A multi-method approach in the physical and mechanical assessment of lava rocks with distinct microstructure

Abstract

Volcanic rocks are common geological-geotechnical units with several heterogeneities. Their microstructural influence on deformation remains unexplored in engineering contexts. This study presents a comprehensive assessment of lava rocks ranging from basalts to trachytes, comprising massive and vesicular textures, varying degrees of fracturing and distinct pore size distributions. A multi-method approach combining X-ray microtomography, thin section analysis, and mercury intrusion porosimetry allowed to characterise the pore space and crystals at distinct scales and define fluid transport mechanisms. Digital rock physics allowed for determining absolute permeability and porosity and developing novel pore network models. Mechanical tests during time-resolved 3D X-ray imaging allowed for fracture development visualisation on varied lavas. Additional physical and mechanical properties were determined following international standards.

Results show that porosity (4 %-29 %) is primarily influenced by pore size and connectivity rather than vesicle shape or orientation. Macrovesicles (> 15 μm) are sub-spherical, while low sphericity indicates vesicle coalescence and fractures, which reduce rock strength and stiffness more than the feldspar content. Vesicular specimens (effective porosities above 10 %) have a permeability controlled by the vesicle size, while the number of microcracks and porosity define permeability in both massive and fractured lavas.

This study highlights the importance of microstructural assessment as a foundation for geological and geotechnical investigations, enabling informed material selection and building conservation strategies. The adopted analysis strategy, which combines techniques often used separately, has proven effective and is suggested for extending to the study of other complex materials.