Mechanical properties of basaltic volcanic bombs: a case study from the Uhlířský vrch volcano, Czech Republic

Abstract

Basaltic eruptions produce a wide variety of volcanic products that show a range of different textures and physical properties, from massive lava to extremely light and aerated reticulite. While the mechanical properties of various lavas and tuffs have already been studied in more detail, volcanic bombs have received almost no attention. We characterised the mechanical and physical properties of bombs from the Plio-Pleistocene Uhl & iacute;& rcaron;sk & yacute; vrch in the Brunt & aacute;l Volcanic Field (Czech Republic) to better define their geomechanical behaviour compared to the wider range of volcanic eruption products. The cylindrical specimens showed a highly variable internal structure which resulted in scattered individual peak strength values. In contrast, the residual strength values were less variable, and thus we devised a system using the Hoek-Brown criterion for the whole bomb mechanical characterisation. This approach assigns different geological strength index (GSI) values to the specimens based on their internal structure. The bombs combine the generally low cohesion and strength of tuffs with a higher internal friction angle and a material constant akin to lava samples. The bombs may therefore be treated as a lava-like material albeit with a pyroclastic origin. From a geotechnical point of view, the presence of bombs embedded in other rock types could affect the general stability of a volcanic structure. Here we identify possible scenarios (i.e. bombs embedded in pyroclastic rock versus bombs buried in lava; bombs distributed randomly versus bombs forming a loosely connected horizon) and describe the possible outcomes. Finally, the cracked and porous structure of bombs locally increases permeability, thereby promoting hydrothermal alteration and further affecting general slope stability. However, the porous structure of bombs could also have a potential positive societal impact as it may allow for local sequestration of carbon dioxide.