Investigation of geomechanical properties of tephra relevant to roof loading for application in vulnerability analyses

Tephra fall can lead to significant additional loading on roofs. Understanding the relevant geomechanical properties of tephra is critical when assessing the vulnerability of buildings to tephra fall and designing buildings to withstand tephra loads. Through analysis of published data and new experimental results on dry tephra (both natural samples from Ascension Island, South Atlantic and synthetic tephra made from crushed aggregates), we discuss the geomechanical properties of tephra relevant to roof loading, which include bulk density, grain size distribution and internal angle of friction. Compiled published data for deposits from 64 global eruptions reveal no clear trend in deposit densities based on magma composition or eruption size. The global data show a wide range of values within single eruptions and between eruptions of similar compositions. Published grain size distributions near to source (≤ 10 km) vary widely but again there are no clear trends relating to magma composition. We used laboratory tests to investigate the internal angle of friction, which influences deposit sliding behaviour. For dry tephra, at the low normal stresses likely to be experienced in roof loads (≤ 35 kPa), we found similar values across all our tests (35.8° - 36.5°) suggesting that any internal sliding will be consistent across a variety of deposits. By considering different magma compositions, densities and grain size distributions, we have provided an envelope of values for deposit parameters relevant to roof loading, in which future eruptions are likely to sit. Finally, we created synthetic tephra (fine- and coarse-grained pumice and scoria) by crushing volcanic aggregates and compared it to samples from Ascension and published data. Our results reveal that synthetic tephra successfully replicated the properties relevant to loading, potentially reducing the need to collect and transport natural samples.