Estimation of natural asbestos content in rocks by fracture network modeling and petrographic characterization

Asbestos may constitute a severe health risk when meta-ophiolites are excavated for large infrastructural projects. For public acceptance, a reliable estimation of the content of Naturally Occurring Asbestos (NOA) is necessary for the design of construction sites, workers' safety and spoil management. In the framework of a research project supporting the final design of a highway tunnel system in NW Italy, SEM-EDS (Scanning Electron Microscopy - Energy-Dispersive Spectrometry) quantitative analyses were performed to provide a direct NOA content estimation by counting and weighing the asbestos fibers in the rocks, after a chemical and geometrical characterization. The direct NOA content estimation was compared with an indirect estimation obtained through a fracture network modeling based on a structural survey on a selected outcrop and statistical analysis of a relative digital image. The fracture intensity, inferred from the fracture network model, was multiplied by coefficients deriving from the semi-quantitative estimation of the geological relations between asbestos mineral occurrence and fracture size, thickness and distribution. A good agreement between the indirect NOA estimation and the average result of the SEM-EDS analysis was obtained. Thus, the statistical analysis of the fracture network may represent a valuable support to the SEM-EDS quantitative analysis based on mineral fibers counting. However, the quality of the indirect NOA estimation depends on the postulates for inferring the coefficients describing the distribution and occurrence of the asbestos minerals within the fractures. This Note discusses the above-mentioned issues, as well as those concerning the procedure for a representative sampling of NOA-bearing rocks and fractures.