Evaluation of the Performance of the ISRM Suggested Methods for Measuring Density and Porosity when Applied to Low-Porosity Rocks

Recent studies conducted in the context of geological disposal of spent nuclear fuel in low-porosity crystalline bedrock have found density and porosity values obtained following the ISRM Suggested Method (ISRM 1977) to occasionally be sporadic. To investigate this behaviour, 137 core specimens of 5 different rock types (granite, granitic pegmatoid, veined gneiss, mica gneiss and sandstone) were submerged in water under NTP conditions and measured repeatedly until the observed saturated mass was constant within the accuracy of the measurement system. These specimens were then fully dried and subsequently saturated again, this time following the Suggested Method (1 hour in water at <800 Pa vacuum) and measured again for density and porosity. Density and porosity values observed following the ISRM Suggested Method were found to be systematically lower than values observed using immersion saturation. Specimens saturated following the ISRM Suggested Method reached on average 40-60 % of the level of saturation reached with the immersion method. This resulted in significant differences in observed density and porosity values when compared to values observed after immersion saturation. Performance of the saturation method proposed in the ISRM Suggested Method should thus be studied in more detail and possibly re-evaluated.