Point load testing for classification of porous effusive rocks

The choice of the ratio K between the uniaxial compression strength (?f) and the size-corrected point load strength (IS,50) for the estimate of ?f from point load tests can have significant consequences on design issues. Several Authors indicated that K ratio departs significantly from the typical suggested values, especially in weak rocks. Experimental evidences from this study confirm that significant variations of K occur even for the same material. Therefore the recourse to literature data of the K ratio for rock materials similar to those tested is possible only if the dependence of K on physical/textural characters of the rock is accounted for. With reference to a scoriaceous lava with widespread and irregularly distributed macropores, the K coefficient was found to be dependent on porosity n, similarly to other porous rocks, through an inverse (e.g. a negative power-law) function. The relationship was successively extended to the same type of rocks from worldwide, obtaining very low confidence regions for the function exponent. The present results indicate that lithology and texture strongly affect both shear and tensile modes of failure.