Strength behaviour in monoaxial loading conditions in effusive rocks: the influence of porosity

Relations between strength index properties from monoaxial tests (uniaxial compression, point load and indirect tensile tests) are extensively used in engineering practice and are object of suggestions by ISRM (1985). The results of an extensive test campaign, that investigated strength and static/dynamic deformability of effusive volcanic rocks under monoaxial loading conditions are presented. The tested lithotypes span a wide range of porosity (n = 1 - 50 %) and consist of an aphanitic groundmass with interspersed macropores and rare minute phenocrysts. The microstructure was observed through optical microscopy, scanning electron microscopy and 3D computed tomography images. The analysis of test results and literature data on similar lithotypes indicate that porosity controls quite strictly both the strength parameters and the ratios between them. The inadequacy of the ratios between point load and uniaxial/tensile strengths proposed in the suggested methods is apparent for n>10%. Some clues on the failure process were highlighted through theoretical and microstructural models in which the strengths obtained from different monoaxial tests are related to porosity