ABSTRACT
The twin circular tunnels of Naples Underground Line 1 extension were constructed at variable depth (maximum depth 45 m bgl) in various ground conditions.
A 1 km long initial stretch of the two tunnels was bored through alluvial sands and silty sands, below the groundwater table, starting from a shaft at about 17 m below the ground level. In this initial stretch the tunnels did not underpass any significant building. After this initial part the line enter into the Neapolitan Yellow Tuff, which is a soft rock usually underlying layers of sandy soils (Fig. 1). Both site and laboratory investigations were carried at the design stage. A limited overview of the average values of the deduced physical and mechanical properties of the sand is proposed in Table 1.
