Ventilation Shafts and Drifts for the Metro Line 6 Project in Naples, Italy-Paper

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The paper describes the construction of ventilation shafts and connecting drifts for the section of Naples Metro Line 6 between Mergellina and Municipio. Located in a densely built urban area with significant architectural and archaeological heritage, the works were designed to limit effects on nearby buildings, utilities and groundwater.<br /><br />Each ventilation chamber connects to the TBM tunnel through a vertical shaft and a short horizontal drift. The 5.5 m-diameter shafts, 24–29 m deep, were excavated using the Vertical Shaft-sinking Machine (VSM). This remotely operated system excavates beneath a stabilizing fluid while precast concrete lining rings are assembled at the surface and lowered into place. It avoids sending workers into the shaft and, unlike conventional methods, does not require lowering the surrounding water table. Low-permeability jet-grouted base blankets and retaining structures further limit groundwater inflow and ground movement.<br /><br />Before drift excavation, the surrounding ground was improved according to its properties: fractured Neapolitan Yellow Tuff was treated by penetration grouting, while loose soils were treated with jet grouting. Some locations required a combination of both. The improved ground formed a stronger, less permeable shell around each drift, enabling safer excavation and reducing water inflow. Drifts were then excavated and supported with steel ribs and shotcrete.<br /><br />Monitoring of nearby structures and utilities was carried out before, during and after construction. At the Torretta site, measured building movements remained within approximately 2 mm, with no settlement trend; vibration levels were also low, with velocities generally within 0.5 mm/s.<br /><br />The authors conclude that combining VSM shaft construction, tailored ground improvement and low-permeability excavation measures enabled complex works below the water table with negligible disturbance to the surrounding urban environment, while improving worker safety.

Keywords

Naples Metro Line 6

ventilation shafts

connecting drifts

Vertical Shaft-sinking Machine

VSM excavation

ground improvement

jet grouting

penetration grouting

groundwater control

urban tunneling