Soil Improvement for Ventilation Adits at the Naples Subway Line 6-Paper

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The paper describes ground improvement for three ventilation adits connecting shafts to the Naples Subway Line 6 tunnel. The adits, 5–15 m long, were to be excavated beneath groundwater pressures equivalent to 18–23.5 m of water head, requiring measures to control inflow and protect nearby buildings and utilities.<br /><br />At Santa Maria in Portico, the adit was excavated in fractured Neapolitan Yellow Tuff. Multi-packer sleeve-pipe grouting injected cement-based and silicate-based mixes into the rock’s cracks, forming a stronger, less permeable shell. This approach proceeded as designed.<br /><br />At Torretta and Vittoria, where the ground included loose soils, the original design relied on surface-installed jet-grouting columns, combined with grouting from the shaft where needed. A field trial was positive, but during work at Torretta, jetting caused repeated soil removal, underground chimneys and surface ground collapses. These events created a serious risk of instability for the drilling rig and raised concerns about effects on adjacent structures.<br /><br />The design was therefore changed: jet grouting was replaced at both sites by artificial ground freezing (AGF). Freeze pipes installed from the shafts formed a frozen, impermeable shell around each adit. Liquid nitrogen was used for both freezing and temperature maintenance; monitoring instruments tracked ground temperatures and water pressure. The frozen shells reached closure after five days at Torretta and 12 days at Vittoria, and were maintained during excavation.<br /><br />The revised approach enabled safe, nearly dry tunnelling while avoiding adverse effects on nearby structures. The case demonstrates how construction-stage observations can prompt design changes that better address unexpected ground conditions.

Keywords

ground improvement

ventilation adits

artificial ground freezing

jet grouting

multi-packer grouting

Neapolitan Yellow Tuff

groundwater pressure

Naples Subway Line 6

tunnelling

soil stabilization