Assessment of NOA risk for the excavation of a highway tunnel system ("Gronda di Genova", Italy): from NOA-oriented geological model to asbestos quantification

<p>For a reliable evaluation of the geo-environmental risk related to the presence of naturally occurring asbestos (NOA) in rocks excavated for large infrastructural projects, a proper procedure has to be followed in order to achieve:</p><p>1) the definition of a detailed geological model, tailored on NOA-related issues;</p><p>2) a representative sampling;</p><p>3) a reliable quantitative determination of asbestos content in rock samples.</p><p>Here we describe the approach followed for the evaluation of the NOA content for the excavation of a complex highway tunnel system (&#8220;Gronda di Genova&#8221; NW Italy), in NOA-bearing meta-ophiolite rocks. The NOA-oriented geological model has been constrained by the individuation of the main &#8220;NOA-related petrofacies&#8221; &#8212;i.e., classes of rocks with common lithological, structural and NOA content features&#8212;, and by the identification of &#8220;homogeneous zones&#8221; &#8211; i.e. geological units into which the NOA petrofacies are distributed. Implementation of Gy&#8217;s theory on sampling was used and here described to maintain statistical validity during sample processing from the primary rock sample to the analytical sample. SEM-EDS procedure for the quantitative determination of NOA content was improved with an error analysis delivering the minimum number of fibers to be measured to achieve the best analytical results.</p><p>The obtained results allowed the prediction of the NOA hazard in terms of risk zonation along the tunnel section and for the evaluation of the amount of asbestos-bearing spoil to be excavated and managed.</p>