Direct and indirect assessment of the amount of naturally occurring asbestos in fractured rocks

Naturally-Occurring Asbestos (NOA) may constitute a severe health risk when metamorphic rocks are excavated for large infrastructure projects. For public acceptance, a reliable assessment of the content of NOA is necessary for the design of the construction site, workers’ safety, and spoil management. Direct measurements of NOA content can be made by SEM-EDS analyses on an adequate number of samples. Such measurements are sufficiently precise but questions about the representativity of the samples may arise, especially when the rock units are structurally and lithologically complex. Therefore, in order to corroborate the direct assessments, alternative and indirect assessments can be performed by measuring the fracture volumetric porosity, provided that almost all the NOA is deposited as veins in the fractures of the rock mass. Statistical sampling and inference procedures can be applied to derive such an index. In this paper, an indirect assessment of NOA is reported and compared with the assessment obtained by direct measurements. The application is to a metamorphic rock formation through which a tunnel is to be driven as part of the ”Gronda di Genova” highway development in Genoa, Italy. Even though significant approximations are made, the indirect assessment is relatively close to the direct measurement.