probability that three basaltic events, each with a 6–10 km plume height could produce proximal tephra loads sufficient to cause roof collapse if roofs were not cleared between eruptive phases.

Finally, I combined my results to develop a GIS tool that assesses susceptibility to roof collapse at a building level. The tool takes model output tephra loading, data on individual buildings, my tephra sliding coefficients and published estimates of collapse loads to identify buildings where tephra loads may exceed the roof failure load for a given eruption scenario.