Experimental relationships among strength parameters in monoaxial loading conditions for porous effusive rocks

Tests on different types of effusive volcanic rocks under monoaxial loading conditions, such as uniaxial compression test (UC), point load test (PL) and indirect tensile test (B), are reported. The considered lithotypes span a wide range of porosity mostly due to macropores dispersed in a continuous ground mass. Strength values are distributed over a wide range and are mainly controlled by porosity. Results were merged with literature strength data on other effusive volcanic rocks. Outcomes from laboratory tests indicate that the ratios between the uniaxial strength (σf) and the point load strength index IS,50 and between σf and the tensile strength (σt) significantly depart from those proposed by ISRM (1985) and decrease as the porosity increases. Conversely the relationship between strength ratio σt / IS,50 and porosity was found to be more or less constant. These results suggest that macroporosity affects in different ways shear and tensile modes of failure (typical of UC and B/PL tests, respectively). This is confirmed by the evidence that porosity has virtually the same influence on B and PL strengths.